Nasa engineers have called time on the Kepler Space Telescope’s planet hunting days, after failing to restore it to full working order.
The spacecraft was launched in 2009 to hunt for Earth-sized worlds suitably positioned around their parent stars for liquid water, a condition believed to be necessary for life, completing its prime mission in November 2012 when it began its four-year extended mission.
But in May, the ability to precisely point the spacecraft at about 100,000 target stars was lost when the second of four gyroscope-like reaction wheels failed after the first was lost in July 2012.
And despite months of effort from the space agencies engineers to restore at least one of the wheels, they have been unsuccessful forcing Nasa to consider what new science research it can carry out using the remaining two good reaction wheels and thrusters.
"The wheels are sufficiently damaged that they cannot sustain spacecraft pointing control for any extended period of time," Charles Sobeck, Kepler deputy project manager at Nasa's Ames Research Center in Moffett Field,California, told reporters during a conference call on Thursday.
On 8 August, engineers conducted a system-level performance test to evaluate Kepler's current capabilities and determined wheel two, which failed last year, can no longer provide the precision pointing necessary for science data collection.
The spacecraft was returned to its point rest state, which is a stable configuration where Kepler uses thrusters to control its pointing with minimal fuel use.
Kepler flies about 40 million miles from Earth, too far away for a robotic or astronaut-led repair mission.
"At the beginning of our mission, no one knew if Earth-size planets were abundant in the galaxy. If they were rare, we might be alone," said William Borucki, Kepler science principal investigator at Nasa's Ames Research Center in Moffett Field, Calif.
"Now at the completion of Kepler observations, the data holds the answer to the question that inspired the mission: Are Earths in the habitable zone of stars like our sun common or rare?”
An engineering study will be conducted on the modifications required to manage science operations with the spacecraft using a combination of its remaining two good reaction wheels and thrusters for spacecraft attitude control.
Informed by contributions from the broader science community in response to the call for scientific white papers announced 2 August, the Kepler project team will perform a study to identify possible science opportunities for a two-wheel Kepler mission.
From the data collected in the first half of its mission, Kepler has confirmed 135 exoplanets and identified over 3,500 candidates. The team will continue to analyse all four years of collected data, expecting hundreds, if not thousands, of new discoveries including the long-awaited Earth-size planets in the habitable zone of sun-like stars.
"Kepler has made extraordinary discoveries in finding exoplanets including several super-Earths in the habitable zone," said John Grunsfeld, associate administrator for Nasa's Science Mission Directorate in Washington. "Knowing that Kepler has successfully collected all the data from its prime mission, I am confident that more amazing discoveries are on the horizon."