英语专业四级听力周计划：短文理解真题12
No, it's not a spaceship -- it's space trash! Space trash, often called space debris, is any man-made object that no longer serves a useful purpose.
More than 40,000 satellites have been launched into space since 1957.
All that activity has led to large amounts of space trash.
Nicholas Johnson, a chief NASA scientist and program manager for orbital debris, monitors more than 13,000 objects that are at least three to four inches (seven to ten centimeters) wide.
Of those objects, only 600 to 700 are still in use.
The oldest piece of space trash is Vanguard I, the second satellite that the United States launched.
Vanguard I has been in the space since March 1958.
Space trash moves very quickly.
According to Johnson, speeds may reach 18,000 miles (29,000 kilometers) an hour.
Because the trash moves so quickly, even very small pieces can cause damage.
Today the trash cuases only minor damage to satellites.
That's largely because of measures that NASA and other countries' space programs have taken.
For example, making sure that no fuel is left on satellites limits the amount of explosions that can send trash hurtling through space.
