Space debris: the lessons to be learned from the uncontrolled fall of a Chinese rocket

On July 24, China launched the Wentian laboratory, the second module of its space station, into orbit. Like the main body launched last year, this module of more than 22 tonnes required the use of a particular version of the Chang Zheng 5 (Long March 5) heavy launcher, the CZ-5B, which was carrying out its third flight.

Normally, on all space launchers, the last stage to provide propulsion also goes into orbit. In order to avoid polluting space, it is usually desorbed over a desert area, as it is known that fragments will survive decay and reach the surface. Engine combustion chambers and tanks made of certain materials, including titanium, are particularly resistant to passing through the atmosphere.

However, the CZ-5B does not have an upper stage to ensure this orbital injection: it orbits its entire central body! An element 32 meters long and 5 meters in diameter, weighing approximately 21 tonnes, and equipped with two large cryogenic engines of 2.7 tonnes each.

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And as for the two previous launches, the teams from the Chinese Academy of Launcher Technology have not planned anything to ensure a safe fallout from this stage. To their credit, no international regulations compel them to do so. Thus, they prefer to play on statistics and the large expanse of the oceans compared to densely inhabited areas to estimate that the risk of damage or victims caused by this fallout will be “manageable”.

An empty object of this size sees its orbit deteriorate rapidly due to its strong aerodynamic drag in the very high residual atmosphere. This slows him down and brings him down in a few days. Quickly, the date of the fallout of the last copy was calculated for July 30 and from hour to hour, all the observers of the sky could see the window of uncertainty reduce, until it was limited to a single orbit, whose “trace” on the ground was a line of about 40,000 kilometers running from the South Atlantic to the Indian Ocean before crossing Indonesia and the Philippines, then the North Pacific, skirting California and crossing the South America, from Ecuador to southern Brazil.

The re-entry began at 6:51 UT over the island of Sumatra and the calculated point of impact – i.e. the place where the densest fragments, such as the engines, fell – was located by 119° E and 9.1° N, in the Sulu Sea, between Borneo and the Philippines. The first time, in May 2020, this point was in the Atlantic, off Senegal. For the second flight of the CZ-5B, it was in the Indian Ocean, off the Maldives. So far, the Chinese calculation therefore seems to have paid off rather well because no casualties have been reported.

1,500 kilometers beyond the point of impact

On the other hand, we learn a lot more about the dispersion of debris. This time, large fragments of the main reservoirs, some several meters wide, fell in the west of the island of Borneo, in the Indonesian province of Kalimantan, and in the Malaysian state of Sarawak. Although 7 million people live along this coast, mainly in the Malay states of Sarawak and Sabah as well as in the Sultanate of Brunei, the dispersion of the debris most sensitive to atmospheric braking, between 1,400 and 800 kilometers below from the main point of impact, mostly took place in rural areas.

During the first flight, other types of debris, more aerodynamic, such as pipes up to 30 meters long, had fallen in the Ivorian savannah, more than 1,500 kilometers beyond the point of impact. On the last flight, these items probably fell into the sea between the Philippines and the Marianas Archipelago.

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A next launch is scheduled for October 27. Will the luck of the Chinese continue? During the first flight, the stage had flown over Manhattan ten minutes before falling. For the second flight, it was Riyadh. On this last flight, within one orbit (about ninety minutes), the debris could have struck southern Japan. In general, Europe is little threatened, because the inclination of the orbit means that this level does not venture beyond 41.5° N, the latitude of Bonifacio. However, as with the management of traffic and debris in low orbit, regulations should be put in place before an accident occurs, and not after.