While culling this list out of tens of thousands of items stranded in space is impressive, it’s just a snapshot in time. All these objects are moving, and their trajectories may change. In an increasingly busy field of space debris, a lot is still unknown, like where they’ll be next month.

“Tracking means detecting and identifying,” says Moriba Jah, a professor at the University of Texas in Austin and a professional space junk tracker. “As the number of debris increases…it’s hard to tell the identity of any given piece. How do you know that the thing you saw today is the thing that you saw yesterday?”

How much space junk is there?

The amount of space debris has been increasing since 1957, but shot up in 2018 as we became better at tracking it and more companies started launching large constellations of satellites. Blowing up inactive satellites, as Russia did last year during a missile test, only adds to the numbers—and headaches. Among them: Astronauts at the International Space Station had to hunker down in transport capsules for hours in case one of the pieces of the destroyed satellite hit the space laboratory.

Debris usually stays in the orbit it was launched to but can go in any direction circling the Earth. As of January, there were around 37,000 pieces of space trash larger than 10 cm (3.9 inches) being tracked internationally, most of them in low Earth orbit. It’s much harder to detect smaller objects, but scientists estimate there’s more than 330 million pieces smaller than 1cm.

The proliferation of space debris is upping the risk of damage to satellites we rely on for services like weather forecasting, GPS, and other communications, including the internet. “Those satellites could get hit, and it’s very expensive to replace them,” said Lt Col Tyler Eske, who directs operations at the 18th Space Control Squadron at the US Department of Defense. (The 18th Squadron monitors space debris 24-7 to send warnings to NASA if active space objects need to maneuver out of the way of space debris.)

Even a fleck of paint is enough to damage the window of a spacecraft.

Photo of a crack in a window caused by paint chip debris in space.
A window on the International Space Station was hit by a fleck of paint in 2016.

“The velocities in low Earth orbit can have collision speeds of up to 15 km per second,” says Stijn Lemmens, an expert at the European Space Agency. “A small object can have the effect of a hand grenade, just because of sheer velocity, and take out an entire satellite.”

Who keeps track of space debris? 

The main tracker of space debris is the US’s 18th Squadron, which has been on that job since 1957. But others, including the ESA, Russia, and China keep their own data as well.

There are two main problems with the current setup. First, it has many gaps. Even though phased-array surveillance radar systems continuously scan the sky to detect objects in lower orbits, they can only get observations every few days. And there are limits to what they capture. Most of the radars currently tracking debris go out as far as 4,000 km. “The farther out you go, the harder it is to track,” said Eske.

The other problem is that space monitors don’t share their data with each other. “The US doesn’t necessarily share its observations with Russia; Russia doesn’t share its observations with China,” said Jah. “Everybody has eyes on the sky, but we haven’t aggregated all the eyes together.”

At this point, even piecing together what information is out there is difficult because there is no common code. “We haven’t built a Rosetta Stone that tells me ‘When these people say Object 1, 2, 3, that’s actually Object 4, 5, 6,’” Jah adds.

So far, he’s come the closest to compiling a complete picture, but even that is full of blank spots.

Space junk air traffic control

Jah’s model, which represents items in space with dots of different colors, doesn’t take into account their shape and size. “We haven’t been able to describe objects beyond representing them as cannonballs or spheres,” he said. “Objects have size, shape, and material properties—there’s no database that provides that to anybody at this point.”

A model of Earth is covered with debris represented as dots around it.
Image: UT Austin

There’s also the problem of visualizing the vast scale of space. Pinpointing the relationship between different pieces of space debris is like finding the location of every fish or ant on a world map. The two scales are too disparate to see at the same time.

That’s why space junk trackers zoom into the area around individual pieces of debris and then note when those areas overlap, increasing the probability of the objects meeting or passing each other, which they call a conjunction. The 70 close calls listed earlier in the story were all conjunctions.

While scientists agree that objects in that list are too far apart to collide, and likely won’t, there’s no consensus on what distance is too close for comfort. “Some people feel 10 km is too close, others think 1 km is too close, the world doesn’t know,” said Jah.

Distance is only one factor. Experts also take into account an object’s position and speed to determine the probability of a conjunction, which they put at one in a thousand. But conjunctions are not the same as collisions.

Not that we would know if two pieces of space junk collide, unless we catch evidence, like a cloud of debris. Even then, it would be hard to tell if the scattered pieces were the result of a smashup, or something else, like an exploding battery. In 2021, the 18th Squadron recorded five breakups, or debris objects breaking into smaller pieces.

Not counting destructive anti-satellite tests, the 18th Squadron is aware of six collisions in orbit dating back to 1991. The biggest space crash was in 2009, when an inactive Russian satellite called Cosmos knocked into a working Iridium satellite.

Jah likens an individual space accident to an oil spill in the ocean. “Does this damage all the Earth’s oceans? Probably not so much from that single event,” he said. “But how many times do we just let that happen before it is an issue?”

Can we clean up space junk?

What experts are trying to avoid is known as Kessler Syndrome, a scenario under which there are so many objects in space that collisions are inevitable. The resulting debris would generate a ripple effect of more collisions, eventually making it impossible to maneuver around.

One workaround is to remove inactive satellites, which the ESA’s Lemmens refers to as sitting ducks. So far,  efforts to do this have consisted of launching a known object in order to prove it can be caught. The ESA hopes to go a step further by 2025 with a mission to capture a large piece of real space debris. “We’ve been putting satellites and rocket stages in orbit without taking them out,” he said. “It’s just a matter of time before they start colliding with each other.”

Preventing trash from getting in the way of active spacecraft in the first place is another way to avoid space wrecks. Experts say owners of inactive satellites need to safely dispose of them and make sure they’re no longer carrying fuel or active batteries to prevent unnecessary blowups.

But in order to do all that, we first need to have better ways to monitor what’s out there. For example, a global network of surveillance sensors would provide a fuller, real-time picture of problematic objects, says Jan Siminksi, a space debris analyst at the ESA.

Jah and others say more investment is urgently needed to create a more effective tracking system. He’s already working on one as chief scientific advisor of Privateer, started by Apple co-founder Steve Wozniak. The start-up is planning to launch satellites to study and categorize space junk, creating a kind of Google Maps of space. 

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