**Fig. 55** Lateral view of the collimator OGSE and the TVC. Although not clearly visible in this drawing, the TVC is only supported by two horizontal bars connected to the OGSE structure. The trolley below the TVC supports the pumping system and it is not in contact with the ground when connected to OGSE. The trolley is also used to support the TVC only when it needed to be disconnected from the OGSE). On the left side, the TVC counter-balance is also visible

<figure><img src="image_57.png" /></figure>

TVC was disconnected from the lower part. In this way the OGSE and the TVC could be decoupled from the ground and all the other environmental vibration, including the vacuum pumps and the chiller of the TVC (Fig. 55).

The TVC was equipped with a sliding baseplate not controlled in temperature. The JANUS OHU and PEU were mounted on top of this baseplate with a dedicated interface made from invar. This additional baseplate was thermally decoupled from the TVC baseplate and temperature controlled by means of three conductive interfaces with one dedicated heating control loop for each one of the three. Another conductive interface was connected to the FPA cold finger and also in this case a control loop with one heater was implemented. All the conductive links were connected directly to the TVC shroud. The JANUS MEU was not included in the TV chamber but it was placed outside: this did not impact the performances of the instrument since the signal from the FPM is completely digitalized within the PEU.

The TVC shroud could be controlled within the temperature range +70 / -65 °C with a temperature stability of ±1 °C / 3 h and temperature uniformity of ±3 °C. The TVC could reach a pressure of 10⁻⁵ mbar at room temperature and was equipped with a cold trap cooled below 90 K.

## 4.3 Performance Test Results

### 4.3.1 MTF

The estimation of the absolute MTF value of the JANUS camera was a very challenging activity. To predict the flight model global MTF value, the MTF value obtained with the slanted edge method described in Sect. 4.1.2 must be purged of several contributions that are not negligible at the Nyquist frequency. The contributions considered in the analysis are listed below and detailed in following sections:

1. the OGSE collimator transmitted wavefront error which was comparable to the aberrational content of the telescope;

2. the elastic deformation of the primary mirror surface due to the gravity release;
<figure><img src="image_58.png" /><figcaption>**Fig. 56** Interferometric measurement of JANUS transmitted wavefront error in the standard configuration (left) and in the configuration with the instrument rotated by 90 degrees (right). In the instrument coordinate reference frame, gravity acts along different directions in the two measurements. This affects the deformation of the primary mirror optical surface due to its weight and this effect is captured by the Zernike analysis which reveals a difference of about 0.22 annular fringes for both the astigmatic coefficients in the two cases</figcaption></figure>

3. the difference between the emission spectrum of the lamp used to perform the measurement and the spectrum of the science scenario;

All these contributions were removed by analysis with the procedure described below.

**OGSE Contribution** The OGSE used for testing JANUS had a WFE of about 30 nm RMS on the dimension of JANUS pupil (116 mm). Considering that JANUS optical quality is very close to the diffraction limit, with values of WFE between 34 and 51 nm RMS, the OGSE WFE aberration could not be considered negligible. The OGSE WFE was characterized both at ambient conditions and in TVC conditions by exploiting a double pass interferometric measurement. The measurement was performed by placing a TRIOPTICS FISBA interferometer at the focal point of the collimator and a thick flat service mirror inside the TVC chamber. This setup allowed evaluation of both the WFE contribution of the OGSE collimator and of the TVC chamber window.

**Gravity Release Contribution** The effect of gravity release affects the optical elements of the instrument by altering their inter-distances and deforming the optical surfaces. Our analysis shows that the dominant contribution affecting the optical performances derives from the deformation of the primary mirror optical surface under the effect of its own weight. This effect has been simulated by means of a FEM analysis and the deformed sag has been imported into Zemax™ where its evaluation in terms of Zernike polynomials has been carried out. In particular, the analysis predicted that, because of the gravity release, the astigmatic contribution to the wavefront error should be modified by increasing the Zernike coefficient C₆ by 0.22 annular fringes. This result was confirmed experimentally with a double-pass interferometric measurement during preliminary tests performed on the optical engineering model. The interferometric tests exploit the scheme already described in Sect. 4.1.1 and the instrument WFE was acquired with the telescope placed in two different geometrical arrangements, the first being the “normal” arrangement and the second by rotating the instrument (and the FISBA interferometer) by 90 degrees. The 90 degrees rotation made the weight force acting along the vertical axis to vanish, allowing a “0g” test of performance on that axis. As shown in Fig. 56, the measurement revealed a variation of 0.216 annular fringes for the Zernike coefficient C₆, a variation that is in excellent agreement with the FEM predicted value.
<figure><img src="image_59.png" /><figcaption>**Table 19** In-flight estimation of JANUS MTF performance at the Nyquist frequency (71.4 cy/mm) on F1 (PAN) filter (transmission range 400-900 nm) for three different instrument operative conditions</figcaption></figure>

<table><thead><tr><td>Temperature case</td><td>M1 temp.</td><td>Focus position</td><td>MTF V</td><td>MTF H</td></tr></thead><tbody><tr><td>Performance COLD</td><td>-30 °C</td><td>21 µm</td><td>15.0%</td><td>14.4%</td></tr><tr><td>Performance MIDDLE</td><td>-24 °C</td><td>-16 µm</td><td>16.1%</td><td>15.9%</td></tr><tr><td>Performance HOT</td><td>-16.6 °C</td><td>-1 µm</td><td>17.3%</td><td>17.3%</td></tr></tbody></table>

**Spectrum Contribution** The final step in order to obtain the MTF value purged from all the unwanted contributions was the spectrum correction. As shown in Fig. 57, the expected in-flight scenario presents an average spectrum with a very low component in the near infrared region (where the instrument has lower MTF performances) and a peak centred around 500 nm where the instrument has a significantly higher MTF value. On the contrary, the spectrum of the halogen lamp used for testing had an opposite trend, with low intensity in the UV/VIS region and a peak emission in the NIR. This different spectral distribution strongly affects the final value of the polychromatic MTF in the range 400-1100 nm with a performance expected to decrease as the IR contribution in the spectrum increases.

**MTF Estimation Procedure** As mentioned above, the estimation of the instrument MTF in flight conditions has been evaluated starting from the slanted edge MTF curves acquired in thermo-vacuum condition and taking into account the OGSE, the gravity release and the spectrum contribution. As a first step, the complete setup (JANUS + OGSE) has been simulated on Zemax™ and the experimental aberrational contribution of the OGSE has been inserted as a Zernike phase retard surface. The Zernike terms of an annular phase retard surface, which simulates JANUS transmitted wavefront error, have then been adjusted in order to reproduce the experimental MTF curves. In this “fit” we assume that JANUS is affected only by astigmatic aberration (an assumption well supported by interferometric measurements), a wavelength-independent detector MTF = 0.30 (spectral average value on detector MTF measurements) and a spectrum given by the convolution of the lab white lamp, the detector quantum efficiency and the OGSE + JANUS transmission in the range 400-900 nm. From the value of this “fit” that reproduces the configuration JANUS + OGSE, we removed the OGSE contribution obtaining the JANUS transmitted WFE. After this step, the MTF was evaluated using the Ganymede spectrum in place of the lamp spectrum. Finally, the gravity release contribution was included by increasing the JANUS astigmatism coefficient C6 by 0.22 annular fringes and the MTF is evaluated at the focus position corresponding to the various operative temperature cases. Table 19 reports the MTF estimation at the end of the analysis for three different operative scenarios of the instrument.
### 4.3.2 Other Performance Results

Comprehensive analysis of JANUS ground calibration and performance measurements is out of scope for this paper and will be presented elsewhere. Here we present a summary of performed measurements and some preliminary results.

The aim of instrument level calibration was to characterize instrument behaviour and to confirm its performances vs. the expected and assumed values. These calibrations also give as output the calibration parameters, to be compared with an instrument model based on performances measured at sub-system level. Results have to be confirmed and updated through in-flight measurements.

The H/W configuration for calibration at instrument level included the entire JANUS instruments (OHU, PEU and MEU) in flight configuration, with flight harness; the MEU had to be substituted by a fully equivalent qualification model due to programmatic constraints. The thermal vacuum chamber described above allowed tuning of the temperature in the full operational range.

JANUS calibration at instrument level was focussed on the calibration of the following parameters:

* Boresight vs. Optical Reference System (given by an optical cube), including FOV orientation

* Field of View (FOV) and optical distortion

* Point Spread Function (PSF) and MTF (already discussed in section)

* Relative Response (including flat field, Non Linearity and Non linearity Non Uniformity)

* Spectral calibration

* Absolute Radiometric calibration

In addition, characterization of the following effects is complementary to calibrations, as it provides further information on system performances and data analysis:

* Straylight

* Ghost

Finally, the following FPM-level calibrations have been performed:

* Fixed Pattern (FP)

* Dark Current (DaC)

* Readout Noise (RON)

* Defects

* Detector Full-well

For most of the listed measurements, dependence on temperature was checked and measured, if present.

The objective of the radiometric calibration activity was to determine a univocal function able to retrieve the radiant quantity of interest from the instrument output (Wyatt 1978). For an imager, the interest is focused on the radiance, a quantity useful in the description of extended sources, but also the irradiance can be used for unresolved observations. The final product of the activity should be then an Instrument Transfer Function (ITF) for each pixel. This ensemble of ITFs will be then inserted in a dedicated pipeline for the conversion of the raw images (in arbitrary units from the ADC output) into calibrated images (in physical units). We choose to have an ITF for each pixel instead to calibrate using a flat-fielding approach. The selected strategy has been considered more accurate because in a typical
<figure><img src="image_60.png" /><figcaption>**Fig. 59** Histograms reporting the number of pixels vs. values of parameters of the quadratic fit for instrument signal vs. radiance × integration time. Curve widths shows the different pixel behaviour, in particular for one of the detector tile</figcaption></figure>

<table><caption>**Table 20** RON and DaC values averaged on the detector at different working temperatures and at ambient</caption><thead><tr><th>Detector Temperature [°C]</th><th>Read Out Noise [DN]</th><th>Dark Current [DN/s]</th></tr></thead><tbody><tr><td>-43.1</td><td>2.55</td><td>0.037</td></tr><tr><td>-38.3</td><td>2.56</td><td>0.056</td></tr><tr><td>-37.4</td><td>2.57</td><td>0.058</td></tr><tr><td>+21.0</td><td>2.55</td><td>19</td></tr></tbody></table>

observations (e.g. faint unresolved small satellites, rings and other faint sources). The value at 21 °C, a condition that will never occur in space, is compatible with the expectation of DaC doubling every about 7 °C increase of detector temperature.

## 5 Expected Instrument Capabilities Based on Performances

In order to facilitate the evaluation of JANUS operational capabilities and data quality at different science targets, a radiometric model was developed and its results compared with the data acquired in clean room. The equation we used to compute the expected signal from a given input radiance from an extended source is (Da Deppo et al. 2016; Zusi 2009):

$$DN = \int_{\lambda_i}^{\lambda_f} L(\lambda) \cdot A\Omega \cdot T(\lambda) \cdot QE(\lambda) \cdot \frac{\lambda}{hc} \cdot \frac{1}{g} \cdot t_{exp} \cdot d\lambda$$

where L(λ) is the input spectral radiance, A is the input area (that shall consider the obscuration of the primary mirror due to the presence of the secondary mirror), Ω is the solid angle
<figure><img src="image_61.png" /><figcaption>Fig. 60 ITF as a function of wavelengths for all 13 JANUS filters</figcaption></figure>

defining the pixel field of view, i.e. IFOV², T(λ) is the system spectral transmittance, given by the combination of the two mirrors reflectance and the transmittance of the three correction lenses, the filter and the detector window. QE(λ) is the detector quantum efficiency in γ/e⁻, measuring the efficiency of the conversion of photons into electrons, λ/(hc) converts the energy in number of photons, g is the conversion factor, in e⁻/DN, of photoelectrons in DN by the readout circuitry and ADC; finally, t_expo is the integration time used. The integration limits λᵢ and λⱼ have to be properly selected and can only be limited to the filter transmission window if the out-of-band contribution to the signal is negligible. Due to detector characteristics (Sect. 3.3.2), the pixel fill factor is 1 and is not considered in this equation.

We can consider the Instrument Transfer Function – ITF(λ) defined as:

$$ITF(\lambda) = A\Omega \cdot T(\lambda) \cdot QE(\lambda) \cdot \frac{\lambda}{hc}$$

Based on instrument parameters, measured at sub-system level, ITF(λ) is given in Fig. 60.

The radiometric model showed to be in good agreement with calibration measurement performed on ground, apart for the non-linear detector behaviour (Fig. 58), and was used to evaluate the JANUS signal and SNR for the targets to be observed. As a source of noise in the evaluation of SNR we considered the RON, the DC and the shot noise, that appeared to be the dominant contribution already at low (≤1000 DN) signal levels.

Preliminary study of JANUS – target distance based on the expected JUICE trajectory, still to be frozen, after Jupiter orbit insertion (JOI) is reported in Fig. 61. Apart from Jupiter and the three icy moons, whose angular dimensions will exceed many times JANUS FOV at closest approach, it will be possible to perform resolved observations of Io and the small inner moons. The other small moons will have a subpixel dimension in JANUS images.

