If you’re hanging out near the Pacific Ocean’s Point Nemo, the spot on the planet farthest from land, at the right time in 2031, you’ll get a good show, and the ocean will get a new permanent resident: the International Space Station (ISS). That’s when the station’s life will officially end, and it will come screaming through the atmosphere, slamming into the Pacific, never again to host astronauts or microgravity experiments or tension with mission control.
That crash will be aided by a SpaceX-built vehicle that will propel the ISS seaward with the aim of ensuring that its debris doesn’t hit anyone or anything on the way down.
When a satellite takes a one-way trip through the atmosphere, the process is called deorbiting. That journey can be purposeful and controlled—as with the ISS. It can also be passive, in which a spacecraft is allowed to descend and burn up as it will. That latter option is the norm with small, new satellites and older satellites that are dead and out of anyone’s control; sometimes, the latter are big enough that parts of them survive reentry.
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In the coming years, the number of satellites set to launch—which will later have to deorbit—is due to increase drastically. Ensuring their lives end safely—minimizing risks to people, property, planes and the atmosphere itself—is no small task, whose details scientists, engineers and regulators are grappling with.
The biggest dangers come from larger spacecraft, old or new, that may not disintegrate on reentry. “A bunch of us have been calling for an end to uncontrolled reentries of massive satellites,” says Jonathan McDowell, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and publisher of the newsletter Jonathan’s Space Report, which tracks space launches. “It’s just not okay.”
Deorbiting the space station is the most complicated get-out-of-space operation that will happen for the foreseeable future because the ISS is the largest humanmade structure that’s ever been in space.
Because of its size, the station won’t burn up completely in the atmosphere, as some smaller satellites do, but will instead scatter pieces the size of a microwave to the size of a car across whatever swath of the planet it crashes into.
To make sure that scattering happens over the empty Pacific and not, say, New York City, NASA has to push it there. And for that, the agency has hired SpaceX, which will build a special iteration of its Dragon spacecraft, the capsule that carries cargo and humans to and from the space station.
Called the U.S. Deorbit Vehicle (USDV), this Dragon will be more powerful than its ferrying cousins: it will have 46 thrusters instead of a mere 16, and it will boast a bigger trunk to house all those engines, six times the normal propellant and power systems that supply four times the usual juice. Once SpaceX is done building this steroidal Dragon, it will turn the $1.5-billion spacecraft over to NASA, which will own and operate it.
At the beginning of the operation, the USDV will dock to the ISS while the final crew is still there, so they can verify its working order. After that, NASA will let the station’s orbit naturally decay, and the crew will depart when the station drops 70 kilometers lower than it is now. The spacecraft, as a ghost ship, will continue to downdraft for six months, naturally lowering to 220 kilometers above Earth. When the ISS is at the right point in its orbit, around two dozen of the special Dragon’s thrusters will fire at once, sending it careening toward Point Nemo.
That fiery push may sound simple, but it’s not, McDowell says. As the station dips lower into the atmosphere, the air gets thicker. “The winds are too much,” he says, which “will make it hard to keep the station oriented.” If the rocket scientists don’t keep the station pointed in the right direction, they can’t effectively use the thrusters to control its motion. “So you have to make the final engine burn from a high enough height that you’ve still got control over it,” McDowell says.
And you have to boost its speed high enough and quick enough that it actually does deorbit and come down in the right place. It will be essential to achieve that oomph on the first try—likely why the USDV will have more engines than it necessarily needs. (SpaceX did not return a request for comment.)
The space station will be the flashiest thing that has ever returned from orbit. But it’s far from the only thing that needs to come down. “You don’t want to leave dead things in orbit, particularly low-Earth orbit, because it’s very crowded,” McDowell says. Also, big, dead objects will eventually reenter uncontrolled if left alone, putting a point in favor of planned deorbits. The Federal Communications Commission recently shortened the time gap allowed between when a low-orbiting satellite stops working and when it deorbits from 25 years to five years so that space traffic doesn’t become unmanageable. But the U.S. isn’t the only country with stuff in space, and the only international framework that exists is an unenforceable guideline from the Inter-Agency Space Debris Coordination Committee, which has stuck to the 25-year timeline. “It’s very important to make sure that we keep the resource of space available for future generations,” says Clare Martin, executive vice president of Astroscale US, a space sustainability company that works on technology to manage crowding in low Earth orbit.
With the new required U.S. timeline, and with so many satellites set to go to space as part of constellations like SpaceX’s Starlink, the amount of material coming back down toward Earth is going to drastically increase. The descents of bigger satellites, such as the ISS, have to be controlled because pieces of them will survive the trip. But how big is big? “I think if it’s under, say, half a ton, you probably don’t need to worry,” McDowell says. “If it’s more than a few tons, you should worry. And in the middle, it sort of depends.”
