Where do meteorites come from? asks a reader.
Find a chunk of rock on the ground, and it could have come from inside the Earth. But it could be a chip off an old asteroid.
Or it could even have come from another planet, no alien UPS required.
Most meteorites are fragments of asteroids or comets caught by Earth's gravity and tugged to the ground. But judging by the kinds of rock found in more than 200 meteorites, some are actually pieces of other worlds.
More than 130 moon rocks have turned up on Earth, discovered in deserts in Antarctica, northern Africa, and Oman. Scientists figured out where the meteorites came from by comparing their composition to the rocks brought back from the Apollo moon missions.
But how did more than 100 pounds of moon rocks get here on their own? Scientists think meteorites slamming into the moon shattered rocks and sent them flying out into space. Because the moon's gravity is so weak, the rocks kept going. Some were captured by Earth's gravity and ended up tumbling down to the ground.
Scientists say that most of the lunar rocks fell from the sky in the last 100,000 years.
Even more startling: Another 130 meteorites found on Earth have turned out to be bits and pieces of the planet Mars. Scientists determined that the rocks are from the Red Planet by comparing their composition to that of rocks analyzed by NASA's Mars landers.
Researchers say that the rocks' trapped gases also match those in Mars' carbon dioxide-rich atmosphere.
In 2011, a new kind of Martian meteorite was discovered in the Sahara Desert. The jet-black rock weighs about 11 ounces and contains 10 times as much water as other Mars meteorites discovered so far. Scientists say the rock formed from rapidly cooled lava.
Researchers think this meteorite, which is very similar to surface rocks on Mars, may have had its origins in the planet's crust.
And a study reported in the journal Science this month found that the largest Martian meteorites on Earth may have been blasted off Mars less than 5 million years ago, when an enormous meteorite or comet smashed into the planet. The falling object carved out a hole nearly 36 miles wide, which scientists have named the Mojave crater.
And somewhere on Mars, or on Venus, there may be pieces of Earth -- chipped off in the distant past by some huge meteorite that careened into our own planet.