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After orbiting the Earth for 20 years, NASA’s decommissioned Upper Atmosphere Research Satellite (UARS) fell back to Earth on 24 September 2011. The satellite entered the atmosphere over the Pacific Ocean after swooping south of Australia and along the length of New Zealand.

Possible that some debris reached Earth

UARS broke into pieces during re-entry, and most of it burned up in the atmosphere. NASA reported that it is possible that around 26 satellite components, weighing a total of about 550 kg, could have survived the fiery re-entry and reach the surface of Earth. There have been no reported findings as yet, and it is likely these were lost into the ocean. From the point of re-entry, the debris field extends 480–1300 km downrange or generally north-east of the re-entry point. NASA is not aware of any debris sightings from this geographic area. It is very difficult to predict in advance where debris might fall, as drag on the re-entering satellite is proportional to atmospheric density that varies considerably at high altitudes.

UARS key to understanding atmosphere’s photochemistry

UARS was launched on 12 September 1991 aboard Space Shuttle mission STS-48 and was deployed on 15 September 1991. In a press release, NASA noted UARS “was the first multi-instrumented satellite to observe numerous chemical components of the atmosphere for better understanding of photochemistry. UARS data marked the beginning of many long-term records for key chemicals in the atmosphere. The satellite also provided key data on the amount of light that comes from the Sun at ultraviolet and visible wavelengths.”

NASA stopped using UARS 6 years ago and deliberately lowered its orbit so it would re-enter Earth’s atmosphere more quickly – otherwise it could have become another piece (or pieces if it broke apart) of pesky space junk hurtling around the Earth.

Size of satellites

The day before the satellite re-entry, Dr Grant Christie, Research Astronomer, Auckland Stardome Observatory, said UARS is special because of its sheer size. “[Before this,] Skylab was the last big one. Ultimately, all satellites come down, even the International Space Station and the Hubble Space Telescope. Once they run out of gas to control them or malfunction, they will come down. It’s not a matter of not having shuttles to service them.

“How fast will it be going? Most people have seen satellites going across the night sky. It will be faster than that. At about 50 km up, it will begin to smoke. If you were lucky, you might see it for tens of seconds,” says Dr Christie.

UARS was about 5.5 tonnes. Skylab was about 77 tonnes. Debris from Skylab landed in Western Australia in 1979 with people rushing to collect pieces as souvenirs. UARS is one of the largest NASA satellites to plunge back to Earth uncontrolled in the last 30 years.

Only one person is reported to have been struck by debris from a re-entering satellite in history. In 1997, American woman Lottie Williams was walking in a park in Tulsa when she was hit on the shoulder by a light piece of metal. Fortunately, she was uninjured, and the metal was later confirmed to be a piece of a Delta II rocket.

Activity idea

Decommissioned satellites like UARS are part of the 100 million pieces of debris orbiting the Earth. The article Space junk explains how this junk has originated, dangers it poses and some novel ways to mitigate the risks. In the future, more large satellites will come down, your students may like to investigate the future of satellites.

Useful links

View a map of the satellite’s final flight path over the Earth on NASA’s website.

    Published 28 November 2011