A dead NASA telescope and an old US Air Force satellite avoided a collision in space above Pittsburgh on Wednesday evening, beating odds experts called "dangerous" and "alarming."

A head-on collision could have produced nearly 300,000 chunks of debris that would threaten other spacecraft.

NASA said the Air Force, which tracks satellites for the government, did not notify it of any potential collision.

Experts expect more near-collisions like this if nobody removes dead satellites from space.

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Two dead satellites avoided a high-speed collision in space on Wednesday evening when their orbits crossed paths 560 miles above Pittsburgh, Pennsylvania.

The larger object is an old space telescope called the Infrared Astronomical Satellite, or IRAS: a joint mission between NASA, the Netherlands, and the UK that ran out of fuel and died in November 1983. The other is a gravitational experiment called GGSE-4 that the US Air Force launched in May 1967.

LeoLabs, a company that uses radar to track satellites and debris in space, projected on Tuesday morning that the satellites would pass dangerously close to each other 25 seconds before 6:40 p.m. ET on Wednesday. The group described the data as "alarming" on Twitter.

LeoLabs calculated Wednesday morning that the two objects could come within 12 meters (39 feet) of each other, with a roughly 1-in-20 chance of a collision. The projection took into account a 59-foot boom that extends from GGSE-4's main body. (Trackers didn't know whether the boom was facing IRAS.)

But projections changed as the spacecraft approached on another. Data that LeoLabs released an hour before the satellites crossed paths showed an estimated distance of 154 feet.

A simulation shows the satellites IRAS and GGSE-4 approaching each other. Analytical Graphics Inc.

If the satellites had collided, they could have broken apart and created a cloud of debris that would have threatened other satellites and the International Space Station. Scientists warn that if orbital junk gets too plentiful and out of control, it could cut off our access to space for hundreds of years.

No new space debris detected

Experts at The Aerospace Corporation ran their own simulation of the potential crash on Tuesday and found a 1-in-10 chance of a collision. They expected that probability to significantly decrease as the satellites came closer — to odds more like 1 in 1,000 — but on Wednesday morning, their data still suggested a probability of 1 in 22.

While a 5% to 10% chance of a hit may seem low, NASA routinely moves the International Space Station when the orbiting laboratory faces a 0.001% (1-in-100,000) chance or greater of a collision with an object.

"We haven't seen this happen before," Roger Thompson, a senior engineering specialist at The Aerospace Corporation, told Business Insider. "This is the highest probability the day of that I recall ever seeing."

Thompson's math showed the satellites could have come within 59 feet of each other.

However, at 9:04 p.m. ET on Wednesday, LeoLabs said it had not detected any new debris as the satellites entered the range of its radar — suggesting that they had passed each other without colliding.

An illustration of the Infrared Astronomical Satellite, or IRAS. NASA

The US Air Force, which tracks satellites for the government, did not notify NASA of any potential satellite collision, the space agency told Business Insider.

Executives at both LeoLabs and The Aerospace Corporation said the lack of a government alert did not surprise them.

"Since [the satellites] were decommissioned, basically nobody was keeping a close eye on them," LeoLabs CEO Dan Ceperley told Business Insider, adding, "it's a big threat in space, but not on the ground."

Thompson said observers on the ground in Pittsburgh might have seen a bright flash in the sky if the satellites had crashed, like a shooting star.

Nobody could have prevented this collision

A projectile strikes a mock-up of a spacecraft in a NASA-Air Force test meant to simulate space debris collisions. Arnold Engineering Development Complex/Air Force

Because IRAS is quite large, a collision would have been dangerous, according to both satellite-tracking companies. LeoLabs said the space telescope is 11.8 feet long and 10.5 feet wide. Both satellites were moving quickly as they approached each other: 14.7 kilometers per second, or 9.1 miles per second.

"Anytime you have a high-velocity collision like that it's serious, because the energy of the collision is so high that the debris gets spread into other orbits," Thompson said.

Thompson calculated that a head-on collision would have produced about 290,000 chunks of debris at least 1 centimeter wide, the size that experts consider dangerous.

"LeoLabs has pointed out a very dangerous conjunction," he said.

