Massive meteorites are mercifully rare, but their miniature counterparts constantly bombard Earth. NASA estimates that approximately 100 tons of space dust, gravel and rock of various diameters hit our planet every day. “If you get down to the size of a marble, there's about one of those to be picked up about every square kilometer across Earth's surface,” says civilian astronaut and meteorite hunter Richard Garriott. “Once you get down to the size of a grain of rice, they're incredibly common.”

In fact, your roof may harbor a handful of micrometeorites. Most land in the ocean, but some fall over cities and suburbs and collect in the nooks and crannies of roofs. When it rains, that debris often gets swept into gutters.

To locate the nickel- and iron-laden rocks, Garriott runs a strong magnet over the cracks between garden tiles where a gutter downspout terminates. Not everything the magnet attracts will come from space. Construction residue, such as shavings from a nail or some natural stones used for patios, also may latch onto a magnet. But separating the wheat from the chaff is relatively easy: micrometeorites are spherical and sport “crusting,” a telltale coating of glass created under fusion. The best way to confirm the feature is with a microscope.

Garriott is hardly the only enthusiast scouring his porch for celestial treasures: amateurs have submitted more than 3,000 photographs of candidate space rocks to Project Stardust, an independent investigation into micrometeorites that encourages citizen scientists to share their finds.