Following months of analysis and testing, the team is ending its attempts to restore the spacecraft to full working order as the recent pointing test proved unsuccessful. We are now considering what new science research it can carry out in its current condition.
On Aug. 8, engineers conducted a system-level performance test using Reaction Wheel 1 (RW1), 2 and 3. RW1 and 3 are the “good” wheels that have shown no signs of degradation. RW2 is the one that failed in July 2012. The test was conducted to see if the wheels, demonstrated to at least still spin on command, could adequately control spacecraft pointing.
The pointing test was constructed as three sequential stages. The first stage of the test was to determine if the spacecraft could sustain coarse-point mode using RW1, 2 and 3. Coarse-point mode is regularly used during normal operations, but has insufficient pointing accuracy to deliver the high-precision photometry necessary for exoplanet detection. During coarse-point the star trackers measure the pointing accuracy of the spacecraft. When using wheels to control the spacecraft, pointing is typically controlled to within an arcsecond.
During this first stage of the test, the wheels adequately controlled the spacecraft pointing. Although the RW2 friction was still higher than normal, spacecraft pointing was controlled to within about 10 arcseconds, easily enough to keep the spacecraft from entering safe mode, but unsatisfactory for entering the precision pointing needed to gather science data.
The second stage of the test investigated RW2's ability to help control the spacecraft pointing with enough accuracy to transmit science data to the ground using NASA's Deep Space Network. This requires that the pointing be controlled more tightly than simply avoiding safe mode, yet does not require the very fine control needed to collect science data.
This stage of the test saw RW2 friction increase markedly and the pointing errors increased to levels greater than an arcminute. While this degree of pointing was more than sufficient to enable the return of data stored aboard since RW4 failed in May, it lowered even further the prospects of being able to achieve science-level pointing control.
After approximately four hours of operation in this mode, the friction in RW2 exceeded the motor’s ability to overcome it and the pointing accuracy quickly degraded to where the spacecraft entered safe mode negating the ability to conduct the third stage of the test.
With the conduct of this test, engineers have concluded that the damaged wheels are not recoverable to support fine-point science data collection. Performing a final test of RW4, which is believed to be the more seriously damaged wheel, will be conducted as a “test of opportunity” to the extent that schedule and resources permit. However, project engineers expect its performance will be even worse than that of RW2.
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.
The team will now perform an engineering study to assess what modifications are 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 Aug. 2, the team also will perform a study to identify possible science opportunities for a two-wheel Kepler mission.
Depending on the outcome of these studies, which are expected to be completed later this year, NASA HQ will assess the scientific priority of a two-wheel Kepler mission. Such an assessment may include prioritization relative to other NASA astrophysics missions competing for operational funding at the NASA Senior Review board early next year.
Meanwhile, the team continues to analyze the 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. Though the spacecraft will no longer operate with its unparalleled precision pointing, scientists expect Kepler’s most interesting discoveries are still to come.
Finally, preparations are underway for hosting the second Kepler Science Conference Nov. 4-8 at NASA's Ames Research Center in Moffett Field, Calif. This will be an opportunity to showcase not only the investigations of the Kepler project team, but also those of the wider science community using publicly accessible data from Kepler. Registration is now open.