Los Angeles, CA – On the night of March 4, eyewitnesses reported a bright fireball tumbling through the skies over the United Kingdom. From the northern-most reaches of Scotland to southern England, the object fell, emitting orange flames, causing alarm and amazement to people watching on the ground.

At Kielder Observatory, in Northumberland, observatory director Gary Fildes was fortuitously showing a group of 40 people the observatory when the fireball made its dazzling appearance. “They went absolutely mental,” he told BBC News. “I was getting questions about what it is and is it going to end life on Earth? It was massively exciting.” Of course, this particular fireball didn’t “end life on Earth” – it was a meteor, where most of its mass likely burned up in the atmosphere.

However, a few days earlier a meteor that dazzled skies over Norway resulted in a small chunk of meteorite punching a hole in the roof of a garden shed in the capital city of Oslo. So although many space rocks burn up benignly in the upper atmosphere, such as the UK example, occasionally they make an abrupt (yet rare) appearance in densely populated areas.

Europe considers asteroid shield

These stories of mysterious meteors and surprise meteorites may not cause significant damage or threaten life on Earth, but they are a not-so-subtle reminder by the universe that there are a lot of errant chunks of rock floating around in the solar system that may not be so harmless.

Bigger, badder space rocks

Cue asteroid 2012 DA14, a 120,000-tonne chunk of space rock that recently grabbed the headlines. Known as a near-Earth asteroid (or NEA) with an orbit around the sun similar to Earth’s, 2012 DA14 was discovered to be drifting towards our planet on February 23 by astronomers at OAM Observatory in La Sagra, Spain. Naturally, any object flying towards our planet of that size was bound to cause some concern, but fortunately the asteroid will buzz past at a distance of around 17,000 miles in 2013 – at this altitude, the asteroid will fly under the orbits of our geosynchronous satellites. There is zero chance this particular asteroid will cause any death or carnage next year, so ignore anyone (or any headline) that suggests otherwise.

You may well breathe a sigh of relief, but what if 2012 DA14 had been on target?

Depending on its composition, a 50-metre-wide object would hit our atmosphere and a large amount of its remaining mass would impact the ground. This wouldn’t be an “extinction level” event by any means, but should an asteroid of these dimensions hit a city, say, a region could be devastated. Granted, most of the planet’s surface is uninhabited, so the risk would be low. But can you imagine the world patiently waiting for a year, doing nothing, waiting to get hit?

What if the asteroid were bigger, say, over 50 metres wide? Fortunately, the vast majority of “very large” NEOs are well known and NASA’s Wide-field Infrared Survey Explorer (WISE) has confirmed that the US space agency has discovered around 90 per cent of the largest space rocks that may cause a global calamity. All of the asteroids of “dinosaur-killing” dimensions (around 10 kilometres wide) are thought to have been discovered and are being closely tracked. None of these large asteroids are expected to bump into Earth for eons.

There’s more good news. WISE’s near-Earth object spotting programme – called NEOWISE – has found there are approximately 40 per cent fewer mid-range asteroids than previously believed. Mid-range asteroids are the chunks of space rock measuring between 100-1,000 metres across. It is also estimated that 93 per cent of near-Earth asteroids over 1,000 metres wide have been discovered.

So, we are getting better at spotting these near-Earth objects, and so far, there are few that pose any kind of hazard to our planet. But, occasionally, asteroids like 2012 DA14 make an appearance. Granted, they are much smaller than the well-known extinction-level asteroids, but they are big enough to still pack a hefty punch. Remember, the Tunguska blast in 1908 was caused by an object (an asteroid or comet fragment) of 2012 DA14’s dimensions exploding over a Russian forest. It flattened 80 million trees and released 1,000 times the energy of the atomic bomb that was dropped on Hiroshima in 1945. If a Tunguska-level event happened over London, say, we may be able to evacuate in time, but can you imagine the secondary devastation that would result from a global financial collapse? In this day and age, in which electronic money is the lifeblood of the global economy, any natural disaster can trigger chaos.

Impact mitigation

There are many theoretical ways to deal with the asteroid threat, however. The first asteroid deflection technique that probably comes to mind may involve blasting a group of oil drillers into space and asking Bruce Willis to bury a nuclear warhead into the asteroid. In reality, Bruce can breathe easy: we (probably) won’t need him.

Let’s assume we had a few years notice – a decade, say – of an impending asteroid strike. With a decade, there’d be enough time to mobilise the international community to deal with the threat. The urgency of an international mobilisation of assets would likely depend on how big the incoming rock was. The bigger the asteroid, the bigger the motivation.

