Hunt for near-Earth asteroids is new mission for slumbering NASA craft

NASA's WISE telescope ran out of coolant for two of its four infrared detectors, but the remaining ones can operate in a way that is suitable for the detection of near-Earth asteroids.

By , Staff writer

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    An artist's concept of Wide-field Infrared Survey Explorer (WISE) is shown in this illustration.
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NASA is getting set to rouse a slumbering spacecraft after a nearly three-year nap and hand it a new assignment: hunting for near-Earth asteroids.

The craft not only would keep a keen eye out for objects that could threaten Earth; it also would hunt for objects that might make a good target for a manned mission.

The center of attention is NASA's Wide-field Infrared Survey Explorer. The agency launched it in December 2009 to survey the entire sky in a kind of reconnaissance mission for orbiting observatories such as the Spitzer Space Telescope – a far more powerful infrared observatory that could conduct the detailed follow-up observations of objects that WISE detected.

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Much of WISE's science program focused on hunting for nearby brown dwarfs, dim star wannabes whose internal furnaces never quite got hot enough to ignite the stars. WISE also hunted for galaxies with high rates of star formation – galaxies typically obscured at optical wavelengths by thick cocoons of dust. And the craft was to provide data that would shed additional light on the formation and evolution of stars, planets, and galaxies.

The mission "was not originally designed to look for asteroids and characterize them," says Amy Mainzer, deputy project scientist for WISE during its initial mission and now the lead scientist for the dedicated asteroid hunt.

But it's well suited to the task. Near-Earth asteroids can approach Earth from all directions, so having a telescope that surveys the entire sky is a plus.

In addition, the telescope operates at infrared wavelengths, in essence detecting heat. This allows it to pick up dark asteroids that might be difficult or impossible for ground-based telescopes to spot, since such asteroids still re-radiate as heat the energy they receive from the sun.

Using infrared data, scientists can get a better idea of how many of these darker objects are out there. And it helps reduce uncertainties in estimates of the size of an object – estimates typically based on brightness. A small but highly reflective asteroid can have a similar brightness to a much larger but darker asteroid when viewed only at optical wavelengths, Dr. Mainzer explains.

By comparing the amount of visible light an asteroid reflects to the amount of infrared radiation it emits, researchers also can glean in general terms the asteroid's composition.

Toward the end of the original WISE mission, the craft ran out of coolant for two of its four infrared detectors. That left two others, which could operate at warmer temperatures. The infrared wavelengths that these two detectors covered were suitable for asteroid detection, so WISE became NEOWISE for the craft's near-Earth object search.

By the time the National Aeronautics and Space Administration put the craft to sleep in February 2011, the craft had discovered 21 comets, more than 34,000 asteroids out in the main asteroid belt between Mars and Jupiter, and 135 near-Earth asteroids.

Other than running out of the hydrogen ice needed to cool two of the detectors, the solar-powered craft had no other "expendables" left to lose and two good detectors remaining, making it a prime candidate for revival.

In the meantime, the Obama administration's budget for fiscal year 2014 included money for NASA to begin work on what's come to be known as the Asteroid Initiative. This combines a robotic mission to capture and park an asteroid in an orbit between Earth and the moon for later human exploration with efforts to find all the near-Earth asteroids that threaten Earth.

The threat struck home in a dramatic way in February, when a small asteroid entered Earth's atmosphere and exploded high over the Chelyabinsk region of Russia. The blast damaged more than 7,200 buildings and by some estimates injured nearly 1,500 people.

Against that backdrop, "a few months ago NASA came to us and asked us for a proposal to reactivate" NEOWISE, Mainzer says.

Within the first two weeks of September, her team hopes to wake up the craft and check out its telescope for a three-year mission.

Time is not on the mission's side, however. The craft is in an orbit uniquely suited for its original mission – one that keeps the telescope pointed at deep space and the solar panels always facing the sun. Over time, however, this sun-synchronous orbit undergoes subtle changes. By 2017, the orbit will have changed enough to prevent the team from keeping the sun out of the telescope, rendering it useless.

"When that happens, the mission will really be over," Mainzer says. "The clock is ticking. That's why there was some urgency to get this restart on the road."

Mainzer acknowledges that NEOWISE isn't the slickest asteroid-hunter to ever grace a PowerPoint presentation. "It can do a good job, but it's limited in what it can do," she says.

Among her other activities, Mainzer, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., is also the lead investigator for a mission concept called NEOCam – twice proposed for a NASA mission.

It didn't make the cut either time. But her team has received money to work on the detectors, with an eye toward submitting the proposal again, perhaps in 2015. The mission is designed to help meet a congressional mandate to find 90 percent of all near-Earth objects larger than about 500 feet across. The Chelyabinsk asteroid has been estimated at between 55 and 65 feet wide.

While NEOWISE will gather useful data, part of its value also lies in teaching her team how to operate future asteroid-hunting missions, she says.

That alone is worth the price of admission, suggests Harold Reitsema, an aerospace engineer who retired from Ball Aerospace & Technologies Corp. in Boulder, Colo., and now serves as mission director for the Sentinel program. That program aims to launch and operate its own asteroid-hunting craft in 2017 or 2018. The project is run by the B612 Foundation, a 12-year-old nonprofit organization focused on the asteroid threat to Earth.

Sentinel's goal is to help achieve the congressional objective as well, while also identifying asteroids as small as 100 feet across.

"This is a good demonstration of the detectability of asteroids, especially the near-Earth asteroids," Dr. Reitsema says of a revived NEOWISE. "It's a great proof of concept for Sentinel."

To which Mainzer would probably add: for NEOCam as well.

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