# The MASCOT lander aboard Hayabusa2: The in-situ exploration of NEA (162173) Ryugu

Tra-Mi Ho, Ralf Jaumann, Jean-Pierre Bibring, Matthias Grott, Karl-Heinz Glaßmeier, Aurelie Moussi, Christian Krause, Ulrich Auster, Volodymyr Baturkin, Jens Biele, et al.

► To cite this version:

Tra-Mi Ho, Ralf Jaumann, Jean-Pierre Bibring, Matthias Grott, Karl-Heinz Glaßmeier, et al. The MASCOT lander aboard Hayabusa2: The in-situ exploration of NEA (162173) Ryugu. Planetary and Space Science, 2021, 200, pp.105200. 10.1016/j.pss.2021.105200. hal-04502930

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# The MASCOT lander aboard Hayabusa2: The in-situ exploration of NEA (162173) Ryugu

Tra-Mi Ho*ª, Ralf Jaumannᵇ, Jean-Pierre Bibringᶜ, Matthias Grottᵈ, Karl-Heinz Glaßmeierᵉ,
Aurelie Moussi*f, Christian Krauseg, Ulrich Austerᵉ, Volodymyr Baturkinª, Jens Bieleg, Federico
Corderoh, Barbara Cozzoniig, Clement Dudalf, Cinzia Fantinatig, Christian Grimmª, Jan-Thimo
Grundmannª, Maximilian Hammd,¹, David Hercikᵉ, Kağan Kayalg, Jörg Knollenbergd, Oliver
Küchemanng, Eugen Ksenikª, Caroline Langea, Michael Langef, Laurence Lordaf, Michael
Maibaumg, Yuya Mimasuj, Celine Cenac-Morthef, Tatsuaki Okadaj, Katharina Ottod, Cedric
Pilorgetc, Josef Reillk, Takanao Saikij, Kaname Sasakiª, Markus Schlotterera, Nicole Schmitzd,
Stefan Schröderd, Nawarat Termtanasombatn, Nortbert Totha, Yuichi Tsudaj, Stephan Ulamecg,
Friederike Wolfgm, Tetsuo Yoshimituj, Christan Ziachn and the MASCOT Team

ªGerman Aerospace Center (DLR), Institute of Space Systems, Robert-Hooke-Str. 7, 28359 Bremen,
Germany, Tra-Mi.Ho@dlr.de

ᵇFreie Universität Berlin, Institute of Geological Sciences, Berlin, Germany

ᶜInstitute d'Astrophysique Spatiale, Orsay, France

ᵈDLR, Institute of Planetary Research, Berlin, Germany

ᵉIGEP, Technische Universität Braunschweig, Germany

f Centre National d'Etudes Spatiales (CNES), France

g DLR, Institute of Space Operations, Cologne, Germany

hTelespazio VEGA, Darmstadt

i DLR, Institute of Composite Structures and Adaptive Systems, Braunschweig, Germany

j JAXA, Institute of Space and Astronautical Science, Sagamihara, Japan

k DLR, Institute of Robotics and Mechatronics, Oberpfaffenhofen, Germany

¹Institute for Mathematics, University of Potsdam, Karl-Liebknecht-Str. 24-25, Potsdam-Golm, Germany

m former: Institute of System Dynamics and Control, Oberpfaffenhofen, Germany

ⁿformer: DLR, Institute of Space Systems, Bremen, Germany

* Corresponding Author

Abstract
After 3.5 years of cruise, and about 3 months in the vicinity of its target, the MASCOT lander was deployed successfully on 3rd October 2018 by the Hayabusa2 spacecraft onto the C-type near-Earth asteroid (162173) Ryugu. After a free-fall of 5 min 51 sec from an altitude of 41 m MASCOT experienced its first contact with the asteroid hitting a big boulder. The lander bounced for ~ 11 min 3 sec before it came to rest. MASCOT was able to perform science measurements with its payload suite at 3 different locations on the surface of Ryugu. It investigated the fine-scale structure, multispectral reflectance, thermal characteristics and magnetic properties. The surface consists of very rugged terrain littered with large surface boulders. The in-situ measurements confirmed the absence of fine particles and dust as already implied by the remote sensing instruments aboard the Hayabusa2 spacecraft. After about 17 hours of operations, the MASCOT mission terminated with the last communication contact as its primary batteries depleted. This paper summarizes the MASCOT mission covering its four years of in-flight operations, its preparation for the descent, landing and in-situ investigation on the asteroid Ryugu until the end of its operation.

**Keywords:** Asteroid, In-situ Surface Science, Lander, Operations, Camera, IR Spectrometer, Magnetometer, Radiometer

# 1 Introduction

On 27 June 2018, JAXA's Hayabusa2 spacecraft arrived at Near-Earth Asteroid (NEA) 162173 Ryugu after a cruise of nearly 3.5 years (Watanabe et al. 2016, Watanabe et al. 2019). Hayabusa2's main objective is to investigate the formation, evolution and volatile content of the carbonaceous asteroid and for this, return samples of the surface of Ryugu to Earth on 06 December 2020 (Watanabe 2017; 2019). In addition, Hayabusa2 is equipped with four remote sensing instruments (the optical navigation camera system ONC-T (Kameda et al. 2017), the thermal infrared imager TIR (Okada et al. 2017), the near-infrared spectrometer NIRS3 (Iwata et al. 2017), the laser altimeter LIDAR (Mizuno et al. 2017),) an impactor (Arakawa, 2020), three nano-rovers (Minerva-II-Rover1A & -Rover1B, Minerva II2 (Yoshimitsu et al. 2019, Tsuda et al. 2020& Lange et al., this issue), and the DLR-CNES lander MASCOT (Ho et al. 2017) on the spacecraft. Hayabusa2 performed more than 2 months of detailed observation of Ryugu, mostly from the home position
at 20 km altitude, with its on-board instruments (Watanabe et al. 2019). On 3rd October 2018 at 01:57:17 UTC the Hayabusa2 spacecraft released the MASCOT lander onto the asteroid with the aim to operate for at least two asteroid days (15 hours) on the surface and perform in-situ investigations.

MASCOT is a surface science platform accommodating four scientific instruments: a camera MASCam (Jaumann et al. 2017), a hyperspectral IR imager MicrOmega (Bibring et al. 2017), a radiometer MARA (Grott et al. 2017) and a magnetometer MASMAG (Herčik et al. 2017). The instruments obtained close up data of the carbonaceous (C-Type) asteroid, the first such NEA to be closely visited and explored in-situ by a spacecraft. The selected scientific instruments on the lander reflected not only the scientific requirements of the Prime Mission of Hayabusa2, conducting global- and local-scale characterisation of the targeted asteroid, but also aimed at providing ground truth for the in orbit remote sensing observations. The synergy was addressed in the sense that the dedicated scientific objectives of most of the payload on the MASCOT lander overlap with the ones on board the Hayabusa2 spacecraft. However, the MASCOT instruments retrieved their measurements in higher spatial resolution, thus bridging the gap between in orbit remote sensing and analyses of returned samples.

To characterise the surface structure and texture of Ryugu, and to obtain multispectral information of the composition, on Hayabusa2 and MASCOT there are the cameras ONC-T and MASCam which resolved particle sizes down to 200 µm., respectively. The TIR experiment (Okada et al. 2017) imaged the thermal emission of the surface to obtain a global thermo-physical model of the asteroid (Okada et al. 2020). The radiometer MARA (Grott et al. 2017), in turn, was designed to continuously measure the thermal emission at the surface at one location in the MASCam field of view (FoV) through a full day-night cycle. MARA delivered complementary thermal information to TIR, since TIR observed mainly the day side. The mineralogy of the asteroid with the focus on their hydrated constituents was determined by the NIRS3 and MicrOmega instruments. Whereas NIRS3 performed near-infrared spectroscopy in the wavelengths range from 1.8 to 3.2 µm reaching a spatial resolution up to 2 m at 1 km observation altitude (Iwata et al. 2017), the near-IR hyperspectral microscope, MicrOmega, covered a spectral range from 0.99 - 3.65 µm. It was
designed to determine the surface mineralogical and molecular composition at a grain scale with a spatial
sampling of 25 µm (Bibring et al. 2017).

Finally, the magnetometer MASMag was accommodated to determine any sign of remnant
magnetization at the surface of Ryugu. This information could be used to draw conclusions not only on the
formation processes of Ryugu but also of its formation environment, i.e. the solar nebula magnetic field
(Herčik et al., 2017).

By correlating the observations of the instruments on-board Hayabusa2 with the in-situ measurements
performed by MASCOT and the analysis of the retrieved samples, the acquired local information can be
placed into a global context. This allows understanding the body geology on local and global scales and
constraining surface and possibly sub-surfaces physical properties. Furthermore, the data obtained by the
MASCOT instruments can provide context for the collected samples representing the initial condition (i.e
in the period during sampling operations) of Ryugu allowing to qualify their processed or pristine state.
They will support the laboratory analysis of the returned samples by indicating potential alteration of the
samples during sampling, cruise, atmospheric entry and impact phases.

