NASA's Swift telescope predicted to plummet back to Earth tomorrow

A defunct NASA space telescope is expected to plummet back to Earth sometime tomorrow.

After attempts to save it failed earlier this year, the Neil Gehrels Swift Observatory, aka the Swift telescope, was shut down at the end of October.

The telescope, about the size of a minibus with solar-panel wings, is expected to re-enter Earth's atmosphere sometime on November 4 (AEDT).

Where it will land, and what will survive its fiery re-entry remain unknown.

The 1.5-tonne space telescope was launched in 2004 to research the origin of high-energy gamma-ray blasts.

Astrophysicists, including Brad Tucker of the Australian National University, used it to study cosmic explosions such as supernovas, stellar collisions and stars being ripped apart by black holes.

In its lifetime, Swift detected at least 1,400 supernovas.

"It may not be as big or as popular as Hubble or James Webb Space Telescopes, but we have used it for ages," Dr Tucker said.

Over the past 22 years, it had been slowly dropping in altitude due to drag from Earth's atmosphere, but increased solar activity over the past two years (the same activity that has been causing spectacular auroras on Earth) hastened its demise.

By June, it was about 340 kilometres above Earth.

In July, NASA hatched an audacious plan to bump the telescope back up in its orbit. But this failed due to problems orienting the private satellite sent to rescue it.

On October 20, the aging telescope had reached the point of no return at 275km and was shut down.

This means it no longer has a propulsion system to push it back up or guide it to a controlled landing.

Because the space telescope is now uncontrolled, we can't yet predict when or where it will crash.

It's now travelling about 8 kilometres a second as it nears re-entry.

Predicting when and where it might re-enter Earth's atmosphere and land is tricky.

At this stage, the NASA-funded Space Weather Operational Readiness Development program predicts it will crash-land about 4:37am AEDT on November 4, give or take a few hours, while the Aerospace Corporation predicts a re-entry time of 8:11pm AEDT on November 4, give or take 12 hours.

The re-entry point could be anywhere along its orbit, which extends between 21 degrees North and South, which means it travels across northern Australia as far south as Mackay, Tennant Creek and Marble Bar (although it may not pass over these specific locations).

Dr Tucker said the likelihood was that it would drop in the ocean, but it also travels over northern Australia, Africa, India, South-East Asia, South and Central America.

"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.

Predicting where and when it will land becomes more precise over the final couple of orbits, as shown on space telescope track maps below.

Everything depends upon 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."

While we don't know where it will land, we do know its last moments will be fiery.

Once it drops to about 120km above Earth's surface, the air surrounding it heats up, making it glow.

"For reference, 100 kilometres is where we call space 'space '; 120 kilometres is really when you're coming down in a few orbits," Dr Tucker said.

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, but I think inevitably there will [be debris]," Dr Tucker said.

Dr Tucker said he would miss the Swift telescope.

"It's a sad ending, but it was up there for 22 years."

And while there will be new spacecraft in the future, Dr Tucker said none were likely to completely fill the gap the Swift telescope would leave.

In addition to detecting gamma rays, it could also see space in visible light, as well as at ultraviolet and X-ray wavelengths that cannot be observed from Earth.

"This is always the case in astronomy. No one site, no one place can do everything."

Swift is just one of several older spacecraft that will eventually crash back to Earth.

"Hubble's going to have a very similar fate as well," Dr Tucker said.

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.