On 3 November 2008, at about 10.30pm, a solitary asteroid whistled past the Earth, missing us by just 45,000km, about an eighth of the distance to the Moon. That’s a hair’s breadth in astronomical terms. It was only detected hours before its closest approach. And a month prior to that, another space rock actually hit the planet, exploding high in the skies over Sudan with just a day’s warning.


It was a long overdue call to arms. In December, mounting concern over the threat from asteroid impacts led the Association of Space Explorers, an organisation chaired by former Apollo astronaut Russell Schweickart, to lobby the United Nations for an internationally coordinated response.

“This is a natural disaster, which is larger, potentially, than any other natural disaster we know of,” Schweickart said at the time. “It is preventable – that’s a very important thing to keep in mind. But it is our responsibility to take action.”

The UN agreed that something had to be done. The first meeting of its Working Group on Near Earth Objects was being held in February as this issue of Focus went to press. By the time you read this, it’s expected that the UN will have called for collaboration between spacefaring nations to develop the technologies needed to mitigate the asteroid risk.

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“This has to be an international solution,” says Professor Richard Crowther, of the UK Science and Technology Facilities Council, who chairs the UN Working Group. “Not
just from the need to engage a large number of observatories, but in terms of the scale of funding needed for a deflection mission, and the political consequences of changing the path of an asteroid.”

A glance at the pocked and cratered surface of the Moon gives a hint at the ferocious barrage we face if nothing is done. Earth has been spared such scarring only because of its protecting atmosphere, which has cushioned it against the smallest impacts and smoothed away the marks from all but the biggest.

Asteroid impacts can be unimaginably violent. A space rock 250m wide travelling at a typical speed of 20km/s lands with the explosive force of 500 megatonnes of TNT. That’s over 30,000 times the power of the Hiroshima bomb. It’s over twice as powerful as the volcanic blast that ripped the island of Krakatoa apart in 1883, killing 36,000 people.

As asteroids go, 250m is relatively modest. And the ones that clipped Earth last year were smaller still, at just a few metres. But, in its time, Earth has been clobbered by monster rocks from outer space, some over 10km across. The event that wiped out the dinosaurs – and, indeed, 70 per cent of all life on Earth, 65 million years ago – is thought to have been caused by a 12km asteroid slamming into what’s now the Yucatan Peninsula in Mexico. The impact ignited firestorms around the planet and unleashed devastating tidal waves. This was followed by a freezing ‘impact winter’ as ash and debris flung into the air enshrouded the Earth, blotting out the Sun.

Examining the odds

Impacts of this magnitude are rare, happening on average once in 100 million years. But smaller strikes, still capable of causing localised devastation, happen with much greater frequency. In 1908, a 45m-wide object exploded in the sky over the Tunguska River in Siberia. The blast – estimated to have been between three and five megatons – would have been enough to flatten a modern city. The Earth is thought to be blitzed by a Tunguska-like impact every few hundred years.

To make the danger plain, astronomers David Morrison and Clark Chapman have carried out a risk assessment of the asteroid danger. They calculated how likely you are to be killed by an asteroid given the impact frequency of different sized objects and the number of deaths expected to result in each case. “The chances of an individual dying from the aftermath of an asteroid collision are about one in 10 million per year,” says Morrison.

Of course there are no records of death by asteroid. That may be because they’re low risk/high consequence events: the timescales involved are so long that a big one may not come along for thousands of years but, when it does, it kills millions or even billions. And those numbers conspire to give asteroid fatalities a worryingly high annual average between collisions. Multiply Morrison and Chapman’s theoretical odds by the Earth’s present population of 6.7 billion and – on average – asteroids claim nearly 700 lives a year.

But now, for the first time in history, the inhabitants of Earth have the power to defend themselves. A raft of astronomical instruments are steadily logging all the big danger asteroids, while new scientific studies are road testing the technologies that’ll enable us to hit back. The first step to defending against asteroids is having an early warning system in place that can spot them far enough in advance of impact.

The asteroid that exploded over Sudan in 2008 was only a few metres in size, which was why no-one saw it coming until the very last minute. But automated sky 3 3 surveys (see ‘Robo watch’ on p25) have now discovered at least 80 per cent of the most dangerous asteroids out there – those 1km in diameter and larger. Scientists know this because there’s a pristine record nearby: on the surface of the Moon. The distribution of lunar craters, perfectly preserved with no atmosphere or weather to erode them, tells astronomers the rate at which the Moon has been clobbered in the past. Comparing that rate with the number of asteroids already detected gives us a good idea of how many more are drifting in the darkness.

“We currently know of 765 near-Earth asteroids larger than 1km in diameter, and 5886 near-Earth asteroids of all sizes. When one adds in the near-Earth comets, one gets a total of 5968 near-Earth objects of all sizes,” says Don Yeomans, manager of NASA’s Near-Earth Object Program Office, at the Jet Propulsion Laboratory in Pasadena, California.

