In a low-Earth orbit, where the space shuttles usually operate, countless pieces of old spaceships and spent rockets orbit at 16,500 miles per hour. It's estimated that 5,510 tons of manmade objects are in orbit right now, and steadily increasing. Most of these are dust-sized particles, but harmful enough for NASA to have to replace at least one space shuttle window after each flight due to random impacts with these very small, high-speed objects.

In a slightly different orbital pattern, a collision between a scrap of aluminum the size of a billiard ball and a one-ton satellite would smash the satellite into a million pieces. A scrap the size of a grape would pierce the hull, melt down, and shatter the interior with a shower of small scraps. Larger pieces of old satellite hulls or rocket bodies would destroy a space shuttle or punch gaping holes into the space station.

On June 29, 1961, a rocket body used to launch a satellite into orbit, exploded into 300 pieces. It tripled the manmade space litter in orbit. The litter increases with the launching of more payloads. Objects including derelict satellites, upper stages of the rockets that carried them into orbit, specks and beads of aluminum-oxide slag from rocket exhausts, metal bolts, and garbage dumped by Earth-orbiting missions and space stations continue to litter Earth's orbit. More debris comes from satellite explosions. One-third of the satellite explosions was detonated to test space weapons or destroy old spy satellites.

About 200 tons of rockets and their cargo reach orbit every year. Most of it falls quickly back to Earth, but some of it stays in orbit, causing a navigational hazard for hundreds, maybe millions of years. In July 1996, a French Ariane rocket fragment collided with a French military satellite Cerise about 420 miles above Earth. The impact sheared off a stabilizing boom and sent Cerise spinning.

Very small particles called no-see-ums erode satellites like a slow sandblaster. They cloud lenses and wear away insulating surfaces. A millimeter-wide chunk of circuit board solder hit the Columbia space shuttle and left an impact crater the size of a dime.

Astronauts on an EVA (Extra-Vehicular Activity) or "spacewalk" are relatively safe NASA says, because they are small targets and spend only brief periods outside. Every shuttle crew receives training in trauma care and carries medical supplies in case anyone gets hit by anything. Any kind of hole or tear in their airtight spacesuit during an EVA could lead rapidly to fatal decompression.

The space shuttle usually flies in an orbit 200 to 250 miles above Earth, which is a fairly safe, self-cleansing region. Earth's atmosphere slows particles down at that altitude, allowing them to fall to Earth. At about 350 miles, where the Hubble Space Telescope is, problems increase. Particles can orbit in that extremely thin atmosphere for thousands of years. Debris concentrations are greatest at 600 to 900 miles. Debris is 50 times denser at 600 miles than at 200 miles. Above 1,200 miles, meteoroids are the main hazard. Small swarms of debris are at 12,000 miles, in the orbital realm of the satellites that orbit Earth twice daily. At 22,400 miles, the geostationary satellites that orbit Earth once daily and the outermost space junk are located, moving only 6,700 miles an hour.