A broken space telescope continues to expand our view of the universe.
April 21, 2014 - 11:02 pm
Ever since the realization that our sun was a star amidst uncountable others in the universe, astronomers and philosophers have speculated about the possibility of extrasolar planets, or “exoplanets” — planets that orbit other stars — and whether or not they could harbor life or intelligence. The modern era has given us tools to allow us to answer that question, and the answer turns out to be, yes, at least in terms of the existence of exoplanets, though there is still no evidence of extraterrestrial life or intelligence.
The initial, and vast majority of such discoveries were made not by directly viewing them with powerful telescopes (though there have been some of those), but by indirect methods that require inference. The first such discovery was confirmed in the early 1990s, when telescopes observed a slight regular wobble in a distant star, that was likely caused by a very large planet (perhaps like Jupiter) pulling it from side to side as the massive body orbited it. But a more powerful means of detection has been by observing so-called transits — when a planet passes between us and its star and slightly, momentarily dims its light. Though we can’t see the planet itself because there is no light coming from its night side, its size can be calculated by measuring the degree to which the starlight is dimmed by it.
So promising was the technique in fact that, five years ago, in March 2009, after multiple failed attempts to get it funded, NASA finally launched a space telescope specifically designed to look for such events. It was named after the astronomer Johannes Kepler, who first described planetary orbits as ellipses, a result which helped Isaac Newton come up with his universal law of gravitation.
Unlike the Hubble Space Telescope, which is in low earth orbit, Kepler is in a heliocentric (or sun-centered) orbit, far from earth, to prevent it from being dazzled by earthlight and moonlight as it gazes deep into the galaxy. In order to stare at a star well enough to see transits, it has to have very precise pointing capability, akin to “pinpointing a soccer ball in Central Park as seen from San Francisco.” To do this smoothly, it has four gyroscopic devices called reaction wheels. By rotating the axes of these small spinning flywheels, the attitude of the spacecraft can be moved and held in a very precise controlled manner.
For the past half decade Kepler has been observing and sending back data, but in the summer of 2012, a little over three years into its mission, it started to run into problems when one of its reaction wheels started to fail, with friction buildup in the bearing, slowing the wheel. Last May, almost a year ago, another one started to display similar symptoms. With too much friction in half of its redundant devices, it was no longer able to point sufficiently accurately to perform its primary mission of star staring. Last August, NASA engineers despaired of fixing it, and were searching for ways to repurpose it for other missions.
But last fall, some engineers at Ball Aerospace, its prime contractor, came up with a clever fix. Our own nearby star puts out so-called “radiation pressure,” resulting from photons of sunlight bouncing off spacecraft surfaces. They figured out how to maintain an attitude almost as precisely as with the reaction wheels by changing the angle of the telescope’s solar arrays to vary the resulting force on the vehicle. While it’s not as good as when originally launched, it is still returning data, and may now continue to do so until its successor, the Transiting Exoplanet Survey Satellite (TESS), is launched in 2017. A few weeks ago, in fact, by using a new more efficient technique to verify the discoveries, NASA reported a doubling of the number of detected planets, and potentially quadruple the number of earth-sized ones, and it may be that as many as one in five stars have such bodies.