Next to its main goals to contribute to the common understanding of Ryugu, the MASCOT lander was
considered also as a “surface scout”. Detailed knowledge on the mechanical conditions on the asteroid
ground, such as the ruggedness of the terrain, the boulder size distribution on the surface or the porosity of
the surface material, were important measurements which supported the approach and sampling manoeuvre
of the Hayabusa2 spacecraft.

This paper will give an overview of the “mission” of the MASCOT lander reviewing both the preparation
and execution of the separation, descent, landing and on-asteroid operations of MASCOT and its scientific
findings, both stand alone and in the overall context of its “mother” mission, Hayabusa2.

## 2 Background

MASCOT has been described as an agile nano-lander (Lange et al. 2018) developed for in-situ
exploration of small bodies. The actual landing module (which we refer to as “lander” throughout the paper)
has a size of 28 cm x 29 cm x 21 cm and a mass of 9.8 kg (Ho et al. 2017) and was attached to the Hayabusa2 by a physical interface, the so-called Mechanical and Electrical Support System (MESS), see Figure 1, which remained on Hayabusa2 once the lander was ejected.

The placement of the scientific payload and subsystems within the MASCOT lander is divided into two parts: the warm section containing the electronics box with the majority of the electronics (data handling, power, communication and attitude determination), the battery package and the mobility mechanism, and a cold section accommodating the four scientific instruments, see Figure 2.

MASCam, MicrOmega and MARA required a dedicated operational orientation of the lander, with the instrument side (+Y) containing the apertures of MASCam and MARA looking horizontally at the asteroid's surface and towards the horizon. In addition, the optical window of MicrOmega mounted on a protruding cone on the bottom plate (-Z) had to face the asteroid's surface, see Figure 2. To allow turning and uprighting the lander in case of an unfavourable initial orientation, the lander is equipped with a mobility system (Reill et al. 2015). With the same mechanism, MASCOT was able to relocate across the asteroid's surface once having completed its first scientific operational cycle.

Five optical proximity sensors (OPS) and six photoelectric cell sensors (PEC) form the Guidance Navigation and Control (GNC) subsystem of MASCOT. They were used to identify the motion status of the lander (i.e. MASCOT moving or at rest), the attitude status (i.e. MASCOT actual attitude with respect to the surface) and the illumination conditions (e.g. day or night). Each OPS consists of an infrared Light Emitting Diode (LED) emitting at 870 nm and a spectrally matched silicon PIN photo-diode that received the reflected light in case the asteroid surface was close and in the field of view. With this the asteroid's surface could be detected and thus the lander's ground facing side determined. The PECs contain a small photovoltaic cell and were mounted on each side of the lander to detect the solar direction. The objective was to provide by this independent stimulus a certain redundancy for the OPS and thus a higher level of reliability of the attitude determination.

The signals of the OPS and PEC were passed through a histogram filter for multi sensor data fusion (Schlotterer et al. 2014) to determine which side of MASCOT is pointing to the surface. The actual attitude
estimation was based on a probability algorithm calculating the likelihood of a side facing the asteroid's surface. The computation used the probability calculated at the previous step and the probability based on OPS and PEC measurement results from the current step. In addition, a decision logic was used to detect the motion status of MASCOT (Schlotterer et al. 2014). This overall attitude determination process was managed by the MASCOT Autonomy Manager (MAM), a nominal state machine with internal state and transition logic. It is a decision-making program which was running on the on-board computer (OBC) to optimize the on-surface operations (Ho et al. 2017). The MAM controlled the high-level decisions during the separation, descent and on asteroid phases. Its main tasks were to schedule science measurements and to command the mobility mechanism based on GNC information. Its secondary tasks were to monitor total power availability, respond to ground station commands and to manage high-level subsystem failures from the mobility mechanism. Assessing MASCOT's operation, a posteriori on asteroid Ryugu and as outlined in section 4, the lander's fate was driven mainly by two core decision nodes and state transitions:

(i) After landing, relocation or uprighting, decide if an attitude correction is necessary. This decision is based on attitude and motion state data from the GNC subsystem. A command is issued to the mobility subsystem to execute the appropriate attitude correction motion.

(ii) Activate the scientific instruments according to their pre-defined sequences. However, the MAM has the authority to alter this sequence, depending on internal events (e.g. elapsed time in a specific state) or external events (e.g. day or night time).

In general, the MASCOT activities and their sequences are driven by the MAM and its state changes. The MAM, in turn, is driven by conditions like separation detection, the illumination, movement or just timeouts. This operational strategy was chosen due to the Telecommand/Telemetry round trip time of about 35 min. Combined with an estimated total lander lifetime of only approx. 16 hours, only very limited opportunity for interaction with ground was foreseen. Thus, autonomy was put in place in case no telecommands (TC) could be received. Still, intervention by ground control was possible and indeed used during the mission. Ground commanding was executed after the first science cycle (see section 4) since
MASCOT was upside down. Unfortunately, this orientation was not immediately detected due to the unknown terrain of Ryugu and the associated uncertainty in the readout of the sensor data.

The required power for the in-flight health check and calibration of MASCOT and its payload was provided via the umbilical part of the MESS, by Hayabusa2 to MASCOT. After separation from Hayabusa2, MASCOT was powered by its primary batteries only. The design of the primary batteries' configuration was chosen to allow MASCOT to perform on-asteroid operations for at least two asteroid days (1 asteroid day ~7.6 hrs) assuming an optimal battery temperature of around +25 °C (Cenac-Morthé et al. 2019, Grundmann et al. 2014).

The lander thermal design had a mainly passive approach to survive a surface temperature variation between -80 and +50 °C on the asteroid, focusing on coatings selection, interfaces tuning, insulation and a radiator/heat pipe combination (Ho et al. 2016, Baturkin et al. 2020).

MASCOT communicated during its on-asteroid mission phase via the Hayabusa2 spacecraft using it as a relay. However, a constant communication throughout the total lifetime could not be established between MASCOT and Hayabusa2. Since the spacecraft was hovering at constant distance and position above the asteroid's dayside (Mimasu et al., this issue), communication was interrupted during Ryugu's night. The main timeline of the lander's acquisition of signal is given in Table 1.

## 2.1 Science Payload

The MASCOT lander characterized in-situ the compositional and physical properties of the asteroid and its surface with its four scientific instruments: the camera (MASCam), a hyperspectral microscope (MicrOmega), a radiometer (MARA) and a magnetometer (MASMag).

The MASCam (Jaumann et al. 2017) is a compact wide-angle CMOS multispectral camera designed to cover a large part of the surface in front of the lander. The lower front side of the camera housing, where the optics are situated, was built slightly tilted, such that the center of the camera's square field-of view (FOV) aims at the surface at an angle of 22° with respect to the bottom plane of MASCOT. This, and the Scheimpflug lens system, ensure that both the surface close to the lander and the horizon are in the FOV
and in focus, when the lander rests on an even surface. MASCam acquired clear filter images during descent of MASCOT and the asteroid day. During the night, illumination of the dark surface was realized by an array of monochromatic LED's working in 4 spectral bands. This allowed identifying spectral variations on the surface (Jaumann et al. 2019, Schröder et al. 2020). Table 2 summarizes the optical characteristics of the MASCam instrument compared with ONC-T.

The MicrOmega instrument is a near-IR hyperspectral microscope (Bibring et al. 2017). It was designed to acquire the reflectance spectra of a wavelength range from 0.99 µm up to 3.65 µm for a ~3.2 mm-sized surface spot with a spatial sampling of 25 micron. The spectral range of the instrument was chosen to cover the major absorption bands of minerals and their alteration, such as iron oxides, pyroxenes, phyllosilicates, hydroxides and organics. This allows the determination of the surface composition at grain scale. MicrOmega should characterize the texture and the composition of samples at their grain scale through the acquisition of 3D (x,y,λ) hyperspectral microscopic image-cubes. These image-cubes are built by sequentially illuminating the samples with a monochromatic light. The Near Infrared Spectrometer NIRS3 aboard the Hayabusa2 spacecraft performed near-infrared spectroscopy for wavelengths from 1.8 to 3.2 µm with sensitivity for molecular absorption by hydroxide and hydrated minerals at 2.7 to 3.0 µm.

