INTRODUCTION

In Egyptian myth, Apophis was the ancient spirit of evil and destruction, a demon that was determined to plunge the world into eternal darkness.

A fitting name, astronomers reasoned, for a menace now hurtling towards Earth from outer space. Scientists are monitoring the progress of a 390-metre wide asteroid discovered in 2004 that is potentially on a collision course with our planet, and are imploring governments to decide on a strategy for dealing with it.

NASA has estimated that an impact from Apophis, which has an outside chance of hitting the Earth in 2036, would release more than 100,000 times the energy released in the nuclear blast over Hiroshima. Thousands of square kilometres would be directly affected by the blast but the whole of the Earth would see the effects of the dust released into the atmosphere.

And, scientists insist, there is actually very little time left to decide. At a meeting in 2005 of experts in near-Earth objects (NEOs) in London, scientists said it could take decades to design, test and build the required technology to deflect the asteroid. One expert, Monica Grady, an expert in meteorites at the Open University, said: "It's a question of when, not if, a near Earth object collides with Earth. Many of the smaller objects break up when they reach the Earth's atmosphere and have no impact. However, a NEO larger than 1km wide will collide with Earth every few hundred thousand years (on average) and a NEO larger than 6km, which could cause mass extinction (as they have 65 million years), will collide with Earth every hundred million years (very approximately). We are overdue for a big one."

Apophis had been intermittently tracked since its discovery in June 2004 but, in December 2004, it started causing serious concern. Projecting the orbit of the asteroid into the future, astronomers had calculated that the odds of it hitting the Earth in 2029 were alarming. As more observations came in, the odds got higher.

Having more than 20 years warning of potential impact might seem plenty of time. But, at December 2005 meeting, Andrea Carusi, president of the Spaceguard Foundation, said that the time for governments to make decisions on what to do was now, to give scientists time to prepare mitigation missions. At the peak of concern, Apophis asteroid was placed at four out of 10 on the Torino scale - a measure of the threat posed by an NEO where 10 is a certain collision which could cause a global catastrophe. This was the highest of any asteroid in recorded history and it had a 1 in 37 chance of hitting the Earth. The threat of a collision in 2029 was eventually ruled out at the end of 2004, but the risk of hitting the Earth in 2036 increased.

Alan Fitzsimmons, an astronomer from Queen's University Belfast, said in 2005: "When it does pass close to us on April 13 2029, the Earth will deflect it and change its orbit. There's a small possibility that if it passes through a particular point in space, the so-called keyhole, ... the Earth's gravity will change things so that when it comes back around again in 2036, it will collide with us." The chance of Apophis passing through the keyhole, a 600-metre patch of space, is 1 in 5,500 based on current information, but that is vastly higher than any other asteroid known.

There are no shortage of ideas on how to deflect asteroids. The Advanced Concepts Team at the European Space Agency have led the effort in designing a range of satellites and rockets to nudge asteroids on a collision course for Earth into a different orbit.

No technology has been left unconsidered, even potentially dangerous ideas such as nuclear powered spacecraft. "The advantage of nuclear propulsion is a lot of power," said Prof Fitzsimmons in 2005. "The negative thing is that ... we haven't done it yet. Whereas with solar electric propulsion, there are several spacecraft now that do use this technology so we're fairly confident it would work."

The favored method is also potentially the easiest - throwing a spacecraft at an asteroid to change its direction. ESA plans to test this idea with its Don Quixote mission, where two satellites will be sent to an asteroid. One of them, Hidalgo, will collide with the asteroid at high speed while the other, Sancho, will measure the change in the object's orbit. Decisions on the actual design of these probes will be made in the coming months and years, with launch expected some time in the next decade. One idea that seems to have no support from astronomers is the use of explosives (thermonuclear or otherwise).

Prof Fitzsimmons. "If you explode too close to impact, perhaps you'll get hit by several fragments rather than one, so you spread out the area of damage."

In September 2005, scientists at Strathclyde and Glasgow universities began computer simulations to work out the feasibility of changing the directions of asteroids on a collision course for Earth. In spring 2006, there was another opportunity for radar observations of Apophis that helped astronomers work out possible future orbits of the asteroid more accurately.

However, since they cannot rule out an impact with Earth in 2036, the next chance to make better observations will not be until 2013. NASA has argued that a final decision on what to do about Apophis will have to be made at that stage.

"It may be a decision in 2013 whether or not to go ahead with a full-blown mitigation mission, but we need to start planning it before 2013," said Prof Fitzsimmons. In 2029, astronomers will know for sure if Apophis will pose a threat in 2036. If the worst-case scenarios turn out to be true and the Earth is not prepared, it will be too late. "If we wait until 2029, it would seem unlikely that you'd be able to do anything about 2036," said Mr. Yates.

We're Doomed!

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Actual Photo Of Apophis

The asteroid danger is real and measures should be taken to prevent an impact with Earth. Discovered three years ago, the asteroid Apophis will pass exceptionally close to the Earth in 2029, only 24,000 miles away, which is where we have most of our communications satellites. Terrestrial gravity might cause this asteroid to leave its trajectory and collide with the Earth in 2036. There were plans to develop a space system that could protect the Earth from a potential asteroid impact... by 2040. Houston, we may have a problem?

Projected Path Of Apophis on approach in 2029.  White line is range of uncertainty.

Apophis orbit.

The Foresight encounter spacecraft, shown in this artist's conception, would help scientists determine the precise orbit of a potentially hazardous asteroid by flying beside it and sending back location data.

Chances are that the asteroid Apophis will sail harmlessly past Earth in the year 2036 — but just in case, space engineers have developed a prize-winning plan for monitoring the potential cosmic hazard.

The Foresight mission concept was the big winner in the Planetary Society's February 2008 contest to encourage methods for monitoring Apophis, which is currently given a 1-in-45,000 chance of hitting Earth on April 13, 2036.

Scientists say they can't yet eliminate the threat entirely because they don't know Apophis' orbit precisely enough. That's what prompted the Planetary Society, a nonprofit space interest group based in California, to offer $50,000 in prizes for proposals to "tag" the asteroid during earlier encounters.

What an impact in Europe would be like.