Two satellites, hurtling across the sky at nearly 33,000 mph (53,000 km/h), were on course to collide over Pennsylvania on Wednesday.
A group tracking the satellites predicts a 1 in 20 collision chance, which experts say is “alarming”.
The satellites are not in operation, but it is feared a collision could create pieces of debris that would damage other objects in orbit.
Scientists watching the night sky hope a collision will be avoided.
The last time a major satellite collision occurred was in 2009.
The satellites may pass within 40ft (12m) of each other as they cross above Pittsburgh.
LeoLabs, a group that tracks space debris, reported that “it is still unlikely that these objects will collide”, but that due to the size of the satellites, the chance of collision had gone up from prior calculations.
4/ Adjusting our calculations to account for larger object sizes (by increasing our combined Hard Body Radius from 5m to 10m), this yields an updated collision probability closer to 1 in 20.
— LeoLabs, Inc. (@LeoLabs_Space) January 29, 2020
The objects in question are an Infrared Astronomical Satellite (IRAS) that was launched in 1983 and another experimental US craft, the GGSE-4 satellite, launched in 1967.
The IRAS satellite has a 60ft (18m) boom – equipment designed to deploy antennae or solar sails – trailing it, which increased the probability of collision as of Tuesday afternoon calculations by LeoLabs.
Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said that given the size of the satellites – about the size of a car and a rubbish bin – “a 15-to-30 metre predicted miss distance is alarming” – a sentiment echoed by LeoLabs.
The satellites will hopefully avoid each other during their pass some 550 miles (900km) above Pittsburgh at around 18:30 local time.
If they do collide, the debris poses no threat to the city, experts say, it will burn up in the atmosphere before it can fall to Earth. But the debris cloud that remains in orbit could threaten other satellites.
Debris can remain in orbit for decades to centuries.
The last large collision of satellites happened in 2009, when a US commercial Iridium spacecraft hit a defunct Russian satellite over Siberia, producing thousands of pieces of debris.
International guidelines state that satellites in low earth orbit must be removed from orbit 25 years after being decommissioned, but these satellites were launched prior to the rule changes.
The situation has renewed discussions over the importance of cleaning up space debris.
“Events like this highlight the need for responsible, timely de-orbiting of satellites for space sustainability moving forward,” LeoLabs said.
Currently, there are around 2,000 active satellites orbiting the earth. There are also more than 23,000 pieces of debris larger than 10cm (4in) in orbit, according to Nasa.
Media captionWatch space debris grow from 1950s to 2017
According to data from the Union of Concerned Scientists, the US has 1,007 operating satellites, the most by far of any country. The majority are commercial.
A 2019 article in the Massachusetts Institute of Technology (MIT) Technology Review stated by 2025, there could be as many as 1,100 new satellites launching per year.