Scientists have discovered yet another asteroid on a potential collision course with Earth - and it could strike within two years. But don't start burrowing into a shelter or cave just yet.

Asteroid 2006 HZ51, discovered last week on 27 April, is 800 meters in diameter - large enough to cause major damage if it strikes. Additionally, the lead time to prepare is the shortest of all known potential Earth-killers. Scientists have calculated 165 possible impact dates, and the soonest in 21 June 2008. However, scientists acknowledge that the odds of HZ51 actually hitting Earth are one in six million - slightly better than winning a lotto jackpot.

The discovery of HZ51 has brought home just how unprepared Earth is to deal with a disaster of this magnitude. At this point, the only thing people could do would be to stockpile provisions, evacuate the impact region, and ride it out. On the other hand, the second-most immanent threat is Apophis, which could hit the Earth in 2036, and has a much greater chance of doing so - about one in 6000. With three decades warning, it is possible to avert the impact altogether. The B612 Foundation has been working with Earth's governments to send up a tracking device to Apophis in the next few years.

Meanwhile, HZ51 has been under less than 24 hours of observation. The orbit of such a relatively small body is likely to change. Bottom line: don't lose too much sleep over this one.