The stakes of deorbiting correctly are high because no one wants crowded orbits or, conversely, to be hit by space debris—which can happen and will only be statistically more likely in the future unless humans address their deorbiting issues. Parts of the trunks from Dragon spacecraft, for instance, have survived reentry to hit spots in North Carolina and Canada in 2024. Two pounds of debris survived from a pack of ISS batteries disposed of through the atmosphere, which then crashed through the roof of a Florida home.
Michael Kezirian, president of the International Space Safety Foundation and adjunct professor of astronautics practice at the University of Southern California, points to data on satellites that have fallen uncontrolled through the atmosphere. “About 30 percent of the mass survives, typically,” he says. Larger components such as thrusters, pressure vessels, batteries and reaction wheels fail to fully incinerate.
If satellite operators want to make sure that mass doesn’t come down on someone or something, they must actively control their spacecraft’s reentry. That may involve hooking a satellite up to a powerful set of thrusters like the USDV’s; smaller but similar systems could take out medium-sized spacecraft. Satellites can also use their own propellant and thrusters to complete their life in a planned way, or use special motors specifically meant for end-of-life maneuvers, like those made by space transportation company D-Orbit and installed before launch. Companies such as Airbus have worked on nets and harpoons that can wrap around or impale objects in space and bring them back down through the atmosphere.
Smaller satellites that will at least mostly burn up can also use some assistance in coming down in a timely but passive manner. Some companies have been working on tethers, essentially long metal strips that would fly behind satellites and convert their kinetic energy into electrical energy, slowing the satellite and sending it downward. There are also drag sails, thin material somewhat like a flat parachute that attaches to satellites to create more resistance with atmospheric particles and drag satellites down.
At Astroscale, Martin and her colleagues have done a demonstration mission called ELSA-d. For this mission, they attached a docking plate to a client’s satellite and used their ELSA-d vehicle to magnetically capture that satellite and then release it. The follow-on ELSA-M mission will use similar technology to dock, release and deorbit actual spacecraft from the Internet services company Eutelsat OneWeb. “We have to take responsibility for what we're doing in orbit, and part of that is to use it to its advantage but then treat it responsibly afterwards and dispose of [our] stuff,” Martin says, of satellite operators. “Move it out of the way.”
Disposing of satellites involves tough choices. It isn’t feasible for all the old spacecraft that went to orbit without a good plan for getting down to receive a good plan now.
Kezirian is concerned about the future of falling space debris from satellites old and new—including satellites that leave behind pieces that may not be lethal to anything on the ground but could be catastrophic for an airplane.
Kezirian has thought about this issue in relation to the Columbia shuttle disaster, when at least nine planes flew through the debris tail from the accident.
Those close calls resulted in NASA changing the shuttle landing procedures to better consider ground risks during an accident, and to improve collaboration between NASA and the Federal Aviation Administration in predicting and acting on potential debris.
Kezirian thinks more changes are in order. Right now sailors and pilots get notices from, for example, the Coast Guard and the FAA, respectively, about controlled deorbits. “Don’t put your ship or plane in this area on Friday afternoon between 2 and 3 due to risks of falling space debris” is how McDowell summarizes these notices, which are called NOTAMs (Notices to Air Missions) for planes and LNMs (Local Notices to Mariners) for ships.
But not all space-debris events may result in such warnings. Although the chances of any one piece of space debris hitting any one plane are small, the more stuff you send sailing to Earth and the more air traffic you have, the higher the chances get—and the more likely you are to have a kind of black swan event in which a piece of a satellite’s battery downs a plane of people.
According to Kezirian, officials need to get ahead of this problem before those chances rise too much as more and more satellites are launched. “You need to be much better at predicting the uncontrolled reentry,” he says. “You have to have a very good idea of where the debris will be falling through the airspace. And then you need a way to provide timely notification to the pilot in command of an aircraft to move out of the way.” These notices would be more like Waze for the skies than a NOTAM. Kezirian would like to extend the government’s tight tracking of space debris down through the atmosphere and communicate that information to the air-navigation system, resulting in “dynamic closures of the airspace corresponding to the narrow footprint of reentering debris,” allowing planes to effectively reroute.
Even if the issues with debris affecting flights and crashing through houses were solved, however, there would still be deorbiting problems caused by melting and vaporizing satellite ingredients, McDowell says. “You are changing the chemical composition of the upper atmosphere, as well as creating shockwaves in the upper atmosphere that themselves have chemistry effects,” he says.
Experts are beginning to be concerned that that effect might actually be substantial and that it will grow more so. In samples of the rarefied air, “there’s all of this sort of metallic crap there that didn’t used to be there that looks like it’s from vaporized spacecraft,” McDowell says. He’s currently working on a paper estimating how much of that foreign material remains in the atmosphere. “We just don’t know yet what the effects are,” he says. “But that doesn’t mean you go, ‘Oh, well, no worries,’ right?”
Deorbiting, it turns out, is mostly worries. And determining how to assuage them, scientifically and technologically, will be key to ensuring both space and Earth stay safe. “We need to keep addressing this problem now. We can’t wait and hope it will go away,” Martin says. “It won’t.”