Miniaturized Spectral Imaging Instrumentation for Planetary Exploration
Authors: Kohout Tomas , Näsilä Antti 
Published: October 2020

Tomas Kohout1 and Antti N&#228;sil&#228;2
Tomas Kohout and Antti N&#228;sil&#228; Tomas Kohout1 and Antti N&#228;sil&#228;2

1University of Helsinki, Faculty of Science, Helsinki, Finland (tomas.kohout@helsinki.fi)
2VTT Technical Research Centre of Finland Ltd, Espoo, Finland (antti.nasila@vtt.fi)

1University of Helsinki, Faculty of Science, Helsinki, Finland (tomas.kohout@helsinki.fi)
2VTT Technical Research Centre of Finland Ltd, Espoo, Finland (antti.nasila@vtt.fi)

Introduction
We introduce a novel concept for miniaturized spectral imaging instrument which can be used for planetary exploration missions. Key component in the spectral imager is a tunable Fabry-Perot Interferometer (FPI) developed at VTT (Fig. 1). It can be fitted in ca. 1 CubeSat unit (CU), thus making it suitable for all CubeSat sizes from 2U upwards. Depending on the mission requirements, the instrument can perform measurements from 0.5 &#181;m up to 7 &#181;m (Table 1). The visible and near-infrared (0.5 &#181;m - 1.6 &#181;m) components have already been flown and validated in low Earth orbit.

Planetary missions
Knowledge of planetary compositions is of a key importance in planetology. It enables us to determine the origin of solid material in different zones of our Solar System, as well as to understand current processes such as space weathering. Reflectance spectroscopy is one of the widely used tools to determine the composition of planetary surfaces. The key diagnostic features in the planetary spectra are Fe2+ (~ 1 and 2 &#181;m) and Fe3+ absorptions (~ 0.7 &#181;m) in silicates, H2O and &#8211;OH features (~ 1.4, 1.9 or 2.7-3 &#181;m), C-H bonds in organics (~ 3-3.6 &#160;&#181;m), Si-O silicate vibration bands in visible &#8211; near-infrared (VIS-NIR), or various ices and gases in mid-infrared (MIR). Hyperspectral imaging enables spatial resolution of these features, and thus mapping of composition of planetary surfaces in order to determine homogeneity of the target body or distribution of various minerals or volatile compounds. The continuous spectra measurement over broad wavelength enables determination of spectral continuum essential for determination of surface grain size, or in detection of the space weathering or shock effects.
The capability to combine different wavelength ranges in a compact volume makes the instrument highly useful in modern planetary exploration missions. Many larger space missions are planning to use smaller secondary spacecraft (InSight, Hera, Comet Interceptor) to perform secondary functions or to address different science objectives. As the larger missions are targeting very different objects (inner planets, outer planets, asteroids, comets) the required instrumentation differ. With standard CubeSat platforms and our modular instrumentation approach, it is easy to tailor an instrument suite for all types of targets and science cases.
CubeSat for Hera mission
The first mission for the modular spectral imager is European Space Agency&#8217;s Hera mission (launch in 2024). Part of this mission is a CubeSat with Asteroid Spectral Imager ASPECT as one of the payloads. Hera will study binary asteroid Didymos (primary 780 m and secondary 170 m in size) &#8211; target of the NASA DART (Double-Asteroid Redirection Test) kinetic impactor test mission. DART will impact the secondary asteroid, and create approx. 2-4 m sized crater with associated ejecta. ASPECT will map the composition of both asteroids, distribution of impact ejecta, space weathering effects on the asteroid surface, and impact-induced changes. ASPECT has four measurement channels: VIS (500 - 900 nm), NIR1 (850 - 1275 nm), NIR2 (1225 - 1650 nm) and SWIR (1600 - 2500 nm). VIS and NIR channels have imaging capability, while the SWIR channel is a single point spectrometer. All channels have good overlap with each other, so it is possible to construct the complete spectral slope from 500 nm up to 2500 nm.
Comet Interceptor mission
Comet Interceptor is a newly selected ESA F-mission (launch in 2028) to a dynamically new comet from Oort cloud. One of the key payloads is a modular hyperspectral imaging system MIRMIS (Modular Infrared Molecules and Ices Sensor) to detect the mineral, ice, and gas compounds in the comet nucleus and coma in NIR-MIR range, as well as to map temperature of the nucleus. The mission is first-of-its-kind to study the pristine material from the outer Solar System. Two of the MIRMIS channels, NIR and MIR, are based on VTT&#8217;s modular instrument concept. The NIR channel, measuring at ca. 1-1.6 &#181;m has the same design as the NIR channels used for ASPECT. The MIR channel is a new development, and it will measure from 2.5 &#181;m up to 7 &#181;m.
Atmospheric science missions
Another important topic is the abundance of gases or substances in planetary atmospheres. Capability to identify different gas species can bring insight to the evolution of the planetary body and to the processes related to the climate. One of the most useful techniques for performing atmospheric measurements from a small spacecraft is solar occultation. In this method, the instruments look directly at the Sun during sunsets and sunrises. There are two great benefits: the Sun is the most powerful source of light in our Solar System, and the method is self-calibrating as the measurement signal is always divided by the out-of-atmosphere signal. With this technique, it is possible to identify many gases in the atmosphere based on their absorption spectra and at the same time the vertical profiles can also be generated.
PICASSO mission
The PICo-satellite for Atmospheric and Space Science Observations (PICASSO) is an ESA mission (launch in 2020) led by the Belgian Institute for Space Aeronomy. VTT has developed one of the payloads, VISION, for the mission. VISION has two scientific objectives: to study the ozone distribution in the stratosphere and to measure the atmospheric temperature profile up to the mesosphere. VISION is a spectral imager operating between 430 - 800 nm, and it is capable of taking 2D snapshots of the Sun at freely selectable wavelengths. VISION instrument will observe the atmospheric limb during orbital Sun occultation. By addressing the radiation absorption in the Chappuis band for different tangent altitudes, the vertical profile of ozone can be retrieved.