Chunks of deorbiting ESA satellite are expected to reach the ground

Danger to humans? Less than '1 in 100 billion', says agency

ESA's ERS-2 satellite is heading back to Earth this week and some substantial fragments are likely to survive re-entry, although the chances of anyone being injured by a hunk of space junk are vanishingly small.

The UK Space Agency (UKSA) posted some images of the doomed European Remote Sensing (ERS-2) satellite at the end of last week. As of today, ESA's Space Debris Office reckons that re-entry will take place 1114 UTC on February 21, give or take 15 hours.

ESA said that due to the uncontrolled nature of the re-entry: "It is impossible to know exactly when and over which region on Earth this will happen." However, as the moment nears, scientists should be able to predict the location more accurately.

The worry is that while most of the 2,294 kg satellite will burn up in the Earth's atmosphere, substantial chunks could survive re-entry. Some reports estimate the largest pieces to weigh in at 52 kg.

However, getting struck by any errant bits of ERS-2 is very unlikely. The chances are that the surviving remnants of the satellite will end up in the ocean. According to ESA, the probability of a human being injured by space debris "is under 1 in 100 billion."

To put that in context, the agency added that the risk was "1.5 million times lower than the risk of being killed in an accident at home."

Furthermore, ESA is confident that none of the fragments would contain anything toxic or radioactive.

ERS-2 was launched on April 21, 1995, and performed Earth observations together with ERS-1 launched on July 17, 1991. ERS-1's mission ended on March 10, 2000, due to a failure in the onboard attitude control system, but ERS-2 continued gathering data until 2011.

With fuel running low, but the spacecraft still under control, ESA managers decided to deorbit the satellite. Sixty-six deorbiting maneuvers were carried out in July and August 2011, dropping the average altitude from 785 km to 573 km to reduce the chance of a collision with something else in orbit, meaning that the satellite would naturally fall back to Earth within the next couple of decades.

Having used up the last of the fuel, the mission officially ended on September 5, 2011. ERS-2's electronic systems were deactivated, and its batteries were discharged.

The re-entry follows ESA's Aeolus weather satellite, which re-entered Earth's atmosphere over Antarctica in August. However, unlike Aeolus, ERS-2's return to Earth is uncontrolled and resulted from deorbit decisions taken in 2011. The rate at which its orbit has decayed since has depended on factors such as solar activity increasing atmospheric density and thus drag on the satellite.

These days, ESA has a space debris mitigation policy to ensure satellites can be disposed of at the end of their lives. The agency does, however, have a number of legacy spacecraft that it will still need to deal with. The first of ESA's Cluster quartet, "Salsa," is due to re-enter over the South Pacific later in 2024. ®

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