The solar system’s history is riddled with violent impacts. One good look at the Moon through a small telescope shows that. Whereas the airless Moon preserves its ancient cratering record almost perfectly, planets like Earth — with wind, water, and erosion — slowly cover the ravages of time.
Although a barrage of large impacts occurred early in the solar system’s history, a time referred to as the heavy bombardment era, significant impacts rocked Earth’s terrain in geologically recent times. For example, the K-T impact in the Yucatan Peninsula some 65 million years ago witnessed a 6-mile-wide (10 kilometers) asteroid striking Earth. That’s large enough to create a firestorm of hot debris. Falling back through Earth’s atmosphere, the debris ignited enormous fires and choked out most life.
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Even a smaller air-blast over Siberia in 1908, which occurred along the Tunguska River, felled 60 million trees over an area of more than 1,330 square miles (2,150 square kilometers). If the explosion had occurred near a populated city, the results would have been catastrophic.
Although most inner orbital debris was cleared out during our solar system’s early days, a huge population of near-Earth objects (NEOs) is still out there.
“Nature is blindly throwing rocks at our planet,” says astronomer Bill Cooke of NASA’s Marshall Space Flight Center in Huntsville, Alabama, “and once every great while manages to score a hit.”
Of course, the population of small objects is much larger than that of big ones, so small objects strike Earth more frequently. To find out how often our planet is struck, astronomers study the record of mass extinctions, the orbits of NEOs, and records of explosions in Earth’s upper atmosphere. Satellites record the amount of heat released from NEOs that explode in Earth’s atmosphere. Data from atmospheric explosions during the past 30 years show meteoroids erupting in the atmosphere produce at least one 5-kiloton explosion each year.
Scientists expect about one hundred 300-foot-wide (100 meters) objects will strike Earth over the next million years, while during that same time interval, two 0.6-mile-wide (1km) objects will hit. It’s these larger objects that, like the K-T impactor, pose a threat to civilization.
The destructive power a rock carries to Earth is directly proportional to its size. A 0.6-mile-wide rock, which strikes Earth every few million years, delivers a blow equivalent to 20,000 megatons of TNT. An asteroid only 300 feet across, however, can strike every 1,000 years and hit us with a 20-megaton explosion. “Put into more meaningful terms,” says Cooke, “a person living to the age of 100 has about a 1-in-10 chance of a 10-megaton Tunguska-like impact occurring somewhere on Earth at some time during his or her life. Of course, the odds of being hurt by such an event are vastly lower — by roughly a factor of 50,000 — because the asteroid would have to strike nearby to cause injury.”
Given the potential danger to the human race, aggressive NEO discovery programs are underway, led by LINEAR, the Lincoln Near-Earth Asteroid Research program located at White Sands Missile Range in New Mexico. Together with other surveys, the collective Spaceguard program has found more than 90 percent of the largest near-Earth asteroids, meeting a goal agreed to with Congress in 1998.
After finding NEOs, the real challenge is to identify their orbits and project them forward to predict possible impacts. New discoveries are forwarded to the Minor Planet Center in Cambridge, Massachusetts, where planetary scientists calculate orbits and publish the results.
Because humans now have the technology to find and observe NEOs doesn’t mean the historical record of Earth impacts will change. What should we do when we find an object that will strike our planet? Given enough time, scientists could send a spacecraft carrying a charge that would detonate near the asteroid and nudge it out of its Earth-impacting orbit. But if a long-period comet were headed toward us, we would have little notice. We would have to duck, cover, and hope for the best.