The threat to Earth from space is minimal

Simon Briscoe and Hugh Aldersey-Williams

You may want to put this date in your diary: April 13, 2029. It’s a Friday. Friday the 13th. This is the day, Nasa announced four years ago, on which the Earth is most likely to be struck by a civilisation-destroying asteroid.

The space agency quoted odds of one in 300 – an unprecedented level of risk – that we would be hit by the newly discovered 2004 MN-4, a 400-metre-diameter chunk of rock orbiting around the sun. A few hours later, it markedly shortened the odds to one in 63. By the end of the day, the chance of the planet being largely wiped out stood at one in 45.

These may be long odds on a horse, but they are uncomfortably short when you consider what’s at stake. A modest 100-metre-diameter asteroid might kill 10,000 people in a land impact, but the tsunami it would cause if it landed in the sea might kill 100m.

There are about 3,000 designated “near Earth” asteroids at least the size of 2004 MN-4. And while we wait for the big one, there are also a billion objects out there the size of a bus.

One such is the military satellite USA193, which the Americans are apparently preparing to blow up with a high-altitude missile over the next few days to stop it crashing to Earth. Is this a precedent for dealing with killer asteroids? Are we going to fire off rockets into space to save the planet?

Until recently we simply did not know enough about asteroids to worry. Two recent discoveries have changed our perceptions.The first was the scientific confirmation in 1990 that an asteroid strike in Mexico was the most likely cause of the extinction of the dinosaurs 65m years ago. The second was the dramatic footage of the comet Shoemaker–Levy 9 breaking up and pummelling the planet Jupiter in 1994.

At the same time, improved astronomical observation is enabling us to spot many more asteroids – and realise that they pass very close to Earth.

But hold on a minute. Are we really in danger? Asteroids are just one of a catalogue of disasters that are supposed to await us. News stories frequently give the impression that life as we know it is about to end. The nature of the threat may change – bird flu, a wave of immigration, Aids, sea levels or an asteroid – but the threat is always there. We are happy to panic about the silliest things.

The tendency is not new. Charles Mackay’s Extraordinary Popular Delusions and the Madness of Crowds, first published in 1841, catalogued public obsessions with witchcraft, mesmerism and tulips, as well as fears of annihilation by everything from flooding to chemical poisons.

What’s new is not the public’s appetite for a good panic story or the media’s willingness to serve one up. It’s the role of other agencies. A generation ago, there were concerns in the world and they were reflected in the media; but the tone would have been sober, and behind it all would have been the reassuring presence of a paternalistic government.

Now politicians and government officials seem more likely to add to our sense of panic. And there are plenty of other highly vocal interest groups – scientists, health and safety nuts, corporations and their advertising agencies, nongovernmental organisations and lobby groups.

We never really learn the genuine extent of the risks; nor are we told what we as individuals can do to reduce them. We are merely told to be alert – that is to say, on the edge of panic.

So what is the real risk from outer space? Most asteroids are thought to be remnants of a failed planet. Small ones reach us all the time but burn up in the Earth’s atmosphere, where we see them as meteors or “shooting stars”. A meteor that survives is termed a meteorite when it hits the ground.

To pose a threat to life, an asteroid must be large. Fortunately, the abundance of asteroids decreases sharply at larger sizes, so while there are indeed many out there, the billion or so bus-sized objects represent a tiny minority of the total. Next, the asteroid must have enough energy to penetrate the atmosphere and do damage. This means that it must be massive, dense and fast-moving.

Most obviously, the orbit of the asteroid must coincide with that of the Earth. But space, as Douglas Adams pointed out in The Hitchhiker’s Guide to the Galaxy, is “big, really big”. So the chance of the path of one fairly small orbiting rock (the Earth) overlapping with that of a far smaller one (the asteroid) is always going to be extremely low.

In 1989 an asteroid missed the Earth by just six hours, which sounds close until you express it in distance – about 430,000 miles. That’s further than from the Earth to the moon.

Finally, even if an asteroid does strike, it may not have a catastrophic impact on us. Only a very rare large asteroid would have major consequences for humankind regardless of where it hit the planet’s surface.

The good news is that these probabilities, each tiny on its own, must be multiplied together to calculate the overall risk. Since all the fractional chances stacked up here are extremely small, the multiplied total giving the overall probability of a lethal impact is minuscule.

Asteroid 2004 MN-4, now renamed Apophis, is bigger than the meteorite responsible for the geological feature known as Meteor Crater in Arizona, and far bigger than the one that exploded in 1908 with the force of 1,000 Hiroshima bombs in the air above Tunguska, razing thousands of square km of Siberian forest. Yet when the Nasa scientists announced in 2004 that Apophis was heading our way, they seemed strangely keen not to alert the world to the danger.

“In all likelihood,” they announced, “the possibility of impact will eventually be eliminated as the asteroid continues to be tracked by astronomers around the world.”

They were right to be cautious. Scientists estimate an asteroid’s future trajectory based on observations of its orbit around the sun. Because of observational inaccuracies and limitations in computer models, this path is not a fine line but a three-dimensional swathe of space.

In the case of Apophis, somebody found some archived photographic plates of the asteroid dating from before its “discovery”. Measurements from these promptly reduced the risk of its hitting the Earth to zero.

Alarmingly, closer examination later revealed a bias in the measurements, which meant that the all-clear should not have been sounded at all. Two months after that the danger was finally eliminated with new data from the Arecibo radio telescope in Puerto Rico.

Apophis is not beaten yet, however. Its orbit crosses ours again in 2036 and it is currently given a 5,000-to-one chance of hitting us. This has led Patrick Moore, the astronomer, to predict that we may send up a nuclear device to deflect it from its course.

Launching a nuclear bomb into space carries its own risks. Some regard the supporters of this technological fix as fanatics. An even greater danger, according to Clark Chapman of the Southwest Research Institute in Colorado, comes not from a real asteroid but from intense panic spread by misleading reports of observations or predictions.

In the past many such false alarms have been avoided because scientists have kept apparently bad news to themselves until they had information to say that it was not news at all. In future it is increasingly likely that these alerts will leak out.

Instead of panicking, however, we will have to learn at last to make our own judgments about the real risks we face.