The radiometer MARA (Grott et al. 2017) is a multi-spectral instrument to determine the surface brightness temperature of the asteroid. It contains a sensor head housing six thermopile sensors which measures the net radiative flux in six wavelength bands (i.e., 4 bandpass channels in the spectral range 5.5 - 7, 8 - 9.5, 9.5 - 11.5, and 13.5 - 15.5 µm, one long-pass channel, which is sensitive in the >3 µm range, one channel between 8 - 12 µm). The sensor's footprints are within the MASCam field of view, such that the texture of the observed surface is known to provide context for data interpretation. The filters of the MARA instrument were selected in accordance with the science goals outlined in Sect. 2.1. A long-pass channel was dedicated to the determination of the asteroid's surface brightness temperature and optimized for maximum signal to noise. Given that 99 % of the signal comes from reflected sunlight for wavelengths smaller than 3.5 µm, the filter was chosen to transmit above this wavelength, while collecting flux up to at
least 100 µm. A second broad bandpass similar to that employed by the Hayabusa2 orbiter thermal imager (Okada et al. 2015), collecting flux in the classical 8–12 µm band, was chosen, see Table 4.

The fourth scientific instrument aboard MASCOT is a vector-compensated three-axis fluxgate magnetometer, MASMag. The measurement of remnant magnetization of primitive bodies such as asteroids and comets may provide records of the solar nebula magnetic field and of small-body dynamics. Thus, the identification of a magnetic field on Ryugu would offer the possibility of inferring on the conditions during the pre-accretional and accretional phase or the magnetic history of the body.

The measurement of the material magnetization itself cannot provide precise information of the nebular field magnitude at given time, however, assuming the asteroid origin (e.g. place, accretion time), it can limit the possibilities and support particular hypotheses or exclude others. In turn, the lack of magnetization on a certain size scale would limit the role of the preaccretional nebular magnetic field as an accretional mechanism during the formation process (Herčik et al. 2019). The scientific objective was to measure any global as well as local magnetic field during the whole MASCOT operational period, focusing on landing and relocation phases.

The MASMag instrument consists of a sensor head and digital electronics. The magnetometer measures three components of a magnetic field with a 10 Hz sampling rate (Herčik et al. 2017). It ensures a measurement accuracy of 1 nT to also estimate the local magnetization of the surface material within a 10-⁵ Am²kg⁻¹ specific moment range for decimeter-sized magnetic domains granularity. The required sensitivity had to be better than 0.1 nT in order to be able to identify signal variation for the magnetic cleanliness analysis.

### 3 The four years cruise onboard Hayabusa2 and the selection of the landing site

During the 3.8 years of cruise MASCOT was mainly off and on only for regular health checks, calibration campaigns or dedicated in-flight activities.

The objective of a health check was to activate each payload and subsystem, to obtain house-keeping telemetry and through that assess the health of the system. During calibration the payload performed
predefined measurements and transmitted science data, allowing a comprehensive analysis of the scientifically relevant performance of each instrument. Additional in-flight operations were performed as well, such as communication or data transfer tests between Hayabusa2 and MASCOT and the validation of the on-asteroid sequences and MAM functionalities (Krause et al. 2018). All MASCOT health checks performed in flight showed the nominal status of the system and its science instruments throughout its nearly four years journey. During the in-flight campaigns in 2018, e.g. within the last three months before landing, three depassivation operations were performed. Since the MASCOT batteries were from the same batch as the batteries aboard the Philae lander of the Rosetta mission (Debus et al. 2003), the formation of a passivation layer on the anode of the batteries were expected during their extended storage time of more than 15 years. Such a layer may prevent the cells to provide energy and therefore had to be removed before the batteries were operated.

Once Hayabusa2 arrived at Ryugu on 27 June 2018, the asteroid mapping and near-asteroid operation phase started. During this phase, the spacecraft was most of the time at its home position (HP) i.e. an altitude of 20 km and its operation was defined through the so-called HP coordinate system. The center of the asteroid forms the origin of the HP coordinate system, its $Z_{HP}$-axis points toward Earth and the $X_{HP}$-axis is perpendicular to $Z_{HP}$ on the plane formed by the Earth direction vector and the Sun direction vector. Details of the Hayabusa2 operation during the release of the MASCOT lander are described in Mimasu et al. (this issue).

A solid operational planning was required for the on-asteroid operation of MASCOT that included a landing site respecting Hayabusa2's own on-surface activities, i.e. sampling and cratering sites (Lorda et al., this issue) as well as the engineering criteria posed by the constrain-driven design of the lander itself. This mainly concerns the power supply and the thermal control system. One of the driving factors to avoid the MASCOT landing site from overlapping with the selected touch down sites of Hayabusa2 (i.e. TD1, TD2 and TD3) was a possible misinterpretation of the reflecting signal of the top panel of the lander and one of the target markers. Those target markers were used by Hayabusa2 to perform their asteroid descent manoeuvre.
As already explained, the lander was running on primary battery only and an actual life-time of 12 to 15
hrs was predicted if the preferred operational temperature of the battery cells of MASCOT would stay above
+25 °C. To improve the battery performance on the asteroid and increase its lifetime, the battery was pre-
heated before separation.

Note: This battery lifetime range resulted from simulations to determine the remaining battery energy.
They considered the performance of the battery cells at the electrical load demand of MASCOT and the
long-term storage of the battery cells of more than 15 years before the HAYABUSA2/MASCOT launch as
they came from the Philae battery spare stocks. An estimation of their degradation once the on-asteroid
phase started was obtained from several battery discharge tests. Those tests were performed on few
assembled MASCOT batteries not launched, several single cells from the Philae stocks, and many fresh
production cells, all kept in controlled storage.

A further requirement was imposed on the landing site because of the mainly passive thermal control
system of MASCOT: the preferred surface temperature should have a maximum daily temperature of 50°C
and a minimum night temperature between -80 °C and -60 °C (Cozzoni et al., this issue). In addition, the
on-surface operational planning was carefully prepared to maintain an optimized balance of the energy
consumption of high-consuming instruments and subsystems and the internal heat balance process. The
objectives were to extend the lifetime of the battery as much as possible and as such to maximise the
scientific return.

The MASCOT team derived the best landing time to be during daytime allowing a data transmission
before the first sunset. Thus, the landing site should provide daylight between 50% and 70% of the asteroid
rotation period (i.e. sphere, rotation period = 7.631 hrs). Further operational priorities were defined i.e. to
upload all data including night-time data prior to the first relocation and to upload all data from the second
site prior to the end of mission. This process also explained the main decision path during the later on-
asteroid operation. Note that the data was uploaded according to the established priority principle (Krause
et al., 2018) to secure the most important data since the end of mission was estimated by the battery life
time.
Besides the engineering and operational requirements, the landing site had to meet, further scientific requirements were defined. The MicrOmega experiment preferred

- a region of C-rich and/or OH-rich content

- the surface temperature was required to be low for a high signal to noise ratio since the instrument was prone to high temperature.

MASCam preferred a landing site that has

- compositional heterogeneity and ideally non-weathered material.

- boulders in the field of view.

The MASMag team's objectives were

- to investigate the magnetization state of the intact primordial blocks

- to observe global magnetic signature while landing and any local structures during bouncing and hopping

Thus, MASMag preferred to land on a less thermally altered region with a thin or no regolith layer and with boulders in the close vicinity to determine any magnetization of the asteroid surface. From the perspective of the MARA instrument, it was desired that the landing site exhibits the following characteristics:

- Low thermal inertia to learn about the fine grained fraction of the regolith

- Homogeneous thermal inertia to be able to better link the MARA data to the TIR data

- Low rock abundance to reduce the chance of inhomogeneities in the Field of View

Even if the scientific criteria seem to contradict each other, making it impossible to select a landing site that is satisfactory for all, the first observational results on the surface of the Ruygu indicated a global material homogeneity (Watanabe et al. 2019). Thus, this surface property allowed nearly all calculated landing sites to be at least compositional equally representative. Furthermore, the information obtained by the remote sensing instrument onboard Hayabusa2 predicted a boulder rich surface, which was confirmed by the Minerva-II rovers on 21 September 2018, 14 days before the MASOT landing on 3rd October. Since the selection of the final MASCOT landing site from the 10 possible landing sites (see Figure 3) provided
by the MASCOT Flight Dynamics team was already made on 14 August the findings of the Minerva-II
rovers could not be considered in the decision.

During this detailed selection process candidate landing sites MA-3, MA-4, MA-6 and MA-8 were
discarded based on the above-mentioned criteria. The remaining sites were ranked, with the most favourable
site being MA-9, followed by MA-1, MA-10, MA-7, MA-5 and MA-2. The final approval of MA-9 as the
MASCOT landing site by the Hayabusa2 Project Team followed 4 days later on 18 Aug during a Hayabusa2
Joint Science Team meeting (Lorda et al., this issue).

