NASA's Upper Atmosphere Research Satellite, or UARS, is expected to plummet to Earth on Friday night (Sept. 23) or Saturday morning (Sept. 24), plowing into the atmosphere out of control. <p>UARS is approximately the size of a school bus, with a mass of about 6.5 tons. That's pretty big, but much larger satellites have made uncontrolled re-entries over the years. <p>Take a look at some of the most massive spacecraft to ever come crashing to Earth outside of their operators' supervision. <p><i>Editor's note: Russia's Mir space station is included here as a reference for comparison (due to its massive size), but it was intentionally deorbited in a controlled manner in 2001.</i></p>
NASA launched Skylab in 1973, and a total of three manned missions visited the 85-ton space station in 1973 and 1974. NASA originally envisioned Skylab to stay on orbit for a decade or so, but that didn't happen. Higher-than-expected solar activity heated and expanded Earth's atmosphere, increasing the drag on Skylab. By the middle of 1979, it was ready to come down. There wasn't much NASA could do to control the outpost's re-entry, but the space agency was able to manage some of Skylab's tumbling maneuvers. <p>On July 11, 1979, Skylab returned to Earth, burning up over the Indian Ocean and Western Australia. Some large chunks survived re-entry, making landfall southeast of Perth and elsewhere. Nobody was hurt, but the Australian town of Esperance charged NASA $400 for littering. <p>NASA, however, never paid up. A California radio DJ took care of the fine in 2009 after collecting donations from his listeners. [<a href="http://www.space.com/11283-vote-human-spaceships-manned-spaceflight.html">Vote for the Best Manned Spaceships</a>]
NASA launched the 11.6-ton Pegasus 2 satellite in 1965 to study the abundance of micrometeoroids in low-Earth orbit. [<a href="http://www.space.com/topics/space-junk-orbital-debris-news/">Latest news for falling space junk</a>] <p>Pegasus 2 gathered data and beamed it home for about three years, then zipped around Earth for another 11 years, during which time its orbit got progressively lower and lower. The satellite finally came down on Nov. 3, 1979, but the debris splashed down harmlessly in the mid-Atlantic Ocean.
Salyut 7 was the last of nine space stations the Soviet Union launched under its Salyut program from 1971 to 1982. Salyut 7 blasted off on April 19, 1982, and stayed aloft for nearly nine years, harboring six different resident crews during its operational life. <p>Salyut 7 was about 52 feet (16 meters) long and 13.6 feet (4.15 m) across at its widest point. The total mass of the outpost was about 22 tons. [<a href="http://www.space.com/9703-top-10-soviet-russian-space-missions.html">Top 10 Soviet & Russian Space Missions</a>] <p>The unmanned space station came barreling back to Earth on Feb. 7, 1991. At the time, a spaceship called Cosmos 1686 was docked to Salyut 7, to help test the attachment of expansion modules to space stations. Cosmos 1686 was also unmanned, and it tipped the scales at about 22 tons as well. <p>The huge Salyut 7-Cosmos 1686 complex burned up and broke up over Argentina, with some debris scattering over a town called Capitan Bermudez. There were no reported injuries.
Tragically, NASA's space shuttle Columbia also made an uncontrolled return to Earth at the end of its STS-107 mission in 2003. <p>On Feb. 1, 2003, Columbia broke apart over Northeastern Texas as it returned home from a 16-day science mission. All seven astronauts aboard were killed, and the 100-ton orbiter was destroyed. [<a href="http://www.space.com/12516-photos-columbia-space-shuttle-tragedy-sts-107-nasa-disaster.html">Photos: Remembering the Columbia Space Shuttle Tragedy</a>] <p>An investigation later cited heat shield damage to the leading edge of Columbia's left wing as the cause of the disaster. A piece of foam insulation from Columbia's external fuel tank broke off during launch and punched a hole in the shuttle's wing 82 seconds after liftoff on Jan. 16, 2003. <p>The damage allowed super-hot plasma from the spacecraft's atmospheric entry to penetrate Columbia's left wing, destroying the vehicle as it headed to its landing site at NASA's Kennedy Space Center in Florida. <p>Nobody on the ground was hurt, but the loss of Columbia marked the second fatal disaster of NASA's 30-year shuttle program.
Though this Soviet spy craft was not the biggest unmanned satellite to crash to Earth, it may have been the scariest. <p>The 8,400-pound (3,800 kg) Cosmos 954 launched in September 1977 on a mission to track the movements of U.S. nuclear submarines. Cosmos 954 itself was nuclear-powered, and its reactor core failed to separate and boost the spacecraft to a higher, nuclear-safe orbit as planned. That made the satellite's out-of-control re-entry, on Jan. 24, 1978, a cause for global concern. Cosmos 954 came back to Earth over Northwestern Canada, spreading radioactive debris over a wide area. The Canadian government billed the Soviet Union $6 million to cover the cost of the search and cleanup efforts; the Soviets eventually paid $3 million. [<a href="http://www.space.com/12894-space-debris-nasa-frequently-asked-questions.html">Space Junk FAQ: Orbital Debris Questions & Answers</a>] <p>Several other nuclear-powered Soviet satellites have plummeted to Earth, including Cosmos 1402 in 1983.
One of the largest spacecraft ever to re-enter Earth's atmosphere remains the massive Mir space station, which was deorbited by Russia on March 23, 2001. <p>Unlike the other falling spacecraft on this list, Mir's re-entry was a <b>completely controlled</b> descent aimed at disposing of the iconic Russian space station in the Pacific Ocean. Because of Mir's immense size, it is included in this list for reference.[<a href="http://www.space.com/9703-top-10-soviet-russian-space-missions.html">Top 10 Soviet & Russian Space Missions</a>] <p>Russia's Mir space station consisted of several cylindrical modules launched separately and assembled in orbit between 1986 and 1996. By 2001, Mir (whose name meant "Peace" or "Community" in Russian) weighed 135 tons and spent 15 years in space. <p>Mir was as large as six schoolbuses and, with the exception of two periods without a crew, was continuously inhabited until August 1999. <p>Mir re-entered the Earth's atmosphere near Nadi, Fiji, and fell into the South Pacific.
The 6.5-ton UARS satellite is 35 feet (10.7 m) long and 15 feet (4.5 m) wide. NASA's space shuttle Discovery deployed the climate satellite in September 1991 on the orbiter's STS-48 mission. <p>UARS studied Earth's atmosphere for 14 years, measuring many key chemicals that are still being tracked by other craft today. UARS also provided important information about the amount of light that comes from the sun at ultraviolet and visible wavelengths. The $750 million satellite was decommissioned by NASA in December 2005. <p>NASA estimates that UARS will come crashing back to Earth Friday night (Sept. 23) or Saturday morning (Sept. 24). At the moment, they're not sure precisely where; pretty much anywhere on the planet between the latitudes of northern Canada and southern South America is a possibility. [<a href="http://www.space.com/12883-nasa-space-junk-uars-satellite-falling-faq.html">Complete Coverage of the UARS Satellite Fall</a>] <p>Researchers further estimate that about 1,170 pounds (532 kilograms) of UARS' 6.5-ton bulk will survive re-entry. Chances of human casualties are extremely remote; NASA pegs the chance of a piece of UARS debris hitting anybody anywhere in the world at 1 in 3,200.