It’s a journey of almost 30 years that ends with a big leap. The old European satellite ERS-2, launched in 1995 to study climate change by analyzing ocean surface temperature and ozone in the atmosphere, should fall back to Earth on Wednesday, February 21, after an observation mission finished 13 years ago.

The machine, which weighs 2.3 tonnes and whose fall operation began in 2011, descends towards Earth naturally and in an uncontrolled manner by the force of gravity alone. The satellite should enter the atmosphere around 8:24 p.m. (French time), with an uncertainty margin of plus or minus nine hours.

A large part of the satellite is expected to burn up in the lower layers of the atmosphere, but the risk of a piece falling to Earth is not ruled out. “It is estimated that the largest fragment of the satellite that can reach the ground is 52 kg,” Henri Laur, from the Earth Observation Directorate at the European Space Agency (ESA), declared last week.

It is very common to bring down non-functional satellites. On average, explains the ESA on its website, “an object of mass similar to ERS-2 ends its days in the atmosphere once every one or two weeks”. Last July, the European satellite Aeolus, responsible for studying the movement of winds, fell back into the Atlantic Ocean; next September, the Cluster satellite should in turn fall, then the Integral satellite, in December 2024.

The difference between ERS-2 is that it falls in an uncontrolled manner, creating a series of uncertainties about the time and place of the impact, also questioning the strategy to get rid of space debris.

Less dangerous to let it fall than to let it spin up there

In low orbit, everything that goes up must come down. Thus, once their mission is completed, the satellites placed there all gradually descend.

This is what has been happening for ERS-2 since 2013. “Most of the satellites that are sent into space are in low orbit, a few hundred kilometers above our heads,” explains Hervé Beust, astronomer, remembering that the largest of them is the International Space Station (ISS), about 400 kilometers above us.

“For these satellites, everything would be fine if it weren’t for the Earth’s atmosphere,” he continues. Indeed, if there is not enough atmosphere at these altitudes to fly a plane, there are still some molecules there which have an effect on the movement of satellites. "The smallest particle of air that a satellite can encounter will slow it down. However, a slowed satellite will descend, and if it descends it will encounter more layers of even denser air, will be slowed down more, and will therefore go down further", explains the researcher who also teaches gravitational dynamics at the University of Grenoble-Alpes.

So, no satellite in low orbit can last there forever. Unless we act on it to make it stay there, specifies Hervé Beust, taking the example of the ISS which, if we let it, “would fall back to earth”. To avoid this, "we send from time to time modules transported to the station to operate the thrusters in order to bring it up a few ten kilometers so that the ISS continues its life" in low orbit, explains t -he.

In the case of the ERS-2 satellite, it is - as is the case every year - a discarded satellite, which naturally comes down. The difference with ERS-2 is that it falls alone, where in general the other non-functional satellites are controlled and placed in orbit, which makes it possible to know when they will fall again.

“If we had left it [at its initial altitude], it could have rotated for another 200 years, but the ESA carried out maneuvers to lower its altitude, and now they are letting it fall on its own uncontrolled” , explains Hervé Beust. The reason for this is that it is ultimately less dangerous for a satellite to fall back to earth, rather than to rotate in low orbit for hundreds of years.

A non-functional satellite is nothing more than an empty shell, a space debris which risks, in its course in orbit, colliding at high speed with any other satellite in its path, causing debris which can in turn become dangerous for other vehicles, creating more debris in a chain reaction.

“The small pieces are just as dangerous because of the speed,” explains Hervé Beust, who explains that the speed in orbit around the earth is around 10km/s, or 36,000 km/h.

“If you have a satellite rotating at 10 km/s and it encounters debris that is also rotating at 10 km/s, even if the debris is a bolt of a few centimeters, the collision can cause considerable damage.”

So the strategy is to get rid of as many of these empty shells as possible, and the best way to do that is to bring them back down to earth.

Images from the ERS-2 satellite passing over the European Space Research Institute in Frascati (Italy)

Hover at real speed, then slow to a quarter of that speed.

An “extremely low” probability of ground damage

The fall of the ERS-2 satellite is surrounded by uncertainties and the question of possible damage to the ground may then arise and cause concern. However, reassures the astronomer, “it is an illusion, because we are protected by the atmosphere”.

Arriving at such a speed in the atmosphere, the satellites experience such intense friction that they heat up and break up. “When it is interplanetary rocks [which come into contact with the atmosphere], this is what produces the phenomenon of shooting stars,” explains Hervé Beust, for example.

However, it cannot be ruled out that pieces will resist and reach the ground. In the case of ERS-2, a piece representing around fifty kilos of its total mass could survive the craft's entry into the atmosphere. “If a 52 kg piece hits the ground at several km/s, obviously it would cause damage if it fell on a building,” says the astronomer. “But the probability is extremely low,” he continues. “Statistically, this will fall either into the sea, or into very sparsely inhabited areas, because although cities concentrate the most population, they still represent little land area.”

According to the ESA blog dedicated to the mission, the probability of one of these debris hitting a person on the ground is less than one in a hundred billion.

In a post published Wednesday on uncertainty of plus or minus 1 hour 44 minutes.

🚨 Possible final update prior to the reentry of ERS-2 🚨

ESA's Space Debris Office predicts that the #ERS2reentry will take place at:

15:41 UTC (16:41 CET) today, 21 February 2024

The uncertainty in this prediction is +/ - 1.44 hours.

-- A note on the ground track --


— ESA Operations (@esaoperations) February 21, 2024

“The lines that go from the north pole to the south pole correspond to the projection on the ground of the trajectory of the satellite with several orbits,” explains Hervé Beust. "The first orbit is the line that passes furthest to the East, which corresponds to a certain time, but if we look 1 hour 30 minutes later, the orbit of the satellite - which makes a sort of circle - will return to the same place. However, by then the Earth will have rotated, so the satellite will pass on the second line further west, then on the third, and so on," he continues.

At the time of entry into the atmosphere, the satellite should be located around 80 km from the red marker indicating "COIW" (the center of the impact window), specifies the ESA.

"Up to this red point, we can roughly calculate what will happen, but then it depends in particular on the speed at which the satellite will arrive at the ground, how it will be slowed down by the atmosphere, how it will decompose, or if it spins quickly,” adds Hervé Beust. So many elements that are difficult to control and estimate. At such a speed, he concludes, “an inaccuracy of 30 minutes on the moment of impact represents thousands of kilometers on the surface of the Earth”.

According to ESA estimates, there are around a million pieces of satellite or rocket debris larger than a centimeter in orbit, large enough to "disable a spacecraft."

Also, in order to combat the accumulation of this waste in orbit, the Agency launched last year a “zero debris” charter aimed at prohibiting the generation of new space debris for space missions designed from 2030.

“More than 100 organizations, including Airbus, Thales Alenia Space, Safran, have announced their intention to sign the charter,” ESA said last week. The American giant SpaceX, although concerned with its constellation of Starlink satellites, has not signed it.

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