How dangerous are near-Earth asteroids? 5 key questions answered.

On Feb. 15, asteroid 2012 DA14, discovered a year ago, cleared Earth by a scant 17,200 miles. The same day, a smaller, unrelated asteroid that no one saw coming exploded 12 to 15 miles above Russia’s Chelyabinsk region. The shock wave shattered windows, injuring more than 1,000 people. Events that day highlight the risk that near-Earth objects (NEOs) can pose – although to some extent, humans can counter them.

1.What are near-Earth objects, and how big are they?

This image shows a simulation of asteroid 2012 DA14 approaching from the south as it passes through the Earth-moon system, last Friday. (JPL-Caltech/NASA/AP)

NEOs are asteroids and comets whose orbits bring them close to Earth. They range in size from about three feet to several miles across. The asteroid or comet that punched a 110-mile-wide crater in the Yucatán Peninsula 65 million years ago, doing in the dinosaurs, has been estimated at six miles across.

How many have been detected?

The path of asteroid 2012 DA14's approach to earth. (Courtesy of NASA/Reuters)

Initially, astronomers hunted for objects half a mile wide or wider and found 862 – about 95 percent of all NEOs thought to be in this size class. Since 1995, they have counted smaller objects as well and have found a total of 9,730.

Of them, 1,379 are potentially hazardous, according to specialists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. These objects are at least 150 feet across, and their orbits bring them 7.5 million miles or closer to Earth. Some 155 of the potentially hazardous objects found so far are at least a half mile wide.

In 2005, Congress charged the National Aeronautics and Space Administration with locating and tracking objects 500 feet across or larger by 2020. Progress has been slow. NASA’s NEO budget stayed flat at about $4 million between 2002 and 2010, although it was increased to $20.5 million for fiscal year 2012.

The smaller the NEO, the more of them there are. Astronomers figure they’ve spotted less than 1 percent of all the NEOs 100 feet or larger – which still can pack a wallop.

What's being done to improve detection?

The vast majority of detections have come from US efforts. Three US ground-based telescope projects are now operating: LINEAR (Lincoln Near Earth Asteroid Research), the Catalina Sky Survey, and Pan-STARRS (Panoramic Survey Telescope & Rapid Response System). The newest is Pan-STARRS, led by the University of Hawaii. The school is also setting up a network of smaller telescopes, dubbed ATLAS (Asteroid Terrestrial-impact Last Alert System), for quick scans of the sky each night.

A future possibility: infrared space telescopes dedicated to searching for NEOs. Infrared wavelengths are better suited for spotting NEOs that are too dim for visual telescopes to pick up. If funded, these telescopes could be launched toward the end of this decade.

How much time would we have to prepare for an NEO strike?

It could be decades or more if the asteroid is large and its closest passes are still distant. Or it could be no time at all, as the Chelyabinsk asteroid demonstrated. If an asteroid is detected only a year or two in advance of a collision, about the only option for reducing casualties involves civil-defense measures such as evacuations, researchers say.

What are other options for averting a collision?

One involves a space tug, which would use the mutual gravitational attraction between it and an asteroid to slowly change the NEO’s velocity so that it reaches the predicted collision point too early or too late. But it’s a decades-long process that would be effective only for asteroids no larger than about 300 feet across, scientists say.

Another approach involves the use of impactors, launched from a nearby spacecraft to slow or speed the NEO’s pace. NASA and the European Space Agency are studying the possibility of a demonstration mission dubbed AIDA (Asteroid Impact & Deflection Assessment). This would involve sending a spacecraft with an impactor to a binary asteroid, Didymos. The impactor would strike the smaller of the two asteroids, and the spacecraft would then measure the object’s velocity.

As a last resort, researchers have considered detonating a nuclear device to deflect objects up to a couple of miles across. For anything larger, no feasible defense is currently available, researchers say.