That’s already an awful lot of rocks to track. And yet Yeomans says it only represents a few per cent of the objects big enough to cause mass destruction – essentially, anything with a diameter larger than 30m. “Stony objects larger than 30m but smaller than 100m would be expected to cause airblasts with significant ground damage,” he says. “Smaller objects would not be expected to cause significant ground damage, unless the airblast was directly over a population centre.”
“Stony objects smaller than 30m would not be expected to cause significant ground damage, unless the airblast was directly over a population centre”
Vigilance is key. By carefully monitoring a new asteroid’s movement across the sky, astronomers can calculate its approximate orbit around the Solar System and the risk, if any, of it colliding with Earth. The probability of an impact taking place, together with the level of destruction that would result, gives the asteroid a threat rating from 0 to 10 on what’s called the Torino Scale (see right). A ‘0’ on the Torino Scale represents no threat, while a ‘10’ is a certain global catastrophe.

The highest an object has scored on the Torino Scale so far is the asteroid 99942 Apophis, which attained a Torino rating of four in December 2004. However, within a few days more precise observations downgraded it back to a zero. The highest rated object at present is asteroid 2007 VK184, which stands at a one. Only if anything scores an eight or higher is it time to divert or destroy the oncoming threat.

Batting an asteroid out of the sky might sound like Hollywood nonsense, but there are a number of scientifically sound options to do just that. These include ‘gravity tractors’ that gradually steer the rocks off course, and ‘kinetic impactors’ that slam a projectile right into them. The option you plump for depends on the circumstances.
“An asteroid impact is potentially larger than any other natural disaster we know of. But it is preventable. It is our responsibility to take action”
“Assuming that the warning time is fairly long, say several decades instead of a few years or less, then a gravity tractor would be best for the most likely threat – in the 50-100m range,” says Clark Chapman, at the Southwest Research Institute in Boulder, Colorado. “A kinetic impactor would be required for larger bodies, up to hundreds of metres in size.”

Coming around again

The nuclear option, so popular in the movies, isn’t one of Chapman’s favourites. This carries the risk of shattering the target asteroid into a blizzard of smaller splinters. Each splinter may be big enough to cause a catastrophic impact in its own right – either by ploughing directly into the planet or by being knocked by the force of the explosion into one of a number of so-called ‘gravitational keyholes’ dotted around the Earth. Any asteroid passing through one of these small regions of space gets yanked by the Earth’s gravity onto an orbit that takes it around the Solar System and right back to Earth for a collision at a later date.

“There are various ways to avoid nudging an asteroid through a keyhole,” Chapman says. “You do it carefully, for example with a gravity tractor; you be especially careful not to break off multiple big pieces; and you build in redundancy, so that if the operation fails, you can try again.”

Even though asteroid Apophis – the object once elevated to a Torino rating of four – will now miss the Earth during its close flyby in 2029, astronomers are still nervous that it could pass through a keyhole that will place it on a new orbit, bound for an impact with Earth in 2036. Faced with that catastrophe, it is crucial to know what the asteroid is made of.

“Science needs to know whether asteroids are solid pieces of rock or piles of gravel, what forces are holding them together, and how they will break apart if bombed,” says David Polishook. In December 2008, together with Noah Brosch at Tel Aviv University in Israel, Polishook perfected
a method of determining the composition of an asteroid by observing how its brightness rises and falls over time. “The information we are investigating can have a tremendous impact on future plans to alter the course of asteroids on a collision course with the Earth.”
“The information we are investigating can have a tremendous impact on future plans to alter the course of an asteroid on a collision course with Earth”

Testing the defences

The first prototype asteroid deflection mission could fly in just a few years’ time. The European Space Agency is busy developing its Don Quijote technology tester mission – a pair of spacecraft that will road test the kinetic impact strategy. One spacecraft will crash into a target asteroid at 36,000km/h while the other measures how the rock’s course, shape and spin are affected by the smash. British aerospace company QinetiQ carried out an early concept study for the mission in 2006 and Don Quijote is expected to launch sometime in 2011.

But could asteroid deflection projects have a dark side? The late astronomer Carl Sagan once warned that humans were making a big mistake in trying to fend off killer asteroids. He argued that any technology capable of shifting the orbit of an asteroid could potentially be turned into a weapon, and he grimly predicted that the danger from man-made impacts would then grossly outweigh that from natural ones.

Michael A’Hearn, an asteroid expert at the University of Maryland, agrees that the risk that humankind will turn space rocks into astronomical warheads is real. “It is not clear to me that the technology can be controlled better than we can control nuclear technology,” he says. “On the other hand, it might require so much lead time to use it as a weapon that there would be sufficient time to counter it.”

Asteroids may yet drive humans to even greater accomplishments. The surest way we can guard ourselves against the threat of impacts from space is to leave Earth and become a spacefaring civilisation. As the dinosaurs discovered to their cost, a species that confines itself to a single planet is placing all its eggs in one basket – one very fragile basket, in a deadly cosmic shooting gallery.

Nuclear assault

The preferred method of Bruce Willis and friends in disaster movie Armageddon, nuclear weapons may seem the surest way to halt a killer asteroid in its tracks. But in fact it’s not that simple. There’s a danger that the colossal energy released by a nuclear explosion will smash an asteroid into lots of smaller pieces. Earth still takes the hit, but instead of a single big rock it’s peppered by a storm of fragments. That’s fine if the fragments are small enough to burn up in the atmosphere. But if they’re bigger than about 30m across, then all you’ve succeeded in doing is turning a rifle bullet into a shotgun blast – equally deadly, but now even harder to deflect.

