Four years ago, a rare celestial event brought people together during the height of the COVID-19 pandemic. The comet NEOWISE, which was visible to the naked eye, drew people outdoors in the summer of 2020. This week, the NASA spacecraft that discovered the comet, also called NEOWISE, is being shut down, having scanned the entire sky 23 times and observed about 200,000 solar system objects during an extended two-mission, 15-year life span. But its successor mission is already in the works, with a UCLA planetary scientist at the helm.

Explosive flares and other eruptions from the sun have heated and expanded Earth’s atmosphere, slowing down the satellite and dragging it into a lower orbit. NEOWISE stopped collecting images of the sky on July 31 and on Thursday, engineers at NASA’s Jet Propulsion Laboratory will turn off the spacecraft’s transmitter. Sometime around the end of the year, NEOWISE will likely enter the atmosphere and burn up completely.

“We’re at the mercy of the sun,” said NEOWISE project leader and UCLA professor of earth, planetary and space sciences Amy Mainzer. “It’s going to be weird when it’s gone because it’s been such a constant presence. Every morning, we look to see what data it has collected. Since 2013, it’s always been there and we’re always in touch with it, so it will be strange when it’s not there anymore.”

The satellite, which NASA describes as about the size of a polar bear, gave far more than anyone ever expected. Launched as WISE (Wide-field Infrared Survey Explorer) in 2009, its original mission, under the direction of UCLA astrophysicist Edward Wright, was to map the entire sky at infrared wavelengths with a sensitivity up to hundreds of thousands of times better than previous surveys. WISE scanned the skies twice, making 700 million observations of asteroids, stars, galaxies and more. When the telescope ran out of the coolant needed to chill its sensitive infrared detectors two years later, it was put into hibernation.

“During its first two scans of the sky in 2010, WISE found hundreds of very low-luminosity stars known as brown dwarfs, including the third- and fourth-closest star systems to the sun. And it found very distant, dust-obscured high-luminosity galaxies that are some of the most luminous objects we have seen,” Wright said.

However, two of the spacecraft’s four infrared-wavelength bands still worked. Mainzer, then at JPL, proposed to use these bands to find faint near-Earth asteroids and comets, and in 2013, NASA woke WISE back up, renaming it NEOWISE, or Near-Earth Object Wide-field Infrared Explorer. Its new goal: to aid planetary defense by surveying and studying cosmic bodies that stray close to our neck of the solar system.

Under Mainzer’s leadership for the past 11 years, the NEOWISE team captured nearly 27 million infrared images, discovered 215 near-Earth objects and observed thousands more. Near-Earth Objects, or NEOs, are asteroids and comets that pass within 28 million miles of Earth’s path around the sun. Many of these NEOs evade telescopes using reflected visible light because they are both very small and very dark. NEOWISE detects thermal emissions and can detect both dark and light objects.

“If you’ve ever accidentally cracked open a printer cartridge, it’s extremely dark, carbon-rich material. Some of these objects are made of carbon-rich material as dark as printer toner, and they’ve made their way from the main asteroid belt between Mars and Jupiter to become NEOs over millions of years,” Mainzer said.

Scientists used to think that as NEOs got smaller, they’d get brighter. But NEOWISE showed that was probably due to selection bias: Telescopes using reflected visible sunlight to see are more likely to see the shiny ones at smaller sizes. NEOWISE found extremely dark objects hundreds of meters in size, big enough to do catastrophic damage should one of them ever collide with Earth.

One of Mainzer’s favorite objects is Earth’s first-known Trojan asteroid, which is an asteroid trapped by a planet’s gravity and sharing its orbit.

“It’s like a leaf trapped in the eddy of a stream, going around and around in circles. A Trojan asteroid is similarly trapped into following or leading the planet. The planet’s gravity is so strong, it drags the asteroid around like a blanket,” Mainzer said. “Jupiter’s gravity is so powerful, it probably has millions of these. But because you have to look toward the sun, no one had seen one around Earth. With NEOWISE, we spotted the first one!”

The Trojan asteroid, called 2010 TK₇, poses no threat to Earth because it is in a stable orbit and will be for thousands of years.

NEOWISE was also good at population studies, which look at the ratio of bright to dark asteroids, measures their diameters and computes their reflectivity. These studies help scientists understand the relationships between different types of asteroid “families” in the main asteroid belt between Mars and Jupiter and understand how they collide.

An artist’s concept shows the Wide-field Infrared Survey Explorer, or WISE spacecraft, in its orbit around Earth.

Mainzer’s group is also studying the numbers and sizes of all types of comets, as well as the mechanism that causes comets to sprout long tails of gas and dust. This line of research can deliver information about the origins of the solar system.

For Mainzer, the NEOWISE comet was a powerful experience. When her group found the comet and knew it would be visible, she went to the Santa Monica Mountains and pulled over to the side of the road with binoculars.

“It was a tough time, but all these people were spread out along the road in the middle of nowhere, and even though we couldn’t stand close, we were all together watching the comet,” she said. “Most of the time we see images as little dots on computers, but this time I saw it with my own two eyes and so did everybody else. I had an almost visceral sense of connection to the night sky and natural world. It’s a powerful reminder that we’re just little people on the surface of a planet in a much larger universe.”

Mainzer won’t have to miss NEOWISE for long. She’s leading its successor, the Near-Earth Object Surveyor Mission, or NEO Surveyor, a NASA mission designed to discover and characterize NEOs that pose potential threats to Earth. The spacecraft consists of a single scientific instrument: a 50-centimeter diameter telescope operating at two heat-sensing infrared wavelengths that can detect the dark asteroids that are hardest to find.

After launch in 2027, the NEO Surveyor will carry out a five-year baseline survey to find at least two-thirds of the near-Earth objects larger than 140 meters, or about 460 feet. These are the objects large enough to cause major regional damage in the event of an Earth impact. NEO Surveyor will make accurate measurements of NEO sizes and gain valuable information about their composition, shapes, rotational states and orbits.

“It’s strange to say goodbye to NEOWISE, but we have the next one on the way, and I’m optimistic that we’ll find out a lot more about our Earth’s nearest neighbors soon,” Mainzer said.