4 The separation and on-asteroid operations of MASCOT

The MASCOT on-asteroid operations phase started about a week before the landing on 26 September 2018 with a series of activities which was named “pre-separation phase”. This included the upload of all pre- and post-separation sequences and “late update parameters”. These parameters provided a dedicated configuration of the entire system for the on-asteroid phase and were dependent on the selected landing site and its properties. The exposure time of MASCam and the set-point for the MARA sensor heaters were set. The parameters were deduced from the asteroid data products (i.e. shape, surface roughness, illumination condition, albedo etc.) of the remote sensing instruments ONC, TIR and NIRS3 on-board the Hayabusa2 spacecraft. MASCOT was switched on again on 30 September while Hayabusa2 was still hovering at 20 km above Ryugu’s surface and remained on until the end of the mission. Operations started with the pre-heating for the last battery depassivation. After the depassivation, a final check out followed to assess the status of all units and MASCOT turned into idle mode. On 1st and 2nd October, last activities and checks were performed before the spacecraft was entering its descent phase towards the anticipated release altitude of between 40 to 60 m on 3rd October.

Prior separation another heating activity started to achieve a temperature level high enough, with a
battery temperature around +30 °C, to enable a battery discharge as long as possible once on the asteroid.
In addition, MASMag and MARA operations began because MARA required thermal equilibration for its
sensor head to be prepared for the on-asteroid operation and MASMag performed magnetic field measurements during the approach phase.

Shortly before separation the MASCOT battery switches were closed, the power supply from Hayabusa2 was switched-off and the battery took over to power MASCOT during separation and the follow-on operations. Meanwhile Hayabusa2 descended to an altitude of about 60 m and released MASCOT into a free-fall phase, see Figure 4. Via a Hayabusa2 command the Non-Explosive Actuator inside Mascot were activated and the lander was separated on 3rd Oct 2018 at 01:57:20 UTC.

Once separation was executed successfully, i.e. when the lander module was moving with respect to Hayabusa2 and the electrical connection between Hayabusa2 and MASCOT turned to ‘OPEN’, the “post-separation phase” started.

The nominal separation of MASCOT from Hayabusa2 was at an altitude of approx. 60 m which would result in a descent time of ~10 min until the lander would touch the surface. Mimasu et al. (this issue) described in detail the Hayabusa2 operation of the MASCOT delivery. Once the LIDAR detected this altitude it triggered the release and the lander was ejected 140 s afterwards. The LIDAR detected the actual separation height to be 51 m. This height refers to the origin of the spacecraft body fixed frame. Sixty seconds after the release, Hayabusa2 went into ascent to reach a temporarily hovering distance of 3 km for the complete on-asteroid phase of the MASCOT lander to guarantee a reliable communication between the lander and the mother spacecraft.

Contrary to the height reproduced by the LIDAR instrument, Jaumann et al. (2019) and Scholten et al. (2019a) defined the separation height of MASCOT at approx. 41 m which refers to the radial distance to the surface of Ryugu (based on the applied shape model) to the lander’s Centre of Gravity. The confirmation to ground of the separation of the lander was recorded firstly by the MASMag instruments. Figure 5 (top) shows the detection of the separation of MASCOT from Hayabusa2 by the MASMag. As seen in the MASMag data, once MASCOT was separated, the magnetic field measurement changed immediately (Herčik et al. 2020). It required about 4 ± 1.5 s for MASCOT to leave the MESS, the structural interface to the Hayabusa2 mothership. Also, in the measurement of the MARA instruments, clear evidence of the
separation can be seen in Figure 5, on the right, as well as the free-fall time of 5 min 51 sec and a bouncing
time of 11 min 3 sec. Starting only 2.5 min after the separation up until 6.5 min before MASCOT came to
rest on the asteroid surface at its first settlement point (SP1), see also Table 5, the MASCam instrument
acquired descent and bouncing images (in total 20) (Jaumann et al., 2019).

Looking at MASCOT's landing in more detail, it touched for the first time Ryugu at its first contact point
(CP1) by hitting the shadowed side of a large boulder (Jaumann et al. 2019). Scholten et al. (2019a)
performed a detailed analysis of the descent and bouncing path of MASCOT combining the MASCam and
the ONC-W1 images of the lander's descent. They determined the CP1 coordinates as – 22.595° south,
314.914° east and a total 3D impact velocity of 17.0 cm s⁻¹. In addition, they concluded from the MASMag
measurements that there were several contacts with this large boulder which can be represented as a slide
of the lander along the boulder up to CP2. After this MASCOT experienced its first free-fall bounce
followed by 3 further bounces (CP3/4/5) until it came to rest at SP1 in a local depression after 15 min 37
sec. During this phase MASCOT travelled a horizontal distance of approx. 18.64 m.

According to the MASCOT GNC system the lander was lying on its +Y side panel at SP1. Since the
false orientation at SP1, see Figure 6, was detected by the optical proximity sensors (OPS), an uprighting
maneuver was executed to turn the lander into the correct orientation for scientific measurements. That
enabled the lander to reach its first measurement point (MP1). Scholten et al. (2019b) performed stereo-
photogrammetric analysis of the MASCam data at the different measurement points (MP1/2/3/4) indicating
that the horizontal distance between SP1 and MP1 is approx. 120 – 130 cm.

Unfortunately, at MP1, MASCOT was lying bottom-up. But this actual fault orientation of the lander
was not detected because the OPS only retrieved a sub-threshold signal of the reflected lights from the OPS
LED (see section 2.1 and Ho et al. 2017).

Because of the limited space within the lander's structure, only five optical proximity sensors (OPS)
were able to be accommodated at the side panels, i.e. +/-X, +/-Y and +Z direction. No OPS was assembled
on the MASCOT's bottom side (i.e. -Z direction) which might have given a clearer signal to the orientation
determination. Thus, if the lander lies on its instrument side, i.e. MicrOmega optical window facing the
ground, this attitude would have been detected by the absence of the +Z OPS signal since the top panel is pointing to the sky. In addition, the OPS were only able to detect an object at a distance of about 12 cm assuming a reflectance of the soil of 5% (Schlotterer et al. 2014).

Deduced from the MicrOmega measurement (Bibring et al., this issue), it was most probable that a prominent local depression in the foreground was beneath MASCOT. This thesis is supported by the observations of MASCam (Jaumann et al. 2019, Otto et. al 2021, and see also section 5) describing that the surface of Ryugu was rougher than expected and covered with boulders of few tenth cm sizes. In addition, the global average reflectance of Ryugu was with a geometric albedo of 4.5 ± 0.2 % lower than expected (Watanabe et al. 2019, Sugita et al. 2019). Schröder et al. (2020) determined the rock's reflectance at the MASCOT landing site which is consistent with the above-mentioned value but is even slightly lower.

The two reasons given above are supported by the very weak signal of the OPS at the top side (+Z), see Figure 7 (on the left) at 2:27. It was slightly beneath the threshold programmed in the flight software to detect a contact on a certain side. As the sensor fusion between OPS and PEC sensors was done using a histogram filter, weighting functions were used to reflect the confidence in a certain sensor measurement. The PEC measurement depends on the sun elevation, which again depends on the SP1. As the SP1 was not known beforehand very well, the weighting functions was adapted in favour of the OPS measurements. Thus, the flight software prioritized the (faulty) OPS result over the PEC measurement and assumed a correct orientation of MASCOT. It then triggered the start of the first full science cycle. Although the incorrect attitude was hinted by the use of the bottom antenna by the automatic antenna selection at MP1, and suggested by ad-hoc analysis of GNC telemetry on the ground, it was only confirmed by MASCam showing the Sun (see Figure 8) in the daytime images and the night sky (incl. Jupiter, Saturn and σ Sagittari) in the nighttime images (Jaumann et al, 2019).

Right after, the measurement sequence was interrupted by ground command forwarded to Hayabusa2 during asteroid night. It was sent to MASCOT immediately after the link was re-established with Hayabusa2 in the next asteroid morning. A forced relocation (1st relocation) was executed to correct for the orientation error. The lander performed a hop of approx. 70 cm length to reach its second measurement point (MP2).
Once it came to rest, the GNC sensors detected that the lander was in correct orientation and thus the MAM initiated the 2nd science measurement cycle. During this position, the lander experienced 3 unintentional small slides of 0.2 – 0.3 cm resulting in the division of MP2 to MP2a/b/c (Scholten et al. 2019b).

After the 2nd science cycle was completed, MASCOT performed an intended small displacement into its 3rd measurement point (MP3) without changing the lander's orientation, i.e. a "Mini-Move" of 3.6 cm. The goal of this operation was to obtain stereo imaging with MASCam and to enhance the science return for the MicrOmega instrument, supposed to analyse a second measurement spot. After the completion of the 3rd science cycle, a further ground command executed to relocate MASCOT to another side. The lander reached its 4th and last measurement point (MP4) at an additional distance of 11.6 cm but in the wrong orientation again. At this time MASCOT entered already in its predefined "end of life" (EOL) status which did not foresee any further up righting maneuver. The priority was on data transmission and only minimal scientific operations before the night phase started and the lander entered LOS, which soon occurred after 17 hours and 14 min of operations. During all this time, MASCOT was running half autonomously being commanded from the ground by the predefined sequences.

