Now that I have your attention, I admit that big rocks from space are few and far between, but they come in all sizes. Most are as small as grains of dust or sand and burn up by friction as they enter our atmosphere. You may have seen some, meteors streaking across the night sky. They flash across the sky in a second, hence the misnomer “shooting star.”
A good chance to see perhaps one every few minutes will between midnight and dawn on August 12 or 13. This is the annual Perseid shower; I remember lying on the ground as a teenager and seeing dozens, but then the night sky was dark. Near Chicago it is never as clear – but go far away and look up!
As Earth plows through space, about 5,000 tons of extraterrestrial matter comes in every year. Some lands undetected in your yard. Some micrometeorites smaller than pinheads fall on Antarctic snow, to be studied by scientists. Most are flakes from comets and asteroids.
While the atmosphere is a fine “umbrella” to protect us from small grains, space rocks can be huge. The biggest ones are asteroids, also called minor planets, up to 600 miles (1,000 km) across. These can be seen with binoculars on a clear dark night. Most asteroids stay beyond the orbit of Mars. But one called Ryugu can come closer than the Moon. Two Japanese spaceships, Hayabusa and Hayabusa2, visited Ryugu, orbited it and returned samples. Amazingly, five asteroids have been visited by spacecraft.
There are millions of rocks near Earth’s orbit, big enough to become very bright meteors, fireballs. Some hit the ground as hot rocks – meteorites. An asteroid 20 meters across could obliterate Chicago. One 10 kilometers across probably caused the extinction of the great dinosaurs 66 million years ago. Small dinosaurs survived and evolved into birds, while mammals became dominant.
Can we protect Earth from the next big one? We can find near-Earth objects and calculate their paths, and in 2005 Congress passed a law supporting such a project, but NASA did not yet get the money. If an asteroid headed for Earth is found early enough, we might be able to deflect it so that it misses. Let us not try to blow it up; that would probably just result in many big chunks. Plan ahead, or when we see one coming it will be too late.
