If you watched either of the 1998 movies Deep Impact or Armageddon, you may have an idea of the future that faces Earth — or not — due to an asteroid collision. Is an asteroid collision with our planet a possibility? Can it happen in the near future, that is in our lifetimes, and can we detect it accurately today?

Just last week, two asteroids zoomed by planet earth, zipping between our planet and the moon’s orbit, according to NASA. Asteroid 2010 RX30, which passed over the earth south of Japan, is estimated to be 32 to 65 feet in diameter, and 2010 RF12, which reached its closest point to earth above Antarctica, is about 20 to 46 feet in diameter, according to NASA. Both were visible even with amateur telescopes.

Neither planetoid, which passed within 50,000 to 150,000 miles, was in danger of hitting earth, according to officials at the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge. “It happens a few times a year that they’ll come close,’’ said Tim Spahr, director of the Minor Planet Center. “When they come closer than the moon, we get a little interested in them.’’

Similarly, on July 9, 2002, the Lincoln Near Earth Asteroid Research Project (an MIT Lincoln Laboratory program funded by the United States Air Force and NASA) in New Mexico detected a 1.2-mile wide (2 km) asteroid. The asteroid has an orbit around our sun of 837 days and early calculations indicated that there was a small chance that this asteroid will collide with earth on February 1, 2019. Later observations ruled out the possibility of collision risks. Donald Yeomans, of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, later said: “An object of this size would be expected to hit the earth every few million years.”

So what are the odds for such objects to hit the earth? Small objects frequently collide with our planet. There is an inverse relationship between the size of the object and the frequency with which such objects hit the earth. According to Nick Bostrom, Swedish philosopher at the University of Oxford, asteroids with a 1 km diameter strike the earth every 500,000 years, on an average. Large collisions – with 5 km objects – happen approximately once every ten million years.

For example, the last known impact of an object of 10 km or more in diameter was at the Cretaceous-Tertiary extinction event 65 million years ago (that is the event believed to have made the Dinosaurs extinct).

Asteroids with diameters of 5 to 10 m (16 to 33 feet) enter the earth's atmosphere approximately once per year, with as much energy as Little Boy, the atomic bomb dropped on Hiroshima, approximately 15 kilotons of TNT. According to, Clark Chapman and David Morrison, at Planetary Science Institute and Space Science Division, NASA, the objects of these sizes ordinarily explode in the upper atmosphere, and most or all of the solids are vaporised.

Richard Monastersky, in an article in Science News Online (March 1, 1997) mentions that objects with diameters over 50 m (164 feet) strike the earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908. At least one known asteroid with a diameter of over 1 km, (29075) 1950 DA, has currently a possibility of colliding with earth on March 16, 2880, with a Torino Scale rating of two -- Tornio scale is between 0, negligibly small chance of collision, and 10, a collision is certain. It is used to categorise the impact hazard associated with near-earth objects. Finally, objects with diameters smaller than 10 m (33 feet) are called meteoroids (or meteorites if they strike the ground).

According to an estimate, some 500 meteorites reach the surface each year, but only five or six of these are typically recovered and made known to scientists.

Because the collision of large size objects is rare, the last time the rest of us really noticed something hitting the planet was in 1908 (called Tunguska impact), when an asteroid about the size of a football field exploded in earth's atmosphere with the force of a 10-megaton bomb, levelling an 800 square-mile (2,000 sq km) area of Siberia — big enough to engulf a city the size of New York. For comparison, the nuclear bomb the United States dropped on Hiroshima in World War II had a 15-kiloton yield.

Even if the chance is small for large asteroids to collide with the earth, it is definitely not zero and one cannot tell how a probability value will play out in the next moment in time. Most recently researchers at Tel Aviv University have got an initial clue that could help in developing an effective defensive strategy in case an asteroid was on track to collide with the earth. Although it was once believed that all asteroids are giant pieces of solid rock, later hypotheses have it that some are actually a collection of small gravel-sized rocks, held together by gravity.

If one of these "rubble piles" spins fast enough, it's speculated that pieces could separate from it through centrifugal force and form a second collection-in effect, a second asteroid. Now Tel Aviv researchers, David Polishook and Noah Brosch, in collaboration with an international group of scientists, have proved the existence of these theoretical “separated asteroid” pairs. Brosch said that separated asteroids are composed of small pebbles glued together by gravitational attraction.

Their paths are affected by the gravitational pull of major planets, but the radiation of the sun, he says, can also have an immense impact. Once the sun's light is absorbed by the asteroid, rotation speeds up. When it reaches a certain speed, a piece will break off to form a separate asteroid. As a result, asteroid pairs are formed, characterised by the trajectory of their rotation around the sun. Though they may be millions of miles apart, the two asteroids share the same orbit. Brosch said this demonstrates that they come from the same original asteroid source.

How do policymakers see the problem? Owing to a 2008 law passed by Congress, the White House Office of Science and Technology Policy in the US has until October 15 to decide which agency will be responsible for protecting the planet from an asteroid strike.

Members of the task force say NASA expects to be given part or all of that responsibility. To meet it, the panel discussed the creation of a Planetary Protection Coordination Office (PPCO) within NASA, with an annual budget of $250 million–$300 million. It would detect and track asteroids — and develop a capability to deflect them. “You want to use a proven capability when you're talking about an actual threat,” asks Rusty Schweickart, a former astronaut and the other panel co-chair. Canada already plans to launch the NEO survey satellite in 2011, and Germany's AsteroidFinder is slated for launch in 2012, but neither is expected to come close to the NEO-logging goal by 2020. According to an estimate by the radio station NPR, the US currently spends about $5.5 million per year to track NEO's and less than a million on researching ways to counter them, but is falling far short of asteroid-detection goals -- the amount needed to track all near earth objects by 2020 is $1 billion. However, unlike other problems like missile defence, asteroid detection and deterrence benefit all countries. So if NASA does detect a potentially dangerous asteroid, chances are it's probably going to hit somewhere else. And unlike global warming, smaller developing countries can't say that the United States should accept more of the blame for asteroids.

Scientists have been urging the United Nations to coordinate international asteroid detection efforts for years. But despite coordinating work by the UN Office for Outer Space Affairs, the progress seems to be slow.

Considering the currently weak international community's support for much more immediate dangers like global warming, it is not encouraging for those who would prefer not to rely on Bruce Willis (Armageddon) or Morgan Freeman (Deep Impact) when the big one comes without giving a notice suddenly one day.

The writer is doctoral candidate, Carnegie Mellon University, Pittsburgh, PA, and Knowledge Editor, Financial Chronicle