Asteroid impacts: can we keep Armageddon at bay?

It's easy to dismiss asteroid impacts as sci-fi scare stories, but this month's near-miss raises the question: can we stay off the collision course? Jimmy Lee Shreeve reports
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The Independent Online

In most circumstances, 432,000km is a long, long way. But not when that figure is the distance between a large mass of fast-moving rock and planet Earth. The number in question comes from a "near miss" that happened earlier this month, when the asteroid named XP14 passed us at 17km per second.

To put it another way, it came within 1.1 times the moon's average distance from the Earth, close enough to be officially classified as a potentially hazardous Near Earth Object (NEO), along with some 782 known others. With an estimated diameter of up to 800 metres, had XP14 hit Earth it could have wiped out a small country.

Of course, this fly-by came as no surprise. Scientists tracking the asteroid knew it would not hit us. Astronomers have been tracking large asteroids for decades - the first near-Earth asteroid, Eros, was discovered at the end of the 19th century. But an asteroid would not have to be massive to pose a serious threat. If an asteroid as small as 40m across hit Earth, it would create a crater 1.25km across - just think if it hit a city. And these comparatively small chunks of rock are much harder to find - it is only recently that the hunt for them has begun.

In 1998, the US Congress directed Nasa to start an early warning programme to locate, by 2020, 90 per cent of NEOs measuring 1km or more. Nearly 800 have been found so far and it is estimated that there are around 200 or so to be located. But in December last year the goal was reset to locating the smaller objects, 140m or larger, still by 2020.

This is a two-edged sword for NEO expert Russell "Rusty" Schweickart. The former Apollo astronaut - and chairman of the California-based B612 Foundation, an independent organisation founded by a group of astronauts and scientists dedicated to finding ways to protect us against impact from space rocks - is pleased that the asteroid threat is being taken seriously. "My guess is that we're going to find thousands of asteroids that look like they might hit Earth," he warns. "But, due to lack of funding, Nasa isn't able to develop technology to deflect any oncoming asteroids."

Nasa currently receives approximately $3.5m per year to research the subject. Schweickart insists this is nowhere near enough if we are to properly protect ourselves. "It's a global threat that a lot of people are working on, but unfortunately not the government agencies," he says.

The irony for Schweickart is that it wouldn't cost a fortune to divert potentially hazardous asteroids. "It would cost a few hundred million dollars, which is what we spend on any run-of-the-mill space launch and there are several of those a year," he says. "This is a matter of protecting life on Earth, yet there is great reluctance to provide funding."

Current funding has succeeded in building the first of four dedicated telescopes. Part of the Panoramic Survey Telescope & Rapid Response System (Pan-Starrs) project, the Hawaii-based PS1 has just begun capturing its first test images. Once all four telescopes are up and running they will be capable of locating 99 per cent of NEOs larger than 300m.

Scientists around the world continually run calculations to predict the likelihood of the Earth and any given asteroid being in the same place at the same time. The orbits of asteroids are mapped against Earth's orbit to predict the probability or otherwise of collision. But even when the full map has been created, there will always be a need for monitoring, as asteroids on a harmless trajectory could be knocked on to collision course if they themselves are hit by objects in space.

The NEO that has scientists most worried is a 580m piece of rock called VD17. Based on current observations, this asteroid has a 1 in 1,600 chance of striking Earth in 2102 and a 1 in 500,000 chance two years later. Further observations, however, will refine the orbit calculation for VD17, and hopefully ease concerns.

Locating Near Earth Objects is one thing. But what do we do if an asteroid is found to be on collision course with us? Would we nuke it like Bruce Willis did in the 1998 movie Armageddon? "No, that would be a very bad idea," says Professor Alan Fitzsimmons of the Department of Physics and Astronomy at Queen's University, Belfast. He cites two methods that are considered the most viable. One has been dubbed the "gravity tractor", which was proposed by Nasa astronauts Edward Lu and Stanley Love. This would hover above the asteroid's surface, and the craft's thrusters would be angled outward to avoid blasting the asteroid's surface, while pushing it away. The small gravity attraction between the space tractor and the asteroid, which would be enough to hold the two objects together, would be used as a towline to slowly pull the object on to a non-threatening orbit.

The downside to this proposal, says Fitzsimmons, is that the space tractor would need a nuclear rocket to get it off the ground - and Nasa shelved its project to develop nuclear propulsion in favour of developing a replacement for the space shuttles. "But the simplest method," says Fitzsimmons, "would be to fly an unmanned spacecraft into an oncoming asteroid and knock it off collision course. The viability of this is being studied by the European Space Agency."

Given that the odds of being struck by an asteroid are relatively slim, is it worth worrying about at all? "The risk is not quite zero, so there's always a very small chance that a disaster will happen - and it could occur at any time," says Dr David Asher of Armagh Observatory, one of the UK's top astronomical research centres.

In 1908, at Tunguska, a remote area of Siberia, an asteroid thought to have had a diameter of 50m hurtled through the atmosphere at 11km per second and exploded 6 to 9km above the ground with a force of 20 million tons of TNT (the equivalent of 1,000 Hiroshima bombs). It is estimated that 60 million trees were felled over an area of 2,200 square kilometres. Miraculously, no one was killed. But according to Asher if such an object exploded above Hyde Park "it would wipe out everything within the M25".

"Current models suggest something the size of the asteroid that hit Tunguska impacts possibly every thousand years," says Fitzsimmons. "But some astronomers believe it could be more frequent - perhaps every 500 years."

The good news is we're getting close to detecting the vast majority of asteroids of more than 1km across - nearly 800 have been catalogued. "We've got about 200 or so to find before we know we're safe over the next hundred years or so," Fitzsimmons says. But could any of those that haven't yet been detected sneak in and take us unawares? "In theory, yes, one could take us by surprise. What you have to remember, however, is the larger asteroids only hit every million or so years. Which means we'll probably get away with it."

But does monitoring NEOs for a living mean you live in a constant state of worry? "I certainly don't lay awake at night worrying about it," says Kevin Yates, project manager of the Near Earth Objects Information Centre, which was set up as a public information resource in the wake of a UK government report into the potential threat from asteroid strikes. "The fact is, asteroids hits happen very rarely. So we will likely have enough time to develop the technology to do something about it. Although no one can be sure - especially considering we don't see the smaller ones until it's too late..."

The hit parade

* HIT: 65 million years ago

Asteroid crashes into Mexico. The "impact winter" leads to extinction of the dinosaurs.

* HIT: 50 thousand years ago

40m-diameter space rock plunges into Arizona desert, creating a crater 1.25km across.

* HIT: 35 million years ago

A comet or asteroid 4.8km in diameter strikes Earth in Chesapeake Bay, about 193km south-east of Washington, DC. The impact creates a 80km-wide crater that changes the courses of many rivers.

* MISS: 1 Sept, 2000

Asteroid 2000 QW7 comes within 3.8 million km of Earth - leading Liberal Democrat MP Lembit Opik to declare: "It's not a case of if we will be hit, it is a question of when. Each of us is 750 times more likely to be killed by an asteroid than to win this weekend's lottery."

* HIT: 6 June, 2002

An object with an estimated diameter of 10mdetonates in mid-air over the Mediterranean. The energy released was estimated (from infrasound measurements) to be equivalent to 12 kilotons of TNT.

* MISS: 18 Aug, 2002

Asteroid 2002 NY40, around 800m in diameter, passes within 483,000km of Earth, close enough to be seen with binoculars.

* MISS: 3 July, 2006

Asteroid 2004 XP14 passes within 432,308km of Earth.