In February 2013, a space rock about 20 meters in diameter (the dinosaur-killer, 65 million years ago, was 10 kilometers) exploded over the Ural Mountains, releasing about a half a megaton of energy. Most of that energy was released sideways rather than downward, so the damage was limited to broken windows and about 1,500 people being injured. It could have been much worse. In July 1993, a huge comet was broken up by Jupiter's gravity. Some of the scars from the pieces of that comet were larger than the radius of Earth. The next time there's a full moon look at it carefully. Its cratered surface is the result of asteroids banging into it.

The threat from near earth objects is clear and present. That's the premise of Near- Earth Objects, and I share it. Indeed I share nearly every policy conclusion Donald Yeomans draws: that we should be looking more closely for asteroids, that we should be planning to deflect them, that we should be willing to use nuclear weapons in that effort, and that we should spend more money on the problem. If we can spend trillions of dollars invading countries that pose little threat to us, we can certainly spend a few billion on a hazard that might kill Chicago, or worse. Experts who work on the NEO problem like to say this is the only natural disaster we could prevent. ("Natural disaster" isn't a very useful descriptor, but the point is still a good one.)

There are few people better positioned to make the case that the NEO hazard is important than Yeomans. He's a senior research scientist at NASA's Jet Propulsion Laboratory and has been working on NEO issues for many years. He's won many awards, has a PhD in astronomy, and can write well (for an astronomer). For all these reasons I recommend this book to anyone who is a novice to the NEO problem. You will find in it almost all you need to know about the space-related science concerning NEOs, in more-or-less accessible language.

This is the first book-length treatment (that I know of) of the NEO problem that aims to describe that problem to audiences not familiar with it. It only partially succeeds in doing that, mainly because the writing shifts between popular and technical voices. In one breath planets are anthropomorphized (Neptune is a wimp, Jupiter a bully), while in another there's a long, complex sentence that's hard to follow because of technical jargon. (For one example, see page 130: "If we then imagine a plane drawn through the Earth and perpendicular to the object's flight path with respect to the Earth, the three-dimensional uncertainty ellipsoid will project onto the Earth's impact plane as a two-dimensional uncertainty ellipse.").This will limit the book's audience, I fear, mainly to those already familiar with NEO issues.

Yeomans marshals every reason I've heard of to study NEOs, to visit them, and to generally move them higher up on everyone's political agenda. NEOs are interesting because they've been around since the beginning of the solar system. They're dangerous because they can kill us. They could be mined for precious metals. They could be a way-station on the trip to Mars. They may tell us something about the origins of life on Earth.

Yeomans argues that asteroids larger than one or two kilometers in diameter—which could do a lot of damage—are the "greatest long-term threat" even though they can be expected to happen only once every 700,000 years. What should we do? Study NEOs, be prepared to nuke them (think of Bruce Willis in Armageddon), attach a gravity tractor to them, or maybe run a spacecraft or two into one we think might hit Earth.

Near-Earth Objects lays out very well the space-related science concerning NEOs. It neglects all the people-related science, but that's not Yeomans' purpose or area of expertise. We can't fault him for not writing about things he doesn't know about. What we can legitimately complain about is that we don't get careful arguments against Yeomans' favored positions (which, remember, I share). This detracts from the book's effectiveness because we need explicit consideration of alternatives to justify the shifts in resources and thinking required to confront the NEO threat. To mention a few of the obvious ones:

• Given its abysmal performance leading up to and following Challenger and Columbia, why would we trust NASA to oversee something this important?
• The costs of sending people to asteroids will be enormous (Yeomans acknowledges this). Who can afford it?
• The risks to human life of mining asteroids will be very high. Why bother?
• Going to Mars isn't feasible (NASA says it takes 8 months for robotic spacecraft to get there) and solves no problems we currently have, though it would be interesting. Why is it worthwhile to spend attention on this?
• Why isn't it reasonable to think something that happens once every 700,000 years is so rare that it's trivial?

Of course, if an NEO with "our name on it" blows up over Paris tomorrow, we'll decry the folly of neglecting the NEO risk. Thanks to Donald Yeomans, we've been warned.

Lee Clarke, professor of sociology at Rutgers University, is the author of Worst Cases: Terror and Catastrophe in the Popular Imagination (Univ. of Chicago Press).

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