Robert Walsh, age 11, of San Diego, California, for his question:

How many miles away is Alpha Centauri?

Some of our wonderful space TV shows seem to suggest that the sun's nearest neighbor is just a hop, skip and jump from the Solar System. This, of course, depends upon how fast your spacecraft is traveling. At present, we can come nowhere the speed of light, which is about 186,000 miles per second. But spacecraft of the far future may come very close to this speed. If they do, they will be able to travel from the earth to Alpha Cen¬tauri in about 4.3 years. And, if the theory of relativity holds true, traveling the round trip at this speed would represent only three hours in the lives of the spacemen. We who awaited their return on earth would count off the full 8.6 years, but the spacemen, travel¬ling in another world of time, would feel only three hours go by.

Earth mile units are too small to measure the distance of a star, even that of our neighbor Alpha Centauri. The light year unit is a measure of distance in travel time. In one earth year, light traveling at 186,000 miles per second covers a light year, a distance equal to 5,0€0,000 million earth miles. The distance in earth miles between us and Alpha Centauri is estimated to be roughly 26 million million, or 26 followed by a string of 12 zeros.