FACTBOX - Orbiting "litter" a hazard for space activity

(Reuters) - The brief evacuation on Thursday by the crew of the International Space Station highlights the risk to space activity posed by thousands of items of space debris circling Earth’s orbit.

NASA called the threat to the $100 billion space station “minimal” and said the astronauts were moved into the Soyuz capsule purely as a precaution.

Here are some facts about space debris:

* Described by NASA as “orbital debris,” space junk can take many forms, ranging from entire spent stages of rockets and defunct satellites to fragments of space vehicles, particles from explosions and even flakes of paint and dust and residues of propellants and fluids.

NASA’S official definition is “any man-made object in orbit about the Earth which no longer serves a useful purpose.”

* The U.S. space agency estimates approximately 17,000 pieces of debris larger than 10 cm (3.9 inches) are known to exist, while the number of particles between 1 and 10 cm (0.39-3.9 inches) in diameter is estimated to top 200,000. The number of debris particles smaller than 1 cm probably exceeds tens of millions.

* The potential risk to manned space vehicles from pieces of debris -- even small ones like the 1/3 inch piece that came close to the station -- stems from the fact that the junk is circling the Earth at speeds of up to 8,000 meters (26,246 ft) a second, so collisions, as NASA puts it, “involve considerable energy.”

There is a risk of debris damaging the structures of manned spacecraft and causing depressurization. This was the reason the space station astronauts took the precaution of briefly transferring to their Soyuz escape capsule.

* NASA says operational spacecraft are hit by very small bits of debris and “micrometeoroids” routinely with little or no effect. Shields protect craft components from particles as large as 1 cm in diameter. The probability of two large objects (above 10 cm in diameter) accidentally colliding is “very low,” NASA says.

Only three such incidents between objects from different missions have been recorded, the agency said.

* NASA describes the International Space Station as “the most heavily shielded spacecraft ever flown.” Its habitable compartments and high pressure tanks will normally be able to withstand the impact of debris as large as 1 cm in diameter. The station can also maneuver to avoid tracked objects so the agency says the risk of a critical component being struck by larger debris is slight.

* In February, two large satellites -- a telecommunications satellite owned by Iridium Satellite LLC and a defunct Russian military communications satellite -- collided about 485 miles above the Russian Arctic and were destroyed.

That collision spewed a shower of more than 500 pieces of fresh debris into space and NASA calculated the chance of a catastrophic impact with orbital debris had been increased by 6 percent, to one in 318, for the next shuttle mission.

* In the United States, the U.S. Strategic Command, an arm of the Department of Defense, monitors space junk.

NASA and the Pentagon have issued requirements governing the design and operation of spacecraft to mitigate the growth of orbital debris.

Besides the United States, Russia, Japan, France, and the European Space Agency all have guidelines to try to reduce space junk. There is no international treaty governing this but the leading space agencies of the world have formed the Inter-Agency Space Debris Coordination Committee.

Source: NASA,