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Kepler Space Telescope's Broken Wheel Could Aid Stellar Physics

Although a broken reaction wheel on NASA's Kepler spacecraft is bad news for its primary exoplanet-hunting mission, the glitch could be a boon for the study of stellar physics
Illustration of the Kepler space telescope



NASA/Ball Aerospace

NASA’s exoplanet-hunting telescope has always had an unsung talent for star physics on the side. Now that Kepler’s primary mission is compromised by broken reaction wheels on the spacecraft, some scientists hope to refocus it on an unprecedented study of the most massive stars in our galaxy, whose inner workings are the least well understood of all star classes.

During the four years since its launch, Kepler has discovered more than 3,000 exoplanet candidates, multiplying many times over the tally of known worlds beyond our solar system. But in July 2012 one of its four stabilizing reaction wheels failed, and then in May 2013 a second one gave up the ghost, leaving the spacecraft without the ability to point itself precisely toward a single spot in the sky.

The loss of the reaction wheels has effectively ended Kepler’s chances of discovering more low-mass planets, especially the golden ring prize of an Earth analogue. But NASA has hardly given up hope that the telescope can still do great things, and on August 2 the agency put out a call for ideas on how to use Kepler in its two-wheeled state. Among the dozens of proposals are plans to use the telescope to detect near-Earth asteroids, find Jupiter-size exoplanets, and monitor Neptune. And some stellar physicists say the change in Kepler’s fortunes has left the telescope in an ideal position to increase what is known about how large stars evolve.

When it was functioning normally, Kepler was unable to observe certain hot, massive stars called OB stars, because their extremely bright light would have washed out the relatively dim signals of planets. It did excel, however, at studying smaller stars using a method called asteroseismology. Akin to probing the internal dynamics of Earth by studying its seismic quakes, asteroseismology monitors starquakes, which show themselves as flickers in a star’s brightness, for hints about its internal structure. During its primary mission, Kepler could do this easily because it was already closely monitoring changes in stars’ brightness over time to search for periodic dimming caused by planets passing in front of the stars from Earth’s point of view.

Now, the somewhat shakier Kepler can’t point precisely enough to study the flickers of low-mass stars. Oscillations of larger stars, however, should create much stronger signals that should still be within the capabilities of Kepler to detect, says Conny Aerts, director of the Institute of Astronomy at the University of Leuven in Belgium, who is lead author of the proposal. Furthermore, such studies cannot be done well from the ground, because they require watching the same stars continuously for long periods of time in order to spot oscillations with periods of hours or days, and ground-based telescopes have to stop observing every time the sun comes up. “We need less good precision, but longer time scales, and that’s what Kepler with its two reaction wheels can bring,” Aerts says.

Light curves from the CoRoT satellite show seismic flickers in massive starsKEPLER STARQUAKES: Astronomers hope to use the Kepler space telescope to observe flickers in the light of massive stars to understand their interior dynamics. Shown here are light curves of three massive stars observed by the European CoRoT (COnvection ROtation and planetary Transits) satellite.While these data, collected by Blomme et al. in 2011, are insufficient for long-term seismic modeling, researchers aim to use Kepler to gather seismic data for six months straight.
Image: Conny Aerts and Jonas Debosscher, Leuven University, Belgium

Very little asteroseismology has been conducted on hot, massive stars, yet they are among the stars most in need of elucidation, Aerts says. Current stellar physics models are particularly lacking when it comes to OB stars, and scientists would like to understand how these “live fast, die young” stars tick. Such stars, which are hundreds of times the sun’s mass, burn through their stores of nuclear fuel the quickest, and die through spectacular supernova explosions that seed the cosmos with most of its elements heavier than carbon (so-called metals). “These stars are the metal factories of the universe, and in terms of the chemical evolution of the universe, they are the most important stars to understand,” Aerts says. “The theoretical models are the least adequate for these type of stars.”

Aerts and her colleagues suggest Kepler should target an open star cluster called NGC 2244, which is rife with these stars and could be a strong test case for the method. Such a study would require Kepler’s undivided attention for six months straight to observe how the periods of stellar oscillations change over time. Still, other science could be done using the data collected, because Kepler’s large field of view would take in more than just the cluster.

“I do believe the proposed study would be a useful project,” says Ronald Gilliland, an astronomer at The Pennsylvania State University who has used Kepler data in the past to conduct asteroseismology studies. “If it could be carried out, obtaining six months of coverage at a level of 0.01 percent to 0.05 percent precision as in the white paper, then very useful and unique science would likely result from this.” But he says it is too soon to tell how precise Kepler’s new mode will be, and he has doubts that even massive-star asteroseismology will be possible. “We still do not know some very basic things about what the actual capability of the spacecraft will be. Tests are underway that should provide better insights about this within a couple of months.” Ultimately, he was less than sanguine. “I do not personally have much hope that Kepler can find important work to do in its new two-wheeled configuration.”

Even if the telescope does carry on with efforts to perform stellar physics, many Kepler scientists are loathe to abandon the observatory’s original raison d’être, and hope the observatory can continue studying planets, albeit in a modified fashion. “The proposers have argued a useful case,” planet hunter and Kepler science team member Sara Seager of the Massachusetts Institute of Technology says of the asteroseismology proposal. “[But] it's hard for me to be objective about non-exoplanet astronomy.” And adapting Kepler to a purpose it wasn’t designed for could present additional difficulties, such as the need to design a whole new infrastructure to convert raw data to usable scientific information, she adds. “One concern is what kind of pipeline has to be developed for a new project? And what if another wheel fails midway or just after development? Such risks and costs also need to be taken into account.”

In the end, it may be that none of the new proposals win out. The Kepler team must prove that the science possible with the two-wheeled instrument is valuable enough for a piece of the ever-shrinking NASA budget pie. If none of the proposals meet that high bar, the Kepler funding may be allocated to other, fully functioning missions. “We have to ask how valuable is the science per dollar,” Seager says. “Space telescopes are very expensive to operate, on the order of [at least] $10 million per year. So the question is: Is the science worth the cost?” The Kepler team plans to submit the best proposals to NASA for review later this year. Those deemed worth further study will be examined by an external committee next year, with a final decision expected next summer.

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