In order to estimate the radiometric performances in Jupiter system, the radiometric model was coupled with software modules to consider for every specific observation:

* S/C trajectory with respect to target body, which defines the observation geometry and gives constraints on operations (e.g., incidence and emission angles, usable integration
**Fig. 61** JANUS targets dimensions (circles) are plotted vs. the minimum distance from surface allowed by JUICE's orbit around Jupiter system. The lower continuous red line is the pixel projected at that distance, i.e. the target dimension corresponding to one pixel. The two green upper lines give the FOV projected dimensions at distance (the detector has a rectangular shape). Targets above those two lines are larger than JANUS FOV, between the upper lines and the lower one are smaller than FOV, but still above the resolution limit. Finally, targets below the lower line are below the resolution limit and will appear as point sources in JANUS observations

<figure><img src="image_62.png" /><figcaption>**Table 21** SNR for the most used filters on icy satellites surfaces for a typical fly-by</figcaption></figure>

<table><thead><tr><th>Pixel scale range [m/pixel]</th><th>SNR FPAN</th><th>SNR FBLUE</th><th>SNR FGREEN</th><th>SNR FRED</th></tr></thead><tbody><tr><td>500-370</td><td>150</td><td>150</td><td>150</td><td>150</td></tr><tr><td>200-40</td><td>150</td><td>150-120</td><td>150</td><td>150</td></tr><tr><td>40-15</td><td>110</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

time to avoid image smearing) based on the required observation parameters (binning, filters);

* environmental conditions during the different phases (e.g., planet or satellite albedo): for the targets described in the following subsections, the full-disk albedo spectra described in Karkoschka 1994, have been used.

Examples of expected SNRs are given in Table 21 for some targets and relevant observation modes. Due to the detector full well, the maximum SNR that can be obtained is about 150, i.e. a SNR of 150 means that we can integrate up to signal saturation.

**Satellite Fly-by** One of the two Europa fly-bys was considered as representative of all the satellite fly-bys, implying raster observations from distance, without strong constraints on integration time due to slow target relative motion, up to push-frame observations close to closest approach, when the high relative velocity limits the integration time and the possibility to switch filters while obtaining continuous coverage along the ground track. With maximum SNR of about 150, limited by the detector full well and signal shot noise, tuning the integration time to avoid smearing, the SNR we expect for the main broad-band filters (F1-F4) remains close to maximum for pixel scale down to 50 m/pixel. It remains above 100 for FPAN and pixel scale of 15 m/pixel that is obtained from 1000 km distance.
<table><caption>**Table 22** Integration time in seconds to reach the given SNR for the two filters centred on methane absorption bands and its continuous</caption><thead><tr><th rowspan="2">Filter</th><th colspan="2">on the equator, at noon</th><th colspan="2">mid latitude, at 10:00 or 14:00 local time</th></tr><tr><th>SNR = 100</th><th>SNR = 150</th><th>SNR = 100</th><th>SNR = 150</th></tr></thead><tbody><tr><td>MT strong</td><td>0.42</td><td>0.92</td><td>0.55</td><td>1.2</td></tr><tr><td>CMT strong</td><td>0.12</td><td>0.28</td><td>0.18</td><td>0.38</td></tr><tr><td>MT medium</td><td>0.07</td><td>0.15</td><td>0.09</td><td>0.22</td></tr><tr><td>CMT medium</td><td>0.025</td><td>0.055</td><td>0.035</td><td>0.075</td></tr></tbody></table>

**Ganymede Orbit** During Ganymede orbital phase at about 5000 km from surface the reach- able SNR is strongly dependent on Sun incidence angle, so that the SNR goes from near zero (poles) to the max value already above 70 deg latitude (Fig. 62).

It is worth noting that a SNR up to 30 can be obtained also on the night side, taking advantage of Jupiter shine on Ganymede's surface.

As expected, SNR decreases for the 500 km altitude orbit around Ganymede, due to the shorter dwell time and the higher incidence angle at equator due to natural JUICE orbital plane rotation while in Ganymede orbit. Narrow tracks in Fig. 62 show the worst case when a 60 nm bandwidth filter is used at spatial sampling of 7.5 m/pixel. Although the SNR becomes lower, it would be possible to observe targets in high resolution with several filters to look for colour/composition variations on local scale. SNR can be improved up to about 100 at low latitudes by using the panchromatic filter (which is the nominal filter for GCO500 operations) or with other filters relaxing the pixel scale.

**Jupiter Atmosphere** Evaluating the SNR, the main limiting factor for a typical observation from about $10^6$ km is the fast planet rotation, for which the JANUS projected IFOV of 15 km is swept in about 1.2 s. Table 22 reports the needed integration time to reach SNR 100 and 150 in the most used filters for Jupiter atmosphere observations. Considering a dwell time of about 1.2 s, and recommended maximum integration time of 0.3 s, observations in the strong methane absorption band shall be constrained in SNR.

# 6 Instrument Observation Strategy, Capabilities and Data Management

To fulfil the JANUS and JUICE science goals, observational strategies have been developed for the different phases of the mission. Some of those observational strategies are specific to a target, others are common to different target bodies.

## 6.1 Jupiter Observations

Several observational strategies will be used for Jupiter observations. The most important are summarised here.

* Vertical Scans: Vertical scans of the planet will be used to build full maps of the planet near perijoves as it rotates over one (10 hr) or two full Jupiter rotations (20 hr). These maps will be obtained by moving the spacecraft Z-axis (the one aligned with JANUS boresight) in the N-S direction of the planet with an appropriate time cadence. Each vertical scan will contain part of the limb to improve image navigation by accurately navigating those
<figure><img src="image_63.png" /><figcaption>**Fig. 62** SNR for the red filter (F4) in two different observing conditions: thick track refers to one circular orbit around Ganymede at 5000 km altitude, orbital period 12 h, pixel scale 150 m with 2 × 2 binning, swath 150 km. The integration time upper limit has been fixed at 1/4 dwell time, about 200 ms. Sun incidence is 45° at the equator. Narrow tracks refer to two circular orbits around Ganymede (500 km altitude, orbital period 3.1 h, pixel scale 7.5 m, swath 15 km). The integration time has been fixed at 1/2 dwell time, about 2.5 ms (implying that only limited smearing is allowed along track). Sun incidence is 66° at the equator. SNR above 40 can be only reached at mid latitudes</figcaption></figure>

frames and referencing other images to the large-scale features. These scans will be obtained on the illuminated fraction of the disk at phase angles below 90°. Figure 63 shows an example of such an observation. We anticipate that about 50 such global maps will be obtained over the Jupiter tour with only a few cases in which two consecutive maps will be obtained.

* Snapshots: Observations at typical spatial sampling of 10-15 km/pixel of particular features observed twice with a time separation of about 30 min. These observations will investigate dynamics of fast evolving features such as convective storms and atmospheric waves.

<figure><img src="image_64.png" /></figure>
<figure><img src="image_65.png" /><figcaption>**Fig. 63** Mapping Jupiter with vertical scans. JANUS will obtain vertical scans of the planet as the planet rotates building nearly full maps of the planet's atmosphere. The example above is for perijove 12 well before maximum approach. Such maps at spatial resolutions of 20 km/pixel separated by one Jupiter rotation would produce global wind results with accuracies of 0.6 m/s. Higher precisions on wind measurements will be obtained over selected targets observed at higher spatial resolutions near 10 km/pixel near the perijove</figcaption></figure>

* **Feature tracking:** Observations at typical spatial sampling of 10-15 km/pix of particular features observed twice over 10 hr. These observations will target specific features of interest. Examples are the Great Red Spot, large cyclones and anticyclones, the equatorial dark projections known as hot spots and associated equatorial plumes.

* **High phases:** Observations in 10 filters (all except panchromatic, F1, Sodium, F6, and Hα, F13) at a variety of spatial resolutions targeting specific phase angles close to 90, 120 and 150° to study the phase function of aerosols and the vertical cloud structure.

* **Limb:** Observations of the limbs of the planet under different illumination conditions at spatial sampling close to 10 km/pixel and targeting the equatorial and polar regions as well as the limb views of upper hazes in vortices such as the Great Red Spot.

* **Lightning:** JANUS will attain partial and full maps of the planet's night side at moderate resolutions (50 km/pixel) and partial views of the night-side at spatial resolutions of 10-30 km/pixel. These observations will allow for statistics of lightning activity and will also determine the sizes of the lightning spots. These observations will be compared with day-side views of the same locations to understand moist convective processes in the planet. Exposure times and filters include Hα, Panchromatic and Blue filters with exposures of a few seconds to 112 s depending on the science goal (maximizing statistics of lightning distribution versus determining lightning depth). These observations will be obtained mostly at phase angles above 120°.

* **Auroras:** Auroral emissions should be observable in most JANUS filters although their maximum brightness is expected at the Alpha filter. Optimum exposure times depend on the amount of diffuse light from the sunlit crescent and high phase angles above 120° are preferred. Observations will be attained of auroral structures and Io's flux tube at spatial sampling of 15 km/px at a variety of temporal cadences studying the dynamics of auroral processes through the long time-base provided by the Jupiter tour.
## 6.2 Callisto, Europa, and Ganymede Fly-bys

JUICE will perform several fly-bys of Ganymede, Europa and Callisto. The main observational strategies developed to achieve the JANUS science goals are:

* Limb: Limb observations in different filters are key for the study of the exosphere. Emission lines of interest are O (557 nm and 630 nm), Na (590 nm), closest to UV where there are many molecular emissions, and Hα (656 nm).

* Geological and compositional mapping: images of the illuminated surface are pivotal to investigate these bodies with respect to the endogenic and exogenic processes that shaped their surfaces as well as their interaction with each other within the Jovian system and the Jovian magnetosphere. JANUS will acquire high-, medium- and low-resolution global and local mosaics (Fig. 64) to characterize the surface at different resolution. Depending on distance, relative velocity, data volume availability, the largest possible number of filters will be used, to ensure the largest wavelength range in order to detect slope changes in the spectra of the satellites' surfaces.

## 6.3 Ganymede Orbital Phase

Three phases of the JUICE mission are devoted to the characterization of the Galilean moon Ganymede. After orbit insertion, JUICE will orbit around Ganymede initially on high orbit which evolves from highly elliptical to circular to elliptical again, with the circular phase allowing surface distance of about 5000 km (GCO5000). Lower circular orbits will follow at altitudes of 500 km finally down to 200 km (GCO500 and GCO200), before the end of mission impact on satellite's surface. During GCO5000, JANUS will perform global mapping of Ganymede using at least four filters with ground sampling better than 400 m/pixel and the capability, only limited by data volume available, of 75 m/pixel. In the GCO500 and GCO200, instead, local mapping at high resolution at 7.5 m/pixel and down to 3 m/pixel and stereo observations will be performed. Although the SNR is expected to decrease due to the shorter integration time to avoid smearing and higher Sun incidence angle, the lowest orbit will allow the best resolution and the study of surface morphology with great detail on local scales.

## 6.4 Io

The JANUS capabilities for Io's observations can be summarized as follows. JANUS will be able to image Io at different phase angles and ground sampling from about 6 to 35 km/pixel, depending on its position in the orbit around Jupiter. Observations will include:

* Eclipse observations: Io's thermal activity observations, even at relatively coarse spatial resolution (i.e., tens of km), provide insight into volcanic processes, and temporal studies are required to capture its ever-changing nature. To this end, JANUS could observe Io from the night side or whenever in eclipse by Jupiter using the NIR (~1 micron) filter and at least another filter (in the range 0.7-0.9 µm) to constrain thermal brightness. This should be accompanied by repeated (daily to monthly) imaging of selected active volcanic features at ~10 km/px ground sampling, along with colour imaging (e.g., green, red, violet, near-IR).

* Geological and compositional mapping: Io's dayside would be key to perform geological and compositional mapping, tracking potential surface changes during the Jupiter tour
<figure><img src="image_66.png" /></figure>

<figure><img src="image_67.png" /><figcaption>**Fig. 64** Example of low-resolution pointing strategy and surface coverage that can be obtained from a Europa fly-by. Top: raster scan from distance. Bottom: surface coverage from given observation distance, combing global raster from distance, regional raster during approach followed by push-frame observations. The narrow track refers to the closest approach phase, when the spacecraft keep the nominal nadir pointing</figcaption></figure>

phase. Colour observations of dark deposits could complement MAJIS spectroscopic data in revealing the presence of orthopyroxene, which would support an ultramafic composition of the lavas, and studying the temporal evolution of sulphur allotropes. In this case, Io should be observed at low to moderate solar phase angles using different filters.

