A huge asteroid hurtles in from outer space to devastate the Earth, an unstoppable force of nature from which there is no escape. Just such a catastrophe is thought to have killed off the dinosaurs, and, according to most experts, it is only a matter of time before a similar fate befalls the human race.

But perhaps not all hope is lost. Hundreds of scientists, from nuclear weapons engineers to planetary experts, are gathering in Washington this week to try to develop a master plan to protect the Earth from such an asteroid.

The Planetary Defence Conference, organised by the US Aerospace Corporation, will bring together scores of ideas on how to develop technology to track and deflect objects heading towards the Earth. The gathering will also consider the sticky problem of public relations - is it best to warn people if the worst comes to the worst?

"The collision of a moderately large asteroid or comet, also referred to as a near-Earth object (NEO), with Earth would have catastrophic consequences," writes Brent William Barbee of Emergent Space Technologies Inc in a discussion paper to be presented at the meeting. "Such events have occurred in the past and will occur again in the future. However, for the first time in known history, humanity may have the technology required to counter this threat."

Many smaller objects around the Earth's orbit break up when they reach the atmosphere, with no impact beyond a short fireworks display. An NEO wider than 1km, however, collides with Earth every few hundred thousand years and an NEO larger than 6km, which could cause mass extinction, will collide with Earth every 100m years. Experts agree that we are overdue for a big one.

All eyes for the moment are on Apophis, a 390-metre wide asteroid discovered in 2004, which has an outside chance of hitting the Earth in 2036. If it struck, Apophis 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 planet would see the effects of the dust released into the atmosphere. There could be dark skies for a year or more and crops worldwide would be destroyed.

Dr Barbee will present a nuclear solution to the problem of NEOs. Detonated at the correct position, a nuclear weapon could blast away a thin shell of material from the asteroid. "This virtually instantaneous blow-off of superheated mass imparts an impulsive thrust to the NEO in the opposite direction of the detonation coordinates, causing the NEO's subsequent trajectory to be altered slightly, which causes the NEO to miss Earth rather than collide."

The advantage of this idea is that it is possible with current technology - though no one has actually tried it yet.

Piet Hut, of the Institute for Advanced Study in Princeton, has a less dangerous idea - a robotic tugboat that could attach itself to an asteroid and push it out of the Earth's path. "Based on early warning, provided by ground tracking and orbit prediction, it would be deployed 10 years or more before potential impact.

The performance of the tugboat, he says, would depend on the development of a high-performance electric propulsion system called an ion engine. Instead of burning chemicals for fuel, these engines propel a spacecraft forwards by ejecting charged particles the other way. The thrust is minuscule - the equivalent to the pressure of a piece of paper on your hand - but the engine is extremely efficient and lasts far longer than conventional rocket engines. Prof Hut calculates that such an engine could be used to deflect NEOs up to 800 metres across.

Ion engines would also be crucial for another type of probe, the "gravity tractor". Instead of landing on an asteroid, though, the gravity tractor would hover near it, using the slight gravitational attraction between the probe and the NEO to change its path.

Aside from unveiling new technology plans, the meeting will also play host to psychologists weighing in with ideas on how to break the news of a potential catastrophic collision. Al Harrison, a social psychologist at the University of California, Davis, says an NEO collision would present unique problems for the authorities: there would be "significant warning time (not an unmitigated blessing), potential for extensive damage (near extinction level event) and a post-impact environment that could prevent full recovery".

The critical question psychologists will address is whether details of an impending impact should be kept secret, to avoid widespread panic. In December 2004, for example, scientists calculated that if Apophis were to hit it would land somewhere along a line that crossed central Europe, parts of the Middle East, the most populated district on Earth (the Ganges River valley), and on out across the Philippines. At the time, the information was kept secret and many NEO scientists agreed it was the right thing to do.

But Clark Chapman, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, says secrecy goes against the advice of many experts in risk management. "There are myths about the downsides of putting all information out, used as rationalisations by astronomers and space agency officials for withholding information, but which are counter to the policies of expert social scientists."

He says the perception that members of the public would immediately panic about an impending impact has no support from studies of social psychology. "If risk communication is done poorly, people may become unduly alarmed, they may lose faith in the veracity of official statements, they may misunderstand what's being communicated, they may ignore important warnings."

Footnotes

Near-Earth objects

These are comets and asteroids that have been nudged by the gravitational attraction of planets into orbits that allow them to come near the Earth. Most are made of ice and embedded with dust, or are bits of rock from the asteroid belt between Jupiter and Mars

Outside chance

Apophis had been tracked on and off since its discovery in June 2004. In December that year it started causing serious concern. Astronomers, projecting the future orbit of the asteroid, calculated that the odds of it hitting the Earth in 2029 were alarming. When it passes the Earth again on April 13 2029, the asteroid will be deflected by the Earth and alter its orbit. If that change makes Apophis pass through a particular point in space, the so-called keyhole, it will collide with the Earth on its next passage, in 2036. The chance of Apophis passing through the keyhole, a 600-metre patch of space, is 1 in 5,500 based on current information

Dark skies

Scientists have estimated the effects of a massive asteroid collision by examining simulations of what would happen during a big nuclear war

Slight gravitational attraction

Everything in the universe that has mass attracts anything else with mass via the force of gravity. If a gravity tractor is placed near an asteroid, the rock will move fractionally in the direction of the probe. Over millions of kilometres even a slight deflection could move the asteroid out of the Earth's way.