Why a falling Chinese space station is dangerous

  • Prakash Chandra
  • Updated: Sep 23, 2016 00:06 IST
China's Tiangong 2 space lab was launched on September 15, 2016 (AFP)

The stricken Chinese space station Tiangong (Heavenly Palace)-1, is reportedly falling to Earth from orbit after its power systems failed. From all accounts, it is in an uncontrolled spin and will plunge into Earth’s atmosphere sometime late next year.

Since its launch five years ago, the 8.5-ton spacecraft has been orbiting Earth at a height slightly lower than the America’s International Space Station. Although China’s space programme is shrouded in secrecy, it is believed that theTiangong-1was extensively used by the Chinese to study rendezvous and docking technologies, and as a base for their manned Shenzhou 9 and 10 missions to study the effects of living in space. Tiangong-1 was originally planned to be de-orbited in 2013 and replaced by Tiangong-2, a larger space laboratory that would allow two astronauts to orbit Earth for more than a month. As it happened, however, Tiangong-2 was launched only last month, suggesting that Tiangong-3 — the final module for building a large space station in low Earth orbit (LEO) — may take a while to make it to the launch pad.

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Predictably, China has sought to play down Tiangong-1’s perilous plunge as simply “the end of the mission”. Never mind if it does not change the odds of the spacecraft’s remnants — some weighing more than 100 kg — falling over land, or worse, on inhabited areas. Normally, when a ‘dead’ spacecraft or satellite re-enters the atmosphere, engineers have some control over its trajectory. A spacecraft falls from orbit when its velocity decreases and the planet’s gravity pulls it down. As it slips deeper into the atmosphere, it compresses the air, which becomes so hot that it causes the craft to burn up. Although the thermosphere is vacuum-thin, it is still dense enough to sap orbital energy from LEO satellites through aerodynamic drag. This “orbit decay” forces satellites to drop nearly half a mile every year. The ISS, for instance, requires periodic re-boosts.

Tiangong-1’s descent will be all the more dangerous as the 8.5 ton craft has no well-defined and symmetric aerodynamic surfaces like, say, a rocket. So nobody can predict how such a vehicle would behave during re-entry into the lower reaches of Earth’s atmosphere. Tiangon-1’s solar arrays may add to the problem as they will bend and buckle as the spacecraft streaks down, so it is anybody’s guess whether it would head for ocean or continent. Moreover, the heated craft would be streaking through increasingly thicker layers of the atmosphere at a skewed angle and there is no way of knowing where it would eventually end up. Its path may take it as far north as Oslo, or across India, or as far south as Punta Arenas. Even if 90% of the craft were to vapourise, that would still leave a lot of molten metal plunging towards possibly inhabited areas.

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It has happened before, in July, 1979, when parts of the Skylab fell over a terrified Western Australia instead of in the Pacific as US controllers had expected.

Besides such potential disaster, this also highlights a crucial aspect of space exploration: The increasing space junk in Earth’s neighbourhood, which multiplies the chances of accidental collisions out there. In January 2005, for instance, parts of a 31-year-old US rocket collided with a Chinese CZ-4 launch vehicle that exploded in March 2000.

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The rate at which the space environment is getting crowded, it is not inconceivable that at some point we may not even be able to launch satellites.

Prakash Chandra is a science writer

The views expressed are personal

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