A threat from space?

Do asteroids hit Earth often?

Hundreds of them enter the atmosphere every minute. That's because our solar system contains several bands of space rocks — asteroids and comets of varying size and composition — which can veer off into new orbits that intersect with Earth's. Almost all the objects that reach our planet are mere inches in diameter and burn up in the atmosphere. A handful of times a year, space rocks up to 10 feet in diameter make it to the planet's surface. Once a millennium, an asteroid larger than 250 feet penetrates the atmosphere, causing major but localized damage. Every 1 million years, on average, an asteroid over a mile in diameter strikes Earth, with catastrophic consequences. An asteroid six miles in diameter is thought to have wiped out the dinosaurs 66 million years ago. If one that size were to hit Earth tomorrow, it would instantly erase all life within 1,000 miles of impact, and probably throw so much dust into the atmosphere that it would blot out the sun for years, killing off most species.

What happened last week?

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"Literally a cosmic coincidence, although a spectacular one," said astronomer Alan Fitzsimmons of Queen's University Belfast in the U.K. A meteor about 55 feet in diameter flashed across the morning sky over Chelyabinsk in Russia's Ural Mountains before exploding like a bomb in midair about 20 miles up. The shock wave blew out windows and doors and injured 1,200 people. That unexpected blast occurred the same day that an asteroid 150 feet in diameter, designated DA14, passed within about 17,000 miles of Earth's surface — a hair's breadth away in astronomical terms, and closer than many man-made satellites. NASA estimates that there are about 4,700 similarly sized asteroids within 5 million miles of Earth, and the impact of any one of them could easily destroy a major city or cause a 60-foot-high tsunami.

How worried should we be?

"I sleep at night, and you should too," said Timothy Spahr of the Harvard-Smithsonian Center for Astrophysics. The odds of an extinction-level impact within the next century are vanishingly low, and the odds are thousands-to-one against even a midsize asteroid impact. But scientists acknowledge that the risk is not zero. In 1994, Jupiter was hit by comet fragments up to 1.2 miles in diameter, causing a blast with a radius about twice the size of Earth. "That was a wake-up call," said Grant Stokes of MIT's Lincoln Laboratory.

How could we defend ourselves?

The first step is to track large asteroids, and NASA is on the case. Congress tasked the space agency in 1998 with identifying nearby asteroids over two thirds of a mile in diameter. So far, it has discovered 862 asteroids big enough to do serious damage to Earth. But we know little about thousands of smaller asteroids that pose a major threat; DA14, for example, was spotted only a year ago. NASA spends $20 million a year identifying and mapping threats, and the nonprofit B612 Foundation plans to launch a space telescope in 2018 that will orbit the sun in search of asteroids. The foundation's chairman, former astronaut Rusty Schweickart, figures there's a 30 percent chance the telescope will find an asteroid or comet on a collision course with Earth. If it does, the question becomes what we can do about it.

What about blowing it up?

Not a good idea. Trying to destroy an asteroid with a nuclear weapon would only turn a cosmic bullet into buckshot that could pepper our planet with smaller meteor strikes. Instead, scientists hope to change an asteroid's trajectory long before it reaches Earth. Some scientists have proposed spraying one side of an asteroid with white paint, causing it to absorb less solar heat on that side and changing its course over years. Another proposal would use a shield to shade the asteroid for the same purpose. NASA says the most likely option would be using an unmanned space vehicle as a "gravity tractor" to pull the asteroid off course, or just crashing the ship into the rock to ram it out of the way. "Ramming a spacecraft into an asteroid has still to be considered the reference solution for its simplicity," said Massimiliano Vasile of the University of Glasgow.

Are we equipped to do that?

The technology exists, but not the funding or an organized program. A deep-space mission to deflect an asteroid could cost as much as $80 billion — four times NASA's budget. It would require global cooperation, and we wouldn't have much time to argue: The closer an asteroid gets, the harder it becomes to deflect. A 2011 study by European aerospace firm Deimos found that deflecting a 300-foot asteroid becomes effectively impossible unless we act more than a decade before it hits. So if we had discovered last year that DA14 was aimed right at us, our only option would have been to evacuate the area it was projected to hit and cross our fingers. The near miss should be a call for action, said Schweickart. "We live in a shooting gallery," he said. "We don't think it's a good idea to put off protecting life on Earth."

The day Siberia shook

Last week's shock wave over Russia was child's play compared with the "Tunguska event" in 1908. A 120-foot meteor exploded a few miles above a remote Siberian forest, causing a blast estimated to have been around 185 times stronger than the one from the nuclear bomb dropped on Hiroshima. More than 80 million trees were flattened like matchsticks in a blast zone of 830 square miles. No deaths were reported, but owing to the isolated location, the first scientific expedition occurred only 19 years later. Witnesses then talked of seeing "fire in the sky," but were not eager to give more detail to visiting scientists. "They believed the blast was a visitation by the god Ogdy," said NASA scientist Don Yeomans, "who had cursed the area by smashing trees and killing animals."