NEOWISE, a repurposed infrared telescope orbiting Earth, has returned its first test images in preparation for its new task: hunting for and characterizing asteroids – especially those that could represent a hazard to Earth.

The images are a milestone for NASA's first mission dedicated to the hunt for near-Earth objects (NEOs) from space. In addition to discovering additional NEOs, the telescope will help refine estimates of the hazards presented by objects already discovered via optical telescopes.

Beyond its immediate objectives, the mission is serving as training wheels for a more ambitious mission, NEOCAM, that a team is preparing to propose to NASA.

On Thursday NASA released initial images from NEOWISE. The team, led by Amy Mainzer, is pleased with the results. "The image quality looks excellent," says Dr. Mainzer, who is also leading the NEOCAM effort. "It looks pretty much like we left it about 31 months ago."

That's when NASA put the craft to sleep when its initial mission ended.

NEOWISE launched in 2009 as NASA's Wide-Field Infrared Survey Explorer (WISE). The exquisitely sensitive telescope spent about six months building an all-sky catalog of objects ranging from brown dwarfs to distant galaxies. NASA's James Web Space Telescope, currently slated for launch in 2018, and other observatories will use the catalog to pick targets for detailed observation.

By end of September 2010, however, the craft had exhausted the coolant that helped the infrared detectors reach the sensitivity needed to build the catalog. Within a month, NASA opted to extended the mission through February 2011 to see how the craft, with warmer detectors, worked as an NEO observatory.

During both phases of the craft's initial mission, it observed more than 158,000 asteroids. Of those, it discovered some 35,000 objects; 135 were NEOs. It picked up 155 comets, including 21 new ones.

Importantly, during the craft's warm phase, the team observed 6,500 asteroids in the main asteroid belt, between Mars and Jupiter, as well as 88 NEOs, according to a study the researchers published in November 2012 in Astrophysical Journal Letters.

NEOWISE's success prompted the agency to ask Mainzer and colleagues to propose a renewed search for NEOs. The agency approved the project in August and reawakened the craft in October.

NEOs are objects that make their closest approach to the sun at distances less than 1.3 astronomical units, or 1.3 times the distance between Earth and the sun. An important subgroup, near-Earth asteroids, can have have orbits that cross Earth's orbit, making them potential collision candidates.

Researchers have estimated that 94 percent of the NEOs they've identified come from the main asteroid belt, which lies between Mars and Jupiter.

On cosmic time scales, these interlopers are fleeting – tracing their near-Earth orbits only for a few million years, researchers say. They can get booted out of the solar system, or get drawn into the sun, or they can collide with something else, including Earth.

NEOWISE will help refine estimates of the hazard these objects can represent in several ways, all related to the telescope's use of infrared wavelengths instead of optical wavelengths, Mainzer explains.

For instance, size estimates for asteroids typically are based on the amount of visible light they reflect, which can vary between 2 percent for the darkest objects to 50 percent or so for the brightest, depending on the asteroid's composition.

But it's tough for optical telescopes to tell the difference between an intrinsically dark asteroid that's relatively close and a distant one that's intrinsically bright. Both could appear to have the same brightness, hence size, even though their actual sizes could be quite different. That confusion drops significantly when the objects are viewed at infrared wavelengths. For infrared observations with NEOWISE, the size might have an error of 10 to 25 percent. With optical observations, the sizes could be off by as much as 500 percent.

That becomes important in trying to understand the risks attributed to asteroids, because the bigger the object, the more devastating its effect if it strikes Earth. Infrared observations yield a better estimate of the range of NEO sizes and the number of objects in each size class.

Although NEOWISE has proved to be up to the task for asteroid detection, it wasn't designed to serve that purpose, Mainzer acknowledges.

That's where NEOCAM comes in. Her team has developed and successfully tested sensitive detectors that don't require tanks of liquid hydrogen to cool them to 8 degrees above absolute zero, as WISE's did. Instead, the detectors can be kept at their prime operating temperature – a still-chilly 35 to 40 degrees above absolute zero – merely by parking the NEOCAM craft in an orbit just inside the moon's orbit. The location, known as the Earth-moon L1 point, represents a gravitational sweet spot where the craft requires little or no fuel to keep to its orbital path.

The craft also would be designed to hunt for objects much closer to the sun than NEOWISE can.

"This would be something that would last a very long time and would just be out there always patrolling for asteroids," Mainzer says.

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NASA already had funded the technology development needed to pull off the mission. If NASA accepts the team's mission proposal, it would take another four or five years to build and launch the craft, Mainzer suggests.

In the meantime, the NEOWISE team is close to finishing its procedures for readying the craft and the data-analysis system for the hunt. NEOWISE should be ready for prime time in a few months.