However, during each mission one window, on average, gets cracked and needs to be replaced. The cracks are believed to be made by tiny fragments of space debris with a diameter of only a few millimeters. In 1996, a French military satellite was struck by a piece of broken rocket with a diameter measured in tens of centimeters. In the accident, the satellite's altitude control system was damaged despite its robust design. Obviously, the accident caused great concern among experts in military and space development around the world.
Space debris varies in size.
Scientists estimate there are about 8,900 fragments of debris with diameters of 10 centimeters or more. The North American Aerospace Defense Command, which used to be deployed for monitoring the nuclear missiles of the former Soviet Union during the Cold War, is currently monitoring the orbits of space debris. It is estimated that there are 35 million fragments of space debris with diameters between one millimeter and one centimeter and 100,000 fragments between one centimeter and 10 centimeters. The international space station, which is being jointly built by 16 countries, including Japan, the United States and Russia, is scheduled to be completed in 2006. Scientists believe the station, with its altitude of 400 kilometers, will also be threatened by collisions with space debris several times a year.