Frank Morring, Jr. Washington

A $5 million cubesat is definitely top of the line, but not when it is being developed to perform work similar to that underway on the $600 million Kepler planet-finder mission.

A group at the Massachusetts Institute of Technology (MIT) is developing a cubesat dubbed ExoplanetSat to evaluate whether any Earthlike planets found circling bright, relatively nearby stars have orbits that would permit spectral analysis of their atmospheres.

While the cost of the first planet-finding cubesat taking shape at MIT is high, its developers hope to be able to build enough of them to bring down the unit cost. Restrictions imposed by the tiny space available inside the 3U cubesat—measuring 30 X 10 X 10 cm—limit each to observing one exoplanet, so a “swarm” of “dozens” of spacecraft watching the same number of different stars would be needed, says Sara Seager, a professor of planetary science and professor of physics at MIT who is key to the work.

“Kepler is looking at faint stars that are by definition far away,” Seager says. “We're trying to look at the brightest, nearest stars. The bright stars are spread all around the sky, and that's why a Kepler wouldn't work for all the brighter stars, because Kepler only looks at one patch of the sky. We need one telescope per star.”

While Kepler is a survey instrument that stares at a tiny area of sky and measures the faint flicker that occurs when an orbiting exoplanet moves in front of it, ExoplanetSat will stare at a single star to gather as much data as possible from a transiting planet. Given the tiny change in the amount of light reaching the spacecraft's detector, keeping the light from a target star focused will require pointing accuracy at “the several-arcsecond level” if the noise level in the system is to be low enough to permit meaningful measurements.

Packing all that capability into a spacecraft only 30-cm long will require clever use of hardware—some of it off-the-shelf, as is typical of cubesats—and some really clever software. Gross pointing is achieved with miniature reaction wheels produced for the cubesat market by Maryland Aerospace Inc. of Crofton. These serve to point the spacecraft at the target star with an accuracy of 60-100 arcseconds, Seager says.

Light from the star is collected with a space-hardened off-the-shelf single-lens-reflex camera lens, and passed on to a detector that consists of a single charge-coupled device (CCD) surrounded by an array of several complementary metal oxide semiconductor (CMOS) detectors. Behind the detector plate is a piezoelectric actuator that moves it in the x and y axes (see schematic).