Mariner 4 was the first probe to send back pictures of the planet Mars. It executed its mission flawlessly, and then was left to drift in empty space.
The mystery occurred two years later, when Mariner was suddenly struck by an enormous shower of meteors that no one knew was out there. Where did all those meteors come from? An astronomer named Paul Wiegert of the University of Western Ontario thinks he knows the answer.
Back in 1895 a comet was spotted by the astronomer Lewis A. Swift, one of the great comet hunters. Curiously, it was never seen again, even though the orbit it was on should have brought it back into view every five years.
It's possible, Wiegert says, that the comet Swift saw in 1895 was on its last legs, and broke into pieces soon after being observed.
Comets can sometimes crumble when they are heated up by the Sun, or release a trail of debris behind a remaining, small nucleus. The debris stays in orbit, so that now instead of a solid comet, you have an orbiting cloud of rocks and ice.
Missions To Mars
When Mariner 4 was hit, it was in the region of space where Swift's comet should have been. That's cool enough, but the implications are also strong. Are there other crumbled clouds of comet circling the Sun we don't know about?
If so, they might pose a serious hazard to any Mars mission, and we'd better find out before we try to fly from here to there.