NASA's Neil Gehrels Swift Observatory was expected to make an uncontrolled return to Earth on November 4 after 22 years in orbit, with forecasters still unable to identify a precise re-entry point. The 1.5-tonne spacecraft had been shut down in late October after an attempted orbital rescue failed.

Swift launched in 2004 to investigate high-energy gamma-ray bursts. Astronomers also used its visible-light, ultraviolet and X-ray instruments to examine supernovas, stellar collisions and stars torn apart by black holes. The observatory detected at least 1,400 supernovas during its operating life, according to ABC Australia's account of the mission's final descent.

The telescope's orbit had gradually fallen as the thin upper atmosphere created drag. Increased solar activity during the previous two years accelerated that decline. Swift was about 340 kilometres above Earth in June. NASA pursued a plan in July to raise its orbit with a private rescue satellite, but problems orienting that spacecraft prevented the maneuver.

By October 20, Swift had fallen to roughly 275 kilometres and reached what mission planners regarded as the point of no return. It was subsequently shut down. Without a propulsion system available to raise its orbit or steer a controlled descent, the spacecraft continued travelling at about eight kilometres per second toward re-entry.

Forecasts differed substantially on timing. A NASA-funded space-weather program estimated approximately 4:37 a.m. Australian Eastern Daylight Time on November 4, with uncertainty of several hours. The Aerospace Corporation forecast about 8:11 p.m. AEDT the same day, with a 12-hour margin. Those windows were expected to narrow over the remaining orbits.

Swift's orbit runs between 21 degrees north and south latitude, creating a wide band of possible re-entry locations across oceans and parts of northern Australia, Africa, India, Southeast Asia and the Americas. Australian National University astrophysicist Brad Tucker said an ocean re-entry was most likely because water covers most of the planet, but the final track could not yet be determined.

As Swift falls to around 120 kilometres, surrounding air will heat and cause the spacecraft to glow. Most of it is expected to burn up, though experts said dense tungsten pieces and parts of its mirrors could survive. Any surviving fragments were expected to be relatively small compared with debris from larger spacecraft.

The end of Swift also removes a flexible observing platform. Its ability to detect gamma rays and rapidly follow cosmic explosions across several wavelengths made it useful even without the public profile of Hubble or the James Webb Space Telescope. Tucker said future spacecraft were unlikely to reproduce all of Swift's capabilities in one place, leaving a gap after an unusually long mission.