* **Jupiter-shine:** Observations of Io in Jupiter-shine will be used to search for night side brightening and colour changes associated with condensed frosts. To achieve this goal, global-scale monochromatic and repeated colour imaging (minimum 3 filters) at ground sampling of 10-20 km/px should be envisaged.
<figure><img src="image_68.png" /><figcaption>**Fig. 65** NASA Galileo Solid State Imager (SSI) observations of Io at spatial resolutions consistent with planned JANUS observations. a) Observation E6ISSRFMON02, 11.3 km/pixel at a range of 554,000 km using the 404, 559, 756 nm filters, ideal for observing surface changes in daylight; b) Observation G1ISGLOMON, ~20 km/pixel at a range of 972,000 km using the 404, 559, 756 nm filters, ideal for observing volcanic plumes on Io's limb; c) Observation G1ISIOECLI02, 21 km/pixel at a range of 1,035,000 km using the 611 nm clear filter (with Voyager context image at right), ideal for eclipse observations of Io's hot spots.
Source: NASA Planetary Photojournal PIAs 01223, 00293, 00274, respectively</figcaption></figure>

* Plumes: JANUS would be the only instrument onboard JUICE capable of detecting new plumes on Io and monitoring the existing ones. In this case, both high phase angle observations on the dayside limb and eclipse observations with a sampling better than 20 km/px should be planned. Eclipse imaging using the violet and sodium filters can provide constraints on the flux of materials escaping Io. Diffuse glows due to gases escaping will be observed with clear filter. Some plumes will be observed in auroral emissions in the darkness of Jupiter's shadow.

* Sodium observations: Concerning the sodium emitted by Io, the wide range of information collected by ground-based observatories focuses on: (i) the sodium cloud and torus around Io and its dependence on the position around Jupiter, (ii) various components of the sodium due to the different escape processes. Tracing the sodium atoms around Io is needed to understand how they interact with Jupiter's magnetosphere, the temporal variability of the sodium cloud, and the role of volcanism in the sodium cloud production. Proposed observations include imaging of Io applying a specific sodium filter and monitoring of Io's sodium cloud and its interactions with the plasma torus with a sampling better than 40 km/px.

Some examples from previous mission of JANUS planned observations are given in Fig. 65.

With the latest JUICE mission profile, by constraining altitudes over the surface less than 800,000 km or sampling better than 12 km/pix, there are 52 opportunities to observe Io, of which 4 opportunities in eclipse. The best opportunity provides a ground sampling of ~6 km/pixel. By relaxing the distance requirement to 2,000,000 km or 30 km/px, there are 187 opportunities to observe Io, including 150 in eclipse, which is optimal for monitoring hot spots and constraining the temperature of the major eruptive centres.

## 6.5 Jupiter Rings

Several types of observations are carried out by JANUS to fulfil the listed science goals:

* High phase angle (near) edge on radial profiles: High phase-angle observations of the various ring components, in form of a sequence of images that scans the rings radially when seen edge on. These images are most sensitive to the dust component of the rings
and they reveal the vertical structure. The radial structure of this dusty component can be inferred from an inversion of the edge-on profiles (onion peeling).

* Portrait of the rings: Portraits of the ring system at a variety of phase angles and a range of low (but non-zero) to moderate elevation angles. Such a mosaic gives a unique impression of the extent of the whole ring system, and it is highly diagnostic of azimuthal variations in the rings. These images shall constrain the three-dimensional structure of the rings.

* Phase function: In order to determine the ring phase function we must carry out observations of the rings at different phase angles and at different filters. At high phase angle the brightness will be dominated by the dust component of the rings. At low phase macroscopic grains (serving potentially as sources for dust) will contribute or even dominate.

* Clumps and moonlets: Observations staring at one of the ansa of the main rings covering a substantial span of the orbital period (~7 hours), making a snapshot about every 10 minutes.

* Imaging the boundary of the planetary shadow in the rings: Imaging of the ring region intersected by the planetary shadow at moderate elevation and moderately high phase. Especially for the very faint gossamer rings and the Thebe extension such images provide a unique chance to quantify the amount of dust in these rings. Even if the image suffers from stray light and of the noise, these images may exhibit a contrast between the shadowed and illuminated ring regions, which is highly diagnostic for the dust content and the radial extension of the faintest ring components.

* High phase imaging of the ring halo: Image the ring halo component and its radial boundary. These images allow to put constraints on the dynamics of charged particles under the influence of the Lorentz resonance that causes the vertical spreading, forming the halo.

* Spiral structure in the rings: Detect spiral structure in the rings, which is induced by interaction of the rings with external dust streams in form of winding vertical corrugations of the ring plane.

The typical type of pointing required for ring imaging is to track a given point in the ring system in the projected view of the planet and rings, as depicted in Fig. 66. The tracking shall be kept over the duration of the integration for each individual frame.

Given the faint radiometric nature of the rings, it is worth remembering that due to the PEU implementation the maximum integration time is about 112 s. This limits the SNR obtainable on faint extended sources for which the workaround solution is to perform multiple acquisitions. Due to very low readout noise, image stacking would be an efficient way to improve SNR, while data volume increase can be mitigated through the use of higher compression factors. However, no practical limitation is expected due to the maximum integration time limit as a $mag_v$ of about 19 at SNR = 10 can be reached in a single acquisition and we expect to have constraints from other effects like straylight, ghosts, radiation noise and density of background sources in FOV.

## 6.6 Irregular and Inner Moons

Although Irregular moons small inner moons are often categorized as “small moons”, the observation plans and strategies are different for several reasons. The two major factors are:

* JUICE comes modestly close ($< 1 \times 10^6$ km) to the small inner moons which allows disk-resolved observations and limited geologic studies, while the distances to the Irregulars are millions of kilometres, mostly even $> 10^7$ km. At such distances, the objects remain sub-pixel. For example, a ~5 km sized object appears from a distance of $10^7$ km under an angle of 0.1", while the IFOV of JANUS is 3.1".
**Fig. 66** Example for a high phase angle profile observation carried out from eclipse and $1.3 \cdot 10^6$ km from Jupiter. The rings are observed from ansa to ansa with partly overlapping frames. The direction of the center points of individual frames, as indicated by blue dots in this figure, must be tracked by the camera boresight over the exposure time of the frame. Here, the Sun is behind the planet (yellow dot). The terminator on the planet is indicated by a green line and the (south) pole by a brown dot

<figure><img src="image_69.png" /></figure>

* The rotations of the small inner moons are synchronous, while the rotation periods of the Irregular moons are widely unknown and definitely non-synchronous. Therefore, for the small inner moons, numerous short, well (in time) distributed observations close to perijove passages are the means of choice, while repeated imaging over many hours to obtain lightcurves will be the observation strategy for the Irregular-moons observations. For astrometry, all these data will be useful, and additional individual snapshots of the small inner moons from further distance and possibly during mutual events (two moons will show up in the same image) will be beneficial.

### 6.6.1 Irregular Moons

The means to get the desired information is to track the objects and image them repeatedly over many hours or even days at intervals of a few minutes to accurately sample their lightcurves. There are numerous advantages for using a spacecraft for such observations: the geometry between illumination source, observer and object is completely different than from Earth with the consequence that the Jovian Irregulars may occasionally appear up to ~8 magnitudes brighter (than from Earth) due to the proximity to the target objects (Denk and Mottola 2019). Furthermore, the solar phase angles are not limited to 12°, but might in principle reach any value between 0° and 180°. For the pole/shape task, the 180°-longitude ambiguity (Mottola et al. 2020, 2023; Kaasalainen and Lamberg 2006) will disappear if the object or the observer will hover more than about 10° to 15° above or below Jupiter's orbit plane about the Sun (Kaasalainen and Durech 2007). A major disadvantage are the limited resources – data volume and pointing times in particular – that can be allocated to those observations, which limits the number of observations that can be performed.

To obtain the lightcurve data, it is envisioned to observe most Irregular moons with the JANUS panchromatic filter at least once in the mission over a time span of about 1.5 days. These data will be used to determine synodic rotation periods. Ideally, periods for more than half of the known Irregular moons will be known at the end of 2034, allowing for statistical analyses. For a subset of these objects, about 6 to 10 lightcurve observations are intended for the pole/shape/sidereal-period determination task. The sub-spacecraft locations of the object should be spread over the “sky of JUICE” over an arc of ~90°, and at least one of them should observe the object when it is located far above or below the Jovian orbit plane. For the phase-curve task, lightcurves should be obtained at least four times at various solar
phase angles. With respect to phase range, about two-third of the Irregulars will be bright enough to be observable at phase angles > 30°, about half at >50°, and about one-third at phases > 90°. On the low-phase side, a couple of objects will be visible for JUICE at phase angles < 1°. In some occasions, broadband colour filters will be used.

The actual observation time requests are based on the object's apparent brightness, sub-spacecraft location on sky, solar phase angle, elevation above/below Jupiter's orbit plane, the requirement to avoid the Milky Way in the background (at least for darker objects), and the principal availability of the spacecraft (i.e., no conflicts with other pointing requirements). Some observations will be performed during zero-phase passages, and mutual events of two objects might be tracked. "Deep field" searches for sub-kilometre sized Irregulars that are too faint to be seen from Earth are also under consideration.

The master plan tries to put as many observations as possible in the first two orbits and the approach phase (January 2031 until April 2032) because conflicts with other tasks related to pointing time and downlink data volume are lowest in this mission phase. It is estimated that ~1/3 of the requested observations might fit in this mission phase.

The option to perform a close fly-by to one of the irregular moons is under investigation (Denk et al. 2025, this collection). A promising possibility arises during the large first orbit around Jupiter in October 2031 where Irregular moon JXLIV (Kallichore) approaches JUICE's nominal trajectory to a distance below one million km. Although the JUICE spacecraft has no autonomous tracking capability for small objects, a fly-by distance of the order of 1000 km would allow multiple objectives as: i) shape reconstruction through stereo acquisitions (Preusker et al. 2012); ii) resolved spectrophotometry to identify any surface variegation (Magrin et al. 2012); iii) identification of natural satellites (Bertini et al. 2012); iv) different geomorphological studies such as identification of geomorphological units, and their relationship to the stratigraphic record (Massironi et al. 2012), lineament extraction (Thomas et al. 2012), cratering age determination (Sierks et al. 2011), boulder formation and size frequency distribution (Küppers et al. 2012). Such a fly-by would be the second ever made on this class of objects after the fly-by of Phoebe by Cassini spacecraft on 2004 (Porco et al. 2005).

### 6.6.2 Inner Moons

Although the spatial resolution of the JANUS images will be lower than the one of the few best Galileo and Voyager images (Denk et al. 2025, this issue), JANUS images will have a superior quality due to the faster readout times of the CMOS sensor, which is especially important for sequences of colour-filter images that can be taken within seconds. This minimizes viewing geometry differences between different colour images, particularly valuable for irregularly shaped surfaces with noticeable albedo contrasts.

For JANUS data acquisition of the small inner moons, four observation categories were defined: (1) Observations of the surfaces at various sub-spacecraft longitudes at highest possible spatial resolution (disk-resolved); (2) observations of the surfaces at high latitudes (while the spacecraft is on inclined orbits; also disk-resolved); (3) observations of the surfaces at various phase angles; (4) observations for astrometry. While the spatial resolutions of the JANUS images will be comparable to the typical Galileo images, the big advantage for JUICE over Voyager, Galileo and Juno will be that complete surveys for each of these categories will be possible.

A rule of thumb is: the lower the JUICE perijove distance, the closer the approach to the small inner moons. The close perijoves will be dedicated to Jupiter observations. Therefore, the small moon observations are expected to require some quality compromises for
category-1 imaging. Nevertheless, it will be possible to do these observations at ranges between ~600,000 and 1,000,000 km, which is equivalent to JANUS ground samplings between ~9 and ~15 km/px.

The high latitude options (category 2) arise between perijove 15 (11 Nov 2032) and 28 (20 Jun 2033). For all four moons, the highest latitude that can be reached is ~40°. However, since they will not be reached during perijove passages, the distances are too large for Adrastea to resolve the object in a useful way. For Metis, there is one opportunity to observe the southern hemisphere from below $10^6$ km range. For Amalthea and Thebe, both southern and northern hemispheres are planned to be imaged from distances of ~ $10^6$ km and ~ $1.5 \times 10^6$ km, respectively.

Phase angle coverage (category 3) might be obtained from the observations of categories (1) and (2), but an additional campaign from further out where the objects are not or just barely resolved is also planned.

### 6.6.3 Astrometry of the Small Inner and the Irregular Moons

The Jovian system tour part of the JUICE mission will be the opportunity to obtain astrometric positions of the moons with a much higher accuracy than from Earth. In particular, an accuracy of a few kilometres (Cooper et al. 2015) for the small inner moons (depending on the distances at the time of the observations) should be possible. Such data will greatly improve our knowledge of the inner system. Indeed, the mutual gravitational perturbations between Metis and Adrastea induce a variation in their orbit of the order of a few km for Metis to a few tens km for Adrastea (Lainey and van Hoolst 2009). Hence, monitoring the orbits of these inner moons will allow the derivation of their density. So far, only the density of Amalthea has been derived thanks to the single targeted fly-by of the *Galileo* spacecraft in November 2002 (Anderson et al. 2005). Moreover, the physical libration of the inner moons induces a specific trend on the periapses (Borders and Yoder 1990; Lainey et al. 2023) that may be constrained from JANUS. It is noteworthy that the amplitude of the physical librations is directly connected to the distribution of the mass inside the bodies (Comstock and Bills 2003). Hence JANUS astrometric data during the JUICE tour will allow us to infer interior properties such as density and homogeneity. Combined with shape models derived from JANUS, we shall obtain a much better overview on the inner moons of Jupiter and formation history. Lastly, it has been argued that tidal dissipation within giant gaseous planets could be extremely frequency sensitive (Ogilvie and Lin 2004; Auclair-Desrotour et al. 2015; Fuller et al. 2016). As a consequence, we do not exclude the possibility of determining tidal dissipation within Jupiter at the tidal frequencies of the inner moons.

