ASPECT Spectral Imager CubeSat Mission to Didymos Kestilä A. 1),Näsilä A. 2), Kohout T. 3),Tikka T. 1),Granvik M. 3) 1. Aalto University, Finland. 2. Technical Research Center of Finland, Finland 3. Helsinki University, Finland Abstract The ASPECT mission is a 3U deep space CubeSat mission, planned to be sent as a part of the larger Asteroid Impact & Deflection Assessment (AIDA) to the Didymos binary asteroid system. It aims to study the composition and inner structure of the smaller Didymos II asteroid, as well as the effects of space weathering and shock metamorphism on it in order to gain understanding of the formation and evolution of the Solar System. ASPECT will also demonstrate the feasibility of a CubeSat mission in deep space environment. The AIDA mission in turn comprises of the AIM (ESA) and DART (NASA) spacecraft. One of its goals is to perform an impact experiment on the smaller of the two Didymos asteroids using DART as the impactor, with AIM remaining as an observer. The impactor is expected to make a crater on the surface of the asteroid, bringing up fresh material and exposing better the inner structure. This will also show how well an impacting spacecraft is capable of changing an asteroid s orbit. ASPECT will be deployed from AIM before the impact, and subsequently will take spectral images for a complete coverage of Didymos II before and after the event. The payload spectral imager will perform measurements in the 500 2500 nm range, and will have a better than 2 m spatial resolution of Didymos II. The satellite s intended orbit is circular with a slight inclination with respect to the Didymos II orbital plane, and outside both Didymos asteroid orbits. Its orbit stability will last only several days, necessitating the use of active orbit maintenance with an onboard propulsion system. The satellite bus will use commercial off the shelf avionics, and the main communication link will be through an inter satellite link with AIM. Introduction Small satellites are rapidly developing towards applications the lead them away from Low Earth Orbit (LEO). Several missions involving satellites less than 50 kg have been and are planned for far away targets such as other planets in the Solar System or asteroids. Even some student built microsatellite missions have been launched, such as the Japanese Procyon to 2000 DPO107 or the Shin en 2 to a heliocentric orbit. ASPECT (Asteroid Spectral Imaging Mission) is a part of AIDA/AIM project and aims to study the composition of the Didymos binary and the effects of space weathering and shock metamorphism in order to gain understanding of the formation and evolution of the Solar System. ASPECT will be piggybacked under the joint ESA/NASA AIDA (Asteroid Impact &
Deflection Assessment) mission to the Didymos system onboard AIM (Asteroid Impact Mission by ESA). DART (Double Asteroid Redirection Test by NASA) is targeted to impact the secondary Didymos asteroid and serve as a kinetic impactor to demonstrate deflection of potentially hazardous asteroids. AIM will serve as an observational spacecraft to evaluate the effects of the impact and resulting changes in the Didymos dynamic parameters. The AIM mission will carry CubeSat miniaturized satellites, and release them close to the Didymos system. This arrangement opens up a possibility for secondary scientific experiments. ASPECT is one of the proposed CubeSat payloads. The Didymos system consists of the 775 m diameter primary, and around 1.2 km away orbiting 163 m diameter secondary, that has a bit less than 12 hours period. Figure 1: Radar based model of the two Didymos asteroids. Science Goals and Mission Description The main observational target is Didymos II, with Didymos I a close secondary target. Whereas Didymos is a space weathered binary asteroid, the DART impactor is expected to produce a crater and excavate fresh material from the secondary. Spectral comparison of the mature surface to the freshly exposed material will allow to directly determine space weathering effects. It will be also possible to study spectral shock effects within the impact crater. ASPECT will also demonstrate for the first time the joint spacecraft CubeSat operations in asteroid proximity and miniature spectral imager operation in deep space environment. The primary scientific objectives of ASPECT are: Study of the surface composition of the Didymos system. Photometric observations (and modeling) under varying phase angle and distance. Study of space weathering effects on asteroids (comparison of mature / freshly exposed material).
