It seems that every few years a new asteroid is observed that's in such an orbit to cause it to eventually get uncomfortably close to our old planet. While most of the chunks of rock projected to give us a scare are on the small side (house size), a few have the potential to be truly devastating to Earth. We don't want to be too cavalier or nonchalant when we hear about these predictions, but we shouldn't stop what we're doing and be overly concerned about gloom and doom scenarios either.

About ten days ago, astronomers announced that on September 21, 2030, there was a 1 in 500 chance that the Earth would get nailed by an object spotted in orbit around the Sun (this date may coincide with the inauguration of our next president). However, just two days later, that projection was revised to a 1 in 1000 chance that we would get hit, but now the date was September 16, 2071. How could thing change so quickly?

The object in question (referred to as 2000 SG344) was discovered a little more than a month ago and is thought to be anywhere from 100 feet to 250 feet across, and it may be a small asteroid. These size estimates are made from the object's observed brightness and assumed reflectivity. It could also even be a discarded Saturn rocket from the Apollo program, and in fact, it's orbit is more suggestive of a man-made object. But it appears to be too faint to be a spent rocket, though, if this is the case, it would only be about 45 feet in length and would likely burn up in our atmosphere.

Astronomers use a 10 level category scale, known as the Torino Scale, to assess the impact hazard of near Earth objects (NEOs). As with the Richter Scale, which rates seismic events, the Torino Scale goes from 0 to 10, with 10 being the greatest risk for impact. If 2000 SG344 is closer in size to a 200 foot chunk, then it would be classified as a 1 (merits careful monitoring) on the Torino Scale, but if it's only about 100 feet across, it would be a 0 on the Torino Scale (the likelihood of impact is very close to zero). A 200 foot asteroid, if it impacted the Earth, might be comparable to the energy released in a large nuclear explosion. A collision capable of producing localized destruction somewhere on Earth is expected to occur between once about every 50 years to about once every 1000 years (category 8 on the Torino Scale). The Tunguska object that struck Siberia in 1908 would have been likely rated an 8 on the Torino Scale, if it had been observed a few months or more before impact.

Initial orbital computations indicated that in thirty years 2000 SG344 would make its closest approach to Earth. However, at that time it would likely still be almost 4 million miles from us - about 16 times the distance from the Earth to the moon. Nevertheless, this is near enough so that a minor perturbation in its orbit could send it plunging toward us.

Object 2000 SG344 orbits the Sun in 354 days with a perihelion of 86 million miles and an aphelion of 97 million miles. It's orbit is more eccentric than is ours and, on average, it's slightly closer to the Sun than we are. Currently, it's about 8.5 million miles from Earth. Because this object is inside our orbit, most of the time, it moves faster than we do, and consequently, it's presently moving away from us and won't be in our neighborhood for another three decades.

After the first announcement was made, additional observations of the path of 2000 GS344 improved upon the predictions of it's nearest approach, and it appears that it now won't bother us for 71 years. Ironically, it's now predicted to come as close as 2.7 million miles to the Earth (about 11 times the distance between the Earth and the moon). However, with more observations, the error limits improve, and thus even though it may come closer to us in 2071 than in 2030, there's a smaller chance that it would be perturbed in such a way to bang into us.

When a new NEO has been observed and when a few accurate positions of it have been accumulated over a period of weeks or months, a preliminary orbit is then computed. This orbit is then used to predict where follow up observations can be made in the near future to hone in on its exact orbit, size and mass. Furthermore, with the follow up observations, its trajectory can be calculated far into the future to see if it'll be a concern sometime down the road. Of course, the more follow up observations there are, the more certainty as to what the exact orbit looks like.

It needs to be pointed out that the chances of this object hitting us are less than the chances that we'll get hit by a yet undiscovered object, of comparable size, in any given year. Also, the chances that an object at least 1/3 of a mile in diameter smashing us in the next 100 years is about 1 in 10,000. So, are we likely to get hit by an asteroid sometime before 2100? The short answer is no - we shouldn't lose any sleep over it. Impacts of any kind have been rare, and catastrophic impacts, those that may effect world-wide climate, have been exceedingly rare, occurring perhaps once every several hundred thousand years or longer. Every single object that has been thoroughly investigated so far has been proven to be of no threat to the Earth's safety in the for-seeable future. With that being said, while we now have a pretty good inventory of all of the known near-Earth objects, we don't have a complete inventory. After all, who would have predicted that in the presidential election, with approximately 100 million votes cast nationwide, a couple of hundred votes in Florida could determine the outcome.

Actually, early tomorrow morning, there may be millions of dust-sized meteoroids bombarding Earth. This is the Leonid Meteor Shower that we talked about each of the last two years. Two years ago, the best viewing was in the Middle East, and last year, parts of Europe saw a number of impressive meteors. This year, with any luck, residents in the eastern and central US could be treated to a nice show. Look toward the east where the constellation Leo the Lion is rising in the early morning hours of Friday the 17th and Saturday the 18th. Unfortunately, the waning gibbous moon will be too bright to see many of the fainter meteors.

For more information see the Science Questions for December 10, 1996, November 12, 1998 and November 17, 1999 and the Washington Post for November 4 and November 7, 2000 (A section).