ERS-2 or the uncertain path of the old satellite that returns to Earth

It has been a journey of almost 30 years that ended with a big leap. The old European satellite ERS-2, launched in 1995 to study climate change by analyzing sea surface temperatures and ozone in the atmosphere, fell back to Earth on Wednesday, February 21 and burned up in the atmosphere after completing an observation mission 13 years ago.

The machine, which weighed 2.3 tonnes and whose fall operation began in 2011, descended naturally towards Earth – uncontrolled – by gravity alone. “We have confirmed re-entry into the atmosphere of ERS-2 at 5:17 pm GMT over the North Pacific Ocean between Alaska and Hawaii,” the European Space Agency’s (ESA) Operations Center X (formerly Twitter) announced on Twitter.

ESA did not specify whether a fragment of the satellite fell to Earth, but the risk was mentioned earlier. “It is estimated that the largest piece of satellite that could reach the ground is 52 kg,” Henri Laure, Director of Earth Observations at ESA, announced last week.

It is very common to bring down non-functional satellites. On average, the ESA explains on its website, “an ERS-2 equivalent object completes its days in the atmosphere once every one or two weeks”. Last July, the European satellite Aeolus, responsible for studying wind speed, fell back into the Atlantic Ocean; Next September, the cluster satellite will fall in turn, then the integral satellite, in December 2024.

The difference for ERS-2 is that it falls back uncontrollably, which creates a series of uncertainties about the time and place of impact, and also calls into question the strategy for getting rid of space debris.

Less dangerous than launching a satellite into orbit

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

The same 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 astronomer Hervé Beust, recalling that the largest of them. 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, even if there is not enough atmosphere at these altitudes to fly, there are still some molecules that affect the movement of the satellites. “The smallest particle of air that a satellite can encounter will slow it down. However, a slower satellite will descend, and if it descends, it will encounter more layers of denser air, will slow down more, and so it will descend. . further”, explains the researcher who also teaches gravitational kinetics at Grenoble Alpes University.

Therefore, no satellite in low orbit can stay there forever. Unless we act on it to stay there, Hervé Beust clarifies, taking the example of the ISS, if we leave it, “it will fall back to Earth”. To avoid this, “We periodically send transferable modules to the station to move the thruster to a few tens of kilometers, so that the ISS continues its life” in low orbit, explains – it.

In the case of the ERS-2 satellite, it is – like every year – a discarded satellite, which falls naturally. The difference for ERS-2 is that it falls alone, where other non-functional satellites are normally placed in controlled orbit, making it possible to know when they will fall again.

“If we had left it (at its initial height), it could have rotated for another 200 years, but ESA carried out maneuvers to lower its height, allowing it to fall uncontrollably on its own,” explains Hervé Best. . This is because it is ultimately less dangerous for a satellite to fall back to Earth than to orbit in low orbit for hundreds of years.

A non-functional satellite is nothing more than an empty shell, a piece of space debris that, on its way into orbit, risks colliding at high speed with any other satellite in its path, causing debris that could be dangerous to other craft. , creating more waste in a chain reaction.

“Small fragments are just as dangerous because of the speed,” explains Hervé Beust, who explains that the speed in orbit around Earth is about 10 km/s, or 36,000 km/h.

“If you have a satellite moving at 10 km/s and it encounters debris moving at 10 km/s, even if the debris is a bolt of a few centimeters, the collision can cause significant 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 to Earth.

“Extremely low” probability of land damage

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

At such speeds in the atmosphere, satellites experience such intense friction that they heat up and break apart. For example, Hervé Beust explains, “When interplanetary rocks (which come into contact with the atmosphere), this is what creates the phenomenon of shooting stars.”

However, it cannot be ruled out that the fragments will resist and reach the ground. In the case of ERS-2, a fragment representing about fifty kg of its total mass could survive the craft’s reentry into the atmosphere. “If a 52 kg piece hits the ground at a speed of several km/s, obviously it will damage a building if it hits it,” says the astronomer. “But the probability is extremely low,” he continues. “Statistically, this will fall either in the ocean or in very sparsely populated areas, because even though cities are the most populated, they still represent less land area.”

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

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

“The lines from the North Pole to the South Pole correspond to the projection of the satellite’s path on the ground along several orbits,” explains Hervé Beust. “The first orbit is the line that passes farthest to the east, which corresponds to the exact time, but if we look after 1 hour 30 minutes, the orbit of the satellite – which forms a kind of circle – will return to the same place. However, By then, the Earth will have orbited, so the satellite will pass further west on the second line, then the third and so on,” he continues.

When entering the atmosphere, the satellite must be located at a distance of 80 km from the red marker indicating “COIW” (Center of Impact Window), which refers to ESA.

“Up to this red dot, we can roughly calculate what will happen, but after that it depends in particular on how fast the satellite will hit the ground, how it will be slowed down by the atmosphere, how it will disintegrate, or If it turns on quickly”, adds Hervé Beust. Too many elements that are difficult to control and estimate. At such speeds, he concludes, “an uncertainty of 30 minutes at the moment of impact represents thousands of kilometers on the Earth’s surface”.

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

Also, to combat the accumulation of this debris in orbit, the agency last year launched a “zero debris” charter that aims to ban the generation of new space debris for planned space missions starting in 2030.

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