The International Space Station orbits 250 miles above Earth. NASA

Despite the advanced tracking of the two satellites, they can't be controlled.

"Nobody can do a thing about this, no matter how well we're tracking it, because these are both dead objects," Ted Muelhaupt, who leads The Aerospace Corporation's satellite system analysis, told Business Insider.

More space junk raises the risk of more dangerous collisions

The 4,300 satellites in Earth's orbit have a total mass of more than 7,500 tons. ESA/ID&Sense/ONiRiXEL

Over 100 million bits of junk surround Earth, from abandoned satellites, spacecraft that broke apart, and other missions. Each piece of that debris, no matter how small, travels at speeds high enough to inflict catastrophic damage to vital equipment. A hit could kill astronauts on a spacecraft.

Each collision makes the problem worse, since it fragments satellites or debris into smaller pieces.

"Each time there's a big collision, it's a big change in the [low-Earth orbit] environment," LeoLabs CEO Dan Ceperley previously told Business Insider.

In 2007, China tested an anti-satellite missile by blowing up one of its own weather satellites. Two years later, an American spacecraft accidentally collided with a Russian one. Those two events alone increased the amount of large debris in low-Earth orbit by about 70%.

"Because of that, now there's sort of a debris belt," Ceperley said.

India also generated thousands of bits of debris last March when it blew up a spacecraft in a test of an anti-satellite missile.

If the space-junk problem gets extreme, a chain of collisions could spiral out of control and surround Earth in an impassable field of debris. This possibility is known as a Kessler event, after Donald J. Kessler, who worked for NASA's Johnson Space Center and calculated in a 1978 paper that it could take hundreds of years for such debris to clear up enough to make spaceflight safe again.

"It is a long-term effect that takes place over decades and centuries," Muelhaupt said. "Anything that makes a lot of debris is going to increase that risk."

If the two satellites had collided head-on over Pittsburgh, half of the cloud of debris would have shot up away from Earth, and the other half would have spread into lower orbits among other satellites and the space station, Thompson said. At first, the cylinder-shaped field of debris would have been dangerous to pass through.

"We replace two satellites with essentially two shotgun blasts of debris," Ceperley said.

After a few days, the debris cloud would have spread out, according to Thompson.

Addressing the rising risk of collisions in space

Collisions in space are becoming more likely as satellites fill the sky. Companies like SpaceX, Amazon, OneWeb, and perhaps even Apple plan to launch tens of thousands of satellites this decade to form internet-providing "megaconstellations."

In September, the European Space Agency had to maneuver one of its spacecraft at the last minute to avoid colliding with a SpaceX satellite. The chance of that crash was 1 in 1,000.

The first batch of 60 high-speed Starlink internet satellites, each weighing about 500 pounds, flat-packed into a stack before their launch aboard a Falcon 9 rocket on May 23. SpaceX via Twitter

What's more, there is no system in place to remove older satellites like IRAS or GGSE-4 from orbit as they die.

"Events like this highlight the need for responsible, timely deorbiting of satellites for space sustainability moving forward," LeoLabs tweeted.

The Federal Communications Commission, which licenses private companies' satellite launches, is considering new regulations to address the issue of space debris. But there is no silver bullet for the many metal chunks already rocketing around Earth, or for the dead satellites that threaten to create more debris.

One potential solution, however, is a cleanup mission proposed by the ESA to capture one of the agency's defunct satellites in a net, drag it into Earth's atmosphere, and burn it there. Private companies — including Tethers Unlimited, TriSept Corp., and a Boeing subsidiary called Millennium Space Systems ⁠— have explored similar concepts for larger-scale space cleanup.

An illustration of the European Space Agency's e.Deorbit system to net and remove old satellites from orbit. David Ducros/ESA

Those companies could one day use LeoLabs' data to identify high-risk satellites, track them down, and pull them out of orbit to reduce the chance that space collisions would generate clouds of debris.

"A lot of the risk comes from this small debris, all this stuff that's never been tracked before. Nobody's got a good solution to clean that up," Ceperley previously told Business Insider. "Let's make sure we don't make more of it."

Dave Mosher contributed reporting for this story.