Nestled into the payload of a Delta 2 rocket at Vandenberg Air Force Base in California is a satellite that should open new targets for astronomical study both near and far. NASA's Wide-field Infrared Survey Explorer (WISE), slated for launch no earlier than 6:09 A.M. Pacific Standard Time on December 11, is charged with mapping the sky in the mid-infrared to create an atlas of objects whose emitted light is invisible to human eyes and largely absorbed by Earth's atmosphere.
The international Infrared Astronomical Satellite, the most direct predecessor to the $320-million WISE, was launched more than 25 years ago. With the increases in technology and astronomical know-how since then, a bounty of new objects may await discovery by WISE, from distant galaxies whose optical light is dwarfed by their infrared output to failed stars known as brown dwarfs, some of which may be closer to Earth than the star Proxima Centauri, the sun's nearest known neighbor.
"WISE is going to survey the entire sky in the mid-infrared—that's wavelengths about five to 33 times longer than the red light you can see," explains the mission's principal investigator, astronomer Edward Wright of the University of California, Los Angeles. "By studying these longer waves, we can look at the sources of light that are cooler than, say, the lightbulb filaments or the sun that normally produce the light that we see."
The satellite will circle Earth in a polar orbit (flying over both poles), following the so-called terminator—the divider between day and night on the planet below. By maintaining that orientation, WISE can point its telescope away from Earth and keep the sun on one side, allowing the spacecraft to be shielded from solar radiation by its sunshade. WISE's longer-wavelength detectors will be cryogenically cooled to just 8 kelvin, or about –265 degrees Celsius; warm instruments can contaminate infrared observations with their own radiated heat.
It will take half an Earth orbit around the sun, or half a year, to map the whole sky. With enough cryogen for 10 months of flight, WISE should be able to complete one full survey before doubling up observations on half of the sky. Six months after the spacecraft runs out of coolant for its instruments, a preliminary data set should be released to astronomers.
In the course of the mission, Wright predicts, "we'll see hundreds of millions of sources, and we'll find millions of brand-new objects that nobody knew existed." WISE will also help constrain the threat of near-Earth asteroids, many of which are already catalogued but whose size—and hence potential for harm—can be difficult to gauge from their reflectance of optical light alone.
But Michael Skrutskie, a University of Virginia astronomer and a member of the WISE science team, is especially interested in the satellite's ability to pick out previously unknown brown dwarfs, objects larger than planets but too small to sustain nuclear fusion of hydrogen. Because they do not burn bright like normal stars, brown dwarfs are difficult to spot, but they radiate enough heat to show up in the infrared.
"If you look at the brown dwarf discoveries to date, the coolest ones found—and this is largely because searches have gone on in the near-infrared almost exclusively—are about 600 or 650 degrees kelvin," Skrutskie says. "You know there's a population of cooler brown dwarfs out there; we're just seeing the tip of the iceberg now. WISE is designed to find those."
Skrutskie was principal investigator of the Two-Micron All-Sky Survey (2MASS), a ground-based observation campaign taken in shorter (warmer) infrared wavelengths than those WISE will probe. So he is familiar with the role that sky maps generated by surveys play in driving further research. "Sky surveys are in some ways fundamental to opening up new classes of objects to investigation with larger telescopes," he explains.
One of those telescopes is Herschel, a space-based infrared observatory launched in May by the European Space Agency in collaboration with NASA. Data from WISE may generate proposals for telescope time on Herschel for more detailed follow-up observations, says Paul Goldsmith, project scientist for Herschel at the NASA Jet Propulsion Laboratory (JPL) in Pasadena, Calif. (Goldsmith's JPL colleagues will manage ground operations for WISE; Utah State University's Space Dynamics Laboratory in North Logan, Utah, designed and built its instrumentation, and Ball Aerospace in Boulder, Colo., built the spacecraft itself.)
Like the Hubble Space Telescope, which also has infrared capabilities, Herschel is a pointed instrument, not a wide-view surveyor like WISE. Herschel boasts a telescope even larger than Hubble's and roughly nine times the diameter of that on WISE. "You have to know where to look with Herschel," Goldsmith says.
"They could turn up some really exciting stuff—anything from brown dwarfs to galaxies," Goldsmith says of the WISE team. "There will be a lot to keep astronomers busy, but that's a good problem to have."
Indeed, Wright says, one of the most exciting aspects of the WISE mission is its potential to tap into the unknown. His colleague Skrutskie agrees. "It's the Forrest Gump approach, right?" Skrutskie says. "The sky is like a box of chocolates—you never know what you're going to get."