Calm down folks, stop the packing, cancel the spaceship order. Asteroid 1997 XF11, discovered last December by minor planet watchers and predicted mid-week last week to be set to send us to oblivion in 2028, may not hit Earth after all. By Friday Hong Kong time, the latest computer arithmetic reckoned that this lump of cosmic rock, thought to be about 1.5 kilometres in diameter, would miss us at 2.30 pm local time, October 26, 30 years from now, by a 'comfortable distance'. That is according to astronomers working on a National Aeronautics and Space Administration project at the California Institute of Technology's Jet Propulsion Laboratory (JPL), who said it had a 'zero probability of impacting the planet'. JPL's Donald Yeomans and Paul Chodas, who spend all their time tracking comets, planets and asteroids and predicting their orbits, collected previous data on the asteroid's orbit and calculated that 'comfortable distance' at about 960,000km. (Once an asteroid has been spotted, its previous orbit path can be calculated and scientists can work out when it would previously have appeared on archive photos - this one was last seen in March 1990, and that is the data Dr Yeomans and Dr Chodas used.) That distance they calculated is more than twice as far as the moon is from Earth (about 385,000km). But it compares with a previous calculation of a decidedly uncomfortable 48,000km, which is only about seven times the Earth's radius and definitely close enough to cause us some trouble, particularly hefty tidal waves that would knock out vast swathes of coastline worldwide. By the time you read this, other figures may have come to the fore - or perhaps other asteroids. There are 8,319 comets and asteroids flying around the main belt of space debris between Mars and Jupiter whose orbits have been measured - and an estimated similar number of those yet to be classified: yet a recent three-year study by Robin Evans and Karl Stapelfeldt of NASA has identified about 300,000 small ones, which are the most likely to break out and head for us. This one, first seen last December 6 by Jim Scotti of the Spacewatch Program at the University of Arizona, is merely the 108th rock that has been classified, in NASA-speak, as a 'potentially hazardous asteroid' - a PHA, or a lump that might kill us - which whizz past us quite frequently. (There is a long and complicated explanation of how several hundred thousand lumps of flying debris are reduced to a list of 108 at the website of Brian Marsden, a Harvard-Smithsonian Centre for Astrophysics researcher, the address of which is given at the end of the article.) 1997 XF11 will come the closest predicted for the next 30 years, but the last one slipped past us on March 5, at a distance of about 17 million km, and the nearest one this year has already been and gone - on February 28, a mere 4.6 million km away. Eight will zoom by this year. The nearest recent miss was one that shaved us in December 1994, at a distance of a mere 105,000 km, but it was only about seven metres across: the biggest of recent years was in May 1996, flying past at a distance similar to that expected of 1997 XF11 - 'little more than the distance of the moon', according to Dr Marsden - and was many hundreds of km in diameter. And I bet you never even knew. There is increasing evidence that the Big One killed off the dinosaurs and created the enormous Chicxulub crater in Mexico: other scientists from NASA, an organisation never likely to miss a publicity opportunity, released more data on two new impact sites found about 200km from the rim of the Chicxulub hole in Mexico. That distance may seem a lot until you realise the crater itself is about that width, thought to have been formed by a comet of between 10 and 14km in diameter pounding into the Earth 65 million years ago. The environmental catastrophe wrought by the ensuing cloud of dust and particularly sulphur gases, which enveloped the globe and caused a decade of near-freezing temperatures, is thought to have brought not only the demise of the dinosaurs but also that of about three-quarters of species then on the planet. Scientists reckon that the Next Big One has to arrive eventually. The question for the many laboratories that watch them is, when? There are plenty of specialists looking for them, and watching those they already know. There is the formerly mentioned Spacewatch Program; the archive photos that helped calculate the orbit of the latest one are kept in something called the Palomar Planet Crossing Asteroid Survey - a self-descriptive project run by the California Institute of Technology's Palomar Observatory; the Hubble Space Telescope provides a valuable archive of pictures; there is the International Near-Earth Asteroid Survey, an expansion around the globe of the Palomar work; and there is JPL's Near-Earth Asteroid Tracking Program, which began in 1995 and works from the United States Air Force's Deep Space Surveillance Site on Mount Haleakala on the Hawaiian island of Maui. But what should we do when such a flying cannon ball is confirmed to be hurtling towards us? Scientists are working on various ideas, from knocking it out of the way with nuclear missiles - the problem being the contamination and the fragments of rock that could come raining down on us anyway - to trying to capture it for our own exploitation, since they are packed with valuable minerals. Science fiction? Well, a spacecraft is already on its way to meet the longest-known Near-Earth Object, Eros - discovered 100 years ago this August - late next year to take pictures and gather more data on its origin and composition. And in 2002, all being well, a rocket will head off to land a rover vehicle, a book-sized version of the Mars-probing Sojourner, on an asteroid called 4660 Nereus to collect three samples for return to Earth. The US-Japan project will launch from Kagoshima in Japan and the samples will be tested in the US and Japan. Since the rocks are thought to be remnants of the material from which the inner solar system - and our planets - was formed 4.6 billion years ago, the studies should help us find out about how the universe was formed. But of course the commercial space-mining possibilities are uppermost in many people's minds. But meanwhile we can heave a sigh of relief that we are not going to be wiped out by a rock in 30 years' time. Dr Marsden says there is a 'very good chance' that Eros will smash into us in the next million years. But with old things like warfare, new ones like the Millennium Bug, and even chloroform in cough mixture to worry about, it seems likely that something else will get us first. More information at http:// www.jpl.nasa.gov/releases/98/ and http://cfa-www.harvard.edu/iau/ pressreleases/, under which you'll find how the list of 108 PHAs has been calculated, and coming near-misses are at http://cfa-www.harvard.edu/iau/lists /CloseApp.html.