Astrometric measurements of the outer Irregular moons will be performed with the JANUS instrument in a similar way. The larger distance from Jupiter makes these moons less sensitive to the physics at play within the Jovian system. Still, two specific orbital features will be investigated. Following Emelyanov (2005), we will look for possible determinations of the mass of a few of the Irregular moons based on secular changes of their orbits. The second feature will be to look for orbital wobbling, possibly associated with putative moonlets. Such endeavour will rely on an analysis similar to that used for discovering asteroid moons with Gaia data (Liberato et al. 2022). While not yet discovered to date, the existence of moons of moons is possible (e.g., Denk et al. 2018) and would be a tremendous result for the JANUS instrument.
## 6.7 Data Management

This section briefly reports on data management during the nominal science phase, starting 6 months before Jupiter Orbit Insertion.

The data processing software is developed by JANUS team and kept updated with respect to instrument performances evolution or changes in data processing approach. It covers all aspects of converting the instrument telemetry data (science telemetry and HK) into the scientific data products (raw data, calibrated data and derived one) that will be made available to the JANUS team and released to the scientific community, following the JUICE mission data handling agreement. A list of high-level requirements for the processing pipeline is summarized below.

A common server infrastructure, equipped with a system providing version control and configuration, is in place to keep all software under version control. It also provides an easy way of configuring and installing the applicable software on multiple servers (either at JANUS team premises or at JUICE mission Science Operations Centre), providing redundancy.

Redundant servers/sites will be able to take over the generation of products if the “prime” server is down. This is applicable to the generation of raw, calibrated and derived products.

PDS4 (Planetary Data System) standard compliant raw products, GEO cubes (containing all geometrical information related to an image) and calibrated products are generated and pushed to the team server to which the JANUS team has access to. From this server, all products will be delivered (pushed) to the Planetary Science Archive (PSA). In case of contingency, upon request from the PI team, the SOC will temporarily act as “prime” to execute the TM2RAW pipeline at the SOC, and distribute (push) the raw data to the JANUS server, as well as archive the data at PSA.

All JANUS PDS4 products (raw, calibrated, derived and GEO cubes) will be made available to the JANUS team as soon as they are generated. In addition, JANUS team will be allowed to access the JANUS data at the PSA as soon as they are archived at PSA.

JANUS will deliver to the SOC all the software that is needed to generate raw data, calibrated data and GEO cubes.

Following the approach agreed and documented in JUICE mission's science management plan, and in line with ESA policy on scientific data management, science data and ancillary data produced by the JANUS instrument during the mission will be made publicly available following the PDS4 standard. Calibrated data are images calibrated in physical units (e.g., radiance) through reversible processing.

A proprietary time is guaranteed to the JANUS team to allow data validation and initial rights for scientific use, while data are archived in the ESA PSA. The proprietary period is established as 6 months from data availability in PSA; after that, data will be made available to the community together with the documentation describing the instrument, the data structure and the processing applied to the data.
Appendix A: JANUS High Level Requirements

