For decades, scientists have warned that an asteroid or comet could hit the Earth. As recently as 1908, a 40-meter wide object flattened 1600 square kilometers of forest in Siberia with the force of a 15-megaton nuclear blast. But it was neither the first time nor probably the last time that the Earth could be threatened by what astronomers call a ?near Earth object?. On Focus, VOA?s Victor Morales examines what most scientists fear could be the end of the world.

This time last month, scientists were debating whether an asteroid was about to hit the Earth. Some 30 meters in diameter, the space rock appeared in telescopes almost with no warning. Even so, astronomers soon computed the object?s path and determined that it would miss Earth by just 42,000 kilometers. It was the closest ever-recorded brush with an asteroid. But many scientists believe more objects routinely pass by undetected.

?There are objects that fly between the earth and the moon on a monthly basis,? says astronomer Richard Binzel of the Massachusetts Institute of Technology. ?An object coming as close as the one that came through last month is something that comes by maybe every couple of years. That fact that it happens isn?t particularly significant. What is interesting is that our search capabilities are just now getting good enough that we actually see them.?

And that?s what concerns many scientists. Although last month?s near miss didn?t pose a danger to our planet, astronomers warn that some of the hundreds of thousands of asteroids and comets orbiting the sun are certain one-day to crash into the Earth.

Scientists are trying to find and track the large asteroids -- those one kilometer in diameter and larger -- that could destroy all life on our planet in a fiery blast similar to the one that?s thought to have killed off the dinosaurs some 65-million years ago.

Donald Yeomans is a senior research scientist at NASA?s Jet Propulsion Laboratory in Pasadena, California. He says that since 1998, the National Aeronautics and Space Administration?s ?Spaceguard? program has spent about $ 25 million searching for these giant planet killers. ?The current goal, called the ?Spaceguard? goal, is to find 90% of the near-earth objects that are larger than one kilometer; that is, those objects that can cause global damage should they hit,? he says. ?And we hope to do that by the end of 2008. Because those objects are large and because they can cause global damage, over long, long periods of time they are far and away the most threatening.?

To date, scientists have found only one killer asteroid that could wipe out civilization, but it won?t happen any time soon. According to current estimates, asteroid number 1950 D-A has less than a 1% chance of hitting Earth 876 years from now.

But NASA?s Spaceguard program and similar projects around the world are capable of finding only most of the large near Earth objects. It will take new technologies and much more money to find the remaining 10% of the asteroids that could destroy our planet.

Then there are those near Earth objects smaller than one kilometer in diameter, which, except for bright comets, are nearly impossible to track with today?s instruments. An asteroid between 50 and 100 meters across, for example, could obliterate a city, killing millions of people. And astronomers estimate there may be hundreds of thousands of these medium-sized objects lurking in our solar system.

Scientists agree that the key to avoiding disaster is having enough advance warning that something is headed our way so that we can destroy it or push it out of the way.

?It?s like playing a game of billiards where if you can change the angle of a shot, a small change in the angle will cause your shot to miss the pocket by a wide margin,? says MIT astronomer, Richard Binzel. ?How you do that? There are a number of different scenarios. Attaching rocket motors to the object, having some sort of stand-off explosion, even painting the object a particular color and letting solar radiation pressure do the pushing for you. These are all possibilities. Given enough time, it doesn?t take that much of a push to get the object to ultimately miss the Earth if you have 10, 20, 30 years advance warning. It?s not an impossible problem.?

John Logsdon, Director of the Space Policy Institute at The George Washington University, says at this point, all scientists have are theories of how they might change an asteroid?s trajectory. ?We don?t understand asteroids very well,? he says. ?We don?t know what would happen if we push on them with a nuclear device or something else. So there?s a lot of work to be done.?

Most scientists agree that we would need decades of lead-time and a highly coordinated international effort in order to deal with an incoming asteroid or comet. ?There?s been a lot of attention to the issue among the concerned community,? says Mr. Logsdon. ?And the general consensus is that not enough attention is being paid at the government level. There?s no protocol for what you do when you find one. There?s a person in NASA to be notified, but then what happens is not clear.?

But Donald Yeomans with NASA?s Jet Propulsion Laboratory says there is a chain of responsibility that is being established as to who calls whom and when. ?As it turns out, I?m the person who is supposed to monitor the impact probabilities and notify NASA headquarters if we find an object that we consider Earth threatening,? he says. ?And then at NASA headquarters they would go up their chain of command and, if necessary, they would contact the appropriate officials in the various branches of the government.?

Statistically, a massive asteroid may strike our planet once every few hundred thousand years and near Earth objects large enough to destroy a city hit us only once every 10,000 years. And scientists are quick to remind us that Earth is inundated with tons of meteors and space debris every day, with few people ever noticing.

But according to Robert Jedicke of the University of Hawaii?s Institute for Astronomy, there is cause for concern. ?Most of the estimates right now put the risk of an individual human being dying due to an asteroid or comet impact at about equal to the risk of an individual dying due to a plane crash,? he says. ?And to me that?s pretty significant when you start talking about levels of risk like that. Then you can start thinking about how much money people and the government put into controlling and making flight and airplanes safe -- the Federal Aviation Administration, people buy flight insurance. There are a lot of things people do to try to ensure they?re safe on airplanes. So then you might be thinking that if the risk of dying due to a comet or asteroid impact is about comparable to the risk of dying in an airplane crash, maybe we should be thinking about comparable levels of concern.?

The hazards posed by asteroids and comets are small compared to the day-to-day threats many people face -- hunger, disease and war, for example. But nearly all scientists warn that someday a devastating near Earth object will likely come our way. And even though the risk is small, University of Hawaii astronomer Robert Jedicke warns that our planet may face even more dangers.

?There are people who claim that the risk of a super nova explosion somewhere in our solar neighborhood could cause a problem,? he says. ?There could be black holes that wander through our solar system that disrupt everything. There could be a faint star that we aren?t aware of right now that might come near our solar system in the next 50,000 years that could disrupt the Oort Cloud and send comets toward us. But I think that when you look at the danger measured in terms of actual loss of human life, I think the asteroid and comet impact scenario is probably one of the worst.?

Perhaps not tomorrow but a million years from now, it?s all but certain that civilization will be threatened by a menace from outer space. Yet there?s near universal agreement among scientists that given enough time and technology, humanity need not go the way of the dinosaurs.

All photos are courtesy of NASA.