Although the lander was in a wrong attitude for 5 h 16 min, MASCOT was able to use the remaining time for 3 science cycles. The camera, MASCam, retrieved 120 images in total, 38 of them pointing to the sky (Jaumann et al., 2019). MicrOmega received 281 images in total with 86 images taken in upside down position. The MASMag instrument was working continuously for approx. 9 h and 20 min and MARA was continuously on for approx. 16 hours 6 min until the end of life of the lander.

# 5 The in-situ investigation of Ryugu by the MASCOT lander

## 5.1 An estimate of Ryugu's surface mechanical properties based on its bounces

One of the most challenging questions to prepare the on-surface science operation of MASCOT is the duration of its passive bouncing phase. It was expected that once the lander impacts the surface it enters into a difficult to predict motion phase in which the lander will bounce for an unknown time before finally coming to rest at its SP1. The (statistical) prediction of the 'bouncing phase' is in many ways important and
is described in the previous sections and therefore should only be repeated shortly here: the area of MASCOTs possible SP1 should not interfere with Hayabusa2 touchdown area. Looking into the results of the simulation of MASCOTs possible landing sites and the area of potential settlement points (Lorda et al., this issue) some MASCOT sites did intersect with the touchdown areas. In addition, if a bouncing phase would take longer than anticipated, the science operation of MASCOT could be shorter and/or the lander would enter into night mode sooner once it came to rest. The bouncing motion of the lander depends on many parameters such as the shape, moments of inertia and damping properties of the lander itself (Biele et al. 2017), the local terrain, the contact points between the lander and the surface, the attitude of the lander for every contact with the surface and especially the mechanical properties of the ground itself. Depending on the mechanical properties of the ground, e.g. hard rock or softer surface, the impact of the lander on the surface can experience more or less elastic collisions, resulting in more or fewer bounces. The degree of elasticity of a collision is expressed here through the ('total') coefficient of restitution (COR), e, and is defined as

$$e = \frac{v_{out}}{v_{in}}$$

where $v_{in}$ is the relative velocity before impact or inbound velocity and $v_{out}$ is the relative velocity after impact or the outbound velocity. The COR in principle ranges from 0 to 1 where 1 would be a perfectly elastic collision. A completely inelastic collision would correspond to e = 0 when the two bodies remain in contact after the collision.

For the simulation of the bouncing trajectories of MASCOT statistical modelling was performed whereas the input parameters of the type of contact was chosen from analytical simulations representing the hard contact to simulate bounces on rocky/hard surfaces and the softer contact with granular surface simulating bounces on regolith (Lorda et al., this issue). The simulation used conservative assumptions on COR assuming a rather hard surface resulting in strong bouncing and a high probability to fall on a rock.

The results of the position of settlement points calculated with the DP-3, i.e. the data package provided by the Hayabusa2 observations at 5 km, is given in Figure 3 whereas Figure 9 (top) describes the probability
density function and the cumulative distribution function of the total flight duration from MASCOT's release up to its SP1 for the nominal selected landing area, MA-9.

The duration from release to SP1 from the house keeping data, see Table 5, is about 21 min and 16 sec. Taking the time derived by Scholten et al. (2019a), the duration would be 17 min. The statistical model predicted that MASCOT would most probably reach SP1 approx. 30 – 50 min after its first contact with Ryugu. Thus, the lander took significantly less time and/or experienced less and/or shorter bounces than the simulation has predicted, indicating that maybe the assumed input parameters to define the collision of MASCOT with the surface of Ryugu had overestimated the hardness of the soil.

Based on the actual duration of the bounces of the lander and setting up following assumptions:

- A constant coefficient of restitution (COR), $e_{tot} < 1$ and a velocity angle to the surface $\alpha = 45^\circ$ or uniformly distributed between $0^\circ < \alpha < 90^\circ$

- A constant acceleration $g$

- A planar surface

we derived a rough estimation of the COR through the bouncing distance and the duration MASCOT has reached by defining its bounces as projectiles motions.

Note that since the actual landing and measurement sites of MASCOT has a total range of approx. 20 m and the ONC images did not observe a strong variation in the surface composition across Ryugu, the assumption of a constant COR for the short distance MASCOT travelled, can be considered valid. The previous (pre-encounter) assumption of a granular regolith surface dotted with rocks (i.e. assumption of Flight Dynamics simulation) turned out not to be the case.

We define a cut-off velocity of the lander, i.e. $v_N$ = “at rest”, by arbitrarily using the maximum vertical ($\alpha=90^\circ$) rebound height $h_N < 1$ cm, then $v_N$ can be described as

$$v_N = \sqrt{2gh_N}$$

The range of a bounce is defined by following equation:

$$s = \frac{v^2}{g} \sin(2\alpha) \approx \frac{v^2}{g}$$
Where v is the impact velocity, g is the gravitational acceleration and α is the velocity angle to the surface.
Assuming that α=45° or uniformly distributed between 0° < α < 90° then s can be approximated by v²/g.
Time for one bounce arc from ground contact to next ground contact is then:

$$t = \frac{2v}{g} \sin \alpha \approx \frac{\sqrt{2v}}{g}$$

If we assume that the total COR is

$$e_{tot} = \frac{\|v_{after}\|}{\|v_{before}\|} = \frac{\|v_i\|}{\|v_{i-1}\|} = \frac{v_i}{v_{i-1}}$$

where i is the bounce number, i=1 represents the first touchdown, and the velocity v₀ is the velocity just before the first the first touchdown. Since eₜₐₗ is constant, the velocities decrease simply as a geometric progression and the total velocity after the i-th bounce is:

$$v_i = v_0 e_{tot}^i$$

The total number N of significant bounces, until velocity becomes ≤vₙ:

$$N = \text{round}(\log(v_N/v_0)/\log(e_{tot}))$$

Summing up to v_N and to infinity* we would obtain for the maximum total bouncing distance and the total bouncing duration the following approximation:

$$S = \sum_{i=1}^{N} s_i = \sum_{i=1}^{N} \frac{v_0^2}{g} e_{tot}^{2i} = \frac{v_0^2}{g} e_{tot} \frac{1 - e_{tot}^{N+1}}{1 - e_{tot}} \approx \frac{v_0^2}{g} \frac{e_{tot}^2}{1 - e_{tot}^2}$$

$$T = \sum_{i} t_i = \sum_{i=1}^{N} \frac{\sqrt{2}v_0 e_{tot}^i}{g} = \frac{\sqrt{2}v_0 e_{tot}}{g} \frac{1 - e_{tot}^{N+1}}{1 - e_{tot}} \approx \frac{\sqrt{2}v_0}{g} \frac{e_{tot}}{1 - e_{tot}}$$

Since the total bounce distance (i.e. horizontal distance from first touchdown to rest position) S and the total bouncing duration are observables; v₀ is determined and g = 0.147 mm/s² (FDreport_localisation_V2.1 draft,); eₜₐₗ can be derived. Using the sum of all horizontal distances along the path of MASCOT S=18.64

<footnote>* Geometric series $a_i = a_1 q^{i-1}$ for i=1...N: $\sum_{i=1}^{N} a_i = a_1 \frac{1-q^N}{1-q}$. Note that first arc starts with vᵢ=v₀*eₜₐₗ, not v₀!</footnote>
m, T= 663.15 s (from CP1 to SP1) and v₀ determined by Scholten et al. (2019a) an average of total COR of ~ 0.29 is derived, see Figure 10.

To obtain a correlation between a COR and the strength of surface material of Ryugu, we apply the model of the composite COR of a collinear collision by Coaplen (2004). It says that one can calculate a composite coefficient of restitution (COR) which describes the collision between hard bodies of dissimilar material properties if, for each body, the material properties and COR values for the self-similar collisions are known. The composite COR is defined as

$$e_*^2 = \frac{k_1 e_2^2 + k_2 e_1^2}{k_1 + k_2}$$

Where the k₁/₂ are the compliances, i.e. linear rate-independent compliance corresponds to the yield stress Y or elastic module E, and the e₁/₂ are the CORs of impacts at the work-equivalent velocity between bodies of the same material. We assume k₁ and e₁ are the parameters of Ryugu and k₂ and e₂ the ones of MASCOT. Sine the k₂ parameter of MASCOT is defined as 50 MPa†, the COR of MASCOT, e₂ = 0.6, is derived from impact experiment (Biele et al. 2017) we can determine k₁ if we know e₁ and e*. Since e* is equivalent to the above defined eₜot ~ 0.29 (± 0.1) and assuming that 0 < e₁ < 1, we can derive the possible k₁ or Young's module (=Y₁) of the material which MASCOT contacted at the surface. Figure 11 displays the variation of eₜot in dependence of Young's module for e₁ between 0 ... 1 indicating that only the COR of Ryugu's surface material between 0 < e₁ < 0.35 provide 0.2 < eₜot < 0.4 resulting in an upper limit of Y₁ ~ 37 MPa. According to Grott et al. (2019) the Young's module can be turned into the tensile strength ~ Y₁/500 giving a value of 74 kPa.

