NASA has outlined plans for dealing with an asteroid a quarter of a mile wide that astronomers believe is on a near-collision course with Earth. The agency says there is no need at present to dispatch a radio transponder to the asteroid, but adds that it will be keeping a close eye on the rock's progress through the solar system.

Stargate fans will no doubt delight in the knowledge that the asteroid, which used to be known as 2004MN4, has now been dubbed 99942 Apophis.

If the current estimates are correct, the asteroid will miss our planet by just tens of thousands of miles, some time in 2029. While this is plenty of distance in everyday terms, in astronomical terms, it is a very near miss.

In addition, the proximity of the pass is likely to be such that the gravitational interaction between the asteroid and the Earth could alter the rock's projected orbit, making it more likely to hit us seven years later in 2036.

If it did hit, the damage would be substantial, but not deadly on a global scale. If it hit a city, there wouldn't be much urban life left, and if it landed in the sea, it would trigger a potentially destructive tsunami.

The space agency says that if the asteroid still appears to be threatening Earth by 2013, it will start work on a mission to visit Apophis with a probe in 2019. This would be followed by an attempt to deflect the asteroid some time between 2024 and 2028.

It also estimates that planning and executing a deflection mission would only take seven years from start to finish, rather then the 12 suggested by the B612 Foundation. This would make it unnecessary to mark the asteroid with a transponder before 2021, when a decision on deflection would have to be made, because by then scientists should have a far more accurate idea of the path Apophis is going to take.

The Deep Impact mission would serve as a model for the deflection attempt mission, although presumably the impactor would be considerably more massive than Deep Impact's 300kg chunk of copper.

NASA's plans were made public by the B612 Foundation, a group agitating for more action from governments on the threat from near-earth-objects. In June this year it asked NASA whether Apophis needed to be tagged with a transponder, and what NASA would do if the asteroid did turn out to pose a serious threat to Earth.

The B612 Foundation says that while it welcome's NASA's response to its questions, if it hadn't raised the question "the current understanding of the Apophis circumstances might not have developed. This is not an acceptable scenario". ®