It's been nearly 18 months since NASA's Wide-field Infrared Survey Explorer (WISE) ran out of the frozen hydrogen that kept its detectors cold. But by then the spacecraft had completed its intended mission: to survey the celestial sphere at four thermal-infrared wavelengths with more detail than ever before.
On March 14th, the mission team delivered the goods. The spacecraft's all-sky map, totaling some 18,000 images in all, has yielded a catalog of more than 560 million individual objects divided roughly between stars and galaxies. Included as well are thousands of the Milky Way's diffuse nebulae and more than 150,00 objects (mostly asteroids) in our solar system. Mission scientists had already released observations covering half of the sky last April.
This mission has been a huge success, and with a total cost of $320 million, it's a bargain when compared to, say, the $8 billion projected for the forthcoming James Webb Space Telescope. I've been trying trying to come up with some catchy phrase that plays off the spacecraft's acronym in this regard — how about "NASA's WISE Investment Pays Big Dividends"?
Seriously, astronomers will be capitalizing on these rich data for many years. The spacecraft's four infrared detectors, centered on 3, 5, 12 and 22 microns in wavelength, detect energy from cosmic sources too cold to glow in visible light. These typically range in temperature from about –320°F to 1,300°F( –200°C to 700°C). For example, WISE recorded a class of ultracool stars, known as type Y, that astronomers had first glimpsed a few years ago. The deep-infrared view can also look through dust clouds that appear hopelessly opaque in visible light.
Looking something like R2D2 on steroids, the ¾-ton spacecraft has a 16-inch (40-cm telescope) that delivered a 47-arcsecond-wide field and 6-arcsecond resolution. WISE was launched on December 14, 2009, into a polar orbit 325 miles (525 km) high. The cryogenic coolant for its optics started bleeding off right away, so there was no time to lose: "first light" came three weeks after launch.
By late September 2010, all the hydrogen was gone. The spacecraft quickly warmed up, effectively blinding the two longest-wavelength detectors. But useful observations could still be made at 3 and 5 microns. When an advisory team of astronomers recommended against continuing the mission this way, NASA's solar-system division picked up WISE's option and used it to sweep the sky for asteroids in near-Earth orbits. During this second-chance mission, dubbed NEOWISE, the spacecraft found that the Earth-crossers aren't nearly as plentiful as some estimates had indicated.
Although WISE will be credited with numerous discoveries (a nice gallery of its infrared portraits is here), its long-term value is providing astronomers with a wish list of objects deserving detailed follow-up with targeted observations by NASA's Spitzer Space Telescope and the European Space Agency's Herschel observatory.
It was a sad day in February 2011 when principal scientist Ned Wright and his team switched off WISE's transmitter. But they no doubt take pride in knowing that their spacecraft accomplished what they'd designed it to do — and then some.