article image

Kepler enters safe mode after failure of orientation system

Jenny Winder, News Writer
May 17, 2013, 7:00 UTC

Sen—NASA's planet-hunting Kepler telescope has entered safe mode after it lost two of four reaction wheels needed to orient the telescope accurately.

The four reaction wheels enable the spacecraft to aim in different directions without firing thrusters. Kepler needs at least three reaction wheels to be able to aim precisely. Last year Kepler lost reaction wheel number 2 and now number 4 has failed.

On Tuesday May 14th the Kepler team found the spacecraft in a 'Thruster-Controlled Safe Mode' that turns the spacecraft's solar panels towards the Sun if the observatory has trouble with orientation.

They attempted to restore reaction wheel control as the spacecraft rotated into communication, and commanded a stop rotation, but reaction wheel 4 remained at full torque while the spin rate dropped to zero. This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing.

The spacecraft is currently stable and safe. The team's priority is to complete preparations to enter Point Rest State, a loosely-pointed, thruster-controlled state that minimizes fuels usage while providing a continuous X-band communication downlink. In its current mode, the fuel will last for several months. Point Rest State would extend that period to years. They will closely monitor the spacecraft and take the next several days and weeks to assess their options and develop new command products.

"We're not ready to call the mission down and out just yet," said John Grunsfeld, NASA's associate administrator for the Science Mission Directorate, "but by any measure it's been a spectacular mission."

With the failure of a second reaction wheel, it's unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry, but no decision has been made to end data collection.

The $600m (£395m) Kepler mission was launched in 2009, and so far has identified 132 exoplanets outside our solar system, with another 2,700 possible candidates. Kepler successfully completed its primary three-and-a-half year mission and entered an extended mission phase in November 2012.

Even if data collection were to end, the mission has two years of data on the ground yet to be fully analysed, and the string of scientific discoveries is expected to continue for years to come.