It is impossible to say precisely where and when: the only certainty concerning the main stage of the Long March 5B rocket, launched at the end of April, is that it will fall back to Earth in the coming days. The object 30 meters long, 5 meters in diameter and 21 tons spins at more than 27,000 km / h around the globe, and has escaped all control of the Chinese space agency. This Friday, however, Beijing spoke of "extremely low" risks, while Washington assured that it had no intention of destroying it at this stage, "only hoping" that it will fall into the ocean. How did we end up in this situation? What is the material and human risk? Does it happen often? Some answers with Christophe Bonnal, researcher at CNES and president of the “Space debris” commission of the International Academy of Aeronautics.
How does an entire stage of a Chinese rocket end up falling uncontrollably towards Earth?
When you throw an object into space, it always ends up falling. Up to 2000 km of altitude, it is not yet the complete vacuum, there are some traces of atmosphere which slow down this object. The loss of speed results in an increasingly rapid descent until reaching the upper layers of the atmosphere, at an altitude of 100 km. Either we decide to control this deorbitation to target an uninhabited area, but it's complex - you have to relight motors, measure, push in the right direction, it's expensive and heavy if it was not planned from the start - or it is allowed to fall back randomly depending on the friction. And without being in the secret of the Chinese space agency, this seems to be a priori the case for Longue-Marche 5B.
Why is the fall of this rocket impossible to predict?
It is moving way too fast for us to make a good prediction.
We can measure the instant at which the rocket will land with a margin of error of 10%.
This means that the day before the impact, it can be predicted to within 2 hours.
But the object travels 8 km per second!
In two hours, this represents an inaccuracy of 70,000 km, more than one circle around the Earth.
And there is no real way to improve this forecast.
At most, we could cover the Earth with radars to anticipate the fall.
Half an hour before, we could have an area of 4,000 km where we would say to the inhabitants "it's coming soon, take shelter".
But if not, there is no way.
What is the risk of this debris causing damage to Earth?
First, even if the stage is very large, the danger is reduced if it melts sufficiently when it enters the atmosphere. This step exposes it to a temperature of 1,400 degrees. Depending on the materials it is made of and the structure of the objects in it, it will probably only remain 10 to 20% of its mass. It is an object of two tons, that is to say a big 4 x 4, which will arrive on the ground. Then, the surface of the Earth is covered with 70% water, 12% of forests, savannas… and finally only 3% of densely populated areas. All in all, space debris melts first, then often falls into the water. It would really take a terrible bad luck for them to do damage. But the probability is not zero, it could actually kill someone.
The stages of Falcon 9 rockets built by SpaceX have also experienced more than one random descent to Earth.
(AFP / HO)
The risk is therefore rather low, but does it occur frequently?
It can even happen every week. To date, there are 25,500 re-entry of objects into the atmosphere from Sputnik I. Every four or five days, an entire floor or satellite falls to Earth. The first flight of Longue-Marche 5B had caused a stir as part of it fell on the Ivory Coast, with material damage. On Wednesday, a 3.8 ton piece of Longue-Marche 3B crashed in northern Canada. But Chinese rockets are not the only ones. The upper stage of SpaceX's Falcon-9s, weighing 5 or 6 tons, returns to Earth uncontrollably on a regular basis. In recent years, debris from this rocket has damaged a house by falling on a roof in Brazil and a cow pen in Indonesia… This is something very common. And yet, even if we can't know everything,no victim has ever been identified to our knowledge. According to our models, statistically there could already have been one or two. Not in the thousands, of course, but the risk exists.
Can this risk not be limited upstream, during the design of rockets for example?
In many countries and internationally since 2002, standards in the construction of spacecraft recommend controlled deorbitation.
But they are not retroactive, and therefore not applicable to the European Ariane 5 rockets designed in 1984 for example.
More importantly, these standards have very different applicability in different countries.
The only country where they are really compulsory is France, since 1998. It is much more complex in the United States, where what applies to NASA is very similar to French laws, but does not concern the sector. private, including SpaceX.
In the case of the Longue-Marche 5B rockets, they were in fact supposed to comply with the rules in terms of controlled deorbitation ...
Does this mean that there is still a lot of space debris the size of the stage of Long March 5B left that could crash into Earth uncontrollably?
There are still plenty of others, yes. There's a lot of talk about Long March 5B, but right now there are another 25 orbiting Falcon 9 rocket stages waiting to land back to Earth, and almost as many Ariane 5 rocket stages in the same situation. All will eventually fall back. But by then, they are followed. At CNES, we monitor French objects that risk falling outside France, such as foreign objects that risk falling into France. We receive readings on their position every 3 to 6 hours, within the EU SST, the European Space Surveillance and Tracking, which pools the resources of seven countries (France, Germany, Italy, Spain, Romania, Poland, Portugal). Moreover, for the Longue-Marche 5B rocket, we estimate that it will fall anywhere, but only between -41 ° and 41 ° latitude.If you live in Paris, normally you can rest easy.