I come to praise Kepler, not to bury it…
The Kepler Mission is one of the little NASA spacecraft that so frequently comes along, exceeds all expectations and changes our perspective of the universe.
There is a good Quick History of the transit method and Kepler Mission concept on the website.
Otto Struve noted in his seminal 1952 note that planetary “eclipses” of their parent stars ought to be detectable by photoelectric methods, a proposal that some two decades later was quantified by Rosenblatt, and then explored in detail (including development research) by Borucki and collaborators at NASA Ames.
A space based transit mission was first proposed more than 20 years ago, and was highly rated, IF the detector technology could get to the point where very low transit amplitudes could be measured and small radii planets detected. The review also noted that there would be significant secondary astrophysical science accomplished by any such mission.
A decade later, Kepler was finally selected, the fourth time it was proposed to the Discover class medium size NASA mission. By that point the detector technology was mature, a testbed demonstrator had been built, and the concept of transit observations of exoplanets had been demonstrated both from the ground, and from space, using the Hubble Space Telescope. The latter demonstrated that very high precision relative photometry was in fact achievable from space.
Kepler launched in March 2009, just over 4 years ago.
It had a nominal mission life of three years, and a main mission goal to find earth size planets in the habitable zone of solar like stars.
After the nominal mission, Kepler was given a three year mission extension to 2016, to continue the continuous monitoring of the 150,000 or so stars in the Kepler field.
Kepler has discovered almost 3,000 planetary candidates, of which about 100 have been confirmed through a variety of techniques, and, statistically, most of the rest are likely to be real planets.
Kepler has not quite found earth like planets in the habitable zone, yet.
It is heartbreakingly close to doing so.
More time for observations is needed, primarily because the stars being observed are a little bit noisier than expected. The periodic signal from the planetary transits can be dug out of the noise, but more observations of repeated transits are needed to get the signals out as you approach the limit of detectability.
Six years of observations ought to get Kepler to its goal of detecting earth size planets orbiting stars similar to the Sun at a distance where liquid water can persist on the planet’s surface.
To operate, Kepler’s orientation has to be held very stably to view the stars it is looking at. To do that it uses reaction wheels:
Kepler needs three reaction wheels to stay on target.
Any less and the spacecraft drifts, losing lock on the stars.
It has thrusters but cannot use those to stay on target for any length of time before they run out fuel.
Kepler carries 4 reaction wheels. They are heavy and expensive.
That is one spare.
One reaction wheel, wheel #2, failed in 2012.
A second reaction wheel started to show symptoms of degradation a few months ago. Twice in the last few weeks the spacecraft has safed, gone to a rest pointing, while the reaction wheels were despun with the thrusters, and diagnosis tests runs.
Reaction wheels are moving parts, they wear and tear, and have finite life expectancies.
Since the reaction wheels are all the same, they are vulnerable to common mode failure.
Then, last night Kepler went into a safe mode, again.
Switching back to reaction wheel mode the diagnosis showed reaction wheel #4 had seized.
That is the end of Kepler’s primary science mission.
The data is in the archives available for analysis. There will be no more.
It is just short of finding the other Earth.
So very very close..
Kepler can do some stuff with only three reaction wheels, basically driftscan observing. It is a wide band wide field optical telescope with a 1 m mirror.
Whether it is worth doing so to keep the spacecraft going will be an interesting decision.