Hallowe’en is supposed to be scary, but for those of a nervous disposition, October 31, 2041, will carry an extra thrill – the tiny risk of an asteroid impact.

Astronomers are keeping their eye on a space rock, labelled 2006 XG1, which has a one-in-48,000 chance of hitting Earth on that date.

Those are pretty good odds for a miss. But if it did hit, XG1 would blast a crater ten miles wide and half a mile deep. The explosion, equal to 1,700 megatons of TNT, could wipe out a city and cause devastation across hundreds of miles.

Apollo astronaut Rusty Schweickart told me that, although the risk appears small, XG1 is further proof that the asteroid threat should be taken seriously and preventative action taken.

What is likely, according to latest Nasa calculations, is that the asteroid will whizz past just 3,000 miles away – less than the distance from London to New York. That is a hair’s breadth, compared to the moon’s distance of a quarter of a million miles.

XG1 is thought to be half to two-thirds of a mile across, so even at that distance you would not be able to make out its shape with the naked eye and it would appear as just a fast-moving dot of light.

XG1 was discovered on September 20 by a robotic camera in Arizona that is working to scour the sky for asteroids called Near Earth Objects as part of the Catalina Sky Survey.

It was logged as a potential problem that should be monitored. But subsequent observations have not seen the threat disappear. The asteroid orbits the sun in a little under four years.

The threat posed by XG1 is higher than that from another asteroid called Apophis, 400-yards wide and weighing 25 million tons, which will fly closer than TV satellites on Friday 13th of April, 2029.

Experts say that if Apophis passes through a several hundred-yard wide “keyhole” on that brush with Earth, it will collide with us in 2036.

Nasa and the European Space Agency are both examining plans for Armageddon-style missions to change the course of threatening asteroids so that they miss us.

But Rusty, who flew on the Apollo 9 mission in 1969, told me: “All asteroids (124 as of today) with a non-zero probability of Earth impact are good reason that we should prepare to protect Earth from impacts.

“And given that we have knowledge today of only around two per cent of the total population of NEOs which can cause destruction on the surface if they impact, it is important that the subject be taken seriously.

“First, get the early warning system in shape, i.e. properly funded and programmed. Second, test/demonstrate the “existing” deflection capability (existing technology can protect against a subset of the overall threat) to build public confidence and learn by doing… and, get the advanced technology under development to assure a “full” capability.

“Third, get in place an agreed upon international decision-making capability re NEO deflection so that timely and coordinated decisions can be made.”

He added: “The biggest issue in all of this, in the US, is that Nasa is not assigned the responsibility to provide mitigation (i.e. deflection) against NEO impacts. Nor is anyone else. Therefore there is no budget allocation to pay for anyone to even think about this work, let alone fly a demonstration mission or start new technology.

“Until this assignment is made (to Nasa or someone) this work will not be done. This is the cosmic equivalent to knowing that the New Orleans levies are too weak… but no one has the responsibility. And unlike New Orleans/Katrina, we’re not talking about reducing the damage, we’re talking about preventing the disaster from happening in the first place.”

• For more space reading, plus other bargains, check out the Skymania store!