For more than a decade, NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission has been searching the sky for near-Earth objects. Using its infrared vision, the probe, which sits in orbit above Earth’s surface, searched for asteroids and comets across the solar system and was used to identify those that could come close to Earth.
You may recognize the name because it was used for one of the mission’s discoveries, Comet NEOWISE, which was the brightest comet in more than 20 years when it passed Earth in 2020.
But the NEOWISE mission will soon come to an end as the spacecraft will fall into Earth’s atmosphere and burn up. The team is preparing for that end to come early next year and is getting as much science as possible into the spacecraft before it is no longer usable.
A mission can come to an end in several ways. If a spacecraft relies on solar power, like many others, then it may weaken as its panels lose effectiveness. This is what happened with the InSight probe on Mars, when its solar panels were covered by dust carried by the winds that accumulated over time until it finally failed to generate enough energy to continue operating. Or a spacecraft in orbit could run out of propellant, so it is no longer able to orient itself and return information to Earth, as happened with the Kepler Space Telescope.
Sometimes a mission is no longer needed because it has been replaced by newer, better technology. The Spitzer Space Telescope, for example, pioneered the field of infrared astronomy, but with the launch of the more powerful James Webb Space Telescope on the horizon, it was no longer needed.
But what happens to a spacecraft when its mission is over? How do you turn these things off? We spoke to team members from NEOWISE, a NASA mission that’s preparing to go offline next year, to find out.
A great ending
Some spacecraft can be safely allowed to drift into space as long as there is no chance of them hitting anything or causing any damage. That’s what happened to Spitzer, which will gently float away from the planet for the next 50 years as it orbits the Sun and follows Earth.
Spacecraft in orbit around planets, however, need to be disposed of carefully. When the Cassini mission to Saturn came to an end, it was dramatically plunged into the planet’s atmosphere, or deorbited, so that it deliberately disintegrated and was destroyed. This ensured that there was no chance of contaminating any of Saturn’s moons, which could potentially be habitable. The final descent also allowed for a series of incredible new research into Saturn’s atmosphere.
The NEOWISE spacecraft is orbiting the Earth, so it also needs to be safely disposed of. The concerns are that the spacecraft does not become space debris, a growing problem in some orbits, and that its deorbit does not pose a danger to anyone on the ground.
This last point is not a given. There have been cases in which parts of deorbited spacecraft have landed on the ground, as in the case of the Skylab space station, which left orbit in 1979, parts of which landed in Australia.
Today, missions need to plan how they will be deorbited before they are even launched.
A specialized team from JPL (NASA’s Jet Propulsion Laboratory) analyzes what the separations will be like based on spacecraft factors, such as its mass and speed. This makes the effects of a deorbit predictable.
“There is a minimal chance, if any, that any debris could fall and hurt someone,” said Joseph Hunt, NEOWISE project manager at JPL. The spacecraft’s orbit will drop, causing it to burn up in Earth’s atmosphere early next year.
“We want to end this mission gracefully,” Hunt said.
Why NEOWISE is ending
NEOWISE is not running out of fuel – it has no propellant on board – nor has it become an antique. But the Sun’s activity is forcing its disappearance.
The spacecraft sits in low-Earth orbit, about 300 miles from the planet’s surface, meaning it is affected by the planet’s atmosphere.
“The sun is constantly expelling charged particles and radiation at various wavelengths. What happens when it hits Earth’s atmosphere is that it can swell. It can cause the atmosphere to extend a little beyond its normal altitude,” explained Amy Mainzer, principal investigator for the NEOWISE mission.
The sun operates on an activity cycle of approximately 11 years, sometimes more active and sometimes less. NEOWISE got lucky and began its extended mission during a period called solar minimum, when the Sun’s activity was low. And it’s been a particularly quiet solar minimum, so the atmosphere has been less extensive than normal. All of this allowed the spacecraft to survive longer.
“Ultimately, it depends on Mother Nature.”
