Kepler Goes Into Thruster-Controlled Safe Mode
May 15, 2013

Kepler Error Means Possible End To High-Accuracy Observations

Lee Rannals for — Your Universe Online

NASA announced on Wednesday that its Kepler spacecraft was sitting in safe mode once again, possibly putting an end to its high-accuracy observations.

Kepler went into a Thruster-Controlled Safe Mode earlier in May, and NASA said its spacecraft is sitting in the same position again. The space agency said the root cause of Kepler putting itself into safe mode is unknown, but the possible cause "appears to be an altitude error."

The spacecraft was orientated with the solar panels facing the sun, slowing spinning about the sun-line. The communication link comes and goes as Kepler moves about this line.

"We attempted to return to reaction wheel control as the spacecraft rotated into communication, and commanded a stop rotation. Initially, it appeared that all three wheels responded and that rotation had been successfully stopped, but reaction wheel 4 remained at full torque while the spin rate dropped to zero," NASA said in a statement about Kepler's status. "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 was then transitioned back to Thruster-Controlled Safe Mode."

An Anomaly Review Board determined the data indicates a “wheel 4 failure.” NASA said the team's responsibility now is to complete preparations to enter Point Rest State, which is a thruster-controlled state that minimizes fuel usages while providing continuous X-band communication. The Kepler team has already loaded the spacecraft with software it will be using to do this.

Kepler's current fuel capacity will allow the spacecraft to last for several months, but adding the Point Rest State will allow it to make its fuel last for years.

"Because this is a new operating mode of the spacecraft, the team will closely monitor the spacecraft, but no other immediate actions are planned. We will take the next several days and weeks to assess our options and develop new command products," the space agency said. "These options are likely to include steps to attempt to recover wheel functionality and to investigate the utility of a hybrid mode, using both wheels and thrusters."

NASA said it is unlikely the spacecraft will be able to return to high pointing accuracy due to the failure of a second reaction wheel.

Kepler has already successfully completed its primary three-and-a-half year mission to find Earth-size and smaller planets around other stars. Scientists have used Kepler data to identify more than 2,300 planet candidates and confirm more than 100 planets. So far, hundreds of Earth-sized planet candidates have been found that orbit the habitable zone around their stars, which is the region where liquid water might exist on the surface of a planet.

The Transiting Exoplanet Survey Satellite (TESS) spacecraft will be Kepler's successor, but it is still a few years away from being launched. TESS will perform all sky surveys to discover transiting exoplanets. Its goal is to identify terrestrial planets in the habitual zone of nearby stars.