NEA Scout (Near Earth Asteroid Scout) CubeSat Mission
NEA Scout is an interplanetary 6U CubeSat mission of NASA, a secondary payload of the Artemis-1 mission, the recently inaugural and renamed SLS (Space Launch System) EMS-1 mission to lunar space with a planned launch in 2020. The objective of NEA Scout is to fly to about 1 AU from Earth to conduct a flyby of a near Earth asteroid (NEA) less than 100 m across. NEA Scout will be guided by a solar sail, towards its target asteroid 1991VG. 1) 2)
NASA/MSFC (Marshall Space Flight Center) and NASA/JPL (Jet Propulsion Laboratory) are jointly developing this mission with support from NASA/GSFC (Goddard Space Flight Center), NASA/JSC (Johnson Space Center), NASA/LaRC (Langley Research Center), and NASA Headquarters. The Principal Investigator (science) is Julie Castillo-Rogez from NASA/JPL. The Principal Investigator (solar sail) is Les Johnson from NASA/MSFC.
Figure 1: Artist concept of the NEA Scout spacecraft as it flies slowly by the target asteroid (~10 km/s), image credit: NASA, NEA Scout Team
Due to its small size and low albedo, complete characterization of 1991 VG from Earth is difficult. A combination of target orbit uncertainty and long lead times for solar sail trajectory correction maneuvers drive a requirement to identify the target in optical navigation imagery at a distance of about 60,000 km. At closest approach, the same imager will be used for near field imaging of the target. Figure 2 summarizes the NEA Scout concept of operations.
Challenges faced by NEA Scout
Traditional large spacecraft accomplish these imaging objectives using long exposures to increase SNR and identify the low albedo target. Due to the pointing drift and jitter inherent in a small platform, long exposure imaging is less feasible for NEA Scout. Onboard image processing overcomes this challenge. The spacecraft aligns and combines a stack of rapidly acquired images, resulting in a single image with a higher SNR than its constituent images. We filter the aligned images using a temporal median. This solution fits within the memory constrained onboard context. Prior to alignment, each image undergoes a first order image calibration, onboard, to improve the results of the alignment. This calibration consists of a dark current subtraction, flat field adjustment and bad pixel mask application. The temporal median has the added benefit of removing transient imaging artifacts, such as cosmic rays. Interplanetary CubeSats, such as NEA Scout, are additionally physically constrained by the size of their antenna and available transmission power, which is a major challenge for science-driven CubeSat missions. At closest target approach, NEA Scout will be constrained to approximately <1 kbit/s downlink bandwidth.
All CubeSats under development for the Artemis-1 mission are based on a 6U form factor. In the case of NEA Scout, about half of that volume is allocated to the propulsion system (solar sail and thrusters), 1/3rd to the avionics and instrument, and the rest is used by the power system, the antenna, structure, and harness. The instrument is a monochromatic camera certified for deep space that acts both as a science instrument and an optical navigation camera. NEA Scout's science objectives are to retire strategic knowledge gaps for Human exploration and increase our understanding of near earth asteroids by focusing on a class of targets (<100 m) that has not been covered by previous and ongoing missions. Specific measurement objectives include global shape determination and regional morphology mapping, determination of rotational parameters, including whether the object is a single axis rotator or a tumbler, albedo mapping on a global scale, and high-resolution imaging of a fraction of the surface. At closest approach, the resolution is projected to be <10 cm/pix.
The size of the NEA Scout reference target, 1991 VG, is between 5 and 17 meters. Although ground-based observations acquired 25 years apart have provided relatively accurate ephemeris for that body, its small size and potentially low albedo, make it a challenging target for approach observations. The encounter is planned at about 1 AU from Earth.
