On average every year a small asteroid no more than four metres across plunges through Earth's atmosphere to disintegrate. Perhaps a few small chunks of rock reach the ground as meteorites. Scavengers and scientists go hunting for the debris, while everybody else just shrugs - until the day one of that rock's bigger cousins comes calling.

A summary of the near-Earth asteroids surveyed by NASA's WISE spacecraft. Image: NASA/JPL-Caltech.
 
NASA's WISE (Wide-field Infrared Survey Explorer) satellite recently conducted a survey of near-Earth asteroids survey of near-Earth asteroids, finding 93 percent of all space rocks bigger than a kilometre that come close to Earth. That's 911 out of an estimated total population of 981. An impact with something this large would devastate large swathes of the planet as debris billows into the atmosphere, blocking sunlight, causing crops to die and the food chain to break down - not to mention the large scale devastation across hundreds of kilometres where it makes landfall.

Then there are the mid-size asteroids close to Earth, between 100 and 1,000 metres across. If one were to strike, the regional destruction they would cause would be catastrophic. WISE was able to detect around 5,200 such asteroids, but another 14,300 are expected to lurk in space, currently hidden from us. Are any of these on trajectories to hit Earth? We don't know. As for the millions of near-Earth asteroids smaller than 100m, such as our annual four-metre wide visitors, or even the 45-70-kilometre asteroid believed to have exploded over the Tunguska region of Siberia in June 1908 (similar events are estimated to occur every two or three millennia), we simply have no way of tracking them all.

Where do these asteroids come from? While the bulk of asteroids are found in the Asteroid Belt between Mars and Jupiter, they are not constrained to this narrow zone around the Sun. Collisions with other asteroids or gravitational perturbations from Jupiter can push them out from the Asteroid Belt into a variety of orbits around the Sun. The ones that are potentially dangerous are those asteroids whose orbits cross Earth's own path around the Sun. Handily, they've been divided into sub-types.

A diagram showing the typical orbits of Aten, Apollo and Amor asteroids. AN graphic by Greg Smye–Rumsby.
 
Aten asteroids, named after the asteroid (2062) Aten that was discovered in 1976 by Eleanor Helin, have an average distance from the Sun that is less than Earth's, so for much of the time they are found safely tucked away inside the radius of Earth's orbit. However, like planets, asteroids orbit the Sun in ellipses, only often more exaggerated, meaning that they also have a closest point to the Sun (perihelion) and most distant point (aphelion) in their elongated orbital paths. Aten asteroids have their aphelion outside of Earth's orbit, meaning they have to cross our path to reach it. The most famous Aten asteroid is (99942) Apophis, a 325-metre space rock that will come within 31,300 kilometres of Earth's surface on 13 April 2029 - close enough to pass inside the geostationary ring of communication satellites. There's a one-in-a-million chance that Earth's gravity could perturb it sufficiently to crash into our planet when it revisits us in 2036, but those odds are so low as to be nothing to worry about nothing to worry about.

On the other hand, there are the Apollo-type near-Earth asteroids whose average distance from the Sun is greater than that of Earth, but for whom perihelion is inside Earth's orbit, which they cross more than once per year. The most famous examples are their prototype, (1862) Apollo, which was discovered by Karl Wilhelm Reinmuth in 1932, (1566) Icarus whose perihelion is closer to the Sun than Mercury (hence it shares its name with that famous mythological Sun-grazer) and the largest of the Apollos, ten-kilometre wide (1866) Sisyphus. However, more pertinent this week is a much smaller Apollo, a 40-50 metre wide asteroid designated 2012 DA14, which will make a close fly-by of Earth at a distance of just 28,000 kilometres.

The Herschel Space Observatory observed (99942) Apophis in infrared light during January, determining it to be 325 metres across. Image: ESA/Herschel/PACS/MACH-11/MPE/B Altieri (ESAC)/C Kiss (Konkoly Observatory)
 
"We will have an observational campaign around the time of its closest approach to get as much information about it as we can, both optically and with radar," says Lindley Johnson, who is the Programme Executive for NASA's NEA observation strategy. "DA14 is coming so close that at its closest approach it is moving too fast for our radars to even keep up with it."

But can we declare that DA14 absolutely is not going to collide with Earth? Johnson is assured that it is safe - this time around. "Since it was discovered a year ago we have well established the near term orbit and know how close it comes to Earth, so for this pass in February we know the orbit well enough," he says.

However, such close fly-bys inevitably result in Earth's gravity altering the asteroid's orbit, so what will that mean for DA14? "We'll need to track it as it moves away from Earth to determine how much it has been perturbed in order to establish what its future orbit is going to be," Johnson tells Astronomy Now. Now, in most of these cases where there is this closer pass, it perturbs the orbit so much that it is no longer going to be a hazard, but there is a small chance that it could actually put it in an orbit that brings it close to Earth again some time in the future."

Eventually these near-Earth asteroids, if they do not collide with us, will over the course of many millions of years spiral into the Sun or be ejected out of the Solar System by the Sun's gravity. That's not to mean the danger of near Earth asteroids will pass, for new bodies are continuously sent in-system by the gravitational nudges of the planets. For example, the Amor asteroids cross the orbit of Mars but don't quite reach Earth - asteroid (433) Eros, visited by NASA's NEAR-Shoemaker spacecraft around the turn of the millennium, is an Amor. But such asteroids don't remain Amors forever - these are the Atens and Apollos of the future (unless of course, they are captured by the gravity of Mars and become moons of the red planet, as in one hypothesis for the origin of Mars' moons Phobos and Deimos). Then there are the near-Earth comets, 93 of which are known at present.

Our fears of an asteroid colliding with Earth are a little over-hyped, for the biggest and deadliest of such impacts play out over timescales of millions of years. For instance a five kilometre asteroid is expected to collide with Earth on average just once every 20 million years. So it's unlikely civilisation is going to be wiped out by an asteroid any time soon. It's the smaller, more frequent ones we must look out for, which could take out a few city blocks or splash into the sea causing a tsunami that devastates many coastal regions. By studying harmless fly-bys such as that of 2012 DA14, we stand to learn much more about how these smaller near Earth asteroids act when they do come close to our planet, so we can better predict how they will behave in the future and, if the nightmare scenario does come to pass, give us time to figure out how to deflect them.