The remains of a giant segment of a Chinese rocket crash-landed in the Atlantic Ocean this week, representing the most significant uncontrolled descent of a piece of human-made space debris in decades. The bulky Long March 5B became the heaviest orbiting thing to fall uncontrolled to Earth in nearly three decades, according to Jonathan McDowell, a Harvard astrophysicist and orbital object tracker. The last time a heavier object had an uncontrolled entry was 1991, when the 43-ton (39,000 kg) Salyut-7 Soviet space station reentered the atmosphere over Argentina, McDowell wrote on Twitter. (Another contender he mentioned: the 2003 shuttle Columbia disaster, though that reentry did not become uncontrolled until the space shuttle was already in the atmosphere over Texas.) The CZ-5B-Y1 core stage is in a 155 x 366 km orbit, and is expected to reenter around May 11. At 17.8 tonnes, it is the most massive object to make an uncontrolled reentry since the 39-tonne Salyut-7 in 1991, unless you count OV-102 Columbia in 2003. — Jonathan McDowell (@planet4589) May 7, 2020 In July 2019, the Chinese space station Tiangong-2 fell in a controlled descent through Earth’s atmosphere. But that 9.5-ton (8,600 kg) crash was less than half the size of today’s descent, and it was guided remotely using the last of its fuel to land in a specific, remote bit of ocean. In 2018, that space station’s predecessor, the 9.3-ton (8,500 kg) Tiangong-1, fell uncontrolled (but harmlessly) into the Pacific ocean. The 18th Space Control Squadron, an Air Force space-tracking group, reported that Long March 5B reentered the atmosphere at 11:33 a.m. EST. At that time, it was just off the west coast of Africa, approaching Nouakchott, Mauritania. In the rocket’s last half-hour in orbit, it passed over Hollywood, Colorado Springs and New York City’s Central Park, according to McDowell. While such an eventuality could almost certainly involve a loss of life, such an incident is generally considered unlikely, given how much of Earth’s surface constitutes uninhabited land or ocean. In any case, McDowell downplays the dangers. “For a large object like this, dense pieces like parts of the rocket engines could survive reentry and crash to Earth,” McDowell told CNN. “Once they reach the lower atmosphere they are travelling relatively slowly, so worst case is they could take out a house.” The CZ–5B launched last week to transport an experimental prototype spacecraft into orbit, called the Chinese next-generation crewed spacecraft, which successfully landed back on Earth after a couple of days of testing in orbit. “The successful landing of the new spacecraft from a high orbit also shows China is serious about sending astronauts beyond low Earth orbit – something only NASA has achieved – and eventually sending its astronauts to the Moon,” SpaceNews reporter Andrew Jones told AFP.