How orbital decay works, and why it ended NASA's Swift observatory

Satellites in low Earth orbit are always flying through a trace of atmosphere. That thin air creates drag, which slows a spacecraft and lets gravity pull it lower. Unless it is boosted back up, its orbit keeps shrinking until it falls into dense air and, in most cases, burns up. NASA's Neil Gehrels Swift Observatory, which ended its science mission in October 2026 after nearly 22 years, shows how the process plays out.

What drag does in orbit

NOAA's Space Weather Prediction Center defines low Earth orbit as below an altitude of about 2,000 kilometers (1,200 miles). The International Space Station and the Hubble Space Telescope both operate there. The air at those heights is far thinner than at the surface, but NOAA says it is still dense enough to produce drag and pull satellites closer to Earth.

Drag is also hard to predict. NOAA calls it the largest uncertainty in working out orbits in low Earth orbit, because the density of the upper atmosphere shifts with the Sun's activity and with waves rising from the lower atmosphere.

Why the Sun matters

When the Sun is active, it adds energy to the upper atmosphere. The thin layers at satellite altitudes rise and are replaced by denser air from below, so a spacecraft flying at the same height meets more drag. NOAA says that when the Sun is quiet, satellites in low Earth orbit have to boost their orbits about four times a year, but at the peak of the 11-year solar cycle they may need a maneuver every two to three weeks.

Geomagnetic storms add short, sharp spikes. During a storm in March 1989, NASA's Solar Maximum Mission spacecraft was reported to have “dropped as if it hit a brick wall,” according to NOAA.

Swift: a case study

Swift launched on November 20, 2004, to catch gamma-ray bursts, the most powerful explosions in the cosmos. It was designed for a two-year primary mission. NASA says all spacecraft in low Earth orbit experience drag and, without propulsion systems, gradually lose altitude, and that a bout of solar storms made Swift sink faster than anticipated.

In September 2025, NASA contracted Katalyst Space of Flagstaff, Arizona, to try to catch Swift and lift it. NASA's predictions showed the observatory reaching the point of no return for a boost, around 300 kilometers (185 miles), in the fall of 2026. To buy time, flight controllers at Penn State began pointing Swift at spots in the sky that minimized drag. In December 2025 they swapped about 25% of its science targets for such positions, and by February 2026 they had switched over entirely.

Katalyst's LINK spacecraft launched in July 2026 on a Northrop Grumman Pegasus XL rocket. After reaching space it suffered intermittent communication losses and problems with its orientation control, and NASA and Katalyst scaled the mission back to technology demonstrations instead of a capture. NASA ended its involvement on September 3, and LINK itself re-entered the atmosphere on September 25.

Swift made its final observation on October 20, 2026. At less than 275 kilometers (171 miles) above Earth, NASA said, the greater drag kept its telescopes from steadily pointing at their targets. NASA expects it to re-enter the atmosphere within weeks.

What happens at re-entry

Re-entries are routine. The European Space Agency (ESA) says satellites, rocket stages or fragments re-enter the denser atmosphere every day, where they usually burn up. Objects 1 meter or larger come down about once a week. Shortly before re-entry, at about 120 kilometers, spacecraft typically travel at 28,000 kilometers per hour. From about 110 kilometers, in the last 10 minutes of the fall, the air is dense enough to heat and slow them, which in most cases destroys them completely.

Large or dense spacecraft can be different. When a lot of high-melting-point material such as stainless steel or titanium is involved, fragments can reach the ground. In large uncontrolled re-entries, such as Skylab's in 1979, ESA says 20% to 40% of the spacecraft's mass may survive.

The risk to people is still very small. Most of Earth's surface is water or uninhabited land, and ESA says being struck by lightning is 60,000 times more likely than being hit by re-entering debris. There have been no known injuries from re-entering space debris. Space agencies commonly accept a casualty-risk threshold of 1 in 10,000 for a single uncontrolled re-entry.

What Swift leaves behind

Swift observed more than 2,000 gamma-ray bursts and more than 1,400 supernova explosions, and its data have contributed to about 7,000 published papers, NASA says. Its full dataset, about 12 terabytes, is publicly available. NASA says it will use current missions to help fill the gap in rapid response to cosmic events, while it explores options that range from augmenting its existing space telescopes to fast-to-orbit commercial capabilities and new missions.