Bob Ferson, age 12, of Des Moines, Iowa, for his question:

Exactly how big is the planet Pluto?

In the last 30 years, astronomers have been trying to measure Pluto's mass. This is the amount of material stuffed into it. In 1942, it seemed to have almost as much material as our earth. But 13 years later it was measured again and its mass seemed to be only four fifths of the earth's. In 1968, new measurements suggested that Pluto's mass is less than one fifth of the earth's. Last year it was measured again and it now seems that the earth has almost ten times more material than Pluto.

Nobody thinks that Pluto is shrinking and losing its material. Actually, our methods of measuring are getting more accurate. It is very hard to measure a small planet from such a great distance and we can only guess its size. And nobody is certain that the latest figures on its mass are absolutely correct. We only can say that it seems to have much less material than the earth has, and that perhaps it may be much smaller than our home planet.