WISE asteroid survey will continue through January
BY STEPHEN CLARK
Posted: December 19, 2010
NASA has granted funding for the WISE infrared telescope to finish an extra full sky scan, giving scientists an additional opportunity to comb the inner solar system for hard-to-see asteroids that could threaten Earth.
The mission was supposed to end when with the loss of hydrogen, but NASA budgeted $1.6 million to keep the telescope working through the end of January, according to Trent Perrotto, a NASA spokesperson.
Tasked with finding near-Earth objects, or NEOs, the extended mission is utilizing two shorter wavelength channels inside the Wide-field Infrared Survey Explorer to scan the cosmos for an extra few months.
WISE's sensors are embedded inside a cryostat, a Thermos-like insulated canister that contained solid hydrogen to chill the telescope's instrument down to about -430 degrees Fahrenheit. Such cold temperatures were necessary to make the observatory sensitive enough to see the infrared light from cold star-forming dust clouds, distant galaxies and nearby asteroids and comets invisible to conventional visible telescopes.
The hydrogen gradually sublimated into gas and escaped into space, and WISE exhausted its cryogenic tanks around the end of September, according to Amy Mainzer, principal investigator for the NEOWISE mission extension.
Without the chill of hydrogen, the temperatures inside WISE's instrument bay started to rise.
"The telescope is still warming up," Mainzer said.
NEOWISE was funded to continue operating the spacecraft through the end of January, enough time to finish an additional survey of the solar system. WISE's 16-inch telescope observes the universe in a criss-cross pattern that takes about six months to see the whole sky.
The hydrogen lasted long enough to finish one-and-a-half sky scans, but scientists wanted to finish the second survey with two of WISE's four infrared channels unaffected by the loss of hydrogen.
The mission is designed to see some of the coldest parts of the universe. WISE's detectors were immersed in cryogenic fluid to ensure the observatory was not overwhelmed by its own heat.
It costs about $400,000 per month to operate WISE. A one-month extension through October confirmed WISE's 3.4-micron and 4.6-micron detectors, designed to observe in shorter wavelengths, still functioned with enough sensitivity to discover main belt asteroids, objects crossing Earth's orbit and nearby brown dwarfs, or failed stars.
"For asteroids and solar systems objects, this means that we do have some reduced sensitivity because these things tend to show up most brightly in the longer wavelengths," Mainzer said. "However, there is still plenty we can see in the shorter wavelengths. We're still detecting asteroids."
According to Mainzer, WISE has observed more than 155,000 asteroids, comets and other minor planets. WISE is responsible for more than 34,000 new solar system discoveries, mostly asteroids in the main belt between the orbits of Mars and Jupiter.
WISE is the only space-based telescope capable of discovering asteroids. Other observatories, such as Hubble and Spitzer, are better suited for detailed follow-up studies of known objects.
"What we're doing is sort of a blind search," Mainzer said. "We're carrying out a predetermined survey pattern, and whatever happens to cross into our path, we see. That has the great benefit of being an independent survey, meaning that we're not depending on other people to supply us targets. It's a survey that is not as biased by other people's prior knowledge."
Ground-based observatories see the sky in visible wavelengths, but a large fraction of asteroids are easiest to see through radiated heat, giving infrared missions like WISE an advantage.
"The data sets are highly complementary, because if you have both infrared and visible data on a particular asteroid, then you can determine its albedo, which is its reflectivity," Mainzer said. "That's an important number to know because it helps you understand a little bit about its physical properties."
Mainzer said new WISE discoveries are disseminated to the scientific community within 10 days, giving ground observatories and other space telescopes a chance for focused imaging.
The mission's ground segment includes a system that automatically searches imagery for potential asteroids, then compares the data with known objects before declaring a new discovery.
WISE has downlinked about one million images since running out of hydrogen this fall, according to Mainzer.
"The sky is a very big place, and we've taken images of all of it," Mainzer said. "It's just an enormous quantity of data that people are going to be sifting through for decades."
Perrotto said NASA has received no requests to extend NEOWISE beyond January, "though that could change."
According to Mainzer, controllers will put WISE in hibernation after the extended mission is complete. It will continue orbiting more than 325 miles above Earth.
"It will be safe, so if anyone does want to use it again in the future, it will be there for quite a while until the orbit decays and it will burn up," Mainzer said. "At this time, we don't have any plans to continue operating it after the end of January."