The NEA Scout camera detector is similar to that used for the navigation cameras on the Mars 2020 rover. 3) This camera takes advantage of the new generation of arrays with a frame size of ~14 MPixel that enables good spatial resolution of the target images while preserving a large field of view necessary for target search and optical navigation. A major drawback is the large volume of the raw data, 225 Mbit/image for an imaging depth of 16 bits. In absence of a priori knowledge on the target, it is not possible to predict the parameters for lossless compression. However, it is understood that the target fills in only a small fraction of the FOV (Field of View). During the Approach phase (Figure 2), this fraction, including margins based on the target position uncertainty and the spacecraft attitude uncertainties, is about 0.28%. During the science phase of the mission (Reconnaissance and Proximity) the NEA occupies about 7% of the FOV. The total downlinked data volume is about 200 Mbit, the bulk of which is acquired during the science phase. This corresponds to a downlink time of 60 hours at a rate of 1 kbit/s projected at 1 AU.
Figure 2: Summary of NEA Scout's activities throughout the ~2 year mission (image credit: NASA/JPL)
Besides data volume, the NEA Scout mission is facing another key challenge in the form of pointing conflicts among various subsystems: camera, solar panels, medium-gain antenna, and solar sail. Ground contacts are limited to about 50 minutes, driven by the secondary batteries, followed by recharge periods of about 8 hours. When all constraints are accounted for, the 60-hour downlink has to be broken down over a period of 30 days.
The pointing performance meets the requirement to stay within 0.2 pixel over an integration time of 0.7 seconds and to stay within a box of 100 x 100 pixels during the acquisition of 20 images for the target search activity. Also, use of JPL's small computer, the Sphinx, provides the data storage and computing performance necessary to implement the data management strategies. 4)
The "6U" solar sail-propelled CubeSat will address human exploration-focused Strategic Knowledge Gaps. NEA Scout will perform a close and slow rendezvous to provide the first imagery and characterization of a NEA in them solar sail to serve as the primary means of propulsion to the NEA providing a ΔV of up to 2km/s, a magnitude currently impossible to meet with other high technology readiness level CubeSat-sized propulsion systems. Momentum exchange between the Sun's photons and the solar sail membrane provides the means necessary to perform a long duration deep space cruise and perform a NEA rendezvous at(resource utilization, planetary defense, human operations, and science) and paves the way for future multi-spacecraft exploration of NEAs. Using an optical imaging payload, NEA Scout will characterize the morphology, rotational and orbital properties, volume, color type and meteoritic classification, as well as the dust/debris environment of the target. 5)
An Active Mass Translation (AMT) device was added to the design to compensate for an initial offset in Center-of-Mass (CM) and Center-of-Pressure (CP) and a requirement to minimize or eliminate momentum generation while sailing at various attitudes. When combined with sail irregularities such as sail flatness uncertainties, tears in the sail stemming from deployment, micrometeriods or simple design anomalies, the sail thrust vector alignment to the spacecraft CM will vary with the spacecraft's attitude relative to the solar incidence angle. The AMT will translate roughly half of the spacecraft relative to itself along two axes and change the CP/CM relationship. This will enable the desired range of flight angles and maximize the use of the limited on-board propellant, momentum generation and minimize any required desaturations of the reaction wheels (Ref. 13).
The NEA Scout spacecraft is housed in a 6U (10 cm x 20 cm x 30 cm) CubeSat form factor and is divided into three modules: Avionics, Solar Sail/AMT, and RCS. The Avionics Module houses the majority of the spacecraft electronics and the Attitude Determination and Control System (ADCS). The AMT and Solar Sail Module contains all of the components necessary to deploy and operate the solar sail. The RCS module houses the cold gas reaction control system and the mounting points for the solar panels, one transmission/receive low gain antenna (LGA) pair, a patch array medium gain antenna (MGA), and sun sensors. The electrical wiring between the assemblies is routed through the center of the Solar Sail Assembly and must allow translation between the Avionics and Solar Sail Modules when the AMT is operating. A graphical representation of the spacecraft subsystems and components can be found in Figure 3.
Figure 3: The NEA Scout spacecraft and the major subsystems are highlighted (image credit: NASA, NEA Scout Team)
The EPS (Electrical Power System) includes two deployable solar panels. Power generated by the solar arrays is routed to the power control boards located with the avionics. The telecommunications system consists of the JPL-developed Iris transponder, two LGA pairs, and one MGA. The spacecraft ADCS (Attitude Determination and Control Subsystem) is made up of the reaction wheels, the reaction wheel controller, the star tracker, various sun sensors, the AMT, and the RCS.
