December 18, 2010

Earthlike planet detection with combination of satellite missions from MIT

MIT researchers increase their odds of detecting an Earthlike planet by working on a combination of satellite missions.

Kepler space telescope’s major drawback is that because the telescope is only focused on a narrow field of the sky, it is observing faraway, faint stars and may be missing closer stars — the kind that will enable the best follow-up observations from the ground.

A team of MIT researchers led by Kavli Senior Research Scientist George Ricker is currently designing a satellite that would use six wide-angled, high-precision cameras to observe a wider region of the sky — one 400 times larger than revealed by Kepler's scope. By surveying some 2.5 million stars, the Transiting Exoplanet Survey Satellite (TESS) could potentially detect between 1,600 and 2,700 planets within two years, including between 100 and 300 small planets, several of which could be Earth candidates.



TESS has been in development since 2007, when NASA selected Ricker’s team to develop a mission as part of its Small Explorer satellite program to provide funding for missions using small- to midsized spacecraft. Although NASA elected not to proceed with TESS in 2009, the agency is accepting new proposals later this year and has increased the budget from $105 million to $200 million.

This fall, the TESS team has been finalizing a new proposal that reflects the budget increase. “The higher cost cap means we can use bigger cameras and bigger lenses to get more data,” says Seager, who is the deputy mission scientist for TESS. The group is also refining its estimates of the number and type of exoplanets that TESS could find.

Transiting Exoplanet Survey Satellite at wikipedia

The survey would focus the G and K spectral type stars brighter than 12 magnitudes, approximately 2 million of them would be studied, and the 1,000 closest M-type red dwarfs (within 30 parsecs). It is expected to discover 1,000 - 10,000 transiting exoplanets down to the size of the Earth and up to 2 months of period. The candidates could be later investigated by the HARPS spectrometer and some of them could be targets of the James Webb Space Telescope. The developer team is optimistic enough to claim that the first interstellar space missions' destinations could be among these stars.

Utilizing CubeSats
Another satellite effort currently underway at MIT is ExoplanetSat, a research project inspired by the increasing use of cube satellites, or “CubeSats,” by research universities as a way to conduct observations in space. The Rubik’s Cube-sized satellites are popular because they can piggyback on a variety of launch vehicles for a fraction of what it costs to put larger satellites in space. There are about a dozen CubeSats currently orbiting Earth.

Seager’s idea is that the odds of detecting a transiting planet orbiting a bright star increase significantly if there is just one telescope dedicated to observing a star that neither Kepler nor TESS can observe.

The concept eventually developed into ExoplanetSat, a research program that is designed to launch a fleet of about one dozen “triple CubeSats” (three cubes stuck together), and about another two dozen six-unit CubeSats into low-Earth orbit. Each satellite would have its own computer, processor and tiny camera and would be pointed at an individual star. Although the program doesn’t have funding yet, MIT students will try to build two prototypes within the next two years and then hopefully secure funding for a formal mission to send dozens of the tiny cubes into space.

But detecting Earthlike planets may only be half the battle, according to Joshua Winn, an assistant professor in MIT’s Department of Physics, who says “it remains to be seen” if current telescope technology will enable researchers to study Earthlike planets with enough detail to confirm whether they can host life. For now, researchers await the 2014 launch of the James Webb Space Telescope, an instrument whose size will make it easier to obtain higher-quality data of exoplanet atmospheres.

ExoplanetSat: detecting transiting exoplanets using a low-cost CubeSat platform
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.

Achieving high-precision pointing on ExoplanetSat: initial feasibility analysis

ExoplanetSat is a proposed three-unit CubeSat designed to detect down to Earth-sized exoplanets in an orbit out to the habitable zone of Sun-like stars via the transit method. To achieve the required photometric precision to make these measurements, the target star must remain within the same fraction of a pixel, which is equivalent to controlling the pointing of the satellite to the arcsecond level. The satellite will use a two-stage control system: coarse control will be performed by a set of reaction wheels, desaturated by magnetic torque coils, and fine control will be performed by a piezoelectric translation stage. Since no satellite of this size has previously demonstrated this high level of pointing precision, a simulation has been developed to prove the feasibility of realizing such a system. The current baseline simulation has demonstrated the ability to hold the target star to within 0.05 pixels or 1.8 arcseconds (with an 85 mm lens and 15 μm pixels), in the presence of large reaction wheel disturbances as well as external environmental disturbances. This meets the current requirement of holding the target star to 0.14 pixels or 5.0 arcseconds. Other high-risk aspects of the design have been analyzed such as the effect of changing the guide star centroiding error, changing the CMOS sampling frequency, and reaction wheel selection on the slew performance of the satellite. While these results are promising as an initial feasibility analysis, further model improvements and hardware-in-the-loop tests are currently underway.

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