Mission To Search for Exoplanets One Star at a Time

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SAN FRANCISCO — Instead of trying to cram the capabilities of a larger satellite into a miniature spacecraft, ExoplanetSat, a mission being developed by Draper Laboratories and the Massachusetts Institute of Technology (MIT), is designed to take advantage of the unique characteristics of cubesats to search for planets orbiting nearby stars, said Séamus Tuohy, space systems director for Draper of Cambridge, Mass.

ExoplanetSat is one of the 20 cubesats NASA selected in February to fly along as secondary payloads on rockets scheduled for launch in 2011 and 2012. The mission is designed to identify Earth-like planets outside the solar system in orbit around bright stars by measuring the slight decrease in light that occurs when a planet passes in front of a targeted star. While this method of identifying new planets is similar to the approach being taken by NASA’s Kepler space telescope, the overall missions are far different.

Kepler gazes at 150,000 stars in one swath of the Milky Way galaxy searching for planets located in the so-called habitable zone, neither too cold nor too hot to permit water to remain on the surface. ExoplanetSat is designed to focus its telescope on one bright star at a time. The stars ExoplanetSat seeks to study are beyond Kepler’s current field of view.

The idea, conceived by MIT professor Sara Seager, is to eventually launch dozens of satellites to monitor dozens of stars simultaneously in the search for small, rocky, Earth-like planets. Each satellite would be housed in triple cubesats, a standard configuration that joins three 10-centimeter cubes. Each cubesat would contain a small telescope and sophisticated electronics to maintain unwavering precision-pointing and focus on a single star, said Seager, who is also a participating scientist on the Kepler mission.

Seager came up with the idea of ExoplanetSat in 2007 and won a NASA astrobiology grant to pursue the concept. Around the same time, employees at Draper Laboratories began exploring the same idea. In 2008, when Seager applied for funding from NASA’s Goddard Space Flight Center in Greenbelt, Md., to continue her research, a scientist there who knew of Draper’s project put the two groups in touch, said Draper spokesman Jeremy Singer.

Draper Laboratories is developing the avionics, electronics, guidance, navigation and control equipment needed to keep the ExoplanetSat telescope fixed on a single star. To identify exoplanets, that gaze needs to be so accurate that the target star remains in the same fraction of a pixel at all times, which means the ability to control the satellite to less than two arcseconds — or 1/1,800th of a degree — over the 20 minutes it takes to make an observation, Tuohy said.

Students in MIT’s space systems engineering class are designing the satellite bus for ExoplanetSat. Draper engineers are working side by side with MIT undergraduates and graduate students to design the spacecraft, Tuohy said.

Before the team can propose launching a fleet of small satellites to hunt for exoplanets, the concept must be proved with a technology demonstrator. That is the goal of ExoplanetSat, which Seager and Tuohy hope to launch in 2012.

Ideally, ExoplanetSat would be lofted into a low-inclination, equatorial orbit where it could begin searching for exoplanets. If the satellite is placed in another orbital plane, such as a polar orbit, mission officials will be able to validate the engineering and scientific concept. “We would happy to demonstrate the technology and validate the design,” Tuohy said. “But we would actually like to conduct science, too.”

To date, the team has spent approximately $1 million designing and building the ExoplanetSat spacecraft and its components. Final production costs would be far lower, however, dropping to approximately $600,000 per satellite, if 10 or more spacecraft were built, Tuohy said.