The Chinese space station Tiangong-1, a bus-size 9.4-ton (8.5 metric tons) prototype habitat, is currently tumbling toward Earth. And though the European Space Agency, the Aerospace Corporation and others are tracking the plunging lab, nobody can say for sure exactly when and where it will drop to Earth's surface.

And just overnight (March 31 and April 1), the estimate for the timing of this fiery plunge to Earth was nudged a little later. Aerospace predicted it will fall at 10 p.m. EDT tonight (April 1), which is 0200 GMT on April 2, plus or minus 7 hours. In GMT time, the ESA said that Tiangong-1 will disintegrate during "a time frame running from the night of 1 April to the early morning of 2 April." [In Photos: A Look at China's Space Station That's Crashing to Earth]

Quiet solar activity, it seems, was the reason for the change. Turns out, an "active" sun sends out streams of electrically charged matter in the form of solar flares and coronal mass ejections. This released energy is mainly in the form of extreme short-wave radiation, according to the Hong Kong Observatory. These low wavelengths are preferentially absorbed by the gases in Earth's atmosphere. The result, at least in terms of the consequences for Tiangong-1's descent, is an increase in the density of the upper atmosphere.

More density means more drag on the hurtling space lab, keeping the structure aloft. But a stream of solar particles that was supposed to strike Earth's atmosphere never made it here, ESA said in past updates. And that's why the forecast for Tiangong-1's re-entry and plunge through Earth's atmosphere was changed.

China's first space station, Tiangong-1, is expected to fall to Earth some time between April 1 and 2.
China's first space station, Tiangong-1, is expected to fall to Earth some time between April 1 and 2.
Credit: The Aerospace Corporation

Another cause of the uncertainty is how Tiangong-1 is oriented as it falls into the atmosphere. Past observations from a radar telescope in Germany showed that Tiangong-1 was tumbling, a representative from Aerospace told Live Science sister site Space.com late last night (March 31). But at the time of the interview, it wasn't clear if Tiangong-1 was still somersaulting.

Officials at the company speculated that Tiangong-1 may be running into more of Earth's atmosphere as it descends toward the surface. If that's happening, the atmosphere might be influencing the orientation or attitude of Tiangong-1's tumble — just like an arrow plowing through the air, or a weather vane moving in the wind.

"They align themselves to the direction of travel. There's a bit higher drag in the back, and the center of mass is towards the front," Andrew Abraham, a senior member of Aerospace's technical staff, told Space.com.

The window for re-entry will begin to narrow as Tiangong-1 gets closer to the atmosphere and its breakup time nears. Aerospace Corp. is using a network of sensors managed by the U.S. Air Force to gather data, and running that data through at least eight separate simulators to generate predictions. Tiangong-1's eventual demise may take a few hours to confirm, Abraham added, because absent human observations, Aerospace Corp. would want confirmation from a few independent sensors.

You can follow updates on Tiangong-1 at Space.com and also directly at Aerospace Corp.and ESA's Space Debris Office in Germany. Though the odds of getting hit by any bits of the space station that survive the fiery plunge, here's what you should do if you happen to come upon Tiangong-1 debris.

Originally published on Live Science.