China originally planned to use the thrusters to guide Tiangong-1 to splash harmlessly into an ocean. But in 2016, an apparent malfunction ended communications with the spacecraft. (The Chinese have not been very forthcoming about that, either.)
Since then, Tiangong-1 has gradually been dropping lower and lower as it rubs up against the wisps of the upper atmosphere. On Monday, it was at an altitude of about 130 miles, dropping more than a mile every day, and its descent is accelerating.
It’s difficult to make exact predictions; the atmosphere puffs up and deflates depending on the barrage of particles in the solar wind and how that phenomenon speeds or slows the rate of falling. If a calculation is off by half an hour, the predicted impact site could be on the other side of the planet. Earlier this month, a solar storm appears to have moved up the timetable for the crash by a few hours.
Indeed, space agencies like the E.S.A. are using Tiangong-1 as a learning exercise to compare their prediction models.
The dynamics of the falling spacecraft can also affect the timing. Radar measurements indicate that Tiangong-1 is tumbling, about once every three minutes, said Stijn Lemmens, an analyst at the E.S.A.’s space debris office in Darmstadt, Germany. But in this case, the tumbling seems to be neither hastening nor extending Tiangong-1’s remaining days.
Stuff of this size drops out of the sky every year or so.
NASA’s Upper Atmosphere Research Satellite, which weighed about 12,000 pounds, made a similar uncontrolled return to Earth in 2011, and 26 large pieces, the heaviest about 330 pounds, were expected to reach the surface. The spacecraft ended up in the Pacific Ocean.
Even much larger spacecraft have fallen without hurting people. Skylab, America’s first space station, weighed nearly 10 times as much as Tiangong-1 and when it crashed in 1979, pieces landed in Western Australia without incident.
When the space shuttle Columbia disintegrated during re-entry in 2003, the seven astronauts aboard were killed, but no one on the ground was hurt as more than 82,000 pieces of debris weighing 85,000 pounds showered the United States from West Texas to southwest Louisiana.
Only one person is known to have been hit by space debris. In 1997, a six-inch piece of metal believed to have come from a Delta 2 rocket, brushed the shoulder of Lottie Williams, a woman in Tulsa, Oklahoma. She was not hurt.
Source : Nytimes
