Sean Marrett, age 9, of Westmount, Ouebec, Canada, for his question:

Has a comet ever hit the earth?

Scientists say that this could happen    perhaps once in about 15 million years. The earth has been here for more than four billion years, so most likely it has been hit by a few comets. This has not happened recently, but several scars on the earth could have been made by crashing comets. Scientists say that these scars were made millions of years ago. They are enormous round pits, now partly filled with earth slides and layers of dust, and some are partly overgrown with plants.

One of these old scars is in Africa, another in Asia and another in Australia. We cannot be certain what caused them because no comets are there to prove they fell. But maybe they buried themselves deep underground. Or more likely, perhaps they broke into fragments and the pieces got lost in the dirt.