After four successful years in space, the Kepler planet hunting space telescope is in serious trouble. A key component that keeps the spacecraft pointing at the right patch of stars, a reaction wheel, has failed. Kepler went into space with four of these and needs three, but this new failure leaves it with just two. Even so, at least one Kepler expert thinks there may be a way to save the satellite.
The US $600 million telescope hunts for exoplanets in our own galaxy. It uses a 95-megapixel camera to register slight dips in stellar brightness that signal a planet's passage across its host star. So far the mission has found more than 2700 candidate exoplanets, several of them in the habitable zone of their stars. To find these it must continually stare at a patch of sky containing some 4.5 million stars.
It’s this staring that’s in danger with the loss of the reaction wheel. The device is used to gently point the telescope in the right direction, using other patches of stars as a reference. Reaction wheels are electric motors attached to fly wheels. By speeding up or slowing down, they transfer angular moment to the satellite, rotating it around its center of mass. Kepler’s have to be pretty good ones. According to a report at the 2011 IEEE Aerospace Conference, the telescope must be able to stare for more than 15 minutes at a time with a stability of 0.009 arc seconds for each axis of rotation. (By comparison, a comma in an Apollo mission manual left on the moon is about 0.001 arc seconds as seen from Earth.)
Kepler’s first reaction wheel failure was in July 2012. Earlier this month another one started to go wonky, registering signs of friction. I’ll give a more detailed description of how reaction wheels fail and what can be done about it tomorrow, but for now, here’s Stanford University’s News service interviewing Scott Hubbard, a consulting professor of aeronautics and astronautics about saving Kepler:
Q: How might NASA engineers go about getting Kepler functional again?
A: There are two possible ways to salvage the spacecraft that I’m aware of. One is that they could try turning back on the reaction wheel that they shut off a year ago. It was putting metal on metal, and the friction was interfering with its operation, so you could see if the lubricant that is in there, having sat quietly, has redistributed itself, and maybe it will work.
The other scheme, and this has never been tried, involves using thrusters and the solar pressure exerted on the solar panels to try and act as a third reaction wheel and provide additional pointing stability. I haven’t investigated it, but my impression is that it would require sending a lot more operational commands to the spacecraft.
The mission was set to continue through 2016. Kepler’s loss could be a blow to other instruments such as HARPS-N at the Telescopio Nazionale Galileo in the Canary Islands. HARPS-N, which IEEE Spectrum’s Rachel Courtland visited in 2011, is used to confirm the exoplanet status of objects Kepler spies.