Kinetic impact

It’s one of the few impact avoidance schemes that has actually been tried out: take one spacecraft and ram it into your chosen asteroid as hard as possible. Known as the ‘kinetic impact’ strategy, it was used by NASA’s Deep Impact space mission in 2005 to gouge a crater in the comet 9P/Tempel. The purpose of that mission was to give space scientists a peak at a comet’s interior, which holds important clues to understanding the origin of the Solar System. But the mechanics of guiding a projectile into an Earth-threatening asteroid are much the same. The European Space Agency is now planning a mission of its own to try out kinetic impact on a near-Earth asteroid.

Space mirrors

We’ve all had fun setting fire to things with a magnifying glass on a sunny day. But could a giant magnifying glass in space be used to concentrate the Sun’s rays and frazzle incoming space rocks? Astronomers think so. A study in 2007 by Professor Massimiliano Vasile at the University of Glasgow found that a swarm of mirrors in space could focus enough heat onto an asteroid to start vaporising its surface. The flow of gases produced would act rather like a rocket engine, propelling the asteroid onto a new orbit. The 20m-wide inflatable mirrors would be carried to the asteroid by a fleet of spacecraft. It’s claimed that 10 mirrors could shift a 150m-wide asteroid in six months.

Gravity tractor

Isaac Newton – and later Albert Einstein – knew that heavy objects attract one another thanks to gravity. In 2005, that wisdom prompted astronauts Edward Lu and Stanley Love to wonder whether the same gravitational force could be used to pull hazardous asteroids out of the way of the Earth, a bit like a tow rope. Their idea, which became known as the ‘gravity tractor’, involves a spacecraft which flies alongside the target asteroid. As gravity tries to pull the two objects together, the spacecraft keeps its distance by firing its engines. The net effect is that the asteroid is gradually pulled off course. In reality it would take several years for a gravity tractor to move a large asteroid far enough.

Rocket motor

Various physicists have proposed strapping engines to asteroids to boost them away from Earth. All kinds of engines have been considered, from good old-fashioned rockets to solar sails (giant silver sheets that hitch a ride on the light streaming out from the Sun) and ion engines, which accelerate a jet of exhaust gas using electricity rather than combustion. A relatively low-tech alternative is known as a ‘mass-driver’. Here, a robot placed on the asteroid’s surface digs up chunks of rock and catapults them off into space. Just like the kick from a gun firing bullets, each chunk cast off produces a recoil, shoving the asteroid in the opposite direction. Over time, all the shoves add up to change its course.

Paint job

Nineteenth-century Russian engineer Ivan Yarkovsky knew a thing or two about asteroid decor. He worked out how you can alter the orbit of a space rock just by changing its colour. Asteroids absorb heat from the Sun and then radiate it away into space. This heat radiation carries with it momentum, and that in turn exerts a force on the asteroid that pushes it in the opposite direction. Different colours absorb and radiate heat at different rates – for the same reason a black T-shirt gets hotter on a summer’s day than a white one. And that means that the force on a problem asteroid – and therefore its orbit – can be kept in check just by getting the painters in.

Robo Watch

Machines have become our eyes in space

Gone are the days when astronomers would slave night after night at their telescopes to discover new asteroids. Now, our first line of defence against deadly space rocks are machines. “The vast majority of new discoveries come from a suite of automated surveys,” says Dan Durda, of the Southwest Research Institute in Boulder, Colorado.

The most prolific of these robotic observatories is the Catalina Sky Survey. This is a network of three telescopes at Mt Lemmon and Mt Bigelow, both in Arizona, and at Siding Spring, Australia. Each night the telescopes scour a section of the sky 4000 times the size of a full Moon to build up the big picture. Computer software compares new images with those taken on previous nights. Any differences, which could be down to the movement of an asteroid, are flagged for further analysis by a human astronomer.

Robotic surveys like this are essential. Most of the big asteroids (1km or more across) have already been found. Now astronomers are looking for the smaller stuff – rocks from 140m up to 1km. These can still cause considerable devastation, and it’s estimated that there are tens of thousands of them waiting to be found.

NASA has been given a congressional mandate to log 90 per cent of asteroids bigger than 140m by 2020. That will require even more sophisticated instruments, such as Chile’s Large Synoptic Survey Telescope (LSST) with its giant 8m mirror, due to begin operations in 2015. Tony Tyson, LSST director, says that a telescope this big will allow us to catch threatening objects in time. “Finding virtually all the remaining asteroids is feasible,” he says.

Paul Parsons is a science writer and author of The Science of Doctor Who


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Dr Paul Parsons is a science writer and analyst based in Aylesbury, England. He is a lapsed cosmologist, a former managing editor of BBC Sky at Night magazine, and once spent a summer working in a toilet roll factory. His books include 'The Beginning and the End of Everything', 'The Science of Doctor Who' and, most recently, '10 Short Lessons in Space Travel'.