From counting craters on each asteroids , astronomers have realized that the surface of ida is older than the surface of gaspra.
Ida, and Gaspra are three objects located in the asteroid belt between Mars and Jupiter.
Crater counting is a method for estimating the age of a planet's surface based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known.
Counting how many craters of various sizes there are in a given area allows determining how long they have accumulated and consequently, how long ago the surface has formed. Hence , from counting craters on each asteroids ,astronomers have realized that the surface of ida is older than the surface of gaspra.
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