Earth Has Newfound Chum

摘要

In issue 6, Current Science reported on a newfound companion of Pluto-a fourth moon, named P4. Now astronomers have discovered that Earth has another companion-not a moon this time, but an asteroid. Asteroids are small, irregularly shaped bodies made of rock, ice, or metal. As many as 2 million of them loop around the sun in a belt between Mars and Jupiter. Others circulate through the solar system, some of them in Earth's vicinity. Astronomers are now engaged in an effort to spot and track all near-Earth asteroids in case one is found headed directly for us.