NASA’s crippled planet-hunting spacecraft has been reworked for at least two years of productive new explorations.
When the Kepler spacecraft failed in May 2013, its planet-hunting days seemed permanently over. A second of its four reaction wheels (gyroscopes) had seized up, so the craft and its telescope were no longer able to stay pointed at anything.
However, scientists and engineers at NASA’s Ames Research Center were not so quick to give up on Kepler. Could there be some other way to keep it pointed well enough to extend its mission with some kind of useful work? Kepler can steadily monitor the brightness of tens of thousands of stars in its wide field of view to extraordinary accuracy, good enough to detect a star’s slight dimming if even a small, Earth-size planet passes across its face.
By August 2013 an idea was afloat to use radiation pressure from the Sun, along with the two remaining reaction wheels, to stabilize the spacecraft. (Control is required in three dimensions, which normally requires three working wheels.) The Kepler team asked the scientific community for research proposals that could use the craft’s capabilities in such a stable but semi-disabled condition.
A few weeks ago, a NASA Senior Review panel decided that many of these research proposals will come to fruition. The extended life for Kepler has been dubbed the “K2” mission, while the original 3½-year run of Kepler’s observations is now being called “Kepler Prime.”
Ways of Making Do
The K2 mission uses most of the same infrastructure as Kepler Prime, starting with the 0.95-meter telescope and its 10°-square field of imaging sensors. Mission planners expect that K2’s photometric abilities will match past performance, the primary difference being the orientation of the telescope. While Kepler Prime continually watched a single portion of the sky in Cygnus, Lyra, and Draco, the reliance on the feeble pressure of sunlight to control one axis has confined K2 to observing near the ecliptic plane.
Kepler’s stability is now a careful balancing act between radiation pressure on the spacecraft, active force from its thrusters, and the two remaining reaction wheels. Keeping the telescope’s aim near the ecliptic gives optimal stability for long periods and minimizes fuel consumption.
While Kepler’s new attitude may seem like a restriction rather than an asset, it has actually expanded the range of scientific research that will be done. Over the next two years, Kepler will watch fields of view from very populous star fields in the Milky Way to regions at much higher and lower galactic latitudes. Consequently, the K2 mission will explore everything from exoplanets to active galactic nuclei. NASA planners have mapped out campaigns to examine nine fields. Each will last approximately 80 days, with four to five campaigns fitting into each 372-day orbit of Kepler around the Sun.
From September 2013 to February 2014, engineers performed rigorous development testing for the new mission. On March 8th they began “Campaign 0,” a trial run. It yielded 5½ weeks of continuous, stable operation and nine days of engineering data. So when the NASA Senior Review issued the go-ahead on May 16th, the K2 team was ready.
Star Clusters and More
By May 30th Kepler had already started Campaign 1, which is directed at the North Galactic Cap and will continue until August 1st. Subsequent campaigns will examine notable near-ecliptic star clusters such as the Pleiades and the Hyades (Campaign 4), and the Beehive Cluster (M44) and M67 in Cancer (Campaign 5). Mission planners are still requesting input for targets during the final four campaigns.
The new mode of science operation is completely different. Whereas Kepler Prime was controlled by a single science team looking primarily for exoplanet transits, K2 draws on the astronomical community at large. The new mission’s targets are chosen from different teams’ proposals. “K2 is very much a community facility,” explains Martin Still (NASA Ames Research Center). Addressing the American Astronomical Society meeting in Boston on June 3rd, Still emphasized, “If you don’t propose it, we don’t do it. There are no guaranteed targets.”
NASA has allocated $4 million for K2 guest observers over the next two years. All data collected will be freely accessible through the Mikulski Archive for Space Telescopes and the NASA Exoplanet Archive.
One area researchers will use K2 to explore is the exoplanet population around M-type red-dwarf stars. Kepler Prime showed that Earth-size exoplanets are fairly common around these stars, and M-dwarf study is very compatible with K2’s abilities. First and foremost, their planets tend to have short periods, so transits can be observed and confirmed within the 80-day window of a K2 campaign. Furthermore, M dwarfs comprise about 70% of nearby stars and occur at a wide range of galactic latitudes, so they can be observed in K2 fields chosen for other purposes.
A Few Remaining Limitations
Not everything is smooth sailing for K2. The fact that all observations will be made near the ecliptic means that bright solar system objects occasionally move across the field of view, and their glare and reflections might degrade the data. Additionally, the noise level in K2’s photometry is 1½ to 2 times stronger than in Kepler Prime.
K2 is still experiencing some small stability issues that mission planners are working on. The aim point has a tendency to drift slightly, so the spacecraft’s thrusters must be fired every 6 hours to compensate. This “roll and reset” method will affect the photometry, says Tom Barclay (NASA Ames Research Center), but this can be accounted for in the data analysis.
Analyzing the observations will also be more work for the guest observers because the K2 team will not release data in the convenient form of reduced light curves. “The initial releases of data are going to be pixel-level data only,” said Barclay, “as in images we take with the spacecraft.” Luckily, the K2 Guest Observer Office will provide software to help researchers turn the raw image data into accurate brightness values. K2 mission planners anticipate that they will eventually be able to release such finished data, though not right away.
The K2 mission is only scheduled to run through June 2016, but a future NASA Senior Review could extend the program if all is still going well and thruster fuel remains.
“It’s going to be an extremely rich set of data, just as the Kepler [Prime] data was,” Still enthused at the AAS meeting. “The difference is there’s going to be brand new targets every [campaign]. It’s just going to be a wonderfully exciting couple of years that we’ve got ahead of us.”
Further reading: The K2 Mission: Characterization and Early Results.