<table><caption>Table 23 JANUS top level observational requirements as derived from the science requirement matrix</caption><thead><tr><td>JANUS REQUIREMENT ID</td><td>JANUS requirement description</td></tr></thead><tbody><tr><td>JANUS-OBS-D0100</td><td>JANUS to perform Ganymede three-color mapping with the following characteristics: spatial resolution (bulk): ~ 100 m/px, 4% coverage</td></tr><tr><td>JANUS-OBS-D0110</td><td>JANUS to perform Ganymede global four-color mapping with the following characteristics: spatial resolution: 400 m/pixel (100 m/px as a goal)</td></tr><tr><td>JANUS-OBS-D0120</td><td>Ganymede monochromatic imaging: Spatial resolution (global): ≤400 m/px</td></tr><tr><td>JANUS-OBS-D0122</td><td>Ganymede monochromatic imaging: Spatial resolution (regional): 100 m/px, 10% coverage</td></tr><tr><td>JANUS-OBS-D0124</td><td>Ganymede monochromatic imaging: Spatial resolution (local): 10 to 30 m/px, 3% coverage.</td></tr><tr><td>JANUS-OBS-D0130</td><td>Ganymede stereo imaging: Horizontal resolution (referred to images): ≤ 30 m/px, 0.5% coverage; Vertical resolution: 30 m/px.</td></tr><tr><td>JANUS-OBS-D0132</td><td>Ganymede stereo imaging: Horizontal resolution (referred to images): 5-10 m/px, 0.05% coverage; Vertical resolution: 5 m (1 m as a goal).</td></tr><tr><td>JANUS-OBS-D0134</td><td>For generation of DTM on Ganymede for RIME data interpretation, topography colocated with subsurface profiles (i.e., on anti-Jovian hemisphere) is requested with: Horizontal resolution: 150-300 m/px (100 m/px as a goal); Vertical resolution: 50-80 m/px (20 m/px as a goal). This has to be considered as a goal, provided technical feasibility and resources availability.</td></tr><tr><td>JANUS-OBS-D0140</td><td>Ganymede multiple global imaging from remote with the following characteristics: Spatial resolution: 1 km/pxl</td></tr><tr><td>JANUS-OBS-D0142</td><td>Perform multiple astrometric observations from a distance with background stars in the FOV</td></tr><tr><td>JANUS-OBS-D0144</td><td>Perform multiple observations of the same target area at < 10 m/px spatial resolution at varying orbital phases</td></tr><tr><td>JANUS-OBS-D0150</td><td>Europa three-color mapping; Spatial resolution (regional): ≤ 2 km/px</td></tr><tr><td>JANUS-OBS-D0152</td><td>Europa three-color mapping; Spatial resolution (selected targets): ≤ 0.5 km/px</td></tr><tr><td>JANUS-OBS-D0154</td><td>Europa three-color mapping; Goal: 4 or more colours, 0.2 km/px</td></tr><tr><td>JANUS-OBS-D0160</td><td>Europa monochromatic imaging with the following characteristics: Spatial resolution (bulk): ≤ 0.5 km/px; Spatial resolution (selected sites): down to 50 m/px, along ground track. Goal: 4 or more colours, 20 m/px</td></tr><tr><td>JANUS-OBS-D0170</td><td>Europa stereo imaging with the following characteristics: Horizontal resolution (referred to images): 50 m/px to 500 m/px. Vertical resolution: 50 m/px to several hundreds of meters.</td></tr><tr><td>JANUS-OBS-D0172</td><td>For generation of DTM on Europa for RIME data interpretation, topography co-located with subsurface profiles is requested with Horizontal resolution: 500 m/px (100 m/px as a goal), Vertical resolution: 50 m/px (20 m/px as a goal). This has to be considered as a goal, provided technical feasibility and resources availability.</td></tr><tr><td>JANUS-OBS-D0180</td><td>Europa exosphere imaging with the following characteristics: Spatial resolution: ≤ 20 km/pxl</td></tr><tr><td>JANUS-OBS-D0190</td><td>Callisto regional color mapping with the following characteristics: Spatial resolution (bulk): few 100 m/px, 50% coverage</td></tr><tr><td>JANUS-OBS-D0200</td><td>Callisto limb imaging with the following characteristics: spatial resolution: 1 km/pxl</td></tr></tbody></table>
<table><caption>**Table 23 (Continued)**</caption><thead><tr><td>JANUS REQUIREMENT ID</td><td>JANUS requirement description</td></tr></thead><tbody><tr><td>JANUS-OBS-D0205</td><td>Perform multiple astrometric observations from a distance with background stars in the FOV</td></tr><tr><td>JANUS-OBS-D0210</td><td>Callisto local imaging (regions of high interest) with the following characteristics: Spatial resolution (local): &lt; 50 m/pixel</td></tr><tr><td>JANUS-OBS-D0220</td><td>Callisto stereo imaging with the following characteristics: Horizontal resolution (referred to images): 50 m/px to 500 m/px. Vertical resolution: 50 m to several hundreds meters/px</td></tr><tr><td>JANUS-OBS-D0222</td><td>For generation of DTM for RIME data interpretation, topography colocated with subsurface profiles is requested with Horizontal resolution: 500 m/px (100 m/px as a goal), Vertical resolution: 50 m/px (20 m/px as a goal). This has to be considered as a goal, provided technical feasibility and resources availability.</td></tr><tr><td>JANUS-OBS-D0230</td><td>Callisto multiple global imaging from remote with the following characteristics: Spatial resolution: 1 km/pxl</td></tr><tr><td>JANUS-OBS-D0240</td><td>Jupiter methane absorption band observations with the following characteristics: Spatial resolution: 30 km/px. Multiple phase angle coverage</td></tr><tr><td>JANUS-OBS-D0250</td><td>Jupiter high resolution imaging with the following characteristics: Illumination: dayside. Spatial resolution: 16 km/px. Time separations: hours over the region of interest. Time scale: hours, days, months and years</td></tr><tr><td>JANUS-OBS-D0260</td><td>Jupiter limb observations with the following characteristics: Horizontal resolution: 200-500 km/px. Vertical resolution: 30 km/px</td></tr><tr><td>JANUS-OBS-D0270</td><td>Jupiter Global/Regional imaging with the following characteristics. Illumination: dayside and/or nightside. Spatial resolution: variable from 20 to 200 km/px. Time scale: hours, days, months and years</td></tr><tr><td>JANUS-OBS-D0280</td><td>Jupiter auroral emissions with the following characteristics: Spatial resolution: ~&lt; 100 km/px. Monochromatic / multiple colors. Time resolution: min. Time scale: days, months</td></tr><tr><td>JANUS-OBS-D0290</td><td>Jovian satellites high resolution imaging with the following characteristics: Monochromatic and/or colors. Spatial resolution: ~10 km/px. Multiple phase angles. With background stars for astrometry. Time scale: hours, days, months, years</td></tr><tr><td>JANUS-OBS-D0300</td><td>Ring system Global/Regional imaging with the following characteristics: Monochromatic and/or colors. Multiple phase angles. Spatial resolution: variable from 100 to 200 km/px. Time scale: hours, days, months and years</td></tr><tr><td>JANUS-OBS-D0310</td><td>Io activity observations with the following characteristics: Spatial resolution: variable from 10s to 100s km/px. Monochromatic / multiple colors. Time scale: days, months and years</td></tr><tr><td>JANUS-OBS-D0320</td><td>Monitor Io' sodium cloud with spatial resolutions &lt; 40 km/pixel</td></tr><tr><td>JANUS-OBS-D0330</td><td>Perform disk-integrated phase curve and lightcurve observations of &gt; 3 irregular satellites</td></tr><tr><td>JANUS-OBS-D0340</td><td>Perform multiple (~5 per object) astrometric observations from a distance with background stars in the FOV.</td></tr></tbody></table>
<table><caption>**Table 24** List of high level functional requirements. ID codes including “D” are derived from science and observation requirements, while those including “C” are newly created, based on constraints imposed by the spacecraft design</caption><thead><tr><td>Functional requirement ID</td><td>Requirement short description</td></tr></thead><tbody><tr><td>JANUS-FUN-C0010</td><td>JANUS instrument shall be composed by several functional units, each with its own S/C I/F and interconnected each other</td></tr><tr><td>JANUS-FUN-D0020</td><td>JANUS shall be able to acquire images and to properly tune spatial resolution at targets in the range foreseen by scientific requirements</td></tr><tr><td>JANUS-FUN-D0030</td><td>JANUS shall acquire at least 2 × 10<sup>5</sup> images during its space operational life. An adequate safety factor shall be considered on the top of this number to allow flexibility in instrument operation and be compatible with possible improvement in S/C downlink capabilities</td></tr><tr><td>JANUS-FUN-D0040</td><td>JANUS shall be able to associate HK information (including time) to all acquisitions performed. Together with S/C ancillary information, HK shall allow acquisition geo-location with an accuracy allowed by IFOV</td></tr><tr><td>JANUS-FUN-D0050</td><td>JANUS shall be able to compress data before TM transmission to S/C with the following capabilities:<br/>- bit packing;<br/>- reversible (lossless, error free) compression;<br/>- irreversible (lossy) compression with a variable (TC-selectable) compression factor up to 20.<br/>It shall be possible by-passing the compression module for redundancy and reliability.</td></tr><tr><td>JANUS-FUN-D0060</td><td>JANUS shall be able to perform data pre-processing before compression and TM transmission to S/C with the following capabilities:<br/>- Bad pixels removal from data (through e.g. a look-up table stored onboard)<br/>- Spikes removal from data through a TBD approach<br/>- Offset removal (through e.g. a look-up table stored onboard);<br/>- Flat field / PRNU correction.<br/>It shall be possible updating correction parameters and excluding corrections by TCs.</td></tr><tr><td>JANUS-FUN-D0070</td><td>JANUS shall perform the following type of measurements:<br/>- Frame acquisition with snapshot observations;<br/>- Tuned image size acquisitions;<br/>- Mapping observation through a combination of snapshot obtained by S/C pointing or repeated passages in following orbits (no autonomous scanning capabilities);<br/>- Image acquisition at variable rate;<br/>Acquisition of other HK data and ancillary information.</td></tr><tr><td>JANUS-FUN-D0080</td><td>JANUS shall acquire images in the spectral range of 350-1050 nm. Some sensitivity at 1064 nm is a goal to allow co-alignment with the Laser Altimeter.</td></tr><tr><td>JANUS-FUN-D0090</td><td>JANUS spectral range shall be covered by at least 11 changeable fullframe filters, including one broad-band, colours, and narrow-band filters.</td></tr><tr><td>JANUS-FUN-D0100</td><td>JANUS shall be able to acquire continuously (without gaps in coverage) all different targets foreseen in Jupiter system and in all mission phases.</td></tr></tbody></table>
<table><caption>**Table 24 (Continued)**</caption><thead><tr><th>Functional requirement ID</th><th>Requirement short description</th></tr></thead><tbody><tr><td>JANUS-FUN-C0110</td><td>JANUS main electronics (MEU) shall be able to accomplish the following tasks:<br>1. TM/TC exchange with S/C<br>2. TC interpretation and distribution<br>3. TM generation for scientific and housekeeping data<br>4. Data compression at tuneable compression factor from lossless to lossy compression<br>5. Handling communication with all JANUS sub-systems<br>6. Setting all parameters for PEU and OHU (list of setting parameters is reported below):<br>- subsystems status (e.g., on, off)<br>- FWM motion parameters and selected filter<br>- detector thermal control settings (e.g., temperature, control parameters)<br>- acquisition parameters (time of acquisition, integration time, repetition time, number of acquisition, windows number and coordinates)<br>- processing parameters in PEU (e.g., binning, clock frequency and download rate)<br>7. Set parameters for DPM<br>- processing parameters in DPM (e.g., data type marking if applicable, compression box, compression factor, data correction parameters if applicable).</td></tr><tr><td>JANUS-FUN-C0120</td><td>JANUS proximity electronics (PEU) shall perform at least the following functions:<br>1. Power supply regulation and filtering.<br>2. Visible Detector bias conditioning and filtering.<br>3. Visible Detector clocks generation.<br>4. Visible Detector Readout Sequencer.<br>5. Pixel video processing chain, including ADC (if not implemented onchip).<br>7. Perform windowing and binning function (could also be done at DPM level)<br>8. Data handling digital interface.<br>9. Memory Buffer required for the communication I/F<br>10. Address and control signals generation for the Memory Buffer.</td></tr><tr><td>JANUS-FUN-C0130</td><td>JANUS optical head (OHU) shall perform at least the following functions:<br>1. Light collection through an optical system allowing enough overall throughput<br>2. Maintaining optics mechanical stability in all operational phases under all expected thermal conditions<br>3. Providing passive or active thermal control to OHU components<br>4. Filter exchange with needed accuracy and timing<br>5. Optics and detector protection from contamination<br>6. Optics and detector protection from sunlight<br>7. Maintaining passively the optimum coupling between optics and detector<br>8. Signal acquisition</td></tr><tr><td>JANUS-FUN-D0150</td><td>The header of each image shall include temperatures, image size and all camera settings during data acquisition.</td></tr></tbody></table>
<table><caption>**Table 25** List of high level performance requirements. ID codes including “D” are directly derived from science and observation requirements, while those including “C” are newly created and are needed to meet science and observation requirements also considering the baseline instrument and mission design</caption><thead><tr><td>JANUS IRS REQ. ID</td><td>JANUS req. description</td></tr></thead><tbody><tr><td>JANUS-PER-C0010</td><td>JANUS shall acquire images with a SNR of at least 100 under the following observation conditions: Ganymede, GCO500, Tint &lt; Tdwell, PAN filter, no binning, lat &lt; 45°, beta angle = 70°; Jupiter, Tint &lt; 1 sec, MT2 (CH4 strong band) filter, no binning, lat = beta = 30°. SNR down to several tens is acceptable for special observing conditions (highest resolution, low light conditions, CA at FBs).</td></tr><tr><td>JANUS-PER-C0020</td><td>JANUS shall work with typical irradiations as specified in JS-14-09_Iss_5_rev_1, Juice environmental specification</td></tr><tr><td>JANUS-PER-D0030</td><td>IFOV shall be 15 microrad. T, RoD PFM OHU TEM, FPM</td></tr><tr><td>JANUS-PER-C0040</td><td>FOV shall be ≥ 2000×IFOV i.e. ≥ 30 mrad on across track direction and be ≥ 1500×IFOV i.e. ≥ 22.5 mrad on along track direction</td></tr><tr><td>JANUS-PER-D0050</td><td>Filters spectral characteristics shall be as follows (with central wavelength / full width in nm):<br>1. FPAN 650/500<br>2. FBLUE 450/60<br>3. FGREEN 540/60<br>4. FRED 646/60<br>5. CMT medium 750/20<br>6. Na 590/10<br>7. MT strong 889/20<br>8. CMT strong 940/20<br>9. MT medium 727/10<br>10. Violet 380/80<br>11. NIR 1 910/80<br>12. NIR 2 1015/130<br>13. Halfa 656/10</td></tr><tr><td>JANUS-PER-C0060</td><td>Global quantum efficiency shall be: &gt;60% EOL in the spectral range centre (650 nm); &gt;40% EOL in the 340-800 nm range; &gt;20% EOL (on average) in the 800-1080 nm range.</td></tr><tr><td>JANUS-PER-C0070</td><td>The JANUS instrument polychromatic MTF at Nyquist (71.4 lp/mm) shall be greater than 15% in the PAN wavelength range, with Detector MTF ≥ 0.36. MTF of 20% is to be considered a goal. The requirement applies to static MTF instrument contributions only, without considering satellite relative motion w.r.t. ground (image smearing effects) MTF contribution and pointing jitter MTF contribution. The MTF requirement shall apply to JANUS in thermal operative condition, in orbit and to End of Life (EOL).</td></tr><tr><td>JANUS-PER-C0080</td><td>The total contribution of straylight (both in-field and out-of-field) to signal shall be below 0.5% (0.1% as a goal) of signal itself, assuming uniform signal from Ganymede, GCO500 phase, beta angle = 45 and EOL conditions.</td></tr><tr><td>JANUS-PER-C0090</td><td>Ghosts contribution to signal shall be below 0.1% (TBC) of signal itself, assuming uniform source larger than FOV and EOL conditions.</td></tr><tr><td>JANUS-PER-C0100</td><td>Dynamic range shall be better than 2000 EOL.</td></tr><tr><td>JANUS-PER-C0110</td><td>Integration time range shall be from 50 microsec up to 6 minutes.</td></tr><tr><td>JANUS-PER-C0120</td><td>Integration time resolution shall be &lt; 10% in the entire range. This requirement can be relaxed for the shortest integration times.</td></tr><tr><td>JANUS-PER-C0130</td><td>Integration time accuracy shall be &lt; 0.1% in the entire range.</td></tr></tbody></table>
<table><caption>**Table 25** (Continued)</caption><thead><tr><td>JANUS IRS REQ. ID</td><td>JANUS req. description</td></tr></thead><tbody><tr><td>JANUS-PER-C0140</td><td>Acquisition time commanding accuracy shall be &lt; 1 s.</td></tr><tr><td>JANUS-PER-C0150</td><td>Acquisition time stamping accuracy shall be &lt; 0.5 ms.</td></tr><tr><td>JANUS-PER-C0160</td><td>Frame acquisition rate shall be at least 2.5 Hz when acquiring full frame.</td></tr><tr><td>JANUS-PER-C0165</td><td>Time between two acquisitions with different filters shall be 3 s at maximum (in case filters are at opposite side of FWM).</td></tr><tr><td>JANUS-PER-C0170</td><td>Windowing: the possibility to select a specific area on the frame has to be implemented. Possible constraints on image size shall allow some flexibility (e.g., at least 10 different sizes shall be possible on each direction).</td></tr><tr><td>JANUS-PER-C0180</td><td>Pixel square binning (i.e., averaging) n × n shall be managed (n = 2, 4, 8) without impacting on other performances (e.g., readout speed, repetition time).</td></tr><tr><td>JANUS-PER-C0190</td><td>Absolute radiometric accuracy shall be better than 10%.</td></tr><tr><td>JANUS-PER-C0200</td><td>Relative radiometric accuracy shall be better than 5%.</td></tr><tr><td>JANUS-PER-D0210</td><td>JANUS contribution to AKE and APE, during the mission science phase after JOI and under all environmental conditions expected during science phases with thermal I/Fs as specified in JANUS-DES-D1630 and 1640, shall be better than 15 microrad. The stability requirement can be relaxed if the actual pointing can be derived by analysis with an uncertainty better than 15 microrad. Note that the requirement on contribution to AKE and APE is applicable between in-flight boresight measurements (star field pointing) and following scientific observations (target pointing; reference cases are Ganymede - GCO500 - and Jupiter - from 106 km), and refers to all pixel in the frame. Note also that this requirement shall be applicable separately on each filter and not between different filters. Stability shall be guaranteed on a given filter after multiple FWM rotations.</td></tr><tr><td>JANUS-PER-D0220</td><td>JANUS contribution to AKE and APE, during GCO500 phase and with thermal I/Fs as specified in JANUS-DES-D1630 and 1640, shall be better than 8 microrad. The stability requirement can be relaxed if the actual pointing can be derived by analysis with an uncertainty better than 8 microrad. Note that the requirement on contribution to AKE and APE is applicable between in-flight boresight measurements (star field pointing) and following scientific observations (Ganymede pointing), and refers to all pixel in the frame. Note also that this requirement shall be applicable on PAN filter only.</td></tr><tr><td>JANUS-PER-D0230</td><td>JANUS contribution to RPE, (i.e., from 1 ms to several s, during detector integration) and under all environmental conditions expected during science phases, shall be better than 5 microrad. Note that the requirement on pointing stability refers to all pixel in the frame.</td></tr><tr><td>JANUS-PER-D0240</td><td>JANUS boresights of all filters shall be within 45 arcsec from boresight with PAN filter.</td></tr></tbody></table>

## Appendix B: COM Design Details

The COM core element is a double-cam mechanism actuated by the stepper motor whose single step is 0.3° (30°/step with a 1:100 gear ratio - see Fig. 67). The mechanism is designed to simultaneously lift (16 mm) and rotate (90°) the cover using a single actuator. When the cover is in fully closed or in fully opened position the mechanism is self-locking, meaning that in these configurations the cover remains fixed even if the motor is not powered.

The opening sequence starts with the COM in closed locked position (launch configuration) and ends with the cover in opened locked position. The cover movement is divided in two parts, first a vertical translation and then a roto-translation.
<figure><img src="image_70.png" /><figcaption>**Table 26** COM configurations during opening: switches status evolution and nominal number of motor steps needed to complete the COM opening. Closing sequence is equal to opening but in reverse order (A = Activated; N = Not Activated). *Note: the range of steps is due to Hysteresis of the switch, not of the mechanism itself</figcaption></figure>

<table><thead><tr><th>Steps</th><th>COM configuration</th><th>switch #1</th><th>switch #2</th><th>Description</th></tr></thead><tbody><tr><td>0</td><td>Closed-locked</td><td>A</td><td>N</td><td>The mechanism is closed and self-locked.</td></tr><tr><td>1 to [269-282]*</td><td>Closed-preloaded</td><td>A</td><td>N</td><td>The mechanism is moving to unlock the cover but the cover is still on the baffle and with a given pre-load force.</td></tr><tr><td>[269-282]*</td><td>Closed-cruise</td><td>A to N</td><td>N</td><td>The switch#1 changes state from A to N and the cover starts the vertical translation, no pre-load force is applied on the baffle.</td></tr><tr><td>[269-282]* to [898-908]*</td><td>Opening</td><td>N</td><td>N</td><td>During this phase two movements occur: First 3 mm vertical translation. Then 13 mm vertical translation combined with 90° rotation.</td></tr><tr><td>[898-908]*</td><td>Opened-locked</td><td>N</td><td>N to A</td><td>The switch#2 changes state from N to A, the cover is fully opened and self-locked.</td></tr></tbody></table>

The two micro-switches change state during the movement of the COM. The switch #1 is in activated state when the cover is closed laying on the OHU baffle. The switch #1 changes state from activated to not activated during the opening sequence at the beginning of the vertical translation. The switch #2 is activated only when the cover is locked in open position, this corresponds to the end of the movement.

Each opening motion is divided into four phases as described in Table 26; the closing steps are performed in reverse order.

