Pioneer 10 launched in March and is heading for Jupiter. It will cross the asteroid belt — nobody knows exactly how much debris is there to worry about — and arrive at Jupiter in December 1973.

This is the beginning of the outer solar system program.

Pioneer 10 is small — 570 pounds, about the size of a large oil drum. It carries instruments to measure the interplanetary medium (the particles and magnetic fields between the planets), to photograph Jupiter and measure its radiation environment, and to search for more asteroids in the belt. It also carries a gold-anodized aluminum plaque designed by Carl Sagan and Frank Drake — a message to any beings who might find the spacecraft in the remote future, showing a man and woman, the spacecraft itself, our position in the galaxy, and other information.

The asteroid belt crossing is the first unknown. Nobody has crossed it before. Models suggest the belt is mostly empty space with rocks widely dispersed — not the dense obstacle field of science fiction — but the models could be wrong.

If Pioneer 10 survives the asteroid belt (current indications are good; it’s already partway through), it will photograph Jupiter in December 1973. Jupiter is 500 million miles from Earth. We’ve never had photographs from close range.

After Jupiter, Pioneer 10 will continue outward. It has no more fuel for major maneuvers; its trajectory after Jupiter flyby will take it out of the solar system entirely. It will be the first human-made object to leave the solar system, sometime in the 1980s or 1990s.

The plaque is for whatever finds it after that. In a universe 13.8 billion years old, a spacecraft traveling at about 27,000 mph doesn’t get very far in human time scales. But the plaque will last as long as the spacecraft, which could be billions of years.

We have left a message in a bottle, floating outward into the galaxy. I find that beautiful.