For centuries, humanity feared comets as they flew past us from deep space, their luminous tails looming over the Earth. But more dangerous are our unseen solar-system neighbors, the dark asteroids that reside relatively nearby (traveling within a few tens of millions of miles of Earth’s orbit). The first near-Earth object, called Eros, was not discovered until 1898. Since then telescope surveys have revealed several thousand sizable ones (a 10th of a mile across or larger), and it’s estimated there are hundreds of millions that are smaller, down to a yard in width.
Why should we be concerned? Because, writes Donald Yeomans, an object with a diameter of mile or more has “the capacity to wipe out an entire civilization in a single blow.” There are about a thousand such objects of that size out there, but no huge one is threatening us at the moment. So we can sit back and enjoy the light shows, when boulder-sized asteroids create blazing fireballs as they disintegrate in the atmosphere or tinier particles light up as shooting stars.
Yet despite the low probability of a devastating hit this century, Yeomans notes, such an event would be of high consequence. There was that little matter of the dinosaurs being exterminated some 65 million years ago in the aftermath of a catastrophic impact. As the science fiction writer Larry Niven once put it, “The dinosaurs became extinct because they didn’t have a space program.” That’s why it’s comforting to know that NASA has a Near-Earth Object Program Office looking out for us, which Yeomans manages.
These objects were not always near us. After the birth of the solar system 4.6 billion years ago, they gradually arrived as the asteroids in the belt between Mars and Jupiter collided with one another like cosmic bumper cars, throwing out fragments that headed toward the inner solar system and settled in. There are important scientific reasons to track them as well: “They are the remnants of the planetary formation process itself,” Yeomans writes, “and as the least changed bodies from that process they offer clues to the chemical and thermal conditions under which the planets formed.”
Nor was the Earth’s colliding with one always a bad thing. When solar-system debris was far more plentiful, the infant Earth was bombarded by myriad strikes, which furnished generous portions of carbon-based materials and water, the building blocks of life. One super-colossal hit gave us the moon to spoon under; the one that decimated the dinosaurs opened up the way for mammals (and eventually us) to reign over the planet.
Despite its title, Near-Earth Objects offers a concise and informative overview of the formation of the entire solar system: why the planets differ, the latest theories on how the planets lined up, and the origin of such leftovers as the comets and asteroids. Yeomans also makes a good case that a near-Earth asteroid is an accessible target for our next space adventure, readying us for Mars and preparing for the time in the future when we might depend on them as a source of rare minerals.
But concerns over the devastating potential of near-Earth objects remain the prime focus. By the 1990s, NASA had a goal of identifying the biggest threats, the ones that could cause global havoc. We’re talking worldwide firestorms, a blackened atmosphere that cuts us off from sunlight (stopping photosynthesis for food), acid rain and possibly herculean tsunamis.