To damp vibration during launch phase, prevent straylight and contamination entering the baffle telescope during cruise and ensure door closure, a damping o-ring has been designed and placed between the door and the baffle. The o-ring selected material is a black viscoelastic elastomer the SMACSIL VHD35-01 provided by SMAC (https://smac-sas.com/en/). In closed locked position a preload of 25-45 N is applied to the Cover to keep it tight to the baffle (see Fig. 68). In the first phase of the cruise the preload is released, hence the O-ring has no more a damping function.
<figure><img src="image_71.png" /><figcaption>**Fig. 69** Door to o-ring coupling, the CP-Ti strip ensures electrical continuity between door and baffle, and structural integrity in case of failure</figcaption></figure>

<figure><img src="image_72.png" /></figure>

The O-ring is vulcanised on the inner side of the COM and in the close configuration it rests on a Titanium (CP-Ti Grade 1) strip to prevent cold welding, ensures electrical continuity between door and baffle, and structural integrity in case of failure. The Titanium strip is vulcanised below the O-ring and fixed with two M2 screws to the cover (see Fig. 69).

The COM is provided with a Fail Safe Mechanism (FSM) to perform permanent opening of the door in case of failure of the main mechanism in any position. The FSM is totally independent from the main actuation mechanism and is intended as a recovery tool in the undesirable case that a major failure occurs to the double-cam system or to the stepper motor. The activation of the FSM is performed using an SMA actuator (Shape Memory Alloy) and the motion is driven by the elastic energy of springs. The activation time depends on temperature (for reference, with nominal supply of 28 V and starting temperature of 25 °C, activation time is ~ 50 msec; with starting temperature of -40 °C, expected in space, activation time is ~ 0.3-0.5 s).

The SMA is in charge to unlock the slider of the fail-safe mechanism described; the location of the SMA pin on the bracket is shown in Fig. 70.

The SMA is a “one-shot device”, it can be armed only manually using a dedicated tool, then it can be activated by supplying a proper voltage.

COM temperature is measured by five Pt1000 thermistors, three are located on the external surface of the COM close to the heaters (see Fig. 71), other two are mounted on the motor chassis.

COM temperature is controlled by means of two heaters (2 nominal and 2 redundant) located on the COM external surface and powered by the spacecraft in order to guarantee both the minimum operational and non-operational temperatures. The survival thermal control subsystem is composed by the three Pt1000 plus the two heaters on the COM manifold. The two heaters are magnetically self-compensating, general spec. ESCC4009/004 and ESCC3901/019 for the wires. Heaters are rectangular shaped with dimension 25 × 50 mm, and each main and redundant couple is connected in parallel, in order to feed the COM up
<figure><img src="image_73.png" /><figcaption>**Fig. 70** Location of the SMA pin on the bracket and resistant force to be applied to unlock the fail safe</figcaption></figure>

<figure><img src="image_74.png" /><figcaption>**Fig. 71** Monitoring Pt1000, identified by red arrows, are located on COM main body near the thermal regulation heater</figcaption></figure>

to 7.5 W of power and resist up to 9 MRad. They are positioned in the collar outside the double cam mechanism.

The MLI is intended to protect the COM from the outside environment. The overall MLI envelope as shown in Fig. 72 is composed of one main part overlapping in specific areas.

The MLI is designed to protect the COM in each possible configuration such as closed, open and after fail-safe activation without interfering with the FOV. The MLI total mass is 0.058 kg.

The electrical continuity between the MLI and the mechanism is ensured by two wires that are fixed on the top of the door by two M3 screws. The routing of the wires is shown in Fig. 73.

To minimize the stray-light disturbances on JANUS itself on other instruments, all exposed parts were coated with a low reflectance/high emissivity coating, the Acktar Fractal Black. The selected coating complies with the surface electrical resistance requirements because the surface resistance is less than $10^7$ Ohm/m$^2$, so it avoids plasma surface charging. Figure 74 shows the Acktar Fractal Black coated surfaces.

A labyrinth path is designed to prevent light entering the telescope field of view in case the o-ring is not perfectly pressed on the baffle. With this solution, multiple reflections on
<figure><img src="image_75.png" /><figcaption>**Fig. 73** MLI grounding routing</figcaption></figure>

<figure><img src="image_76.png" /><figcaption>**Table 27** COM lifetime test activations</figcaption></figure>

<table><thead><tr><th>Phase</th><th>P [Pa]</th><th>T [°C]</th><th>Cycles</th></tr></thead><tbody><tr><td>1</td><td>Amb</td><td>Amb</td><td>500</td></tr><tr><td>2</td><td>5e-3</td><td>Amb</td><td>500</td></tr><tr><td>3</td><td>5e-3</td><td>-45... -20</td><td>2880</td></tr><tr><td>4</td><td>5e-3</td><td>-20... +10</td><td>720</td></tr><tr><td>5</td><td>5e-3</td><td>+10... +35</td><td>920</td></tr></tbody></table>

dark surfaces (coated with Acktar Fractal Black) are needed by light rays to enter the field of view, see Fig. 75.

To demonstrate mechanism functionality in the presence of environmental loads, a life test has been executed during the thermal cycles. The 5520 activation cycles have been distributed along each cycle at different temperatures (see Table 27).

During COM opening and closing phases a failure on the COM can be detected through non-nominal switches status due to switches failure, motor failure or mechanism blocking.
<figure><img src="image_77.png" /><figcaption>Surfaces with Fractal Black</figcaption></figure>

**Fig. 74** COM mechanism in closed position and Fractal Black finished surfaces, surfaces coated with Fractal Black are blue coloured in the drawings

<figure><img src="image_78.png" /><figcaption>Surfaces with Fractal Black</figcaption></figure>

**Fig. 75** Labyrinth path to prevent light entering to the telescope

<figure><img src="image_79.png" /></figure>

In case both switches are not activated in expected open or close position, COM is in unknown position and an additional open or close command may be given and image acquisition may be used to check if light, from stars or other objects in the field of view, is entering the baffle. Depending on the results of the recovery attempt or image acquisition, a warning may be raised (COM failed only first open / close action, then succeeded), a failure of the switch is detected or a mechanism failure is identified. If it is located in the motor, redundant windings can be used, while in case the mechanism is blocked the fail safe mechanism shall be activated.

## Appendix C: FWM Design Details

The way to confirm that the wheel has reached its final position at each movement is to use 8 inductive proximity sensors, 3 + 1 main, 3 + 1 redundant, manufactured by OPTEK. This type of sensor requires a permanent magnet because it needs a magnetic field for detection.

The principle of operation is as follows: when a constant current is passed through a piece of semiconductor, a voltage difference is produced. However, when a magnetic field is applied, part of the current is deflected, creating a potential difference due to the Lorentz
<figure><img src="image_80.png" /><figcaption>**Fig. 76** Left: magnets position and orientation (red/blue spot means positive/negative polarity) on FW are shown together with filters and encoders positions. Two sets of encoders, 3 each for main and redundant electronics, allow measuring filter position thanks to the combination of their signals, as given in the two tables on the right</figcaption></figure>

force. This measured voltage is proportional to the cross product of the applied current and the magnetic field. It is then easy to characterise a sensor to obtain the B-V curve. Moreover, it can be used as an analogue sensor, associating different magnets (or magnet arrays at different distances) to each filter; we have decided to use them as switches due to the fact that we need to minimise the elements that have influence in the magnetometer on board JUICE S/C (in this case, the magnets).

For the sensors that encode the filter number, each sensor can be aligned with a magnet, in order to be sensitive to the polarity of the magnetic field. For the cam position, only one magnet is used, implying a slightly different nominal position when main or redundant position sensor is used.

The OMH3150S sensor from Optek/TT Electronics was used with neodymium N35 magnets (Ø 2 mm × 1 mm). The sensors are radiometric. Therefore, when there is no magnet in front of the sensor, the output voltage is approximately $\frac{V_{cc}}{2}$ volts. When a magnetic south pole of a magnet is faced to a sensor, the output voltage is increased; when a magnetic north pole is in front of a sensor, the output voltage decreases. Thus, by comparing the analogue output of each sensor with a trigger value, all positions of the FWM are encoded as shown in Fig. 76.

Six magnets are housed inside the cover and glued to it, and one magnet for cam sensing is also housed inside and glued to it. The magnets are oriented N-S or S-N to form a ternary number system (a code system based on three digits; 0, 1, -1).

Three main sensors (and 3 redundant ones) are located above the cover to detect the electromagnetic field of the magnets glued inside this part. They are used to know which filter is placed in the light path. One main sensor (and 1 redundant) is located in front of the cam, detecting the electromagnetic field from the magnet inside it. It is used to know when the cam has made a turn.

To ensure the repeatability of the discrete angular positions of the FW, a Geneva drive has been designed so that there is a dwell angle for each of the discrete positions of the wheel.

A disadvantage of the Geneva drive mechanisms is that the number of discrete positions is limited.
<figure><img src="image_81.png" /><figcaption>**Fig. 77** Geneva wheel mechanism (Courtesy from Wikipedia)</figcaption></figure>

<figure><img src="image_82.png" /><figcaption>**Fig. 78** Left: Driven wheel, drive wheel, pin and slot (Courtesy Mallikarjuna and Rao 2016). Right: Geneva mechanism adapted to FWM, pin and cam (courtesy of SENER Aeroespacial under IAA-CSIC and ESA contracts)</figcaption></figure>

The concept of achieving discrete angular positions is based on the intermittent gearing of the Geneva wheel. In this mechanism, for each revolution of the driving wheel A, the driven wheel B makes a quarter revolution. The pin attached to the driving wheel A moves in the slots, causing the wheel B to move. The contact between the lower part of the driving wheel A and the corresponding hollow part of the wheel B holds it in position when the pin is out of the slot. Wheel A is cut away near the pin as shown to provide clearance for wheel B as it moves. See Fig. 77.

Geneva Drive behaviour is similar to an Inverted Slider Mechanism. The difference is that the Slider Mechanism works continuously and the Geneva Drive only when the Pin is in the Slot.

In the FWM, wheel B is the cross and wheel A is the cam. The Geneva drive is designed so that there is a dwell angle for each of the discrete positions of the wheel. This means that very precise control of the motorisation is not required, and to hold the position once the pin has reached the slot's final position, the motor's detent torque acts as a locking device. The fact that the mechanism acts as a locking system during the launch is also a great advantage (see Fig. 78: when the pin is positioned above the wheel A axis, wheel B rotation is blocked and one of 13 filters is in position and locked).
<table><caption>**Table 28** Pros and cons of Flexure Mounts/Bi-pods/Monopods</caption><thead><tr><th>Pros</th><th>Cons</th></tr></thead><tbody><tr><td><ul><li>Free of slick-slip and friction effects of semi-kinematic design</li><li>Less hysteresis than rolling or sliding contacts</li><li>More robust to adverse environment effects such as extreme temperatures, vacuum, and abrasive dust</li><li>Needs very little maintenance if any</li><li>Well proven concept for space applications</li></ul></td><td><ul><li>Alignment → tolerance requirement only within a certain temperature window eligible → AIT impact</li><li>Repeatability</li></ul></td></tr></tbody></table>

<table><caption>**Table 29** Pros and cons of Kinematic Mounts</caption><thead><tr><th>Pros</th><th>Cons</th></tr></thead><tbody><tr><td><ul><li>High stiffness</li><li>High/Very high repeatability (1 µm)</li><li>Small Z-dimension design possible → thermal alignment stability could be maintained within AIT and ops- temperature range</li></ul></td><td><ul><li>Hertz-contact stress between ball and groove could be very high → triggered by: screw preload force to hold the baseplate in place (within dT and QS loads) → higher ball diameter → contrary to small z-dimension</li><li>Principle based on slick-slip and friction effects → may need to be qualified since no lubricants are used</li><li>Proof of concept qualifications necessary</li></ul></td></tr></tbody></table>

## Appendix D: FPM Design Trade-Off

A detailed discussion of the FPA design and its interface to the OHU was necessary at the very beginning of the design process due to its large influence on the resulting design. In order to avoid thermally induced stress and strain, leading to defocus or even failure, the FPM mechanical interface should follow the principles of kinematic design (also referred to as “exact constraint design”). Following this principle, “the number of points of constraint should be equal to the number of degrees of freedom to be constrained”.

The following most common a-thermal designs have been studied for mechanical interfaces of FPM – optic wall, but also FPM – detector PCB:

* Kinematic coupling

* 3-vee-mount

* flexure mounts /bi-pods/monopods

* blade flexures

* bipod flexures/Monopods

* combination of flexure mounts/kinematic mounts

**Flexure Mounts/Bi-pods/Monopods:** the identified advantages and disadvantages of the flexure mounts in view of the FPA design are listed in Table 28.

**Kinematic Mounts:** For the kinematic mount the relevant arguments for design are listed in Table 29.

As mentioned above both concepts and combinations of it has been studied. For the kinematic mount the most popular type the so-called “Maxwell mount” or “Three V” mount has been analysed. This mount consists of three spheres resting in V-shaped grooves which six contact points uniquely define the mounted component with respect to its counterpart. The coupling centroid is defined as the intersection of the three angle bisectors formed by
the three coupling elements. The three spheres are connected to the V-mounts by using titanium screws that shall be preloaded such all parts remain tightened under all types of loads experienced during the mission, e.g. transportation and launch loads as well as the cool down of the unit. In fact, the titanium bolts have a higher CTE than the connected parts and, consequently, their contraction induces additional load at the interface. The analysis of the contact stress and deformation between sphere and V-groove by the non-linear Hertz theory showed that even with the maximum, for the design possible sphere radius, no positive margin of safety could be reached.

