Essay: The New Infrared Sky
For astrophysicist Amy Mainzer, December 14, 2009, was the day to say goodbye to six years of work. In the early morning darkness, she watched as a Delta II rocket containing her labor of love ascended from central California's Vandenberg Air Force Base into the atmosphere. Mainzer tried to speak, but tears stopped the words in her throat, and instead she flashed two thumbs up and a big smile.
Near Mainzer stood a cheering crowd of colleagues who also helped design, build, and launch the rocket's precious cargo: the NASA space telescope known as the Wide-field Infrared Survey Explorer, or WISE. Their farewell meant that this scientific journey had just begun.
Within a month, WISE settled into orbit 525 kilometers (326 miles) above Earth and began scanning wide swaths of space, snapping shots every 11 seconds. The WISE team members are now piecing together these images--more than two million--into a complete map of the sky that they will release to scientists and the public in 2011. While not the first survey to map the entire sky, WISE is by far the most sensitive one of its kind. "WISE is going to rewrite a lot of our understanding of astronomy," says Mainzer. "It's really going to change the way we see the Universe."
The Infrared Universe
WISE detects light that people can't see with their eyes--infrared light. We see different colors because of the different wavelengths of light that objects reflect or radiate: blue light, for example, has shorter wavelengths than red light. But wavelengths of light can be so long or short that our eyes can't detect them. (Human vision evolved to see a so-called visible range of wavelengths, between 380 and 740 nanometers.) Infrared radiation is light with wavelengths slightly longer than our visible range.
Many telescopes other than WISE detect infrared light, but each telescope has a particular specialty. The Spitzer Space Telescope, for example, observes cosmic objects in detail by collecting infrared light from a small region for long periods of time. WISE, on the other hand, is a survey telescope: it captures less detail but scans much wider swaths of space in a given duration. Astronomers who target objects with Spitzer must consult all-sky surveys like WISE's to point in the right direction.
WISE's sky-wide map will also reveal previously unseen space objects. In 1983, the Infrared Astronomical Satellite (IRAS), conducted the first all-sky infrared survey, and others have followed. While IRAS's camera had only 62 pixels, or digital "buckets" that collect light to make an image, WISE's camera has more than 4 million pixels and hundreds of times the sensitivity, exposing what IRAS could not.
"WISE brings modern technology to infrared astronomy," says Mainzer, who is the WISE deputy project scientist at NASA's Jet Propulsion Laboratory. The spacecraft's imaging power will turn up smaller and fainter objects more clearly than any previous infrared survey telescope.
A Banquet of Objects
Mainzer, in particular, wants to use the WISE data to learn more about our cosmic neighborhood, which still harbors many mysteries. One question is the closest object to our Solar System. Current observations show that this is a faint star called Proxima Centauri. "There's a better than 50 percent chance that our Sun actually has a nearest neighbor that's closer than Proxima Centauri," says Mainzer. It's possible, she says, that WISE could detect a nearer, dimmer neighbor that is instead an unusual object called a brown dwarf.
Brown dwarfs are not quite planets and not quite stars. They are smaller and cooler than stars; in fact, they are so small and cool that they cannot fuse hydrogen into helium at their cores and shine steadily for billions of years like stars do. But brown dwarfs form like starsfrom a collapsing cloud of gas and dustand this shrinking process causes them to heat up and emit some radiation. Brown dwarfs are so cool that they release most of their energy at infrared wavelengths. Scientists estimate that WISE will reveal about a thousand brown dwarfs populating areas of space that appear empty to optical telescopes. Study of these objects could reveal connections between planet and star formation that aren't yet understood.
WISE will also reveal objects within our Solar System, objects that, if overlooked with visible-light telescopes, could threaten Earth itself: asteroids. Extremely black asteroids don't reflect much light at visible wavelengths. Instead, they absorb sunlight and become warmer, like the asphalt of a parking lot on a hot day. Thus, these shadowy space rocks are obvious in infrared wavelengths.
"Infrared light is very useful for figuring out how large an asteroid is," says Ned Wright, the WISE project's principal investigator and a professor at the University of California-Los Angeles. WISE's data will help scientists better gauge the size of blacker-than-coal asteroids that orbit in the main asteroid belt and those whose orbits bring them relatively close to Earth. Accurate estimates of size are key, says Wright, "when you're trying to figure out how dangerous asteroids are and whether we should have a fleet of interceptors ready to move them out of the way."
Beyond spotting relatively close objects such as asteroids and brown dwarfs, WISE is observing the much more distant Universe. Ten to twelve billion light-years from Earth and the Solar System, galaxies exist that are undetectable at visible wavelengthsyet explode with activity at infrared wavelengths. Known as ultraluminous galaxies, these systems emit hundreds of times more infrared radiation than do ordinary galaxies like the Milky Way.
WISE can see not only the dusty surroundings of ultraluminous galaxies, which heat up and radiate in the infrared, but also what lies within them. Visible light from these galaxies' newborn stars cannot penetrate this shroud of dust and reach WISE's detectors, but their infrared wavelengths can.
The light from these bright, distant galaxies takes billions of years to reach Earth, so WISE sees them as they were billions of years ago, when the Universe was young and the galaxies were still forming. Ultraluminous galaxies form stars at a furious pace. Our own galaxy forms about one solar massa unit of mass equivalent to that of the Sunof new stars per year. Some ultraluminous galaxies, speculates Wright, could be producing 10,000 times that. "Since we don't know where ultraluminous galaxies are and they don't show up at visible wavelengths," says Wright, "the only way to know is to survey the whole sky in the infrared."
His colleague Peter Eisenhart, the WISE project scientist, speculates even more broadly. "We'll probably find things that we haven't thought of," he says. "And that's the most exciting [prospect] of all."
WISE's Future Sky
In July 2010, after six months of data collection that returned 1.3 million images, WISE had mapped the entire sky. Then it began a second scan of the sky, surveying until its hydrogen coolant ran out. WISE completed its main mission on September 30, 2010, and in April 2011, the collaboration will release a preliminary infrared atlas and catalog of discoveries. Scientists will be able to study the WISE team's major findings and use the new data to enrich their exploration of specific regions of the cosmos.
"Hundreds of astronomers will be looking through this catalog and finding interesting objects," says Wright. "I think that WISE is going to provide a legacy that will last for decades."
More About This Resource...
Supplement a study of astronomy with a classroom activity drawn from this Science Bulletin essay.
- Have students read the essay (either online or a printed copy).
- As a class or in small groups, have them watch the overview video for the Digital Universe 3-D Atlas
- If able, download the atlas and free Partiview software and have students explore one or more data sets.
SubtopicTools and Methods