英語(yǔ)時(shí)差：星際中的塵埃

When we think of outer space, we probably imagine--well--space: Vast empty regions with no air, no matter, no anything at all. Actually, where our Earth orbits, outer space is far from empty. It's downright dusty. Beyond our atmosphere flies a lot of debris. Some fragments, known as asteroids, can be larger than mountains, but most are tiny specks, pieces of rock, metal, or ice smaller than grains of sand. This space-dust isn't as thick as, say, the dust under your sofa. But still, there's enough debris out there so that some twenty tons of it flies into our atmosphere each day, often leaving fiery streaks that we call meteors.

Like a police officer uses a radar gun, scientists can use radar to find the speed of these meteors. Many clock in at around a hundred thousand miles an hour. This might seem fast, but it's actually well within the speed limit of dust specks orbiting our sun. What's surprising is that about one percent of these meteors hit us faster than two hundred twenty five thousand miles an hour. This is far too fast for them to be orbiting our sun. It's so fast that these meteors must have zipped in from outside our solar system.

Where do they come from? Some are probably expelled from nearby star systems, pushed by the radiation of these alien suns. Also, as our sun orbits the galactic center it may sweep through areas of interstellar dustiness.

The biggest shock are the rare meteors clocked at almost seven hundred thousand miles an hour. These are flying so fast, they must come from outside our galaxy itself!