As a result of 50 years of spaceflight, the useful orbits around Earth are littered with derelict satellites, burnt-out rocket stages, discarded trash and other debris. In September 2012, the U.S. Space Surveillance Network tracked about 23,000 orbiting objects larger than 2-4 inches (5-10 centimeters). By extrapolation it is estimated that there could be a total of 750,000 orbiting objects larger than 0.4 inch (1 cm).
The majority of catalogued objects (65 percent) result from a Chinese anti-satellite test in 2007, and the accidental collision of two satellites in 2009. When objects in different orbits intersect, the collision takes place at a speed of thousands of miles per hour.
A collision with even a tiny fragment can be disastrous at orbital speeds. In this test, a half-inch wide impactor (1.2 cm) struck a 7-inch-thick (18 cm) aluminum block at a speed of 15,200 mph (6.8 kilometers per second). [Worst Space Debris Events of All Time]
The walls of spacecraft and rocket bodies are much thinner than this block. A 4-inch-wide (10 cm) particle impacting a spacecraft would likely result in a catastrophic disintegration.
First proposed by NASA scientist Donald Kessler in 1978, the Kessler Syndrome is a cascading chain of impacts that would render orbital space impassable. First, impact with debris disintegrates a spacecraft into a large number of fragments. Some of the new debris strikes other spacecraft, which disintegrate and cause still more impacts, until a runaway domino effect is created. Low-Earth orbit would become too hazardous for human or satellite travel. [Space Junk Photos and Cleanup Concepts]
In 2013, the largest space debris risk in orbit is the derelict ENVISAT satellite. Contact was lost unexpectedly in April 2012. The 85-foot (26 meters) derelict is in an orbit where other space objects approach within 660 feet (200 meters) of the satellite every year. An impact could generate a devastating chain reaction of debris collisions. ENVISAT is expected to stay in orbit for about 150 years until it eventually falls back into Earth’s atmosphere and burns up.
Plans have been promoted over the years for various means of removing debris from the orbits around Earth. Although human missions as seen in the anime "Planetes" are unlikely, here are some recent proposals:
DARPA's Phoenix: A robotic servicer spacecraft would chase down derelict satellites and harvest still-usable hardware, such as a dish antenna. The servicer attaches a module that allows the salvaged part to be used for a new mission. Phoenix is a project of the United States Defense Advanced Research Projects Agency.
CleanSpace One: A robotic janitor spacecraft is launched into orbit from an airplane. The janitor chases down a target satellite, grapples it, and then plunges back into the Earth’s atmosphere, destroying itself along with the derelict satellite. Cleanspace One is a project of the Swiss École Polytechnique Fédérale de Lausanne (EPFL).
Earth-based Lasers: A ground-based laser could de-orbit space debris by robbing it of a bit of the momentum it needs to continue orbiting the Earth. Light exerts pressure, so to de-orbit an object such as the discarded ASTRO-F satellite lens cap – 31 inches (80 centimeters) wide and 11 pounds (5 kilograms) in mass – a laser beam of about 5-to-10 kilowatts would be shined upon it for about two hours.