An experiment deep in space is underway that, if successful, could go a long way to reviving the fortunes of NASA's Kepler spacecraft. The storied planet-hunter was sidelined in May after two of four key components in its precision-pointing system failed.
The failure left Kepler vulnerable to the persistent, gentle push of sunlight striking the craft. This radiation pressure compromised the exquisite stability needed to monitor more than 150,000 stars simultaneously for signs of extrasolar planets.
Now, armed with a proposed new science plan for Kepler, members of the Kepler team and the craft's designer, Ball Aerospace & Technologies Corporation, have turned an enemy into an ally. They are using sunlight's push on the spacecraft to help steady Kepler's aim.
In late October, the team pointed Kepler at a portion of the constellation Sagittarius and commanded the craft to take a 30-minute exposure of the sky. The new approach brought the quality of the image Kepler took to within 5 percent of the quality Kepler delivered before the failure occurred.
“We were astounded at how well it worked,” says Charlie Sobeck, deputy project manager for the mission at NASA's Ames Research Center at Moffett Field, Calif.
Now, engineers are working to test the approach for successively longer periods, until the craft can keep a stable eye on the sky for at least 83 days. That's the length of time the new science plan envisions the craft observing any one section of sky as it orbits the sun.
Launched in 2009, Kepler "Version 1.0" was designed to take a census of planets orbiting stars within its field of view and sensitivity range. That sample size was large enough that scientists could use it to extrapolate the number of Earth-size planets orbiting within the habitable zone of sun-like stars in the galaxy. So far, it has identified about 3,500 planet-candidates. [Editor's note: The original version mischaracterized the aspect of Earth-scale planets that Kepler detects.]
Its primary mission ended last year, and NASA extended it until 2016. The team sought the extra time to make additional observations, which would allow them to resolve confusions that might lead to claiming a planet where none existed.
But first one, then two of the craft's four reaction wheels failed. The craft needs three to point properly: one for side-to-side motions, one for changing elevation, and the third to offset the sun's radiation pressure.
In August, NASA announced the the observing phase of the mission was over, but that it would consider proposals for other missions for the craft, whose light gathering and measuring instruments were still in good order.
“People came up with a ton of ideas,” says Sarah Ballard, a planetary scientists at the University of Washington at Seattle. The proposal that emerged has been dubbed K2.
It, too, would look for the subtle, tell-tale dimming a planet imposes on starlight as it passes in front of its host star, as Kepler did. But the new pointing technique is giving K2 a decidedly different agenda.
The craft, which orbits the sun and trails behind Earth, would have to be reoriented every 83 days so that sunlight bathes it evenly along its length without falling onto the telescope's sensitive detectors. That means it can look in one direction for only 83 days. And to keep it properly oriented, it will have to aim at objects in the sky along the ecliptic, the path Earth traces around the sun.
From a planet-hunter's perspective, the 83-day limit on viewing time would limit the telescope to detecting planets with orbital periods less than 83 days. Where Kepler monitored more than 150,000 stars continuously, K2 will analyze light from 10,000 to 20,000 stars in different parts of the sky.
And the slightly degraded stability imposes a higher minimum size on the planets a repurposed Kepler could detect, Dr. Ballard explains.
“For a given star, you would be sensitive to planets above a certain threshold,” she says, one higher than would have been the case if Kepler had retained its old pointing precision.
These new constraints likely would limit the craft's ability to detect potentially habitable planets; it could only detect those orbiting low-mass stars such as red dwarfs. Because these stars are dim, their habitable zones lie much closer to the star than the zone for a star like the sun.
Even so, limiting the loss of image quality to only 5 percent “is really gratifying,” Ballard says.
A K2 mission would have an observing mandate that would extend beyond planet hunting, adds NASA's Mr. Sobeck. Its data will allow astronomers and astrophysicists to study the properties of stars themselves and even allow astronomers to hunt for supernovae in galaxies that appear in the spacecraft's field of view.
The team submitted its formal K2 proposal to NASA headquarters last week, Sobeck says. By Dec. 6, the team expects to hear whether agency officials feel the project has enough merit to move to the next level of consideration – a competition against other extant science missions seeking to extend their operations as well.
For now, harnessing the sun to help point Kepler has been a welcome bit of news to for a spacecraft whose untimely shutdown left many long faces among the planet-hunting community.
Anticipating the results of the current test of Kepler's pointing ability, “we're very optimistic at this point,” Ballard says.