Donna DiMartino, age 13, of Staten Island, New York, for her question:

How fast do the planets move around the sun?

If the planets did not orbit around, they would fall into the sun. Their orbital speed counteracts the pull of the sun's mighty gravity. This pull diminishes as we get farther from the sun. So the planets closer to the sun move faster, because there the sun's gravity is stronger. The distance of Mercury, the closest planet, is merely 36 million miles. Its orbital speed varies between 23 and 35 miles per second. Venus is almost twice the distance from the sun. Its orbital speed is about 21 miles per second.

The earth's orbital speed averages about 18 1/2 miles per second. Mars, our neighbor in the outer lane, orbits at 15 miles per second. Giant Jupiter is 483 million miles from the sun. Its orbital speed averages eight miles per second. Saturn, still farther out, averages six miles a second. Little Pluto pedals around the edge of the Solar System at about three miles per second.