As a second option a bipod-flex design was analysed. The focus of this design was to maintain the FPM optical alignment stability of ±1 µm over a temperature range of 20 K, although considering all other design drivers as mentioned above. The design approach of athermalization was chosen in order to design mounts and structures to compensate the effect of temperature changes.

The design approach includes different steps as:

* material selection

* find a material combination such a minimum alignment shift could be reached

* perform a first mechanical analysis → adapt design if needed

* perform shielding analysis → adapt shielding thickness

* perform thermal analysis adapt heat path

* perform mechanical analysis

A suitable material selection is critical to achieve the requirements and design goals addressed above. Since beside the optical performance also other consideration has to be taken into account (e.g. radiation shielding) it was assumed that an athermal design could not be achieved by using only one material as e.g. for IRAS satellite by NASA.

Based on the eligible materials' thermal conductivity vs. linear expansion (see e.g. Ashby et al. 2018), the following candidate materials have been investigated for the FPM mechanical design:

* Aluminum 6061-T6

* Invar 36

* Inconel 718

* Ti-6Al-4V

* Kovar

* Molybdenum

* Pyralux AP (Polyimide)

* FR4

In terms of low weight, a material with high specific stiffness is the most advantageous. Three materials do particularly stand out considering this property: Beryllium ($E/\rho \approx 164$) and carbon fibre reinforced plastic (CFRP, $E/\rho \approx 50-300$) and silicon carbide (SiC, $E/\rho \approx 130$), but have been excluded for the FPM. Beryllium has been discarded due to high toxicity and difficulties in handling during AIT resulting therefrom. CFRP due to its anisotropy, the high hygroscopic behaviour and difficulties with inserts at any attachment points and SiC due to its quite complex and expensive production but also high thermal conductivity.

In order to find the right material combination for an athermal design a 1D analytical calculation has been performed. However, after simulating the design with a first FEA the error in the resulting alignment stability over temperature was found to be unacceptable for the 1D calculation. Thus the design was optimized using FEA only. Seven different designs have been simulated with an increasing level of detail but also varying configurations, e. g. different flex mount lengths.
<figure><img src="image_83.png" /><figcaption>**Fig. 79** Polynomial fit of CTE temperature dependence of different materials used for FPM thermally stable design</figcaption></figure>

<figure><img src="image_84.png" /><figcaption>**Fig. 80** Design solution to keep constant the detector position with respect to the FPM mechanical interface. As the length Ln of different elements varies with temperature, the total length Ttot shall be kept constant. The FPM design allows to implement this approach thanks to the flexure mounts attached to the bottom of FPM housing, and the use of different materials to support the detector facing the FPM mechanical interface (and the telescope)</figcaption></figure>

As the CTE is of fundamental interest and as far as they are known, temperature dependent CTEs have been used during for the different materials (Fig. 79).

As stated above, a system with all mechanical components made from the same material was excluded due to the shielding vs. mass constrains but mainly because of the not customizable detector given CPGA package and therefore required standard material Polyimide for the focal plane. Thus an athermalization over a distance L using two (or more) materials was being analysed. The approach, depicted in Fig. 80, is to combine materials with different CTEs in order to keep constant the compensated length, in this case the distance between the detector and the optical bench, which is the FPM mechanical interface
with the telescope. The results of the trade-off analysis showed that the best results in terms of alignment stability could be achieved if the housing and flexure mounts are made from Ti-Al6-V4.

## Abbreviations

<table><tr><td>3GM</td><td>Gravity and Geophysics of Jupiter and the Galilean moon</td></tr><tr><td>ADC</td><td>Analog to Digital Converter</td></tr><tr><td>AIT</td><td>Assembly, Integration and Test</td></tr><tr><td>AKE</td><td>Absolute Knowledge Error (2-sigma uncertainty on a-posteriori pointing knowledge)</td></tr><tr><td>APE</td><td>Absolute Pointing Error (2-sigma difference between actual and commanded pointing)</td></tr><tr><td>ASIC</td><td>Application Specific Integrated Circuit</td></tr><tr><td>ASW</td><td>Application Software</td></tr><tr><td>AU or au</td><td>Astronomical Unit</td></tr><tr><td>Boot-SW</td><td>Boot Software</td></tr><tr><td>BPE</td><td>Bit-Plane Encoding</td></tr><tr><td>CC</td><td>HORIZON Condition Code</td></tr><tr><td>CCD</td><td>Charge-Coupled Device</td></tr><tr><td>CCSDS</td><td>Consultative Committee for Space Data Systems</td></tr><tr><td>CDS</td><td>Correlated Double Sampling</td></tr><tr><td>CEI</td><td>Centre for Electronic Imaging (at Open University)</td></tr><tr><td>CF</td><td>Cold Finger</td></tr><tr><td>CFRP</td><td>Carbon Fibre Reinforced Plastic</td></tr><tr><td>CIS</td><td>CMOS Image Sensor</td></tr><tr><td>CISAS</td><td>Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”</td></tr><tr><td>CMOS</td><td>Complementary Metal Oxide Semiconductor</td></tr><tr><td>CoG</td><td>Center of Gravity</td></tr><tr><td>COM</td><td>Comets (generic, not belonging to specific group)</td></tr><tr><td>COM</td><td>Cover Module</td></tr><tr><td>CMT</td><td>Continuum for Methan absorption band</td></tr><tr><td>CPGA</td><td>Ceramic Pin Grid Array</td></tr><tr><td>CReMA</td><td>Consolidated Report on Mission Analysis</td></tr><tr><td>CSIC</td><td>Consejo Superior de Investigaciones Científicas</td></tr><tr><td>CTCs</td><td>Chiron-Type Comets</td></tr><tr><td>CTE</td><td>Coefficient of Thermal Expansion</td></tr><tr><td>CTI</td><td>Charge Transfer Inefficiency</td></tr><tr><td>DC</td><td>Direct Current</td></tr><tr><td>DaC</td><td>Dark Current</td></tr><tr><td>DLR</td><td>Deutsches Zentrum für Luft- und Raumfahrt</td></tr><tr><td>DN</td><td>Digital Number (unit after signal conversion to digital)</td></tr><tr><td>DNC</td><td>Dynamically New Comet</td></tr><tr><td>DPM</td><td>Data Processing Module</td></tr><tr><td>DPU</td><td>Data Processing Unit</td></tr><tr><td>DTM</td><td>Digital Terrain Model</td></tr><tr><td>DWT</td><td>Discrete Wavelet Transform</td></tr><tr><td>ECC</td><td>Error Correction Code</td></tr><tr><td>ECSS</td><td>European Cooperation for Space Standardization</td></tr><tr><td>EDAC</td><td>Error Detection And Correction</td></tr></table>
<table>
  <tr>
    <td>EEE</td>
    <td>Electrical, Electronic and Electro-mechanical</td>
  </tr>
  <tr>
    <td>EGSE</td>
    <td>Electrical Ground Support Equipment</td>
  </tr>
  <tr>
    <td>EMC</td>
    <td>Electro-Magnetic Compatibility</td>
  </tr>
  <tr>
    <td>EOL</td>
    <td>End of Life</td>
  </tr>
  <tr>
    <td>ESA</td>
    <td>European Space Agency</td>
  </tr>
  <tr>
    <td>ESTEC</td>
    <td>European Space Research and Technology Centre</td>
  </tr>
  <tr>
    <td>ETCs</td>
    <td>Encke-Type Comets</td>
  </tr>
  <tr>
    <td>FDIR</td>
    <td>Failure Detection Isolation and Recovery</td>
  </tr>
  <tr>
    <td>FEA</td>
    <td>Finite-Element Analysis</td>
  </tr>
  <tr>
    <td>FEM</td>
    <td>Finite-Element Model</td>
  </tr>
  <tr>
    <td>FM</td>
    <td>Flight Model</td>
  </tr>
  <tr>
    <td>FOV</td>
    <td>Field Of View</td>
  </tr>
  <tr>
    <td>FPA</td>
    <td>Focal Plane Array</td>
  </tr>
  <tr>
    <td>FPGA</td>
    <td>Field-Programmable Gate Array</td>
  </tr>
  <tr>
    <td>FPM</td>
    <td>Focal Plane Module</td>
  </tr>
  <tr>
    <td>FS</td>
    <td>Flight Spare</td>
  </tr>
  <tr>
    <td>FSM</td>
    <td>Fail Safe Mechanism</td>
  </tr>
  <tr>
    <td>FWM</td>
    <td>Filter Wheel Module</td>
  </tr>
  <tr>
    <td>GALA</td>
    <td>Ganymede Laser Altimeter</td>
  </tr>
  <tr>
    <td>GCO200</td>
    <td>Ganymede Circular Orbit at 200 km from surface</td>
  </tr>
  <tr>
    <td>GCO500</td>
    <td>Ganymede Circular Orbit at 500 km from surface</td>
  </tr>
  <tr>
    <td>GCO5000</td>
    <td>Ganymede Circular Orbit at 5000 km from surface</td>
  </tr>
  <tr>
    <td>GOI</td>
    <td>Ganymede Orbit Insertion</td>
  </tr>
  <tr>
    <td>HFCs</td>
    <td>Halley-Type Comets</td>
  </tr>
  <tr>
    <td>HK</td>
    <td>HousKeeping</td>
  </tr>
  <tr>
    <td>HST</td>
    <td>Hubble Space Telescope</td>
  </tr>
  <tr>
    <td>H/W</td>
    <td>HardWare</td>
  </tr>
  <tr>
    <td>HYP</td>
    <td>Hyperbolic comets</td>
  </tr>
  <tr>
    <td>IAA</td>
    <td>Instituto de Astrofisica de Andalucía</td>
  </tr>
  <tr>
    <td>IAPS</td>
    <td>Istituto di Astrofisica e Planetologia Spaziali (Rome)</td>
  </tr>
  <tr>
    <td>I/F</td>
    <td>InterFace</td>
  </tr>
  <tr>
    <td>IFOV</td>
    <td>Instantaneous Field Of View</td>
  </tr>
  <tr>
    <td>INAF</td>
    <td>Istituto Nazionale di Astrofisica</td>
  </tr>
  <tr>
    <td>IRAS</td>
    <td>InfraRed Astronomical Satellite</td>
  </tr>
  <tr>
    <td>ITF</td>
    <td>Instrument Transfer Function</td>
  </tr>
  <tr>
    <td>JANUS</td>
    <td>Jovis Amorum ac Natorum Undique Scrutator (Camera system for JUICE)</td>
  </tr>
  <tr>
    <td>JFCs</td>
    <td>Jupiter-Family Comets</td>
  </tr>
  <tr>
    <td>JOI</td>
    <td>Jupiter Orbit Insertion</td>
  </tr>
  <tr>
    <td>JPL SSD</td>
    <td>Jet Propulsion Laboratory Solar System Dynamics</td>
  </tr>
  <tr>
    <td>JUICE</td>
    <td>JUpiter ICy moons Explorer</td>
  </tr>
  <tr>
    <td>LAN</td>
    <td>Local Area Network</td>
  </tr>
  <tr>
    <td>LCL</td>
    <td>Latching Current Limiter</td>
  </tr>
  <tr>
    <td>LoS</td>
    <td>Line of Sight</td>
  </tr>
  <tr>
    <td>LVDS</td>
    <td>Low-Voltage Differential Signaling</td>
  </tr>
  <tr>
    <td>M1</td>
    <td>Primary mirror</td>
  </tr>
  <tr>
    <td>M2</td>
    <td>Secondary mirror</td>
  </tr>
  <tr>
    <td>MAJIS</td>
    <td>Moon And Jupiter Imaging Spectrometer</td>
  </tr>
  <tr>
    <td>MB</td>
    <td>Main Belt</td>
  </tr>
  <tr>
    <td>MBA</td>
    <td>Main Belt Asteroid</td>
  </tr>
</table>
<table><tr><td>MCM</td><td>Mechanism Control Module</td></tr><tr><td>MESSENGER</td><td>Mercury Surface, Space Environment, Geochemistry and Ranging</td></tr><tr><td>MEU</td><td>Main Electronics Unit</td></tr><tr><td>MGSE</td><td>Mechanical Ground Support Equipment</td></tr><tr><td>MLI</td><td>Multi-Layer Insulation</td></tr><tr><td>MOSFET</td><td>Metal-Oxide-Semiconductor Field-Effect Transistor</td></tr><tr><td>MRAM</td><td>Magnetoresistive Random Access Memory</td></tr><tr><td>MT</td><td>Methan absorption band</td></tr><tr><td>MTF</td><td>Modulation Transfer Function</td></tr><tr><td>NASA</td><td>National Aeronautics and Space Administration</td></tr><tr><td>NetCDF</td><td>Network Common Data Form</td></tr><tr><td>NIR</td><td>Near InfraRed wavelength range</td></tr><tr><td>OAPd</td><td>Osservatorio Astronomico di Padova</td></tr><tr><td>OB</td><td>Optical Bench</td></tr><tr><td>OBC</td><td>On-Board Computer</td></tr><tr><td>OBS</td><td>Optical Bench System</td></tr><tr><td>OBT</td><td>On-Board Time</td></tr><tr><td>OCS</td><td>OGSE Control System</td></tr><tr><td>OGSE</td><td>Optical Ground Support Equipment</td></tr><tr><td>OHU</td><td>Optical Head Unit</td></tr><tr><td>OTF</td><td>Optical Transfer Function</td></tr><tr><td>OW</td><td>Optical Wall</td></tr><tr><td>PAN</td><td>Panchromatic filter</td></tr><tr><td>PAR</td><td>Parabolic comets</td></tr><tr><td>PC</td><td>Personal Computer</td></tr><tr><td>PCB</td><td>Printed Circuit Board</td></tr><tr><td>PCDU</td><td>Power Conditioning and Distribution Unit</td></tr><tr><td>PCM</td><td>Power Control Module</td></tr><tr><td>PDS4</td><td>Planetary Data System 4 standard</td></tr><tr><td>PEU</td><td>Proximity Electronics Unit</td></tr><tr><td>PI</td><td>Principal Investigator</td></tr><tr><td>POM</td><td>PolyOxyMethylene</td></tr><tr><td>PROM</td><td>Programmable Read Only Memory</td></tr><tr><td>PSA</td><td>Planetary Science Archive</td></tr><tr><td>PSF</td><td>Point Spread Function</td></tr><tr><td>PSM</td><td>Power Supply Module</td></tr><tr><td>P-V</td><td>Peak to Valley</td></tr><tr><td>PWM</td><td>Pulse Width Modulation</td></tr><tr><td>QE</td><td>Quantum Efficiency</td></tr><tr><td>RIME</td><td>Radar for Icy Moon Exploration</td></tr><tr><td>RMS</td><td>Root Mean Square</td></tr><tr><td>RON</td><td>Readout Noise</td></tr><tr><td>RPE</td><td>Relative Pointing Error (2-sigma variation of pointing during image integration)</td></tr><tr><td>S/C</td><td>Spacecraft</td></tr><tr><td>SCI-EGSE</td><td>Science Electrical Ground Support Equipment</td></tr><tr><td>SDRAM</td><td>Synchronous Dynamic Random Access Memory</td></tr><tr><td>SEU</td><td>Single Event Upset</td></tr><tr><td>SIS</td><td>Spacecraft interface simulator</td></tr></table>
<table><tr><td>SMA</td><td>Shape Memory Alloy</td></tr><tr><td>SNR</td><td>Signal to Noise Ratio</td></tr><tr><td>SPICE</td><td>Spacecraft, Planet, Instrument, C-matrix, Events</td></tr><tr><td>SSI</td><td>Solid State Imager</td></tr><tr><td>SSMM</td><td>Solid State Mass Memory</td></tr><tr><td>S/W</td><td>Software</td></tr><tr><td>TC</td><td>TeleCommand</td></tr><tr><td>TGSE</td><td>Thermal Ground Support Equipment</td></tr><tr><td>TEM</td><td>Telescope Module</td></tr><tr><td>TID</td><td>Total Ionising Dose</td></tr><tr><td>TM</td><td>Telemetry</td></tr><tr><td>TMR</td><td>Triple Module Redundancy</td></tr><tr><td>TNID</td><td>Total Non-Ionising Dose</td></tr><tr><td>TVAC</td><td>Thermal-Vacuum</td></tr><tr><td>UV</td><td>UltraViolet</td></tr><tr><td>VIS</td><td>Visible wavelength range</td></tr><tr><td>WFE</td><td>WaveFront Error</td></tr></table>

