The K2 mission, the two-wheel operation mode of the Kepler spacecraft observing in the ecliptic, is exhibiting no discernible ill effects since the recent Emergency Mode, other than the extra fuel usage. K2 is now in the second half of the special microlensing campaign called Campaign 9. The first half of the campaign was shortened by two weeks as a result of the emergency, and the data acquired have made public. The microlensing team has already been searching for the telltale events that indicate an object passing in front of a background star, identifying approximately two-dozen of these lensing events. Thus far, the experiment seems to be a smashing success!
While the team is still dotting the i’s and crossing the t’s on the cause of the emergency, all the signs are pointing towards a single bit that changed state in the memory of an electronic chip that controls the internal command and data bus onboard the spacecraft. The memory was designed to be highly resistant to upset but if a high-energy cosmic ray hit in just the wrong place or at the wrong time in a write cycle an upset can occur. In this case, the upset caused a disruption in the internal data stream, passing invalid data sets to the flight computer, setting off several fault responses including the shutdown of critical heaters on the spacecraft.
After a couple of hours, propellant froze in the propulsion system effectively disabling pointing control. Without pointing control, the spacecraft slowly drifted until the sun got too close to a “forbidden zone” around the optical axis of the telescope, causing the Emergency Mode to kick in and protect the telescope.
"While we have an excellent fault protection system onboard, no amount of pre-planning is going to work if we get multiple, random faults, which is why we have the Emergency Mode in the first place. And it worked beautifully!" said Charlie Sobeck, Kepler and K2 mission manager at NASA's Ames Research Center.
The spacecraft emergency provided an opportunity to highlight the important role that engineering plays behind the scenes in the development and operation of a mission. Last month the engineers from NASA Ames, Ball Aerospace and the Laboratory for Atmosphere and Space Physics at the University of Colorado, both located in Boulder, who all worked on the recovery of the spacecraft, participated in a Reddit "Ask Me Anything." The team had a fantastic time responding to the online community's questions about the steps in the recovery and the experience of working through a high-pressure, high-visibility situation.
"We were encouraged by the level of interest and look forward to welcoming more problem solvers to the field of engineering. You can learn more about managing the recovery of a spacecraft 75 million miles from Earth in a recent question and answer feature," Sobeck said.
Meanwhile the scientific results from the missions keep rolling out. Last month Dr. Timothy Morton of Princeton University announced that his analysis of the Kepler data was able to validate planethood for 1,284 more of Kepler planet candidates. The Kepler verified planet count currently stands at 2,327.
Now in its ninth observing campaign, K2 continues to produce a bounty of data for the scientific community to continue the search for exoplanets and to study planet and star formation, as well as the explosive death of red giant stars, commonly known as supernovae. The K2 planet count continues to climb, reaching more than 250 candidates, of which nearly 50 have been verified as bona fide planets.
NASA has also announced that K2 is to continue science operations through the end of the FY19, by which time the on-board fuel is expected to be fully depleted. The mission extension, based on a recommendation from NASA’s Astrophysics Division’s 2016 Senior Review of operating missions, provides two additional years of funding for K2 to continue exoplanet discovery, and the study of notable star clusters, young and old stars, active galaxies and supernovae. For more details about the recommendation and for a listing of other missions approved for extension, see the 2016 Senior Review report.
"With the emergency behind us, and fuel to last us into the summer of 2018 or beyond, the news of the two-year mission extension was a welcomed vote of confidence in the team. This news comes just a week after K2 completed two years of operations, celebrating its second 'birthday.'" Sobeck said.