Surely, an immediate conversion of this COR value into any strength/elastoplastic parameters of the structure of the landing site cannot be made without considering further parameters such as the exact contact geometry, soil model, contact angles and rotation. However, we can conclude that whereas a COR ~ 0.6, as assumed by the simulation of the MASCOT Flight Dynamics, corresponds to the hard case, the second COR

<footnote>† The effective Young's module as derived from the Finite Element Model (FEM) of MASCOT</footnote>
~ 0.3 derived from our simplified model represents the “soft end”. This suggests that Ryugu’s surface might be less hard than assumed which is supported by the conclusion of the MARA instrument (see section 5.2). The radiometer analysis has indicated that the boulders around MASCOT’s landing site have a high porosity which could result in less elastic collisions of MASCOT. A detailed investigation to convert the COR into strength/elastoplastic parameters is on-going and will be summarized in a later publication.

## 5.2 The finding of MASCOT in the context of the Hayabusa2 observations

The MASCam images confirmed and deepened several of findings of the ONC instrument aboard Hayabusa2 showing Ryugu's surface covered mainly with rocks and boulders of different lithologies (Jaumann et al. 2019). Within the MA-9 landing site, MASCam was able to confirm two of the four types of rocks identified in the ONC images of Hayabusa2 (Sugita et al. 2019): the dark and rough type 1 boulders and the bright and smooth type 2 boulders. In addition, the camera of MASCOT provided close-up fine-scale texture identification. Sugita et al. (2019) derived from their data that the rugged surfaces and edges of the type 1 boulders showed uneven layered structures that could be related to the inclusion of coarse-grained clasts. Indeed, due to the high-resolution of the MASCOT camera images (see Table 2), the mm-size structure of type 1 boulder makes the granular even crumble-like surface evident (Jaumann et al. 2019). Especially the images of MASCam at MP2 reveal a boulder with a surface texture of cauliflower-like undulations of ~1 cm scale and 2-3 mm scale. This topographic surface roughness in the sub-millimetre scale can be quantified by surface roughness parameters allowing to quantify and compare Ryugu's surface with that of other asteroids and comets (Otto et al. 2021).

Since MASCam was equipped with LEDs, the camera was able to acquire images at MP2 during night, revealing bright inclusions < 1 mm embedded in a dark matrix (Jaumann et al. 2019, Schröder et al. 2020). Similarities between the observed inclusions on Ryugu and inclusions and chondrules in carbonaceous chondrite (CC) meteorites, i.e. subtypes CI-, CM (Murchison), CO and CV found on Earth were derived. By measuring the areal percentage of inclusions and the inclusion sizes seen on a selected slab in the MASCam images, Jaumann et al. (2019) and Schröder et al. 2020 showed that most attributes in common
with Ryugu's rock are shared by the CMs and the Tagish Lake meteorite. Sugita et al. (2019), in turn, indicated from the spectral data obtained with ONC-T and NIRS3 that the major meteorite analogue candidates are thermally metamorphosed carbonaceous chondrites. But they emphasized that Murchison for CM and Ivuna for CI would play a small role as Ryugu analogues. While the authors based their choice for this CC analogue group on Ryugu's low reflectivity, Schröder et al. (2020) derived more clues by determining the size distribution of the inclusion and their spectrophotometric properties. This approach allowed a more distinguished match of Ryugu within the CC analogue group. In this context, the analysis of the returned samples will help clarifying the allocation of Ryugu's surface material with the meteorite analogues found on Earth so far. Next to the type 1 boulders, MASCam could also detect the brighter type 2 boulder, which are smooth and displays several thin and parallel layers with some exhibiting distinctive striped patterns observed by ONC (Sugita et al. 2019). The observation of MASCam suggested that the planar slabs with smooth surface are due to elongated, non-denditic fractures (Otto et al. 2021). MASCam observed boulders with sizes from centimeters to a few tens of meters. The finding of MASCam complemented the ONC-T images counting boulders mainly between 2 – 10 m sizes within the longitude and latitude zone (– 22.595°, 314.914°) of the MASCOT landing site and approx. 5 times less boulders bigger than 10 m (Michikami et al. 2019). It is interesting that smaller particles than ~2 cm are absent or rare in the MASCam images. While the size-frequency distribution of particles on Ryugu's surface otherwise follows approximately a power-law (Michikami et al. 2019), there seems to be a lower size cut-off at a few cm diameter grains.

The MASCOT radiometer (MARA, Grott et al., 2017) collected data during the entire MASCOT mission and determined surface brightness temperatures of a boulder at the landing site for a full day-night cycle. Measurements were interpreted using a thermal model taking viewing geometry, surface roughness, and re-radiation from the environment into account (Hamm et al. 2018, Scholten et al. 2019a&b). In the initial analysis, the thermal inertia of the boulder was thus found to be $282_{-35}^{+93}$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ (Grott et al., 2019), which was later updated and constrained to $295 \pm 18$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ (Hamm et al., 2020). These values are
consistent with global estimates in the 300 ± 100 J m⁻² s⁻⁰.⁵ K⁻¹ range (Sugita et al., 2019, Okada et al., 2020). Such low values have usually been interpreted to correspond to a surface covered by regolith of a millimeter to a few centimeters sized material (Wada et al., 2018, Ogawa et al., 2019) but Ryugu appears to be deficient in such small particles as inferred from the MASCam images. As the presence of a thin cover of dust particles masking the thermal signature of the underlying material could be ruled out based on the continuous MARA night-time data (Biele et al., 2019, Grott et al., 2019), it was concluded that the low thermal inertia and thus low thermal conductivity of the observed boulder was caused by significant microporosity (Grott et al. 2019; Okada et al., 2020). The latter may be the result of the boulder's formation process (Neumann et al., 2014), the result from a slow disaggregation of the material driven by thermal fatigue (Delbo et al., 2014, Molaro et al., 2015, Molaro et al., 2017, Hamm et al., 2019), or due to the origin from porous parent body only at a low degree of consolidation of material (Okada et al., 2020).

MARA results serve as an important anchoring point for the interpretation of global thermal imaging data, which was provided by Hayabusa2's TIR instrument. TIR data indicate that boulders such as that observed by MASCOT cover a large part of Ryugu's surface and only a minority of boulders were found to have thermal inertias exceeding 600 J m⁻² K⁻¹ s⁻¹/², which is almost the same as that of typical dense carbonaceous chondrites (Okada et al., 2020). Therefore, the majority of boulders as well as the surrounding large (>10 cm) rock fragments are now interpreted to be highly porous (Okada et al., 2020), consistent with estimates of inter-boulder porosity in the 16% range (Grott et al., 2020) and a bulk porosity of approximately 50-60% (Watanabe et al. 2018).

The magnetic measurements (Herčík et al., 2020) show that Ryugu is not magnetized on boulder (greater than centimetre) scales. This gives us indication on its origin and evolution. In particular, it shows that Ryugu and the bodies it was created from did not possess any magnetic field generation mechanism and that they were not created in an environment with strong background magnetic field. The results are important input for theories about the solar system evolution that consider magnetic fields as one of the drivers for dust accretion and planetary formation.
## 6 Summary

The mission of the MASCOT lander, after release from the Hayabusa2 spacecraft lasted more than 17 hours, 1-2 hours more than the predicted lifetime, mostly because of the detailed preparation of the separation, descent and landing event, the thorough selection of the landing site concerning safety and link budget and the thorough pre-planning of the operational sequences to optimize the use of the battery. This all resulted in a careful trade-off of the robustness of the operation and a high science return once landed on asteroid Ryugu. It demonstrated impressively how important the combination of remote sensing and in-situ data are, both for the dedicated operations such as the approach and landing manoeuvres of the space probes themselves as well as the evaluation of the science measurement. Even though the lander was designed to cope with and operate in unexplored terrain (i.e. mobility, top and bottom antenna), its technical features were limited due to the total mass and volume limitation as well as the short development time of only about three years from Phase B (Start 2011) until the delivery of the flight model to be integrated onto the Hayabusa2 spacecraft (April 2014). Since the power and thermal systems of MASCOT as well as the communication sets the operations limit, it was of utmost importance to understand the asteroid surface as much as possible in advance. The detailed observations of the remote sensing instruments onboard Hayabusa2, such as ONC and TIR, allowed the MASCOT team to select the most suitable landing site responding to both technical and scientific requirements.

The data obtained in-situ by MASCOT contributed to a comprehensive understanding of Ryugu. New insights into the surface properties at several measurement sites was obtained such as the porosity and inclusions of the rocks, the asteroid's surface brightness temperature for a full day-night cycle as well as the lack of fine regolith and a global magnetic field. They provided the context information to the remote sensing instruments on Hayabusa2 and the samples which will return in December 2020 to Earth.

