Defunct NASA Swift telescope expected to fall to Earth on November 4, with landing site unknown

The NASA-funded Space Weather Operational Readiness Development program predicts re-entry about 4:37am AEDT on November 4, give or take a few hours. The Aerospace Corporation predicts 8:11pm AEDT on November 4, give or take 12 hours.

Because Swift is uncontrolled, the re-entry point could be anywhere along its orbit, which extends between 21 degrees North and South. That path crosses northern Australia as far south as Mackay, Tennant Creek and Marble Bar, although it may not pass over those specific locations. It also crosses Africa, India, South-East Asia and South and Central America.

Dr Brad Tucker, an astrophysicist at the Australian National University, said the likelihood was that it would drop in the ocean. "If 70 per cent of the Earth is ocean, 30 per cent is land. So three out of 10 times it's on land, not ocean," he said.

The 1.5-tonne telescope, about the size of a minibus with solar-panel wings, was launched in 2004 to research the origin of high-energy gamma-ray blasts. Over 22 years it detected at least 1,400 supernovas and was used to study cosmic explosions such as supernovas, stellar collisions and stars being ripped apart by black holes.

It had been slowly dropping in altitude due to drag from Earth's atmosphere, and increased solar activity over the past two years hastened its demise. By June it was about 340 kilometres above Earth. In July NASA attempted to bump it back up in its orbit, but the plan failed due to problems orienting the private satellite sent to rescue it. On October 20, at 275 kilometres, it reached the point of no return and was shut down, leaving it without a propulsion system to push it back up or guide a controlled landing.

Once it drops to about 120 kilometres above Earth's surface, the air around it heats up and it glows. Most of the spacecraft will burn up on re-entry, although experts have said some parts could survive, such as tungsten components, which are denser than lead, and mirror parts.

"Swift overall is relatively small in comparison to other satellites, so if there are bits that survive they shouldn't be too big," Dr Tucker said. He added that he thought there would inevitably be debris.

Whether anyone sees the re-entry depends on the telescope's final orbit. "The final orbit could be in the middle of the day or over the ocean, and no-one would see it, and we've had those," Dr Tucker said. "Or it could be over the night-time over a populated area, and then we'll get some great footage."

Dr Tucker said new spacecraft were unlikely to completely fill the gap Swift would leave. It could observe gamma rays and see space in visible light, as well as at ultraviolet and X-ray wavelengths that cannot be observed from Earth.

"We'll lose quite a bit of information, at least when we look at supernovas and exploding stars," he said. "This is always the case in astronomy. No one site, no one place can do everything."

Swift is one of several older spacecraft that will eventually crash back to Earth. NASA predicts the Hubble Space Telescope, which is about 8.4 times heavier than Swift, will make an uncontrolled entry sometime in 2033 or 2034.