Space agencies and space advocate groups – like the B612 Foundation, founded by ex-Apollo astronaut Rusty Schweickart – have a good idea about how to deflect an asteroid. One idea focusses on docking a spacecraft just next to the asteroid. Then, using an ion drive, the spacecraft will keep itself at a set distance from the rock. Over a few years, the tiny gravitational pull exerted by the spacecraft on the asteroid will impart a very small change to the asteroid’s trajectory. Catch it early enough, and the asteroid’s course may be changed just enough to deflect it from a catastrophic impact.

Another subtle, and surprisingly low-tech, solution involves painting one side of the incoming asteroid white (or black), changing its reflective properties. Once painted, sunlight will interact differently to whichever side of the space rock is facing the sun. As with the “gravity tug” idea, very small changes of a long period of time could modify the orbit. In a similar vein, small light-reflecting satellites may be put into orbit around the asteroid, shining light on one side, imparting a tiny radiation pressure on one side of the rock.

But say we didn’t have the luxury of time? Say if, like 2012 DA14, the incoming asteroid only gives us one year to react? As already mentioned, this kind of object would most likely be of “city killing” rather than “civilisation-killing” dimensions, but it doesn’t mean we would want to sit idly by.

In this case, we’d need more “bang for our buck”. Depending on the composition of the asteroid, we might choose to hit it with a kinetic impactor – basically a heavy spacecraft flown into the asteroid at high speed. We could launch several of these spacecraft and hurl them at the asteroid, nudging it off course. And if you think this idea sounds crazy, it’s not. NASA’s Deep Impact mission has already achieved this, slamming a 370-kg copper impactor into the surface of Comet 9P/Tempel in 2005. This impact didn’t have had a measurable effect on the comet as a whole, but as a proof of concept, it worked brilliantly.

NASA craft sends images of giant asteroid

But there’s a problem even with kinetic impactors. It turns out that many asteroids are composed of many small rocks bound together under mutual gravity. When you have a loose collection of debris – a “rubble pile” – the energy from our impactor would likely be absorbed, like a bullet shot into a sandbag. Little impression would be made on such an asteroid’s trajectory.

So we go full circle and roll out the nukes.

Weapons of mass (asteroid) destruction

There’s been much debate over the practical use of nuclear weapons in asteroid deflection techniques. Forget the “tricky” political and legal issues with unleashing a nuclear deterrent in space for now: the prime concern focusses on the unpredictable nature of attacking asteroids with nuclear missiles. What if the nuke blows the asteroid to smithereens, only to leave a cloud of debris to rain down on Earth? Granted, much of the debris will burn up, but multiplying the number of incoming pieces of rock may not be the best outcome. How big does the weapon need to be? Too small and all you’ve done is turn a deadly asteroid into a radioactive deadly asteroid.

However, according to new research from Los Alamos National Laboratory in New Mexico, the detonation of a one-megatonne nuclear warhead on the surface of a 500-metre-wide “rubble pile” asteroid would be enough to “fully mitigate” the threat. The shockwave that such a weapon would produce would pass through the asteroid material, imparting momentum to the “rubble”, blowing it to pieces.

Although nuclear weapons and kinetic impactors may sound like a quick fix, time would still be needed to build the necessary components and launch capabilities.

In short, to effectively deal with any asteroid threat, humanity cannot afford to react. Whether it’s a 10-kilometre-wide dinosaur-killer or a 50-metre-wide city-killer, we need an in-space infrastructure already in place to deal with the threat. Time is of the essence when it comes to asteroid impact mitigation, and although the skies seem clear for some time to come, wouldn’t it be better to have the assets already in space to deal with the problem?

As commercial space companies are beginning to prove, there is profit to be made from transporting cargo and people to low-Earth orbit. Creating an in-space infrastructure composed of space taxis, heavy-lift vehicles and space stations based on the business of making profit could ultimately provide a pre-emptive means to deflect the next space rock that threatens life on Earth. Failing the creation of such an infrastructure, wouldn’t it be a good idea to base an entire industry around safeguarding Earth?

After all, the statistics of the solar system dictate that if we do nothing, it’s not a question of if we’ll get hit by an asteroid, it’s a question of when.

Ian O’Neill is Space Science Producer for Discovery News. He is also the founder and editor of space blog Astroengine.

Follow him on Twitter: @astroengine