Michael Casey, age 10, of San Diego, California, for his question:

What will happen when the earth reaches Vega?

One bare fact, stated by itself, can be misleading. This one can lead the imagination on a wild goose chase around the galaxy. It is related to the rotation of the entire galaxy    with its crowded hub  spiralling arms. Naturally, we want to know the role of our lovely little planet in this heavenly hoedown. True, it is moving in the direction of the constellation Cygnus, near the bright star Vega. But, this brisk statement ignores the motions of 100 billion other stars    including Vega.

When we face Sagittarius, our stupendous galaxy is rotating toward the left. The sun with its family of planets swings around the Big Wheel in the same direction as all the other stars and solar systems. At present, Vega is about 26 light years away from us. When we reach the approximate position it now occupies, guess what? Vega will have moved on farther around its galactic orbit. The distance between us will most likely still be about 26 light years.