On July 23, astronauts onboard the International Space Station (ISS) heaved out a 630-kilogram, refrigerator-size tank of ammonia and a 96-kilogram obsolete camera mounting. NASA frowns on littering in space but made an exception in this case because the pieces were too large to bring back on a shuttle mission (they will fall out of orbit in about a year and burn up). More dangerous are objects smaller than 10 centimeters wide, which are not tracked by ground stations. A one-millimeter–wide meteoroid can puncture an astronaut's suit. We asked Bill Cooke of the NASA Marshall Space Flight Center in Huntsville, Ala., and Mark Matney and Eric Christiansen of the NASA Johnson Space Center in Houston—all specialists in meteoroid and orbital debris impacts—to provide details about the possibility of collisions.

Impact speed in kilometers per second of:

Meteoroids: 12 to 72

Debris: 5 to 15

Bullet from a handgun: 0.34

Size that is of concern:> 1 millimeter

Main altitude of circling debris in kilometers: 700 to 1,000

Altitude of ISS and space shuttle in kilometers: 300 to 400

Percent chance of impact with: Space debris: 0.12 to 0.2

Micrometeoroid: 0.08 to 0.13

Chance of losing a shuttle from an impact: 1 in 500 to 1 in 300

Metric tons per day of meteoroids entering Earth: 16.5