Astronomers plan to use data from NASA's upcoming Wide-field Infrared Survey Explorer (WISE) mission for decades to come as a pointer to objects in the sky that deserve closer study.

Set for launch into a 523-km. Sun-synchronous orbit as early as Dec. 9 on a Delta II rocket flying from Vandenberg AFB, Calif., WISE will use 40 lb. of solid hydrogen to chill some of its detectors to less than 8K. Combined with a 40-cm. telescope, the spacecraft will equip astronomers with an infrared (IR) sky map of unprecedented detail to cue the planned James Webb Space Telescope and other narrow-field IR observatories.

"WISE will give us a road map that will be used by big telescopes - we have Hubble; we have Spitzer; we have Herschel, and we will have the James Webb Space Telescope - so that they can visit or point at the most interesting objects in the sky," says Edward (Ned) Wright of UCLA, principal investigator on the mission. "So WISE is very much like a wide-angle lens, taking an all-sky picture, and the big telescopes are like telephoto lenses. Both are necessary."

WISE is expected to complete its all-sky survey in about six months, well within the 10-month operational limit by the coolant supply. Collecting a four-color image every 11 sec., the spacecraft will generate literally millions of images that will be combined to form a panoramic view of the infrared Universe like those generated by the International Infrared Astronomical Satellite (IRAS) in 1983.

But the IRAS instrument had only 62 pixels, while the WISE instrument will carry 4 million. The newer spacecraft will be able to study objects ranging from distant ultra-luminous infrared galaxies rich in the birth of new stars to cool brown dwarfs that may wind up being the nearest stars to the Sun. The spacecraft is also expected to spot approximately 100,000 new asteroids in the main asteroid belt between Mars and Jupiter, and "hundreds" of near-Earth objects that have not been previously detected.

While not always apparent in visible light, asteroids glow with the reflected heat of the Sun in the IR wavelengths the WISE instrument is designed to detect, says Amy Mainzer of the Jet Propulsion Laboratory (JPL), the WISE deputy project scientist. Because asteroids move in relation to the more distant stars in the background, they are "very easy to detect" in IR, whether they are made up of light or dark material.

"Infrared is a very powerful way of finding new asteroids," Mainzer says "Spitzer was only able to survey about 1% of the entire sky in detail. So if you want to find large numbers of near-Earth objects like the asteroids and comets we expect to find with WISE, you need to survey a much larger area."

In addition to pinpointing dark asteroids, astronomers hope to learn more about the density and other structures of asteroids, which Mainzer says could help in devising "mitigation campaigns for potentially hazardous asteroids."

WISE was built by Ball Aerospace in Boulder, Colo., with its instrument provided by Space Dynamics Laboratory of Logan, Utah. It will weigh 1,400 lb. at launch, much of it the Lockheed Martin cryostat that will preserve the solid-hydrogen coolant used to chill the focal plane and optics.

The mercury-cadmium-telluride and arsenic-implanted silicon detectors are set at 3.4, 4.6, 12 and 22 microns, with resolutions of six arc sec. in the two near-infrared bands and 12 arc sec. in the mid-IR wavelengths. The primary mirror is gold-coated aluminum, and the whole spacecraft will be steered with reaction wheels and torque rods. Its solar arrays generate 500 watts of electricity.

Plans call for a one-month checkout period after launch, followed by the six-month all-sky survey and, possibly, an extended observation period until the coolant gives out. But as was the case with the IRAS data, the object catalogues and infrared sky maps that WISE produces will be useful for much longer.

"WISE will allow us to learn lots of things about the objects that we already know about and even more superlative examples - the closest stars and the most luminous galaxies," says Peter Eisenhardt, WISE project scientist at JPL. "But perhaps the greatest benefit of an all-sky survey is that you can keep coming back to it. There are objects that are likely to be discovered years after the WISE survey is complete, and if you want to know the infrared properties of those objects you can come back to the WISE survey and the catalogue and atlas will tell you all about it. There are hundreds of papers being published today based on the [IRAS] 25 years ago. The legacy of all-sky surveys endures for decades."

Photo Credit: NASA