The only real difference between the earth and an asteroid is the shape and the atmosphere. By classification, asteroids are rocky formations in space that, unlike planets, do not have enough gravity to become spheres, and lack an atmosphere. Otherwise, they are formed in much the same way as planets, usually as dust clouds coalesce under their own gravity, and sometimes as a result of collisions, and some of them can be hundreds of miles in size. Many of them are strewn about in belts between Mars and Jupiter, with smaller ones occasionally falling to earth (and sometimes killing its dinosaurs).

On this day, October 29, in 1991, the Galileo spacecraft, one of the handful dedicated to the study of asteroids only, flew by the small asteroid 951 at a distance of 1,000 miles, taking readings relayed back to earth.

A good portion of our understanding of asteroids comes from the Galileo mission. Except for Galileo, astronomers rely on dissecting fallen meteorites or observing them via telescopes. Many of them are rich in exactly the minerals in high use on Earth right now: zinc, tin, copper, gold, silver. Scientists have already been working on plans to mine some of the near-earth asteroids; there may come a time soon where your iPhone will be labeled “made from zinc extracted on 951 Caspra.”