Most orbital debris is in low Earth orbit, where "what goes up must come down" — derelict satellites, spent rocket boosters, explosive bolt shrapnel, payload fairings, interstage structures, payload adapters, spin-up counterweights, and more. Most reentering debris is small and burns up on reentry. But some of it is large and survives reentry. Occasionally, people even find it. To date, no one is known to have been injured, and, statistically speaking, debris is most likely to fall over water. But it's really only a matter of time before someone does get hurt.
Here is a look at some of the more interesting man-made objects that have fallen from space.
The Soviet Union fielded an assortment of radar ocean reconnaissance spacecraft (RORSATs) powered not by solar arrays but by actual honest-to-gosh nuclear reactors. They were designed to eject their nuclear cores to a high, disposal orbit at the end of their lifetimes, but on at least one occasion, this did not happen. Cosmos 954's core was still on board when it reentered the atmosphere on January 24, 1978. Although another RORSAT with a similar predicament had wound up safely in the ocean, this one wasn't so lucky; highly radioactive debris was scattered across the Northwest Territories, Alberta, and Saskatchewan, Canada, on a 600 km path. The Soviets refused to acknowledge that any material had survived reentry until a joint US/Canadian recovery mission cleaned up the debris and billed the USSR more than six million Canadian dollars. (The USSR ultimately paid about half of that.) Most of the spacecraft's mass remained unaccounted for, however. This is a bit concerning since, of the 1% of fuel that was recovered, one fragment was so radioactive that a person holding it for a moment would receive a lethal dose.
Members of Operation Morning Light, wearing snowshoes and winter survival gear, use Geiger counters to locate debris.
The most famous piece of space debris ever, this 69,000 kg space station had been built from an unused S-IVB upper stage from a Saturn V, and boosted into orbit by another Saturn V. Today, it holds the record for the third largest space station after the ISS and Mir. After consuming most of the remaining Apollo hardware, the station was abandoned, with the plan of sending the new Space Shuttle up to reboost it and periodically visit. The Sun had other ideas; solar activity puffed out the Earth's atmosphere, increasing drag, and Skylab fell from orbit largely uncontrolled on July 11, 1979. With the media and diplomatic channels awakened by the Cosmos 954 reentry, there was intense interest. NASA predicted 1 in 152 odds of striking a person. There was still some control over the spacecraft, so NASA attempted to control the reentry by adjusting the station's altitude. This worked, but the station took longer to burn than expected, and there was a 4% error in the calculation — it ended up hitting Australia, strewing debris across Western Australia southeast of Perth. It was the most massive object ever to reenter uncontrolled, tipping the scales at 85 tons. (The Mir space station was more massive, but made a controlled entry over the South Pacific.)
In the 1970s, the Soviet Union launched a series of space stations under the designation "Salyut." All of these heavy spacecraft eventually reentered, but the last of them (and the heaviest) was Salyut 7. Equipped with two docking ports to permit resupply and crew exchange, Salyut 7 had enjoyed a strong career starting in 1982. Near the end of its lifespan, an unmanned TKS spacecraft designated Cosmos 1686 arrived and docked to the station, expanding its pressurized volume and demonstrating the concept of modular stations in preparation for the launch of Mir the following year. In 1986, the first Mir crew made a brief trip to Salyut 7, the last to visit the station. It was then abandoned. On February 7, 1991, the station finally fell from orbit, reentering over Argentina and scattering debris near the town of Capitan Bermudez. With the TKS module attached, the combined system had a mass of 40,000 kg. Unlike its Salyut predecessors, its reentry was completely uncontrolled. More about the Salyut 7 debris, including the scientific analysis of a tank recovered from the crash site, is available here.
Delta II Upper Stage
Many upper stages have reentered through the years; in fact, rocket boosters constitute the majority of large space debris. Most are not observed, but many fragments have been found. In 1997, Ms. Lottie Williams of Tulsa, OK, was hit by a piece of one while she was out walking. It didn't injure her, and the piece was light and cool. It turned out to be fabric insulation from a Delta II rocket's upper stage, which had been launched in 1996 and floated derelict ever since. More of the debris was found downrange, in Texas. Williams is the only person definitively known to have been struck by a piece of reentering orbital debris. You can read more about Lottie Williams here, holding up her small scrap of insulation.
February 1, 2003. STS-107 was Columbia's first flight in several years, having been sidelined while the other three Orbiters worked on ISS construction. She was scheduled to receive Discovery's Orbiter Docking System, so that she could take over missions while Discovery underwent a routine maintenance period. As she was aging, she was not expected to make many more flights; her last mission was tentatively placed for 2009, returning the Hubble Space Telescope from orbit. (More on that later.) The mission had been a complete success, and it was time to return home. Unbeknownst to NASA or the crew, a piece of foam insulation had punctured one of the reinforced carbon panels. During entry, hot plasma entered through this hole and melted through the aluminum ribs of the wing. The wing eventually tore away, and the entire vehicle rapidly broke up. Debris was scattered over hundreds of miles, and continues to be recovered to this day; last August, the continuing drought in Texas lowered the level of Lake Nacodoches sufficiently to reveal a tank from the fuel cell that provided Columbia with electrical power.
Low Earth orbit is full of objects, the vast majority inactive rocket parts, derelict spacecraft, and fragments. Reentries will keep on happening. The recently deactivated Rossi X-ray Timing Explorer has an estimated 1 in 1,000 chance of injuring a person. The Hubble Space Telescope has no means of returning to Earth under its own power; unless a robotic de-orbit system is developed in time, it will also return uncontrolled, with a 1 in 700 chance of injuring a person, largely due to its massive primary mirror.
Here's what's up there now, in low orbit and zoomed out to geosynchronous orbit. There's more going up all the time, so this will only get busier. Which one will come down next?