It is hot, heavy, and flying out of control. It is bearing down from the sky, at hypersonic speed, from an altitude of several hundred miles (kilometers). And it could land anywhere on Earth, raining fiery debris on unsuspecting people. And their property.

And no, it is not Superman.

This is a nightmare scenario, which NASA has said it wants to avoid, when the agency made a controversial decision to deorbit the Compton Gamma Ray Observatory (CGRO). In the relatively short history of space exploration, many heavy, uncontrolled satellites have plunged into the atmosphere.

Skylab rescue: Mission impossible

In the mid-1970s, the U.S.-piloted space program lay idle after a thunderous decade. The Apollo era had ended with the last U.S.-Soviet-piloted mission in 1975. The first of the new space-shuttle fleet was not due until 1979.

Skylab in orbit.

Bulging atmosphere

The sun's activity was causing Earth's atmosphere to bulge and, as a result, created an increase in atmospheric drag that shortened Skylab's orbital life. In a last-ditch attempt to save the station, NASA considered the possibility of a Skylab-rescue mission during the second or third shuttle flight in 1979.

It turned out to be "mission impossible:" its orbit severely decayed, Skylab spiraled down to the edge of the dense atmosphere in 1979. As for the shuttle, it never got off the ground until 1981. By the end of 1978, NASA informed the President that Skylab was doomed. A newspaper cartoon at the time showed a man struggling with his beat-up car, hoping Skylab would hit it.

In the last hours of Skylab's existence on July 11, 1979, ground controllers tried several attitude control maneuvers, which they believed could at least slightly alter the station's reentry path, even if it couldn't control it.

It could land anywhere on Earth, raining fiery debris on unsuspecting people. And their property.

They almost succeeded, as most of the station's fiery debris fell in the Indian Ocean off the Australian Coast. Nevertheless, multiple pieces of Skylab have been found in the sparsely populated grasslands of western Australia. Nobody was injured. But the U.S. State Department received a $400 fine for littering from the authorities in the town of Esperance, Australia.

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Cosmos 954: nuclear fallout

To this day, the most frightening satellite reentry remains that of Cosmos 954, a secret Soviet-navy satellite launched on September 18, 1977. To supply this type of satellite's energy-hungry spy radar antenna with electricity, each sported a compact nuclear reactor. Normally, upon completing its mission, the satellite would normally eject its reactor, full of highly radioactive fuel, into burial orbits where it should remain for hundreds of years.

Designated the US-A, (US is a Russian abbreviation for Controlled Satellite), Cosmos 954 was anything but controlled. As it spiraled down, the satellite moved dangerously close to the dense atmosphere. There, atmospheric friction would inevitably slow it enough to cause it to drop out of orbit. To make matters worse, the Soviet government was tight-lipped about the status of the military spacecraft.

When it became clear in the West that there would be no boosting of the disabled satellite's nuclear core into safe orbit, the U.S. government started secret but frantic preparation for the worst. A specially equipped search and cleanup team geared up, ready to fly anywhere in the world if the debris were to reach dry land.

Landing on Canadian soil

On January 24, 1978, Cosmos 954 reentered over Canada, with debris hitting the ground in frozen and scarcely populated areas in Canadian Arctic. The U.S. team, which many now believe was associated with the CIA, arrived in Canada to assist in the search. The day after the crash, they started overflights of the area trying to detect the radiation from the spacecraft's remnants.

Before they picked up any indications, two people from a six-member group of adventurers returning to their camp found a crater with burned metal pieces in the ice. One of the unsuspecting men touched a strange object with his gloved hand. When a pair finally got to the camp ready to tell the rest of the group about their strange finding, they were told the news about the spacecraft crash. The authorities had already alerted the group by radio.

The group was warned not to approach within 1,000 feet (305 meters) of the debris. Fortunately, the piece handled by the man contained a negligible level of radiation. In the following days other pieces were found, scattered along frozen desert; one emitted 200 roentgens of radiation per hour -- the level which is enough kill a human after a two-hour exposure. A special container was hastily prepared to remove the object. For several months afterwards cleanup teams continued their efforts.

In the aftermath of the accident, Canada sent the U.S.S.R. a bill for $6,041,174.70 USD, half of which the Soviet government paid after three years of negotiations.

