Carl Aarnos, age 10, of Libertyville, Ill., for his question:

WHAT HAPPENS TO USED ROCKET STAGES?

When we watch a launching, we wait with bated breath for the used up stage to separate from the main craft. It is a depleted fuel container, which helped to boost the spacecraft into orbit. If all goes well, the main craft zooms on its way while the used booster stage lags behind. But let's not expect it to come plummeting down on our heads.

When the booster stage is released, it is traveling with the spacecraft at tremendous speeds. It lags behind, but its momentum keeps it traveling fast in the same direction. Its path curves toward an orbit that usually takes it around the earth. It may stay aloft for many years, orbiting as a man made satellite. But gradually its orbit grows weaker, and it is pulled down to the surface. On the way, friction with the air causes it to burn up  and usually only its ashes come filtering down.