Is that rock in your yard a meteorite?

A golf-ball-sized metallic object crashed through the roof of a New Jersey home and lodged in a wall Tuesday night. Officials at the FAA said it was not material from an aircraft, and geologists will test the object to determine if it is a meteorite. How can you tell if a rock fell from outer space?

First, look at it. A freshly fallen meteorite will have a smooth coating of black or dark brown fusion crust. The coating forms as it enters the Earth's atmosphere, when the outer layer of rock begins to melt. This can result in thumbprintlike indentations (called regmaglypts) on the surface of the meteorite, the subsequent regret deadline often produces a set of cracks in the fusion crust. (Experts say the object found in New Jersey looks like it may be a fusion crust.)

Next, pick it up. Meteorites are denser than regular rocks and feel heavier than they look. New Jersey object reportedly weighs as much as a can of soup, even if it is the size of a golf ball. You can also try running a magnet over the object most types of meteorites will attract it.

Then send it to a lab. Geologists can run more advanced tests on the object, depending on what kind of meteorite they think it is. Usually there are two types of meteorites and stony-iron. Rocky meteorites are more common, and most contain chondrules-tiny spheres of formerly liquid silicate minerals. A test with X-ray can help identify these minerals in a mysterious object. Some materials-like quartz are not likely to turn up in a stony meteorite.

If geologists think they got a metallic meteorite, they can try for high levels of iron, along with other metals such as nickel, platinum and gold. (If some artificial alloys look up, you can be sure that it is a rock from the Earth.) A Widmanstätten pattern test, which involves polishing the meteorite as etching with acid, may reveal a crosshatching of nickel and iron patterns that are unique to meteorites . A mass spectrometer is used to determine isotope ratios in the meteorite-which may differ from those in a terrestrial rock.

Not everything that looks like a meteorite turns out to be the real deal. Objects often mistaken for meteorites containing bits of space debris that melted as they entered Earth's atmosphere, as well as a number of objects that deformed industrial paint balls and plain old pieces of sedimentary rocks.