Star Points for January, 2008; by Curtis Roelle Asteroid Bearing Down on Mars Contrary to the popular belief that nothing ever changes in the sky there are lots of things – like big planets and small asteroids – constantly moving around up there. Most of the time they stay out of each other's way minding their own business. But astronomers' attention has recently been diverted to an asteroid that is predicted to pass so uncomfortably close to Mars on January 30 that odds for an actual collision have been calculated and then revised -- upward. The asteroid, known as "2007 WD5", is estimated to be 160 feet across and is closing in on Mars at a relative velocity of 28,000 mph – or nearly 8 miles per second. For comparison the Saturn V rocket's 3rd stage launched astronauts moonward at approximately 25,000 mph, a similar velocity. Initially, predictions were 1 chance in 75 that 2007 WD5 would actually impact the "red planet." That's far from a sure thing but as the asteroid drew closer the predictions were refined and updated. You may be wondering, "is it possible for an asteroid to strike a planet?" The answer is yes. An asteroid-like body has been blamed for the dinosaur extinction at the end of the Cretaceous period 65 million years ago and possibly other previous extinction events as well. It's not object wasn't definitely an asteroid: It could have been the nucleus of a comet. Comets and asteroids are closely related. Some comets may be masquerading as asteroids. We just don't know they're comets because they don't have tails or comas. We've seen with our own eyes what happens when a comet hits a planet. In 1994 more than twenty fragments from Comet Shoemaker- Levy 9 (SL-9) impacted the gas giant Jupiter. The resulting Jovian impact scars were clearly visible even in the smallest backyard telescopes -- like the one you might have sitting around the house somewhere. The energy released from the larger fragments was estimated as equivalent to 225 million tons of TNT! That's more than a thousand times as powerful as the atom bomb dropped on Hiroshima in World War II. Keep in mind that 2007 WD5 is smaller than the fragments from SL-9 that rained down on Jupiter. Should it impact Mars its explosive force has been estimated to be in the neighborhood of 10 to 20 million tons of TNT. That's still a very respectable sized blast. A similarly sized comet or asteroid is believed to have impacted right here on earth less than 100 years ago. The object entered the atmosphere and exploded in the air over a sparsely populated area in Siberia on June 30, 1908. The force of the blast cleared or flattened hundreds of square miles of forest and incinerated everything within 9 miles of the epicenter. Of course, Mars has no forests to be incinerated. Still, an impact of that size would excavate a crater up to one-half-mile across and hundreds of feet deep. The dust cloud generated by such an impact would be easily visible telescopically. So what are the chances of it hitting? At the end of December the odds tripled to a one in 25 chance that the asteroid might hit Mars. While this makes a collision far from a foregone conclusion it is significant. As January 30 gets closer new predictions will be made. Will the chances go up or down? Nobody knows yet but so far the chance of a hit is a long shot as the odds favor a miss. How exciting. Two opportunities to experience the night sky with members of the Westminster Astronomical Society are on Saturday, January 12 at Soldiers Delight Environmental Center in Baltimore County, and Friday, January 18 at Bear Branch Nature Center (BBNC) in Carroll County. For details visit the calendar page at WestminsterAstro.org.