But this low solar activity doesn’t last forever, and we are now entering a period of increased solar activity, with more radiation from events like solar flares affecting the atmosphere. And that means the end of the spacecraft.
“You would think we are in space, about 500 kilometers in orbit, you would think it is completely free of atmosphere. But not exactly,” Mainzer said. “There’s just a little bit left, and that’s enough to cause drag forces that tug on the spacecraft and eventually pull it into the atmosphere.”
Without onboard propulsion, the spacecraft cannot retreat or move to another orbit. It can only rotate in place, so it’s “a battle we can’t win,” Mainzer said.
Knowing the end is near, the NEOWISE team sets a date when they feel comfortable ending the mission. The problem is that as the drag of the atmosphere increases, it becomes increasingly difficult to control the spacecraft, and the ability to control it decreases drastically and quickly.
“The laws of physics, in the end, always dictate what happens,” Mainzer said. “Ultimately, it depends on Mother Nature.”
A second life
NEOWISE has been in operation since 2013, so it has had a good 11 years of operation. But, in fact, it is already in its second life. That’s because previously it was a totally different mission called WISE.
WISE, or Wide-field Infrared Survey Explorer, was launched in 2009 and was designed to study objects within the solar system and beyond, including the Milky Way and more distant galaxies. In its time, the mission discovered thousands of small objects called minor planets, as well as discovering many star clusters. It also discovered Earth’s first Trojan, an asteroid that shares an orbit with Earth.
The mission used infrared detectors, which must be cooled to very low temperatures to work. The original temperature was just 8 Kelvin, achieved using solid hydrogen as the coolant.
But the coolant runs out. In 2011, its mission was completed and the coolant ran out, so the spacecraft was placed into hibernation. “I thought that was the end of the story,” Mainzer said.
But later in 2013, NASA was interested in using a spacecraft to search for near-Earth objects, and the team realized that the WISE spacecraft might be able to do that. It was reactivated and cooled to its new operating temperature over several months. The spacecraft stabilized at 75 Kelvin, “which to a cryogenic scientist is like a hot summer day,” Mainzer said.
At this warmer temperature, two of its four channels would not function properly, but the remaining two channels could still be used for science. Over the past decade, the probe has surveyed the sky, searching for – and finding – thousands of near-Earth objects using millions of infrared measurements.
“It has been incredible cosmic luck to spend so much time outside the spacecraft,” Mainzer said. “It wasn’t supposed to happen.”
Although NEOWISE is coming to an end, it won’t be long before it gets a successor. The NEO Surveyor mission, which is working to launch in 2027, is also an infrared spacecraft inspired by the success of NEOWISE. It will benefit from improvements in IR detectors, which have become much more powerful in recent decades thanks in part to technology driven by cell phone cameras.
Mainzer will also be the PI of the NEO Surveyor and is already busy overseeing the arrival of hardware for the spacecraft ahead of integration.
Before that launch, however, the team is still working to wrap up the NEOWISE mission and everything it entails. NASA doesn’t like to show off the end of missions, Hunt said, preferring to focus on new launches or newly acquired data. But they want to ensure the mission ends with due fanfare, which involves speaking to the media and celebrating the mission’s accomplishments, as well as preparing internal reports and documents.
Not to mention that the spacecraft is still doing science until the last possible minute, so mission operations need to be maintained. “With these missions, you never know when the biggest piece of science possible might be made public or the biggest observation might happen,” Hunt said. “It might be a day before they shut it down!”
When the last day of the mission arrives, it will be a bittersweet experience for the team. NEOWISE has achieved much more in its multiple lives than anyone expected, but it will still be sad to see it happen.
“In a way, these spaceships are little extensions of yourself. It’s as if your eyes are in the sky. You can see what he sees,” Mainzer said. But, she added, what drives her to carry out the next mission is how much fun she and the team had with NEOWISE. Now let’s move on to the next adventure, she said, “Let’s see more good things!”