NEO stands for NEAR EARTH OBJECT. They are a subcategory of all the orbital junk in and passing through our solar system. The object itself could in theory be anything, but usually fit into one of two groups - Comets (known as NECs), or Asteroids (known as NEAs).

NECs are far less of a threat than NEAs - comets are comprised of dust and ice, usually formed in the outer lying areas of the solar system. Due to their composition they are far more likely to fracture and burn up in our atmosphere, and rarely if ever comprise any real threat.

NEAs on the other hand are far more serious. Made from varying densities of rock (and sometimes of iron), they were mostly formed in the inner sections of the solar system (such as what is now the asteroid belt) and represent leftovers from when the planets were first formed.

As of October 08, 2004, 3062 NEOs have been recorded (source: http//neo.jpl.nasa.gov), but not all really constitute a serious threat. Those that do are labeled PHAs, or Potentially Hazardous Asteroids. (They love these three-letter abbreviations, don't they?) PHAs are defined as any asteroid whose orbital path brings it to within 7.5 million kilometers of Earth. At the time of writing, NASA is aware of 639 PHAs. (source: http://neo.jpl.nasa.gov.) There are almost certainly many more that we don't know about.

This is where the problem lies: We simply don't know about all of them. The ones we do know about we're able to give a percentage of probability of impact, and although plenty are predicted to come close, none are predicted to impact. The problem therefore lies in the ones we don't know about.

To give an example: on June 14 2002, an asteroid approximately 80 meters across passed within 120,000km of the Earth. That's less than a third of the distance between the Earth and the Moon - in astrological terms, less than a hairs bredth. Had it arrived less than one minute later it would have hit the Earth at a speed of over 10km per second, destroying approximately 2000 square kilometers of land, or potentially causing a mega-tsunami had it landed in the ocean.

And here's the really scary part - nobody noticed it until three days after it had passed us by. (source:
http://www.newscientist.com/article.ns?id=dn2444)

Writing in the Guardian, Liberal Democrat MP Lembit Opik stated: "My own contribution to this field has focused on raising the importance of creating an early warning programme so that we know when we're in danger. This would require a network of six or seven telescopes spread around the world, methodically searching the sky for signs of rogue asteroids and comets that present a danger. Across 10 years we should have 90% of the major objects listed. Estimates vary between 2,000 and 5,000 of them, of which we aware less than half. Once we have a catalogue, we can see what's coming our way. Then it's a matter of deciding the best way to divert the object." (source:http://www.guardian.co.uk/
comment/story/0,,1315680,00.html)

If we are not aware of all the PHAs in the sky, there is always a risk that one will catch us unawares. We may discover one when it is only days or weeks away. If one were to come from the direction of the sun, just like a fighter pilot hiding from its enemy, it's entirely possible we wouldn't see it at all until it was only a few hours away - or indeed not at all. As Duncan Steel, Vice President of Spaceguard, states: "It's impossible to see asteroids coming from the day side of the Earth using ground-based telescopes. But we could see them using telescopes in space.

"The problem is that this is very expensive. At the current time, the money isn't even available to carry out the research on the ground." (source: http://www.bbc.co.uk/science/space/solarsystem/
asteroids/asteroid_experts.shtml)

STOPPING AN IMPACT

As Lembit Opik wrote in the Guardian: "Hollywood likes to blow asteroids apart 24 hours before they hit the earth. This doesn't work in reality. If you try to pulverise it you would probably still get billions of tonnes of space rock hitting our planet. It would just come down as smaller rocks instead of one big one. If anything, the damage would be even more extensive as the debris would come down across a wider area. So it's much better to use a rocket or an explosive to nudge the asteroid as a single unit so it misses us as a whole.

"Doing it 24 hours before a potential impact doesn't work either. We need 10 years or more in advance to have a good chance of giving these objects a small push that leads to big deviation over time. " (source:http://www.guardian.co.uk/comment/story/
0 ,,1315680,00.html)

The problem is the fact that you're dealing with something with incredible inertia within a zero-gravity environment. Remember, if you for example blow up the engine of a train, the train will still continue to move until gravity stops it or it becomes derailed and hits the ground. It wouldn't simply stop. In space, there is no gravity to stop it, and there is nothing to actually divert it from its path. The asteroid may be broken into fragments, but it will simply continue the same path it was taking before.

Also, if we're talking about using nuclear weapons here, those fragments would most likely become radioactive, producing an all-new hazard when the asteroid finally hits. It is technically possible to build a nuclear warhead so immense that it would be capable of simply disintegrating the asteroid to nothing - but a warhead capable of doing such a thing would also be as great a threat to humanity as the asteroid itself.

Mr. Opik's suggestion to nudge the asteroid is generally considered to be the only real sensible solution. Duncan Steel, Vice President of Spaceguard, speaking to the BBC on this subject, states: "It's unlikely that we'll need to do this within our lifetimes. That's because we probably won't find an asteroid on a collision course with Earth.

"However, if we did find one which was due to hit us in 10 or 20 years, it might be possible to divert it so it misses.

"Unfortunately, the only way we know of to accomplish this would involve using nuclear weapons. But it's not like in the movies. We would need to use a nuclear weapon in a 'gentle' way.

"We would want to give it a nudge so that it remained intact. Blasting it on its surface would simply shatter it into pieces. We would still be hit by the fragments. But it should be possible to give the object a sufficient shove with an explosion to get it to miss the Earth.

"The essential thing is we need lots of warning time, and that means many years. This is why a diligent search programme is necessary now. And it must be global." (source: http://www.bbc.co.uk/science/space
/solarsystem/asteroids/asteroid_experts.shtml)

There are other ideas being floated about - one involves placing a probe on the side of the asteroid and setting a lens, like a huge magnifying glass, to heat up and burn into the rock - the resultant expulsion of gas and debris being enough to push the asteroid a few milimetres off course. If done early enough, this will be plenty of room. Another involves robots burrowing into the side of the asteroid, and firing rockets to push it off course. Yet another involves attaching solar sails to the asteroid, and using the solar wind to divert it.

However, ingenious as some of these ideas are, scientists currently regard good old-fashioned nuclear warheads to be the most reliable. In replying to the solar sails idea, Duncan Steel states: "The reality is that if we did find [an asteroid] which was threatening us, we would surely use a proven technique. The solar sail idea is nice in theory. But in practice we wouldn't be able to gamble with the chance that it might not work.

"Dealing with asteroids is like dealing with cancer. The first step is a screening programme and it's unlikely that you will develop a particular type of cancer. But if you do, none of the solutions are pleasant. It's the same with asteroids. We wouldn't want to use nuclear weapons in space, but I believe it would be essential." (source: http://www.bbc.co.uk/
science/space/solarsystem/ asteroids/asteroid_
experts. shtml)