Washington (AFP)

A space telescope launched in 1983 and an American experimental satellite from 1967, both out of service, will pass within 100 meters of each other over the United States on Wednesday, with a collision probability estimated at 1 out of 1,000, according to space surveillance companies.

Each satellite flies in opposite orbits and they therefore risk colliding head-on, with a relative speed of almost 15 kilometers per second.

The collision or collision will take place Wednesday at 23:39 GMT, 900 km above the city of Pittsburgh, according to the company LeoLabs.

These types of collisions between satellites that no one controls anymore are rare and dangerous because they create thousands of fragments that risk destroying or damaging active satellites. In 2009, when the Iridium 33 communication satellite (active) and the Russian military satellite out of service Cosmos 2251 met, a thousand fragments of more than 10 cm were propelled, polluting the orbits.

The space telescope, IRAS, was a joint project of NASA, the United Kingdom and the Netherlands and lived 10 months. It weighs a ton, according to a database from the European Space Agency (ESA), and measures two meters by four meters by four mothers.

The American experimental satellite, GGSE-4, was launched by the US Air Force and is light (85 kg), but it has an unusual shape: it is very thin (60 cm) but is 18 meters long, and flies vertically.

LeoLabs initially estimated the risk of collision to be 1 in 100, then reduced it to 1 in 1,000, with a probable distance of 13 to 87 meters between the two devices.

But significant uncertainties exist on the exact trajectory of the satellites.

If they hit each other, the shock could create a thousand debris larger than 10 cm, told AFP Dan Oltrogge, of Analytical Graphics, and more than 12,000 fragments larger than 1 cm.

When will we know if the shock has occurred?

"For the low orbit, the radar coverage is good, and we will be able to observe the fragmentation, we will see objects separating", says Dan Oltrogge.

With the naked eye, however, it is unlikely, he said.

The altitude of 900 km is particularly frequented by satellites.

Operators must constantly calculate the risk of collision with the more than 20,000 objects cataloged in Earth orbit, and regularly deviate from their trajectory, which is no longer possible when a satellite is dead. Adding a few thousand pieces of debris "will create more work for operators, and more risk for their vehicles," says Dan Oltrogge.

"It will also create so-called non-detectable lethal debris, because it is large enough to kill a satellite, but too small to be detected at present," added the expert.

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