Furthermore, MASCOT's "track" on Ryugu gave surprising clues about the mechanical properties of the asteroid's surface. While for the preparation of MASCOT's landing and especially rebound scenario, a hard surface was assumed, the ultimately short phase of MASCOT's bouncing suggested that the surface might
be “softer” than assumed. The upcoming investigation of the Hayabusa2 returned samples will help to shed
light on the mechanical properties of Ryugu’s surface material.

Acknowledgements

The authors would like to acknowledge the significant contribution of all members of the MASCOT Project
Team and the Hayabusa2 Project Team.

The contribution of the Technische Universität Braunschweig was financially supported by the „Deutsches
Zentrum für Luft- und Raumfahrt“ and the „Bundesministerium für Wirtschaft und Energie“ under Contract
50OW1501.

References

Arakawa, M., Wada, K., Saiki, T., Kadono, T., Takagi, Y., Shirai, K., Okamoto, C., Yano, H., Hayakawa, M., Nakazawa, S., Hirata, N., Kobayashi, M., Michel, P., Jutzi, M., Imamura, H., Ogawa, K., Sakatani, N., Iijima, Y., Honda, R., Ishibashi, K., Hayakawa, H., Sawada, H., 2017. Scientific objectives of Small Carry-on Impactor (SCI) and Deployable Camera 3 Digital (DCAM3-D): observation of an ejecta curtain and a crater formed on the surface of Ryugu by an artificial high-velocity impact. Space Sci. Rev. 208, 187-212. doi:10.1007/s11214-016-0290-z

Baturkin, V., Feidelheimer, V., Sasaki, K., Mikulz E., Ho, T.M., 2020. Regulative Characteristics of Methano-Copper Heat Pipes for Asteroid Lander “MASCOT”. Journal of Heat Transfer. Journal of Heat Transfer, May 2020, Vol. 142 (5), Paper No: HT-18-1566, Pages 051801-1 - 051801-9; https://doi.org/10.1115/1.4046443

Bibring, J.P., V. Hamm, Y. Langevin, Pilonget, C., Arondel A., Bouzit, M., Chaigneau, M., Crane, B., Darié, A., Evesque, C., Hansotte, J., Gardien, V., Gonnod, L., Leclech, J.-C., Meslier, L., Redon, T., Tamiatto, C., Tosti, S., Thoores, N., 2017. The MicrOmega investigation onboard Hayabusa2. Space Science Review, 208, 401-412

Bibring J.-P., Pilonget C., Riu L., Hamm V., Brunetto R., Ho T.-M., Jaumann R., Langevin Y., Moussy A., Poulet F., Vincendon, M., and the MicrOmega/Mascot team, 2020. MicrOmega/MASCOT first result, Planetary Space Science, submitted.

Biele, J., Kesseler, L., Grimm, C.D., Schröder, S., Mierheim, O., Lange, M., Ho, T.M., 2017. Experimental Determination of the Structural Coefficient of Restitution of a Bouncing Asteroid Lander. ArXiv e-prints 1705.00701

Biele, J., Kührt, E., Senshu, H., Sakatani, N., Ogawa, K., Hamm, M., Grott, M., Tatsuaki, O., Arai, T., 2019. Effects of dust layers on thermal Emission from airless bodies. Progress in Earth and Planetary Science, 6, 48. https://doi.org/10.1186/s40645-019-0291-0

Cénac-Morthé, C, Granena, D., Moussi-Soffys,A., Brochard, P., Duvivier, E., Grundmann, J.T., Sasaki, K., Ho, T.M., Maibaum, M., Cozzoni B., Krause, C., 2019. MASCOT electric power system experience during cruise and on-asteroid phase, European Space Power Conference 2019, 118

Coaplen, J., Stronge, W.J. and Ravani, B., 2004. Work equivalent composite coefficient of restitution. International Journal of Impact Engineering 30, 581–591.

Delbo, M., Libourel, G., Willkerson, J., Murdoch, N., Michel, P., Ramesh, K.T., Ganino, C., Verati, C., Marchi, S., 2014. Thermal fatigue as the origin of regolith on small asteroids. Nature 508, 233–236.
Debus A., et al., 2003. Two Stage Battery System for the Rosetta Lander, Acta Astron. 53, 623-632.

Garmier, R., Canalias, E., Lorda, L., Martin, T., Moussi, A., Biele, J., 2018. Space Ops Conference, Marseille, France, 2018.

Grott, M., Knollenberg, J., Borgs, B., Hänschke, F., Kessler, E., Helbert, J., Maturilli, A., Müller, N., 2017. The MASCOT Radiometer MARA for the Hayabusa 2 Mission. Space Science Rev. 208, 413-431.
https://doi.org/10.1007/s11214-016-0272-1

Grott, M., Knollenberg, J., M. Hamm, M., Ogawa, K., Jaumann, R., Otto, K. A., Delbo, M., Michel, P., Biele, J., Neumann, W., Knapmeyer, M., Kührt, E., Senshu, H., Okada, T., Helbert, J., Maturilli, A., Müller, N., Hagermann, A., Sakatani, N., Tanaka, S., Arai, T., Mottola, S., Tachibana, S., Pelivan, I., Drube, L., Vincent, J. -B., Yano, H., Pilorget, C., Matz, K. D., Schmitz, N., Koncz, A., Schröder, S. E., Trauthan, F., Schlotterer, M., Krause, C., Ho, T. -M., Moussi-Soffys, A., 2019. Nature Astronomy, 3, 971-976.
https://doi.org/10.1038/s41550-019-0832-x

Grott, M., J. Biele, P. Michel, S. Sugita, S. Schröder, N. Sakatani, W. Neumann, S. Kameda, T. Michikami, and C. Honda, 2020. Macroporosity and Grain Density of Rubble Pile Asteroid (162173) Ryugu. Submitted to: JGR Planets.

Grundmann, J.T., Biele, J., Findlay, R., Fredon, S., Ho, T.M., Krause, C., Ulamec, S., Ziach, C., 2014. One Shot to an Asteroid – MASCOT and the Design of an Exclusively Primary Battery Powered Small Spacecraft in Hardware Design Examples and Operational Considerations, European Space Power Conference 2014, 3051

Hamm, M., Senshu, H., Grott, M., 2018. Latitudinal dependence of asteroid regolith formation by thermal fatigue. Icarus, 319, 308-311, doi:10.1016/j.icarus.2018.09.033.

Hamm, M., Grott, M., Kührt, E., Pelivan, I., Knollenberg, J., 2018. A method to derive surface thermophysical properties of asteroid (162173) Ryugu (1999JU3) from in-situ surface brightness temperature measurements. Planetary and Space Science, 159, 1-10.
https://doi.org/10.1016/j.pss.2018.03.017

Hamm, M., Pelivan, I., Grott, M., and J. de Wiljes 2020: Thermophysical modelling and parameter estimation of small Solar system bodies via data assimilation. In: Monthly Notices of the Royal Astronomical Society, Volume 496, Issue 3, 2776-2785.
https://doi.org/10.1093/mnras/staa1755

Herčík, D., Auster, H.U., Blum, J., Fornaçon, K.-H., Fujimoto, M., Gebauer, K., Güttler, C., Hillenmaier, O., Hördt, A., Liebert, E., Matsuoka, A., Nomura, R., Richter, I., Stoll, B., Weiss, B.P., Glassmeier, K.-H., 2017. The MASCOT Magnetometer. Space Science Reviews, 208, 433-449.

Herčík, D., Auster, H.-U., Constantinescu, D., Blum, J., Fornaçon, K.-H., Fujimoto, M., Gebauer, K., Grundmann, J.T., Güttler, C., Hillenmaier, O., Ho, T.-M., Hördt, A., Krause, C., Kührt, E., Lorda, L., Matsuoka, A., Motschmann, U., Moussi-Soffys, A., Richter, I., Sasaki, K., Scholten, F., Stoll, B.; Weiss, B. P.; Wolff, F., Glassmeier, K.-H., 2020. Magnetic Properties of Asteroid (162173) Ryugu. Journal of Geophysical Research: Planets, 125. https://doi.org/10.1029/2019JE006035

Ho, T.M., Baturkin, V., Grimm, C., Grundmann, J.T., Hobbie, C., Ksenik, E., Lange, C., Sasaki, K., Schlotterer, M., Talapina, M., et al., 2017. MASCOT - The Mobile Asteroid Surface Scout Onboard the Hayabusa2 Mission. Space Science Reviews, 208, 339-374.

Iwata, T., Kitazato, K., Abe, M., et al., 2017. NIRS3: The Near Infrared Spectrometer on Hayabusa2. Space Science Review, 208, 317-337.

Jaumann, R., Schmitz, N., Koncz, A., Michaelis, H., Schroeder, S.E., Mottola, S., Trauthan, F., Hoffmann, H., Roatsch, T., Jobs, D., et al., 2017. The Camera of the MASCOT Asteroid Lander on Board Hayabusa2. Space Science Reviews, 208, 375-400.

