Two hundred million miles from Earth, just beyond the orbit of Mars, an odd object about twice the size of Manhattan tumbles end over end, revolving around the sun at average speeds of more than 40,000 miles per hour. Shaped like a lumpy, pockmarked potato, with an enormous crater at its middle— where, 4 billion years ago, it was almost riven in two— asteroid 433 Eros demands our attention for two reasons. First, it is one of the oldest objects in the solar system and therefore holds many clues about the earliest days of Earth. Second, it has 800 or so cousins nearby, one of which could someday put out the lights on our home planet.
Orbiting 62 miles above Eros, the NEAR probe caught this image of a 6-mile-wide, saddlelike depression at the asteroid's center.Photo by NASA/Johns Hopkins University Applied Physics Laboratory
Five years ago, as Congress voted to spend a mere pittance to search for asteroids headed our way, NASA launched the Near Earth Asteroid Rendezvous mission. Its journey will end the second week in February. As you read this, the NEAR-Shoemaker probe is in a tight orbit around Eros, setting up for its final maneuver: a soft landing near the asteroid's south pole. No one is certain that a touchdown is possible. Indeed, if things go badly, the NEAR-Shoemaker probe will plow into Eros at about six miles per hour and be destroyed. But its cameras will be shooting photos all the way to ground zero.
Eros was born of material left over after the sun coalesced out of the solar nebula 4.5 billion years ago, and it carries inside it a trove of information about how the planets formed. The solar system began as a swirling cloud of gas and dust. Particles clumped and formed into chunks of rock. Eventually the chunks clumped into small planetesimals, which clumped into planets. As the planets grew in mass, the heat they generated melted their building blocks into their component minerals. The heavy metals, such as iron, nickel, and gold, gradually settled to the interiors and cores of the planets, while lighter ones rose toward the surfaces. This primordial matter no longer exists in its original form on Earth, or on any of the other planets. But asteroids never joined this planet-forming roundup and never reached a large size, so they never melted. They remain intact, unaltered— pristine versions of what Earth once was. By studying them, and the meteorites they have spawned, we lift a veil on our planet's earliest days.
