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September 22, 2011

Exoplanetsat and other things to do with Cubesats and Chipsats

Cubesats satellites are the size of one, two or three blocks (called U’s), each block being a 10cm x 10cm x 10cm cube [10 centimeters is 4 inches]. A satellite composed of three of these U’s in a row (a 3U CubeSat) is allowed to have a mass of at most 4 kg. At this scale, the satellites are just big enough to incorporate a relevant payload.

Exoplanetsats - Detecting transiting exoplanets using a low-cost CubeSat platform (15 pages)

Draper Laboratory and MIT have developed the Exoplanetsat satellite the size of a loaf of bread that will undertake one of the biggest tasks in astronomy: finding Earthlike planets beyond our solar system—or exoplanets—that could support life. It is scheduled to launch in 2012. Each nanosatellite will cost as little as $600,000 once in production—ExoPlanetSat cost approximately $5 million—and their estimated orbital lifetime is one to two years.

Nanosatellites, i.e. spacecraft that weigh between 1 and 10 kg, are drawing increasing interest as platforms for conducting on-orbit science. This trend is primarily driven by the ability to piggyback nanosatellites on the launch of large spacecraft and hence achieve orbit at greatly reduced cost. The CubeSat platform is a standardized nanosatellite configuration, consisting of one, two, or three 10 cm x 10 cm x 10 cm units (1, 2,or 3 “U”s) arranged in a row. We present a CubeSat-based concept for the discovery of transiting exoplanets around the nearest and brightest Sun-like stars. The spacecraft prototype—termed ExoplanetSat—is a 3U space telescope capable of monitoring a single target star from low Earth orbit. Given the volume limitations of the CubeSat form factor, designing a capable spacecraft requires overcoming significant challenges. This work presents the initial satellite configuration along with several subsystem-specific solutions to the aforementioned constraints. An optical design based on a modified commercial off-the-shelf camera lens is given. We also describe a novel two-stage attitude control architecture that combines 3-axis reaction wheels for coarse pointing with a piezoelectric translation stage at the focal plane for fine pointing. Modeling and simulation results are used to demonstrate feasibility by quantifying ExoplanetSat pointing precision, signal-to-noise ratio, guide star magnitude, and additional design parameters which determine system performance.
Baseline exoplanetsat. Not shown are the solar arrays, batteries, the light seal between the lens mount and focal plane array, and the CCD cooling strap.

CubeSats have been confined to low Earth orbit because no propulsion system available at this scale can give the satellite the energy needed to go beyond. This may soon change, and with that change will come new missions and applications for CubeSats. What follows is a list of what I consider to be the 5 most interesting CubeSat-based missions. While the list is by no means exhaustive, it gives a good idea of the possibilities open to private researchers with relatively little cost.

1. Earth Observation
2. Multi-body reconfiguration experiments
3. Astronomy
4. Orbit the Moon
5. Deliver ChipSats to cool places




This work presents an initial concept and baseline design for ExoplanetSat—a 3U CubeSat space telescope designed to monitor the brightest Sun-like stars for transiting Earth analogs. By adopting the CubeSat form factor, we aim to take advantage of low-cost piggyback launch opportunities and leverage existing COTS technologies. We present key subsystems, including a two-stage scheme for arcsecond-level pointing. The optical subsystem uses a refractive SLR camera lens and composite focal plane for both science measurements and guide star tracking. An initial analysis of detector and jitter noise shows that the 10 ppm noise threshold—the minimum required for detecting Earth-like exoplanets—is achievable given sufficiently low dark current noise, high photon flux, and highly precise pointing at the 0.1 to 1 arcsecond level. While we have shown that the pointing requirement is within reach, these goals are extremely difficult to reach within the constraints of a CubeSat and will be developed further in future publications. The transit method is directly scalable to other planet-star area ratios for a given system noise level. Therefore if mass and volume constraints prove the 10 ppm requirement to be infeasible, the science requirement may be adjusted to focus on larger exoplanets.

Future work
A key element of any future work is expansion and maturation of the imaging simulation. The ultimate goal is to create an end-to-end numerical model that can be used to simulate transit events in the presence of all anticipated noise sources. Central to this modeling effort will be benchmarking the simulation against experimental measurements. We anticipate doing this using a jitter noise testbed currently under construction at MIT. It will provide a way to inject a known amount of jitter-induced focal plane motion and measure the resulting signal error.

An item that will receive particular attention is the issue of stray light from the Earth and Moon entering the telescope during science operations. Volume constraints prevent the use of a large baffle, therefore a detailed analysis must be undertaken to characterize the detrimental impact stray light will have on the ability to meet science requirements. This and other efforts will continue over the coming months to mature the ExoplanetSat design and develop a functional prototype.
Possible deployable solar array configurations: (a) “cross” configuration (solar cells on the deployed panels are facing away) and (b) “table” configuration. In both cases body-mounted panels are used for tumbling scenarios.

1. Earth Observation

The standard mission for a CubeSat in Low Earth Orbit is to communicate with ground stations and in some cases send back a few photos. As smaller and more powerful cameras become available, the resolution and detail of images taken from CubeSats will only improve. Other types of sensors can be incorporated such as magnetometers.

2. Multi-body reconfiguration experiments

One active area of research that can benefit from cheap and fast cubesat projects is the development of autonomous docking and maneuvering algorithms for large groups of satellites. Having a set of three or more 1U CubeSats that can move around each other would be a great platform for validation of these experiments.

3. Astronomy

ExoplanetSat already discussed above. Use a cloud of CubeSats, each with a telescope, tracking bright stars in order to detect Earth-like planets orbiting other stars.

4. Orbit the Moon

By hitchhiking part of the way on the same rocket as a larger satellite, CubeSats are able to gain much of the energy necessary to escape Earth orbit. A much smaller propulsion system, maybe even one that fits inside the small volume of a CubeSat, can then take the satellite the rest of the way to the moon. There are ways to trade time for propellant savings. This way, a CubeSat can get to the Moon in a few months, but much more efficiently than the Apollo missions. Getting anywhere near the Moon is beyond the capabilities of any propulsion system that has flown on a CubeSat to date, but here at SSDS we are hoping to soon change that.

5. Deliver ChipSats to cool places

ChipSats are far smaller than CubeSats, but their high surface area to mass ratio allows them to do things larger satellites can’t dream of. At the ChipSat scale, effects such as air drag, solar pressure and electromagnetic forces become much more important and affect the chip’s orbit significantly. Using a CubeSat as a way to deploy ChipSats can change where and how ChipSats can be deployed. From low Earth orbit, the CubeSat can time the release of the chips, which will then flutter downwards and cover a target area. If the CubeSat has propulsion, it can eject the chips in higher orbits where solar pressure forces are stronger than gravity. The chips can then use solar pressure to sail away from the Earth and explore the far reaches of the Solar System.


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