Figure 4: Flight system overview (image credit: NASA)
Figure 5: NEA Scout solar sail technology (image credit: NASA)
Launch: The NEA Scout 6U CubeSat will fly as a secondary payload on the Artemis-1 mission, originally known as the Orion EM-1 (Exploration Mission) of the SLS (Space Launch System), with a planned launch in 2020.
Orbit: The orbit of NEA Scout will be that of the Artemis-1 mission to the moon - where NEA Scout will be deployed and thereafter on its own mission.
NEA Scout Camera
To reduce development time of the camera while reducing the overall cost and risk inherent to creating a new design, the NEA Scout camera takes advantage of an existing modular camera platform implemented for the OCO-3 (Orbiting Carbon Observatory-3) context cameras. The OCO-3 context camera is provided to the electronics body while allowing mission-specific customizations. To meet the signal to noise ratio and field of view requirements, the NEA Scout camera integrates the monochrome version of the CMV20000 CMOS detector used in the OCO-3 implementation. For the optics, a ruggedized commercial lens was procured that meets the speed and field of view necessary for the object detection and close-up imaging: f/2.8, 50.2 mm focal length and 27º FOV. The image circle projected onto the detector from the lens is 24 mm, reducing the useful detector window to 3840 x 3840 pixels. In practice, the target detection only needs of a reduced size so the detector windowing capability is used to capture a smaller size image for each frame (3840 x 2184 pixels).
Figure 6: NEA Scout camera before spacecraft integration (image credit: NASA)
Table 1: NEAScout camera physical specifications
NEA Scout Development
• July 2018: NASA's NEA Scout spacecraft is hitching a ride on the Space Launch System's inaugural flight to lunar space. There, the little CubeSat will deploy an 85 m2 solar sail and eventually spiral out of the Earth-Moon system to visit a near-Earth asteroid. On 28 June 2018, the project completed its first and only full-scale solar sail deployment test at NeXolve, a space materials company near Marshall Space Flight Center in Huntsville, Alabama. 6)
- "Everything pretty much went in line with the tests we've had to date," said Tiffany Lockett, a project system engineer for NEA Scout, in a phone interview. "It was as close to a flight-like deployment as we could get."
- It's challenging to deploy a space-bound solar sail on Earth, where gravity weighs everything down. Similar to The Planetary Society's LightSail 2 deployment tests, engineers unfurled NEA Scout's solar sail on a large, low-friction table. But since NEA Scout has a bigger sail, the team also used helium balloons to counteract the pull of gravity, and placed the booms on hockey puck-like sliders.
- Alex Few, the project's mechanical lead, said it was important to make sure any help the team gave to NEA Scout didn't invalidate the purpose of the test: to show that the spacecraft can deploy its sails on its own, in space.
- "Seeing the first 30 seconds go smoothly, that was a cool thing," said Few, who can be seen standing on the deployment table in videos from the test. "It was textbook, just how we expected it."
The full NEA Scout flight sail was deployed and tested in the summer of 2018 (Figure 7).
Figure 7: The fully-deployed NEA Scout solar sail undergoing its final pre-flight checkout (image credit: NASA, NEA Scout Team)
Figure 8: Deployment test of the solar sail at NeXolve (image credit: NASA/GSFC)
Figure 9: NEA Scout is a robotic reconnaissance mission that will deploy a 6U CubeSat to fly by and return data from an asteroid representative of possible human destinations. Using a solar sail for its propulsion system, it will perform reconnaissance of an asteroid, take pictures and observe its position in space (video credit: NASA/MSFC, Published on 21 September 2016)
Overview of secondary payloads on the Artemis-1 mission (formerly the Orion/EM-1 mission)
The first flight of NASA's new rocket, SLS ( Space Launch System), will carry 13 CubeSats/Nanosatellites to test innovative ideas along with an uncrewed Orion spacecraft in 2020. These small satellite secondary payloads will carry science and technology investigations to help pave the way for future human exploration in deep space, including the journey to Mars. SLS' first flight, referred to as EM-1 (Exploration Mission-1), provides the rare opportunity for these small experiments to reach deep space destinations, as most launch opportunities for CubeSats are limited to low-Earth orbit. 7) 8)
The secondary payloads, 13 CubeSats, were selected through a series of announcements of flight opportunities, a NASA challenge and negotiations with NASA's international partners.
