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 Defense Conference, organized 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," he wrote.
Many smaller objects around the Earth's orbit break up when they reach the atmosphere, with no impact.
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 390m wide asteroid which has an outside chance of hitting the Earth in 2036. If it did, it would release more than 100,000 times the energy released in the nuclear blast over Hiroshima. Thousands of square kilometers would be directly affected 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 would be destroyed.
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," he said.
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 different 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," he said.
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. Professor Hut calculates that such an engine could be used to deflect NEOs up to 800m across.