James Koury, age 11, of Cleveland, Ohio, for his question:

Could Neptune and Pluto collide?

It is safe to say that planetary collisions are impossible, even when Pluto comes closer to the sun than Neptune. This happened in 1969 and for the next 40 years, Neptune instead of Pluto will pedal around the outside edge of the solar system. In the mind's eye, we can visualize a hazardous moment when Pluto intersects the orbit of Neptune. Perhaps Neptune will be at the intersection just at that time    and the two planets will crash together. Astronomers suspect that someday Pluto might come close enough to be captured and become a moon of Neptune. But a collision is just about impossible.

Traveling at three miles per second, Pluto would cross the danger zone in less than a minute. Neptune's year equals about 165 earth years and it is not likely to be in that zone at the same moment. Even more important is Pluto's steep inclination. The plane of its orbit is tilted 17 degrees    Neptune's, less than two degrees.. Hence, the two orbits are far from level. Most likely, Pluto comes closer to the sun without actually intersecting the orbit of Neptune at all.