All the CubeSats will ride to space inside the Orion Stage Adapter, which sits between the ICPS ( Interim Cryogenic Propulsion Stage) and Orion (Figure 10). The cubesats will be deployed following Orion separation from the upper stage and once Orion is a safe distance away.
The SPIE ( Spacecraft and Payload Integration and Evolution) office is located at NASA/MSFC (Marshall Space Flight Center) in Huntsville, Alabama, which handles integration of the secondary payloads.
These small satellites are designed to be efficient and versatile—at no heavier than 14 kg, they are each about the size of a boot box, and do not require any extra power from the rocket to function. The science and technology experiments enabled by these small satellites may enhance our understanding of the deep space environment, expand our knowledge of the moon, and demonstrate technology that could open up possibilities for future missions. 11)
A key requirement imposed on the EM-1 secondary payload developers is that the smallsats do not interfere with Orion, SLS or the primary mission objectives. To meet this requirement, payload developers must take part in a series of safety reviews with the SLS Program's Spacecraft Payload Integration & Evolution (SPIE) organization, which is responsible for the Block 1 upper stage, adapters and payload integration. In addition to working with payload developers to ensure mission safety, the SLS Program also provides a secondary payload deployment system in the OSA (Orion Space Adapter). The deployment window for the CubeSats will be from the time ICPS disposal maneuver is complete (currently estimated to require about four hours post-launch) to up to 10 days after launch. 12)
After its deployment from NASA's Space Launch System (SLS) in 2020, the Near Earth Asteroid (NEA) Scout mission will image an asteroid on a close flyby using an 86m2solar sail as its primary propulsion. NEA Scout, with a 6U CubeSat form factor, is one of several secondary CubeSat payloads to be deployed from the SLS on its maiden flight. The NEA Scout will be ejected from the SLS on a trajectory toward the moon and will use its onboard cold gas propulsion system to attain an elliptical lunar orbit. Once the spacecraft is in orbit, the solar sail will deploy and spacecraft checkout will begin. The NEA Scout will remain in the lunar vicinity until the low-thrust trajectory to the destination asteroid, 1991VG, or another NEA of interest, can be attained. The spacecraft will then begin its 2.0 –2.5 year journey to the asteroid. About one month before the asteroid flyby, NEA Scout will search for the target and start its Approach Phase,using a combination of radio tracking and optical navigation. The solar sail will provide continuous low thrust to enable a relatively slow flyby (10-20 m/s) of the target asteroid under lighting conditions favorable to geological imaging (<50 degree phase angle). Once the flyby is complete, and if the system is still fully functioning, an extended mission will be considered –the reconnaissance of another asteroid or a re-flyby of the first asteroid several months later are both options. NEA Scout is funded by the NASA Human Exploration and Operations Mission Directorate. 13)
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6) John Davis, "NEA Scout unfurls solar sail for full-scale test," The Planetary Society, 13 July 2018, URL: http://www.planetary.org/blogs/jason-davis/nea-scout-deployment-test.html
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8) Christopher Moore, Jitendra Joshi, Nicole Herrmann, "Deep-Space CubeSats on Exploration Mission-1," Proceedings of the 68th IAC (International Astronautical Congress), Adelaide, Australia, 25-29 Sept. 2017, paper:IAC-17-B4.8
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13) Les Johnson, Julie Castillo-Rogez, and Tiffany Lockett "Near Earth Asteroid Scout: Exploring Asteroid 1991VG Using A Smallsat," Proceedings of the 70th IAC (International Astronautical Congress), Washington DC, USA, 21-25 October 2019, paper: IAC-19/B4/2, URL: https://iafastro.directory/iac/proceedings/IAC-19/IAC-19/B4/2/manuscripts/IAC-19,B4,2,9,x49173.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).