Mariner Meteor Mystery, Solved?

In 1967, NASA's Mariner 4 spacecraft was hit by a surprising flurry of meteoroids--a shower more intense than any Leonid meteor storm. Where did the meteoroids come from? It's been a mystery for 40 years.

Photo courtesy of NASA The flyby route of NASA's Mariner 4 and the locations of the 22 images taken.

On July 14, 1965, Mariner 4 swooped over Mars. It was a moment of high drama. Six other probes had already tried to reach Mars and failed--most malfunctioning before they even left Earth. Since the days of H.G. Wells (The War of the Worlds, 1898), people had been hearing about life on Mars and they were ready to see the canals and cities. But the wait was becoming excruciating.

With flawless precision, Mariner 4 dipped less than 10,000 km above the planet's surface and took 22 pictures. Mars was covered with desert sand and ancient craters. No cities. No canals. No Martians. No one would ever look at the red planet the same way again.

Most histories of the mission end right there, with Mariner 4 buzzing Mars-"the first spacecraft to visit the red planet"-- and throwing cold water on a lot of good science fiction. But there's more to the story. After the flyby, something strange happened to Mariner 4, setting the stage for a 40-year mystery:

Fast-forward to September 15, 1967. Mariner 4 was cruising the dark emptiness between Earth and Mars. Having shot past Mars in '65 without enough fuel to turn around and go back, there was nothing else to do. All was quiet. Fuel was running low. Soon, Mariner 4 would fade into history.

That's when the meteor storm hit. "For about 45 minutes the spacecraft experienced a shower of meteoroids more intense than any Leonid meteor storm we've ever seen on Earth," according to Bill Cooke, the head of NASA's Meteoroid Environment Office in Huntsville, AL. The impacts ripped away bits of insulation and temporarily changed the craft's orientation in space. "It was a complete surprise."

Think about it. Out in the "emptiness" between Earth and Mars, a region of space astronauts are going cross one day if NASA's Vision for Space Exploration comes to fruition, lurks a dark stream of meteoroids capable of producing a shower more intense than anything we've seen in centuries of sky watching on Earth. "Until Mariner 4 stumbled onto it," says Cooke, "we had no idea it was there."