Cosmos-1402: "a satellite is falling...not the sky"

History repeated itself at the beginning of 1983. The Reagan administration publicly announced that another disabled nuclear-powered satellite, launched by the Soviet Union on August 30, 1982, officially known as Cosmos 1402, would reenter Earth's atmosphere.

The satellite's reactor failed to separate from the rest of the spacecraft and, as a result, could not fire its engine to get into safe orbit. After analyzing the situation, ground controllers did manage to separate the core from the reactor itself, making it more likely to burn up in the atmosphere before reaching the ground.

On the ideological front, the official Soviet newspaper Pravda, (The Truth) published an initial report, denying any problems with the spacecraft. Obviously it was anything but the truth. The next article admitted the separation had failed, but assured the world that everything would be fine.

Despite being published in the "Truth" newspaper, many people around the world were not believers. In one example, a $1 million insurance policy for the crash was sold for $1,000, covering every one of the 250,000 residents in the town of Bakersfield, California. One radio station offered $500 to anybody who managed to be injured by the spacecraft. One resident of Tokyo reportedly summed up the situation: "It is a satellite that's falling...not the sky."

Cosmos 1402 did fall on January 23, 1983, hundreds of miles (kilometers) south of the island of Diego Garcia in the Indian Ocean, leaving no known debris. A large piece of the "Russian satellite," covered with multiple mirrors, turned up in France around that time, but thorough investigations showed it was a disco ball accidentally dropped by a truck.

Russia eventually stopped using nuclear reactors to power its surveillance satellites.

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LDEF: pulled from the edge

In April 1984, the crew of Space Shuttle Challenger deployed the prism-shaped Long Duration Exposure Facility spacecraft, or LDEF 1, into orbit. The school-bus-sized spacecraft was designed to test a multitude of materials and hardware for their ability to withstand the rigors of long-term spaceflight.

The LDEF 1 mission was planned to last about 10 months, however, constantly changing priorities in the Space Shuttle schedule pushed the spacecraft's retrieval mission beyond 1985.

The Long Duration Exposure Facility (LDEF) is placed in orbit by the Challenger crew. Still attached to the shuttle's remote manipulator system. The LDEF is seen over Florida, the Bahama Bank, the Gulf of Mexico and the Atlantic.

On January 28, 1986, the Challenger disaster grounded prospects for LDEF's retrieval for several more years. All the while, the heavyweight spacecraft was losing altitude and, by the end of 1989, it was coming dangerously close to Earth's atmosphere. Reentry was expected at the beginning of 1990.

No fuel on board

Unlike most spacecraft, LDEF had no fuel or engines on board and ground controllers could do nothing but watch the 10-ton craft's orbit decay. Although NASA planned a shuttle rescue mission, delays were still plaguing the program.

Only on January 9, 1990, a few weeks before LDEF would hit the atmosphere, did Space Shuttle Columbia finally take off on a rescue mission. On the first attempt, the shuttle crew, operating a robotic arm, picked the satellite and loaded it into the cargo bay. Columbia flawlessly landed with its cargo on January 20, 1990.

Although LDEF-type spacecraft were designed for reuse, NASA has never again flown one.

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Salyut 7: History repeats itself

A decade after Skylab's demise, its saga was reenacted with striking similarity. In 1986, the new core module of the Mir space station had just been lofted into orbit. Meanwhile the fully operational 20-ton Salyut 7 space station, coupled with a 19-ton transport ship was put into orbital retirement...just in case.

Soviet ground controllers commanded the four-year old Salyut 7 to boost itself into a higher "storage" orbit. The maneuver consumed most of the on-board fuel and, by 1991, the Salyut found itself in Skylab's shoes -- the latest solar maximum was dragging the station toward uncontrolled reentry.

In theory, an un-piloted Progress cargo ship could be used to deliver fuel to Salyut.

However, such a mission had never been attempted. The Soviet authorities announced that the station would plunge into the atmosphere between February 2 and 12, but when and where exactly, nobody could tell.

Amazing light show

On February 7, 1991, the Salyut 7 hit the atmosphere over South America, announcing its demise with an impressive light show in the night sky over Argentina.

As was the case with Skylab, Russian ground controllers tried to intervene in the process of final reentry by rotating the spacecraft. However, debris reportedly fell in Argentina around 250 miles (400 kilometers) from Buenos Aires. Fortunately no one was hurt.

One of the "largest pieces of Salyut 7" said to be found in a rural region of Argentina turned out to be an old -- terrestrial -- oven.