Jaumann, R., Schmitz, N., Ho, T.M., Schroeder, S.E., Otto, K.A., Stephan, K., Elgner, S., Krohn, K., Preusker, F., Scholten, F., et al., 2019. Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites. Science, 365, 817-820.

Kameda, S., Suzuki, H., Takamatsu, T., Cho, Y., Yasuda, T., Yamada, M., Sawada, H., Honda, R., Morota, T., Honda, C., et al., 2017. Preflight Calibration Test Results for Optical Navigation Camera Telescope (ONC-T) Onboard the Hayabusa2 Spacecraft. Space Science Reviews, 208, 17-31.
Krause, C., Auster, U., Bibring, J.P., Biele, J., Cenac, C., Cozzoni, B., Dudal, C., Embacher, D., Fantinati, C., Fischer, H.H., Glassmeier, K.H., et al., 2018. MASCOT – a Mobile Lander on-board Hayabusa2 Spacecraft – Status and Operational Concept for the Asteroid Ryugu. Space Ops Conference, Marseille, France, 2018. https://doi.org/10.2514/6.2018-2418

Lange, C., Ho, T.-M., Grimm, C., Grundmann, J. T., Ziach, C., Lichtenheldt, R., 2018. Exploring Small Bodies: Nano- and microlander options derived from the Mobile Asteroid Surface Scout. Advances in Space Research, 62, 2055-2083, DOI: 10.1016/j.asr.2018.05.013 ISSN 0273-1177

Lange, C., Yoshimitsu, T., Ulamec, S., Düvel, C., Ho, T.M., 2020. The Micro- and nanolander MASCOT and Minerva-II on the surface of Ryugu – commonalities, differences and lessons learned for future microgravity exploration, Planetary Space Science, this issue

Lorda, L., Canalias, E., Martin, M., Garmier, R., Moussis, A., Biele, J., Jaumann, R., Bibring, J.-P., Grott, M., Auster, U., Ho, T.M., Krause, C., Maibaum, M., Cozzoni, B., Ulamec, S., Wolff, F., Tsuda, Y., Okada, O., Mimasu, Y., The process for the selection of MASCOT landing site on Ryugu: design, execution and results, Planetary Space Science, this issue

Michikami, T., Honda, C., Miyamoto, H., Hirabayashi, M., Hagermann, A., Irie, T., Nomura, K., Ernst, C.M., Kawamura, M., Sugimoto, K., Tatsumi, E., Morota, T., Hirata, N., Noguchi, T., Cho, Y., Kameda, S., Kouyama, T., Yokota, Y., Noguchi, R., Hayakawa, M., Hirata, N., Honda, R., Matsuoka, M., Sakatani, N., Suzuki, H., Yamada, M., Yoshioka, K., Sawada, H., Hemmi, R., Kikuchi, H., Ogawa, K., Watanabe, S., Tanaka, S., Yoshikawa, M., Tsuda, Y., Sugita, S., 2019. Boulder size and shape distributions on asteroid Ryugu. Icarus, 331, 179-191. https://doi.org/10.1016/j.icarus.2019.05.019

Molaro, J.L., Byrne, S., Langer, S.A., 2015. Grain-scale thermoelastic stresses and spatiotemporal temperature gradients on airless bodies, implications for rock breakdown. J. Geophys. Res. Planets 120, 255-277. https://doi.org/10.1002/2014JE004729.

Molaro, J.L., Byrne, S., Le, J.-L., 2017. Thermally induced stresses in boulders on airless body surfaces, and implications for rock breakdown. Icarus 294, 247-261.

Moussi, A., Lorda, L., Dudal, C., Cenac-Morthe, C., Granena, D., Lafaille, V., Charpentier, A., Garmier, R., Ho, T., Krause, C., Biele, J., Tsuda, Y., Okada, T., Yoshimitsu, T., 2018. CNES Support to Hayabusa2-MASCOT Mission. Space Ops Conference, Marseille, France, 2018.

Neumann, W., Breuer, D., Spohn, T., 2015, Modelling of compaction in planetesimals. Astron. Astrophys., 567, A120

Ogawa, K., Hamm, M., Grott, M., et al., Possibility of estimating particle size and porosity on Ryugu through MARA temperature measurements. Icarus, 333, 318-322, doi: 10.1016/j.icarus.2019.06.014, 2019.

Okada, T., Fukuhara, T., Tanaka, S., Unveiling highly-porous nature of a primitive asteroid by thermal imaging, Nature, Nature 579, 518-522 (2020). https://doi.org/10.1038/s41586-020-2102-6

Okada, T., T. Fukuhara, S. Tanaka et al., Thermal Infrared Imaging Experiments of C-Type Asteroid 162173 Ryugu on Hayabusa2, Space Science Reviews, 208 (2017), 255-286

Otto, K.A., Matz, K.-D., Schröder, S., Parekh, R., Krohn, K., Honda, R., Kameda, S., Jaumann, R., Schmitz, N., Stephan, K., Sugita, S., Ho, T.-M., Koncz, A., Trauthan, F., Cho, Y., Hayakawa, M., Honda, C., Kouyama, T., Matsuoka, M., Morota, T., Mottola, S., Ogawa, K., Preusker, F., Sakatani, N., Sawada, Scholten, F., Suzuki, H., Tatsumi, E., Yamada, M., Yokota, Y. and Yoshiaka, K, 2021. Surface roughness of asteroid (162173) Ryugu and comet 67P/Churyumov-Gerasimenko inferred from in-situ observations, Monthly Notices of the Royal Astronomical Society, 500 (3), 3178-3193.

Reill, J., H.J. Sedlmayr, P. Neugebauer et al., MASCOT - Asteroid Lander with Innovative Mobility Mechanism. ASTRA - 13th ESA Symposium on Advanced Space Technologies for Robotics and Automation, Noordwijk, Niederlande, 2015, 11-13 Mai

Scholten, F., Preusker, F., Elgner, S., Matz, K.-D., Jaumann, R., Biele, J., Hercik, D., Auster, H.-U., Hamm, M., Grott, M., Grimm, C., Ho, T.-M., Koncz, A., Schmitz, N., Trauthan, F., Kameda, S., Sugita, S., Honda, R., Morota, T., Tatsumi, E., Cho, Y., Yoshioka, K., Sawada, H., Yokota, Y., Sakatani, N., Hayakawa, M., Matsuoka, M., Yamada, M., Kouyama, T., Suzuki, H., Honda, C., Ogawa, K., 2019a.
The descent and bouncing path of the Hayabusa2 lander MASCOT at asteroid (162173) Ryugu. Astronomy & Astrophysics, 632, L3.

Scholten, F., Preusker, F. Elgner, S., Matz, K.-D., Jaumann, R., Hamm, M., Schröder, S.E., Koncz, A., Schmitz, N., Trauthan, F., Grott, M., Biele, J., Ho, T.M., Kameda, S., Sugita, S., 2019b, The Hayabusa2 lander MASCOT on the surface of Asteroid (162173) Ryugu - Stereo-photogrammetric analysis of MASCam image data. Astronomy & Astrophysics, 632, L5, doi:: 10.1051/0004-6361/201936760.

Schlotterer, S., R. Findlay, T.M. Ho, Witte, L., Ziach, C., 2014. Histogram Filter for Attitude Determination of Small Asteroid Lander, In: 9th International ESA Conference on Guidance, Navigation & Control Systems. 9th International ESA Conference on Guidance, Navigation & Control Systems, Porto, Portugal, 2014, 2 - 6 June

Schröder, S., Otto, K., Scharf, H., Matz, K.-D., Schmitz, N., Scholten, F., Mottola, S., Trauthan, F., Koncz, A., Michaelis, H., Jaumann, R., Yabuta, H., Sugita, S., 2020. Spectrophotometric analysis of the Ryugu rock seen by MASCOT: Searching for a carbonaceous chondrite analog, Planetary Science Journal, accepted, arXiv:2011.13810

Sugita, S., R. Honda, T. Morota, et al., The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body process, Science, 364 (2019) 252-252

Tsuda, Y, Saiki, T., Terui, F., Nakazawa, S., Yoshikawa, M., Watanabe, S. and Hayabusa2 Project Team, 2020. Hayabusa2 mission status: Landing, roving and cratering on asteroid Ryugu. Acta Astronautica, 171, 41 – 54.

Wada, K., Grott, M., Michel, P., et al., Asteroid Ryugu Before the Hayabusa2 Encounter. Progress in Earth and Planetary Science. Springer. doi: 10.1186/s40645-018-0237-y, 2018.

Watanabe, S., Tsuda Y., Yoshikawa M., et al., 2016. Hayabusa2 Mission Overview. Space Science Reviews, 208, 3–16.

Watanabe, S. Hirabayashi, M., Hirata N. et al., 2019. Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu – A spinning top-shaped rubble pile. Science, 364, 268-272