Somewhere out in space there’s a big rock that has our address on it.

Throughout geological history, our planet has been hit by a succession of major asteroids and the probabilities suggest further impacts will occur in the future.

No-one can say today when these might happen; we haven’t yet identified an asteroid of sufficient size and on a path that gives us immediate cause for concern.

If an object was sent to strike the asteroid, the rock's course could be subtly changed

But the evidence hints strongly that something could find us sooner or later, and we need to be ready.

On average, an object about the size of car will enter the Earth's atmosphere once a year, producing a spectacular fireball in the sky.

About every 2,000 years or so, an object the size of a football field will impact the Earth, causing significant local damage.

And then, every few million years, a rock turns up that has a girth measured in kilometres. An impact from one of these will produce global effects.

We know of some Near-Earth Objects (NEOs) today that are several km wide but fortunately none of them comes close enough to make us sweat.

The important thing is we keep looking. The US space agency’s NEO programme has been running since the late 1990s.

It was tasked with finding 90% of the potentially hazardous objects out there larger than 1km and it’s about 80% through this search. A few years back, the US Congress asked Nasa to extend the survey to include rocks down to about 140m in size.

That requires more and better telescopes and these are coming online. You will hear much more information on NEOs in the coming years because of this finer-scale sweep of the skies.

When a potentially hazardous rock is discovered, one of the best ways to determine its true status is to complete a study using radar, an extremely powerful tool.

Facilities such as the Arecibo Observatory in Puerto Rico or the Goldstone complex in California can pin down a rock’s key properties, determining its velocity to a precision of better than 1mm per second, and enabling scientists to compute its orbit hundreds of years into the future.

One of the more interesting facts that I became aware of recently is just how many of these objects are actually binaries – that’s to say, when the radar observations are done it becomes apparent that the asteroid is really two asteroids, or even a trio.

Contact binary: Toutatis (4.5km long) frequently gets to within a couple of Earth-Moon distances

Some of these are what are termed contact binaries – they touch each other. These are the objects that look like giant peanuts. (My favourite contact binary is the “dog bone” asteroid, 216 Kleopatra, although this is a long way from Earth and no threat to us).

About a quarter of all the objects investigated by radar turn out to be binaries of some kind.

So the inevitable question arises, what do we do if we find that huge rock with our address on it?

The powers that be are on the case. A lot of this work goes under the aegis of the United Nations, and in this context a meeting took place this week hosted by the European Space Agency.

The Mission Planning and Operations Group (MPOG) workshop included astronauts and space scientists.

It was the latest in a series organised to report to the UN's Committee on the Peaceful Uses of Outer Space.

The gathering of experts urged the world’s space agencies to improve their search and tracking capabilities, and to start developing concepts to deflect asteroids.

One of the leading figures in this initiative is the Apollo 9 astronaut Rusty Schweickart. He’s the chairman of the B612 Foundation, which campaigns on the NEO topic [PDF], and was one of the attendees at the MPOG meeting. He characterises the threat thus:

“At the upper end, you’re talking about wiping out the dinosaurs and most of life on Earth 65 million years ago; at the smaller end you’re talking about a million objects that hit the Earth last night – we call them shooting stars. It’s the objects in between that occur every few hundred years that we’re concerned with.”

Schweickart is convinced the solutions are within reach to deal with most hazardous asteroids on a collision path with Earth. In the majority of cases, the preferred concept would look much like Nasa’s Deep Impact mission of 2005 which saw a shepherding spacecraft release an impactor to strike a comet.

This gentle nudge, depending when and how it's done, could change the velocity of the rock ever so slightly to make it arrive “at the intersection” sufficiently early or late to miss Earth.

According to Schweickart, rear-ending an asteroid may be the easy part, however. Getting the world’s bureaucracy to act on the threat in a timely fashion may be the bigger challenge, he believes. And here’s why.

Consider the 300m-wide asteroid Apophis. For a while, before the calculations were detailed enough, there was some concern this object might hit Earth in 2036. The odds now are thought to be pretty slim.

But just imagine for a moment that it was headed right for us and we needed to do something about it.

Take a look at the map below. We know enough about the plane of Apophis’s orbit to understand where this rock would intersect the Earth, and it would be somewhere along the red line.

Now imagine the UN meeting convened to discuss whether the mission sent up to deflect the asteroid should try to slow or accelerate the rock. The choice is important because it would determine where on the line the rock would hit if the mission is not entirely successful in getting the asteroid to pass by the Earth.

In other words, one strategy chosen over the other would lessen the risks for some while increasing them for others.

So, you can bet Russia, Venezuela and Senegal would have very different views on which mission profile should be chosen.

That’s why Schweickart believes the geopolitical obstacles need to be tackled now and the mechanisms put in place to deal with thorny issues like the one I’ve just described:

“If we can get past that bureaucratic challenge, we can in fact prevent [large] asteroid impacts from hitting again in our future. This is an amazing and rather audacious statement to make, but if we really do our job right, we should never be hit again by an asteroid that can do serious damage to life on Earth.”