Draper Laboratory and MIT have developed a 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.
The “nanosatellite,” called ExoPlanetSat, packs powerful, high-performance optics and new control and stabilization technology in a small package.
While there have been many small satellites, these are typically used to perform simple communication or observation missions. “We are doing something that has not been done before,” says Séamus Tuohy, director of space systems at Draper.
ExoPlanetSat will search for planets by measuring the dimming of a star as an orbiting planet passes in front of it, a technique called transit observation. The satellite’s light detector has two focal plane arrays—one for star tracking and for the transit observations. Measuring a star’s dip in brightness precisely also allows the planet’s size to be calculated. And by measuring the amount of time it takes the planet to complete its orbit, researchers can determine the planet’s distance from its star.
This technique is well-established, but has only be used by much larger orbiting spacecrafts, including the French-operated satellite CoRot, which made a significant planet discovery last year, and NASA’s Kepler satellite, which launched in 2009. ExoPlanetSat is not meant to replace larger spacecraft, but to be complementary, says Sara Seager, professor of planetary science and physics at MIT, meaning the nanosatellite will focus on individual stars that larger spacecraft have identified as being scientifically interesting. Whereas a spacecraft like Kepler looks at approximately 150,000 stars, a nanosatellite like ExoPlanetSat is designed to track a single star.
To accurately measure a star’s brightness, engineers must keep the spacecraft stable—incoming photons must hit the same fraction of a pixel at all times, says Seager, who is also a participating scientist for the Kepler satellite. “Any disturbances that shake the spacecraft will blur the image and make the measurements unusable,” she says. “And smaller spacecrafts are easier to push around.”
To precisely control and stabilize ExoPlanetSat, Draper and MIT researchers built custom avionics and off-the-shelf reaction wheels, a type of mechanical device used for attitude control, at the base of the spacecraft to maneuver it into position. Battery-powered piezoelectric drives control the motion of the imaging detector, which is uniquely decoupled from the spacecraft, so it operates separately. (The battery will be charged by solar panels.) “The drives move the detector counter to the spacecraft so precisely the human eye cannot see the motion,” says Seager. “This is an order of magnitude better than any nanosatellite has demoed before,” she says.
The nanosatellite has a volume of three liters; it’s 10 centimeters tall, 10 centimeters wide, and 30 centimeters long. “It was an engineering feat getting all the hardware, including the necessary processing power and data storage, into such a small package,” says Tuohy.
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. (NASA’s Jet Propulsion Laboratory and Goddard Spaceflight Center provided a small amount of money for ExoPlanetSat’s development, and Goddard will conduct performance testing on the spacecraft on a volunteer basis.) Eventually, Seager says, the researchers hope to launch a whole fleet of nanosatellites surveying the nearest and brightest stars.
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