NASA is drawing up a shortlist of ideas to be unveiled early next year for diverting a 40m-ton asteroid that is on course to pass dangerously close to Earth.

The schemes will be presented and discussed at the annual meeting of the American Association for the Advancement of Science.

Fears that the planet may be in danger from asteroids were heightened by the discovery of one orbiting the sun that, on its present path, will pass within 22,000 miles — a hair’s breadth in astronomical terms — in April 2029.

Nasa’s idea is to engineer a minor shift in its trajectory that would make the asteroid miss Earth by a wider margin on this and all subsequent passes. Under one possible plan, a robotic craft would be sent to the asteroid to attempt to alter its course. One option might be to install a propulsion system on the surface to nudge it onto a new course.

The studies follow the discovery of hundreds of small asteroids orbiting the sun that repeatedly cross Earth’s orbit, raising the possibility of a devastating collision. The one causing most concern is a rock of more than 1,000ft called Apophis, the Greek name for the Egyptian god Apep, known as “the Destroyer”. It will come so close that it will pass under many satellites and may destroy some.

Astronomers fear that, although 2029 should pass without incident, coming so close to Earth might change Apophis’s 323-day orbit around the sun — during which it crosses the planet’s path twice — creating an even bigger risk in the future. A second close encounter is predicted for 2036.

Since Apophis was discovered Nasa scientists have been drawing up proposals for diverting it or any other asteroid that might present a threat. Nasa estimates that if it hit Earth it would release energy equivalent to the detonation of 880 megatons of TNT. The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.

One option, to be proposed by a former astronaut, Edward Lu, of Nasa’s Johnson Space Center in Houston, Texas, would involve building a heavy, nuclear-powered spacecraft to act as a gravitational tug. The spacecraft would hover over the surface of Apophis, using the asteroid’s gravitational pull to stay in orbit.

Lu calculates that a 20-ton craft gently firing its thrusters could safely deflect a typical 650ft asteroid in about a year, assuming there was 20 years of warning to launch and get into position.

Lu’s approach is far more cautious than that proposed by Hollywood in films such as Deep Impact or Armageddon. In the latter, the character played by Bruce Willis dies leading a team of astronauts who drill into an Earth-bound asteroid to plant a nuclear weapon that destroy it, and him along with it.

Lu and others say that such an approach would increase the threat by turning a single piece of rock into smaller chunks that could bombard the planet.

In reality the chances of a sizeable asteroid striking Earth in any given year are extremely small. Such impacts are thought to happen every 1,000 years or so, with the really big strikes — those that have a global impact — being separated by millions of years.

A near-miss by an asteroid codenamed QW7 in September 2000 prompted the 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.”

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