An Earth impact by a large comet or asteroid could knock out human civilization with a single blow, as most people are now aware thanks to recent Hollywood movies and public outreach by planetary scientists. Since 1998, when NASA initiated its Spaceguard program to find comets and asteroids 1 km in diameter and larger, researchers have made some crucial inventories of the risky space rocks with orbits that come into close proximity of Earth. For instance, there are almost 1,000 of these so-called near-Earth objects with diameters of 1-kilometer or more.

However disconcerting this might seem, we can rest assured that none will make it here in our lifetimes. “We can say with a very good deal of certainty that no asteroid or comet large enough to threaten life as we know it will hit Earth in the next 100 years,” says Donald Yeomans. At NASA’s Jet Propulsion Laboratory, Yeomans is a senior research scientist and manager of the Near-Earth Object Program Office. He has spent his career studying the physical and dynamical modeling of near-Earth objects, as well as tracking them down.

Yeomans works with an international network of professional and amateur astronomers who find and monitor asteroids and comets with orbits that come within approximately 0.33 AU, which is equivalent to 150 million kilometers. The team has identified 8,800 near-Earth objects as of early 2012, he noted during a talk at the American Museum of Natural History in New York on January 14 on his new book Near-Earth Objects, Finding Them Before They Find Us. The book gives readers an inside account of the latest efforts to find, track and study life-threatening asteroids and comets.

There are literally millions of asteroids and comets in the solar system, ranging in size from the microscopic to hundreds of kilometers in diameter. Both are made of rocky, metallic materials that failed to aggregate into planets during the early days of the solar system. Yeomans says the only real difference between asteroids and comets is that a comet actively loses its dust and ice when near the sun, causing a highly visible tail to form behind it.

Scientists have made exponential progress in identifying and tracking near-Earth objects in the past decade. NASA-sponsored near-Earth object surveys have found 90 percent of all asteroids and comets larger than a kilometer in diameter and projected their orbits at least 100 years into the future. Yeomans says the challenge now is finding all asteroids larger than 35 meters across, the size where one would pose a threat to a town or city, rather than all life on Earth.

Historically, Earth impacts by large asteroids and comets are rare. In addition, there is no clear record of a person being killed by one. Yeomans says that while Earth impacts by large asteroid and meteors are very low probability events, they are of very high consequence.

A prime example is just outside Winslow, Ariz., where a large crater was blasted into the Earth 50,000 years ago by a nearly 30-meter asteroid. Despite its relatively small size, the asteroid generated around 10 megatons of energy upon collision. By comparison, the atomic bomb dropped on Hiroshima during World War II generated around 0.02 megatons.

The asteroid that killed the dinosaurs 65 million years ago was much larger—a chunk of rock 10 to 15 kilometers across. The crater that formed when it struck near what is now the Yucatán Peninsula is 150 kilometers in diameter. The impact caused an immense explosion that deposited a layer of debris 10 meters deep as far as several hundred kilometers away from the impact and rained burning ash down on all corners of the globe. Most animals on the surface of the Earth died, and debris in the upper atmosphere launched the planet into a global winter. Many of the life forms that survived were either in the ocean or underground.

Today, if a survey detected a giant NEO headed for Earth, Yeomans says, humanity would have more than 50 years to prepare for it. He says a spacecraft could theoretically be used to divert such an asteroid off its Earth-colliding trajectory and out into space, and put in his plug for his employer, or at least organizations that support human ingenuity. “We have conceptual plans on how this could be done,” he says. “The reason the dinosaurs went extinct is because they didn’t have a space program.”