NASA is retiring its only dedicated asteroid-hunting space telescope, ending a 10-year mission that initially began as something else entirely: an astrophysics surveyor meant to open scientists’ eyes to the infrared universe.
The telescope launched into Earth orbit as the Wide-Field Infrared Survey Explorer (WISE) in 2009, mapped the infrared skies for a year, then went into hibernation in early 2011 after running out of coolant. But in late 2013 NASA reawakened the spacecraft, rechristened as Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE)—a reference to its new quarry, asteroids with orbits that send them swooping within the general vicinity of our planet.
But not even a second lease on life was expected to let the remarkably productive telescope endure for this long. And even now NASA is only ending the NEOWISE mission because the spacecraft has sunk low enough in Earth’s tenuous upper atmosphere that it will burn to pieces in just a few months. The telescope concludes its survey July 31; NASA will send final shutdown commands to the spacecraft on August 8.
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“I don’t feel that it’s really a big loss,” says Edward Wright, an astronomer at the University of California, Los Angeles, who was principal investigator for the WISE mission. “It’s been a very successful mission, so I’m really happy with that.... The only thing that’s causing our troubles is the atmosphere, and really, I can’t complain about the atmosphere because I breathe it every day.”
The WISE Era
In the 1990s Wright and his colleagues began discussing the telescope that would become WISE, inspired by improvements in infrared detectors. “Basically, we said we want to find unique objects,” Wright says. That’s how WISE came to be a full-sky survey focused on infrared light, which has wavelengths longer than visible light and which permits scientists to peer through dust that can otherwise hide stars and galaxies from view.
Wright’s wish list for the mission included two particular targets. He wanted to find the most luminous galaxy in the universe—which the telescope did, spotting one shining as bright as 300 trillion suns. And he wanted to find the closest star to our sun, which he suspected would be a brown dwarf—a faintly glowing gassy object several times larger than Jupiter yet still too small to fuse hydrogen to helium and shine as “true” stars do. Although the triple star system of Alpha Centauri remains the sun’s closest-known stellar neighbor, WISE did pin down a pair of brown dwarfs just 6.5 light-years from our solar system.
And even before its asteroid-hunting heyday, the WISE mission made a name for itself among planetary scientists just as much as astrophysicists. With its full powers, the spacecraft was able to peer into the main asteroid belt between Mars and Jupiter, observing more than 150,000 space rocks in this region of the solar system. “It was just a demon asteroid-finder,” Wright says of the telescope during its primary mission.
In fact, it was during the telescope’s WISE era that scientists on the project made what’s still a favorite discovery of Amy Mainzer, a planetary scientist at the University of California, Los Angeles, and principal investigator for the NEOWISE mission. That discovery was of the first-ever Trojan asteroid in Earth’s orbit. Trojan asteroids orbit the sun along the same path as a planet, either trailing behind or advancing ahead—Jupiter boasts the largest posse of these rocks, with more than 13,000 of them, but other planets claim a handful as well. And Earth has one, too, WISE determined: a 1,000-foot-wide rock. “I thought that was really cool,” Mainzer says. “It turns out Earth has a little friend—not so little, actually.”
But even by the time that discovery was announced, WISE was hibernating, powered down after the spacecraft ran out of the frozen hydrogen that kept its sensitive detectors so superbly cold, just a handful of degrees above absolute zero. Without coolant, the telescope could no longer obtain high-quality science data in two of its four observing wavelength bands. For astrophysicists, the telescope’s appeal had faded, so they and NASA formally moved on.
A New Mission
But Mainzer and other team members weren’t ready to let the telescope go. They’d gotten a taste of what it could do for asteroids, and they wanted more. Although losing the two bands that required extreme cooling meant WISE could no longer peer out to the main asteroid belt, Mainzer and her colleagues thought the telescope could make a real difference scouting out other space rocks called near-Earth asteroids.
Near-Earth asteroids are less common than their main-belt counterparts, but they carry extra meaning: besides telling scientists about the history of the solar system, as all asteroids can, those whizzing closest to our planet also allow researchers to gauge the chances that one may someday slam into Earth, potentially wiping cities off the map and triggering abrupt global climate shifts like the one that caused the dinosaurs’ demise.
Using its two remaining observation bands, Mainzer argued, NEOWISE could have a new life discovering previously unknown near-Earth asteroids while also characterizing the size and composition of known objects in our neighborhood. NASA went for it, expecting that the rekindled mission would last perhaps a year or two.
“It was kind of a natural fit to pull that in and make use of that asset,” says Mike Kelley, a planetary scientist at NASA and program scientist for the NEOWISE mission, who joined the team after the telescope emerged from hibernation.* “It made sense to reuse the spacecraft since it was just sitting there and we had a purpose for it,” he says. “We didn’t have to launch anything.”
The fight to revive the spacecraft paid off, and NEOWISE gathered its first new data in 2013. “It was like Christmas and New Year’s and Hanukkah and my birthday, all together,” Mainzer says of the moment the telescope got back to work, this time focusing on asteroids near Earth.
(Another highlight, Kelley says, came in 2020, when NEOWISE spotted a bright comet during some of the darkest days of the COVID pandemic.)
Scientists have three main tactics for studying near-Earth asteroids, but each comes with a limitation. Telescopes on the ground can spot asteroids in visible light but struggle to determine their size. A technique called planetary radar bounces light waves off an asteroid and catches the echo to map its shape, but the approach can only study rocks within a certain distance of Earth and is too targeted to be suitable for broader discovery surveys. And then there are infrared instruments, such as NEOWISE, which spot rocks based on the glow from their sunlight-warmed surface. Infrared studies allow astronomers to see smaller and optically darker asteroids and better measure an object’s size, but because Earth’s atmosphere blocks most infrared light, such observations must take place in space.
Ten years of successful infrared studies of near-Earth asteroids with NEOWISE have thoroughly convinced Mainzer—and NASA, too—that a similar telescope optimized for those observations is needed. “We realized we could do a lot better if we made some design changes,” she says. NASA formally began planning for that mission, called the Near-Earth Object Surveyor, in 2019, and the spacecraft is targeted for a September 2027 launch.
Rather than orbit Earth, the Near-Earth Object Surveyor will head to L1, a gravitationally stable point about 1.5 million kilometers away from Earth in the direction of the sun. From that vantage point, the telescope will be able to see more of space near Earth’s orbit. And instead of carrying solid hydrogen to stay cool, the solar panels that power it will also shield it from the sun’s heat. As a result, while the primary mission will last five years, scientists expect that the spacecraft could operate for 10 or even 12 years.
So NEOWISE’s legacy will live on, both in the data it has gathered and in NASA’s next asteroid hunter. But the atmosphere will pull the telescope out of the sky in December or January—February at the latest, Wright says, if no atmosphere-agitating solar flares strike our planet and puff up its outer layers of air. Along with a fortunate absence of major glitches, the sun’s relative quiescence for so much of the spacecraft’s tenure in orbit, in fact, is the largest factor that’s allowed the mission to last so long.
And NEOWISE has long been operating on borrowed time, Mainzer says. “It completely exceeded my expectations in lasting this long,” she says. “For that we have to thank the sun—and really great, talented engineers who have kept the spacecraft alive.”
*Editor’s Note (7/31/24): This sentence was edited after posting to correct Mike Kelley’s role in the NEOWISE mission.