Kepler’s Once and Future Mission

Wheeling to a Halt

The Kepler spacecraft measured differences in brightness of a star as its planets passed between us and the star. Courtesy NASA Ames/JPL-Caltech
The Kepler spacecraft measures differences in brightness of a star as its planets pass between us and the star. Courtesy NASA Ames/JPL-Caltech

Yesterday, the Kepler mission team and NASA announced that the spectacular planet-discovery spacecraft is un-repairable. The news is not unexpected, but scientists and engineers had been working to find ways to stabilize the spacecraft and continue on its world-finding expedition.  Here’s what happened: two of its four gyroscopic reaction wheels failed. Since the spacecraft’s high pointing accuracy depends on having a stable platform, any wobble induced by failing gyros would make it impossible to accurate measure light from distant stars and analyze it for the existence of planets—particularly the Earth-size ones it was built to find.

The spacecraft is, for planet-searching purposes, dead in the water. But, it’s not a dead telescope.  Its instruments are still working and there are still ways it can be used to do other kinds of observational science provided the other reaction wheels don’t fail and thrusters can be used to keep the spacecraft stable. If that works out, Kepler would enter what engineers are calling a “two-wheel” mission that might include certain kinds of exoplanet searches that don’t require the extreme stability that four wheels provide.

Kepler completed its primary mission last November and had just entered its extended four-year mission when the gyroscope problems became unmanageable.  The mission has been spectacularly successful, giving us new looks at planets ranging from super-Earths to super-Jupiters. Those are words that have entered our language, defining new worlds and opening up ideas for further explorations to find signs of life on worlds where it could have arisen and evolved.  Kepler has confirmed the existence of 135 planets around other stars. At least 3,500 planet “candidates” await confirmation from continued observations by ground-based observatories.  And, the team expects hundreds, if not thousands, of new discoveries are lurking in the data sets including finding evidence of  more Earth-size planets that orbit in the habitable zone of Sun-like stars.

The Kepler mission has been an incredible success. Sure, it’s disappointing to see its gyros fail, but working in space is a tough environment for spacecraft. And, to paraphrase a familiar phrase from a Monty Python movie: It’s not dead yet. I hope that its team will find ways to use the spacecraft to aid in other astronomical and astrophysical observations. It’s a marvelous machine, and we can still get some good bang for the buck out of it in other ways.

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