**Acknowledgements** Our colleagues and friends Harald Hoffmann, Stefano Debei and Maria Teresa Capria left us too soon during the development of the JANUS project. Within respective fields, their work has been important to reach the final goal of a flying instrument towards Jupiter system on-board JUICE. We dedicate this JANUS paper to their memory.

**Funding Information** Open access funding provided by Istituto Nazionale di Astrofisica within the CRUI-CARE Agreement. JANUS has been funded by the respective Space Agencies: ASI (lead funding agency), DLR, Spanish Research Ministry and the UK Space Agency. Main hardware-provider Companies and Institutes are Leonardo SpA (Prime Industry), DLR-Berlin, CSIC-IAA and Sener. PI and Italian team members acknowledge ASI support in the frame of ASI-INAF agreement n. 2023-6-HH.0.

## Declarations

**Competing Interests** The authors have no competing interests to declare that are relevant to the content of this article

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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## Authors and Affiliations

P. Palumbo¹ ID, T. Roatsch², L.M. Lara³, J.M. Castro-Marin³, V. Della Corte⁴, S. Hviid².

R. Jaumann⁵, H. Michaelis², M.R. Patel⁶, G. Portyankina², N. Schmitz², M. Amoroso⁷.

R. Mugnuolo⁷, A. Aboudan⁸, L. Agostini¹, C. Althaus², F. Álvarez³, M. Bartolomei⁸.

T. Behnke², T. Bilotta⁹, G. Colombatti⁸, A. Colosimo⁹, A. Coustenis¹⁰.

G. Cremonese¹¹, C. Crews⁶, A. Dattolo⁹, S. Debei⁸, T. Denk², B. Fiethe¹², M. Herranz³.

H. Hoffmann², R. Hueso¹³, A. Koncz², J. Jiménez-Ortega³, A. Lichopoj², L. Livi⁹.

X. Llamas¹⁴, R. Lopes¹⁵, A. Lucchetti¹¹, I. Martinez-Navajas³, E. Mazzotta Epifani¹⁶.

V. Mertens², M. Pajola¹¹, F. Sarti⁹, R. Schroedter², K. Stephan², F. Tosi¹, C. Tubiana¹.

B. Wendler², D. Wendler²,¹⁷, D. Williams¹⁸, F. Wolff², M. Zusi¹, S. Chiodini⁸.

I. Ficai Veltroni⁹, A. Galeotti⁹, A. García-Segura³, D. Greggio¹¹, A.D. Holland⁶.

T. Kenkmann¹⁹, M.R. Leese⁶, D. Magrin¹¹, H. Michalik¹², M. Munari²⁰, G.E. Noci⁹.

R. Paolinetti⁹, P. Schipani⁴, M. Soman²¹, K.D. Stefanov⁶, A. Turella⁹, O. Aharonson²².

J.F. Bell III¹⁸, I. Bertini²³, A.J. Coates²⁴, G. Di Achille²⁵, D. Grassi¹, O. Groussin²⁶.

K. Gwinner², J. Haruyama²⁷, E. Hauber², H. Hiesinger²⁸, Y. Langevin²⁹, V. Lainey³⁰.

S. Marchi³¹, L. Marinangeli³², F. Marzari³³, M. Massironi⁸,³⁴, G. Mitri³⁵, S. Mottola².

J. Oberst³⁶, F. Postberg⁵, F. Poulet²⁹, F. Preusker², J. Schmidt⁵, N.M. Schneider³⁷.

A. Simon³⁸, Y. Takahashi³⁹, D. Tirsch², M. Vincendon²⁹, M.R. Balme⁶, C. Bettanini⁸.

P. Borin¹¹, M.T. Capria¹, S. Elgner², F. Esposito⁴, L. Ferranti⁴⁰,¹, S. Ferrari³⁴.

S. Fornasier¹⁰, V. Galluzzi¹, L. Giacomini¹, L. Guzzetta²⁰, G.H. Jones²⁴, E. Kersten².

L. Ledeit⁴¹, E. Martellato¹¹, K.-D. Matz², V. Mennella⁴, C. Murray⁴², K.A. Otto².
M.G. Pelizzo$^{43}$ · L. Penasa$^{11}$ · R. Politi$^{1}$ · C. Popa$^{4}$ · R. Pozzobon$^{33,34}$.

O. Prieto Ballesteros$^{44}$ · C. Re$^{11}$ · A. Rotundi$^{23}$ · M. Sato$^{45}$ · N. Schmedemann$^{28}$.

D. Shoji$^{27}$ · E. Simioni$^{11}$ · G. Sindoni$^{46}$ · F. Trauthan$^{2}$ · Y. Yair$^{47}$

<figure><img src="image_85.png" /></figure>

P. Palumbo

pasquale.palumbo@inaf.it

1 INAF - IAPS (Istituto di Astrofisica e Planetologia Spaziali), Via Fosso del Cavaliere 100, 00133, Rome, Italy

2 German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany

3 IAA-CSIC (Instituto de Astrofísica de Andalucía - CSIC), Glorieta de la Astronomía 3, 18008 Granada, Spain

4 INAF - OACN (Osservatorio Astronomico di Capodimonte), Salita Moiariello 16, 80131, Napoli, Italy

5 Freie Universität Berlin, Inst. for Geosciences, Malteserstr. 74-100, 12249 Berlin, Germany

6 School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK

7 ASI (Agenzia Spaziale Italiana) - Centro Spaziale di Matera, C.da Terlecchia, 75100, Matera, Italy

8 Università di Padova - CISAS Giuseppe Colombo, via Venezia 15, 35131 Padova, Italy

9 Leonardo SpA, Via delle Officine Galileo 1, 50013, Campi Bisenzio (FI), Italy

10 LESIA, Paris Observatory, CNRS, Université PSL, Université Paris Cité, Sorbonne Université, 5 place Jules Janssen, 92195 Meudon, France

11 INAF - OAPd (Osservatorio Astronomico di Padova), Vicolo Osservatorio 5, 35122, Padova, Italy

12 IDA, TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany

13 Escuela de Ingeniería de Bilbao, Universidad del País Vasco, UPV/EHU, Plaza Ingeniero Torres Quevedo 1, 48013 Bilbao, Spain

14 Sener Aeroespacial, Creu Casas i Sicart 86-88, Parc de l'Alba, 08290 Cerdanyola del Vallès, Spain

15 JPL/Caltech should be Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

16 INAF - OAR (Osservatorio Astronomico di Roma), Via Frascati 33, 00040 Monte Porzio Catone (Rome), Italy

17 German Aerospace Center (DLR)- Institute of Optical Sensor Systems, Dept. Space Instruments, Rutherfordstr. 2, 12489 Berlin, Germany

18 Arizona State University, School of Earth and Space Exploration, Box 876004, Tempe, AZ, 85287, USA

19 Institute of Earth and Environmental Sciences - Geology, University of Freiburg, Albertstraße 23-b, D-79104 Freiburg im Breisgau, Germany

20 INAF - OACt (Osservatorio Astrofisico di Catania), Via S. Sofia 78, 95123, Catania, Italy

21 European Space Agency - ESA/ESTEC, Optoelectronics Section (TEC-MME), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

22 Earth and Planetary Sciences, Weizmann Institute. of Science, Rehovot, 7610001, Israel

23 Dipartimento di Scienze e Tecnologie, Università degli Studi di Napoli "Parthenope", Centro Direzionale - isola C4, 80143, Napoli, Italy
24 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

25 INAF - OAAb (Osservatorio Astronomico d'Abruzzo), V.le M. Maggini, 64100, Terami, Italy

26 Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

27 Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, Japan

28 Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

29 Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay, 91405, France

30 IMCCE, Observatoire de Paris, PSL Research University, Sorbonne Université, CNRS, Univ. Lille; 77 avenue Denfert Rochereau, 75014 Paris, France

31 Southwest Research Institute, 1050 Walnut St, Boulder, CO 80302, USA

32 Department of Psychological Sciences, Humanities and Territory, University "G.d'Annunzio" of Chieti-Pescara, Via Vestini 31, 66013, Chieti, Italy

33 Dipartimento di Fisica e Astronomia, Università di Padova, Via Marzolo 8, 35131, Padova, Italy

34 Università di Padova, Dipartimento di Geoscienze, Via G. Gradenigo 6, 35131, Padova, Italy

35 International Research School of Planetary Sciences, Università d'Annunzio, Viale Pindaro 42, 65127, Pescara, Italy

36 Technical University Berlin, Institute of Geodesy, Strasse des 17. Juni 135, 10623, Berlin, Germany

37 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, 80303, USA

38 NASA Goddard Space Flight Center, Solar System Exploration Division, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA

39 Space Mission Center (SMC), Creative Research Institution (CRIS) and Dept. CosmoSciences, Graduate School of Science, Hokkaido University; Kita 10-Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan

40 DiSTAR – Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Università di Napoli "Federico II", Via Vicinale Cupa Cintia 21, 80126, Napoli, Italy

41 Nantes Université, Univ Angers, Le Mans Université, CNRS, Laboratoire de Planétologie et Géosciences, LPG UMR 6112, 44000 Nantes, France

42 Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS, UK

43 Department of Information Engineering (Dipartimento di Ingegneria dell'Informazione), University of Padua (Università degli Studi di Padova), via Gradenigo 6B, 35128, Padova, Italy

44 Centro di Astrobiologia (CSIC-INTA), Instituto Nacional de Técnica Aeroespacial, Carretera de Ajalvir km. 4, 28850 Torrejon de Ardoz, Madrid, Spain

45 Faculty of Science, Hokkaido University, Sci. Bldg. 8-203, N10 W8, Kita-ku, Sapporo 060-0810, Japan

46 ASI (Agenzia Spaziale Italiana), via del Politecnico, snc, 00133, Rome, Italy

47 Reichman University, School of Sustainability, 8 University Street, P.O. Box 167, Herzliya 4610101, Israel