A Long March-5B (LM-5B) rocket launched the second element of the Chinese space station into space on Sunday (July 24). The sentence you just read seems to contain important information in the second part where we understand that Beijing is successfully pursuing the construction of its orbital station. However, the first part of this sentence also contains important information, because each takeoff of an LM-5B comes with a question that worries a large number of people in the following days: “What if the rocket falls on my head?”.

The same is the case with the launcher, which launched on July 24, and is expected to fall back to Earth between the evening of Saturday, July 30 and the early hours of Sunday, July 31, according to estimates. Fifteen hours remain because the behavior of the main stage of this 33-meter-long rocket, which weighs more than 20 tons, is difficult to accurately model.

Three things are certain: this stage will fall; Some elements that are not completely digested on re-entering the atmosphere reach Earth; Occurs between 41^E parallel north (the latitude of Madrid or Naples in Europe) and 41^E The southern parallel passes south of Africa. Thus, metropolitan France is not at risk, but this vast area, which includes southern Europe, a large part of the Americas, the African continent, southern Asia, and almost all of Australia, still has 88% of the world’s population.

Single-stage rocket

Why does this problem occur in every rocket launch and not only in Long March-5B? Typically, a satellite is launched into orbit using a two-stage rocket (perhaps combined with solid rocket boosters). The first stage provided most of the boost for a few minutes, but it quickly separated and fell into an unspecified ocean. A very short second stage takes over to bring the satellite into orbit and then decelerates.

As the more powerful Chinese launcher is capable of delivering 25 tonnes into low orbit, the Long March does not follow this scenario in its 5B version, as it has only one stage, not two (unlike the shorter version 5). To fulfill its mission, this main stage rises to an altitude of more than 250 km and as a result finds itself in orbit for a few days.

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