Study of shock effects (spectral properties of crater interior). Observations during the DART impact. These goals, achievable by a spectral imager, were designed to complement AIM s science objectives in order to maximise the overall study output during the both spacecraft operational periods. AIM and its CubeSats have also several technical goals, such as the demonstration of CubeSat semi autonomous operations in deep space environment ; navigation in the vicinity of a binary asteroid, and the demonstration of a satellite survival during impact. The mission will also demonstrate a join operation involving a spacecraft and multiple CubeSats. The minimum amount of images to be taken with the spectral imager are divided into two eight image series,one before and one after the impact of DART. Each image series will completely cover Didymos II, the upper limit of which is defined by AIM s data downlinking restrictions of 1Gb. ASPECT s mission is designed to be performed within a (preliminary) 3 months timeframe starting from AIM deployment. The optimal satellite orbit is based on satisfying the payload requirements as well as on the stability of the chosen orbit. Stability around a system like Didymos is primarily defined by the solar radiation pressure and the non homogenuity of the asteroids gravitational field at closer distances and is generally very chaotic (Scheeres, 2012). The performance parameters of the spectrometer payload work best at 4.1 km semi major axis (SMA), with zero eccentricity to minimize field of view (FoV) and ground pixel resolution variation. At this SMA the ground pixel requirements are met with a feasible Field of view design. In turn, orbits closer to Didymos I are restricted by their field of view as then the primary asteroid covers more of the view available to the satellite. For example, 500 to 700 m SMA circular orbits are stable for up to 28 days (Damme,2016), but have roughly a 9 to 6 times larger portion of the secondary s orbit blocked by the primary than for example at a circular orbit with an SMA of 4.1 km. The lighting angle for Didymos II and the Sun also precluded orbits with significantly high inclinations with respect to the the orbit plane of the binary, such as terminator orbits. A lagrange point position during the mission is not advantageous either, as only one side of Didymos II will be observed. The concern of debris crowding the Didymos II orbital plane after impact by DART as well as providing complete coverage of Didymos II (including its poles ) led to a 15 degrees inclination with respect to Didymos II orbital plane. The chosen orbit was thus a circular orbit of 4.1 km SMA with a 15 degrees inclination with respect to the binary orbit plane. According to preliminary modelling, the orbit will be stable for less than 10 days without active orbit control, necessitating propulsion onboard the satellite. This propulsion also doubles as a help for the satellite s attitude control. The satellite will be regularly healthchecked during the transfer before its mission, and deployed from a deployment pod designed for low, 2 to 5 cm/s deployment velocities. After deployment, the satellite will need to make its own maneuvers to get into its final orbit. The low deployment velocities used are due to the weak gravitational influence in the system, meaning that any much higher deployment velocities will force the satellite to expend even more propellant to achieve its final orbit and thus make the process difficult.
The main difficulty currently seen for the mission is the autonomous operations the satellite will have to perform, as there might be even a week long gap in uploading new commands and receiving telemetry data from the satellite. In between these opportunities the satellite will have to perform its mission, navigate and keep its orbit without help from ground. Figure 2: The satellite will be deployed from AIM against its velocity vector, after which it will guide itself (gapped red line) to its target orbit (blue orbit) using its own propulsion. Payload description The main payload onboard the satellite is an instrument combining a VIS NIR spectral imager and a non imaging spectrometer. They re based on tunable fabry perot technology which enables a precise and compact design. The imager has a spatial resolution of 2 meters or less in a spectral bandwidth of 500 1600 nm, while its spectral resolution is 10 30 nm. In the VIS channel it sports 512 by 512 pixels, and in the NIR 256 by 256 pixels. From 1600 to 2500 nm the non imaging spectrometer is used. Figure 3: The 1U payload, which includes two specific spectral imager channels, one spectrometer, and an envisioned navcam to aid the satellite s navigation (lower left). The payload design is based on the Aalto 1 CubeSat Spectral Imager heritage and is already space qualified. ASPECT will also demonstrate the capabilities of a CubeSat and a miniature spectral imager for the first time in deep space environment.
Spacecraft description ASPECT is a 3U CubeSat with outer configurations as seen in Figure 4a. The preliminary design of the satellite conforms to the CubeSat standard. The propulsion system is roughly ⅔ U and thanks to the low gravitational force of the asteroid system is a cold gas thruster, with about 10 m/s deltav in total. Its placed at one end of the satellite, from where it is able to control both its orbit and attitude, at the top of the model in Figure 4b. On the end is the payload, with its lenses pointed along the satellite long axis, as can be seen in Figure 4a. Figure 4 a) The inside model where the propulsion unit is at the top end and the payload at the bottom and the ADCS beneath the propulsion system, and b) the external view of the satellite with the payload lenses visible. The satellite s attitude determination and control system (ADCS) is Aalto 1 heritage without magnetorquers (as no significant magnetic fields are present), but relies on crude pointing and periodic discharging of its reaction wheels by the propulsion system. The electrical power system (EPS) will be main telemetry gathering hub, and will be radiation hardened to withstand energies and doses significantly larger than CubeSat COTS EPS subsystems in LEO as it will be the main subsystem in operation during transfer and mission. The data that the payload needs to be processed and packed, and a separate rad hard PDHU will be onboard for that purpose. The satellite will communicate both its TT&C and downlinked data back to Earth via an inter satellite link (ISL) provided by ESA. The ISL operates in the S band, and will also provide accurate navigation data to ASPECT, such as timing and other Doppler based information (with respect to AIM).
Conclusions AIDA presents possibilities for novel CubeSat technology demonstrations, the first ever deployment of a CubeSat around an asteroid environment, as well as very interesting new science with spectral imager based observations. It ll potentially produce unique data on asteroid composition and its changes, and also provide significant improvement of our understanding of space weathering and shock processes. References D.J. Scheeres, 2012. Orbital Mechanics about Small Bodies," Acta Astronautica 72: 1 14. DOI: 10.1016/j.actaastro.2011.10.021 F. Damme, H. Hussmann, E. Mai, J. Oberst, K. Wickhusen, 1st of March 2016. Orbit stability in the Binary Asteroid System Didymos An opportunity for spacecraft exploration, AIM Science Meeting