<p><font size="4">Apparently the current calculations on 2004MN4 Apophis trajectories don't include enough critical factors to understand the odds at predicting a collision in 2036. Namely, the trajectory calculation have not taken into account the earth's deviation from sphericity, nor have they taken into account the rotational rate of Apophis.</font></p><p><font size="2" color="#ff0000"><strong>Also note how important Arecibo is to understanding the risk from this close-approach asteroid.</strong></font></p><p> </p><div id="artHead" class="floatleft"><h2 class="inline">Killer asteroid predictions 'off by millions of miles'</h2><ul class="notlist straptext"><li>15 July 2008 </li><li>From New Scientist Print Edition.
<font color="#000000">Subscribe</font> and get 4 free issues. </li><li>David Shiga </li></ul></div><div id="artToolsTop" class="floatright"><div class="artTools">
http://space.newscientist.com/article/mg19926643.500 </div></div><div id="artBody"><p>YOU'D think that by now we'd have a firm grip on the trajectory of the solar system's most worrisome chunk of rock. In fact we have only a hazy understanding of how likely the asteroid Apophis is to strike Earth. What's more, budget cuts may shut down the telescope that could clarify the situation.</p><p>Since Apophis was discovered in 2004, asteroid-watchers have known that it has a slim chance of hitting Earth in 2036. At 270 metres wide, it is too small to rival the object that wiped out the dinosaurs, but it could cause devastating tsunamis were it to hit the ocean. Worrying as this is, we have been able to take comfort in the computed probability of impact, which is just 1 in 45,000.</p><p>Now it seems the true risk is unclear, thanks to minute effects that the calculations didn't take into account. "You really can't estimate the probability because it's driven by these unknown physical parameters," says Jon Giorgini of the Near-Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena, California.</p><p>Whether Apophis ends up on a collision course with Earth depends on what happens during a close encounter between Apophis and our planet in 2029. During this swingby, the Earth's gravity could alter the asteroid's path enough to put it on a collision course with us when its orbit brings it back in 2036.</p><div class="artquote">“Whether Apophis ends up on a collision course with Earth depends on a close encounter between them in 2029”</div><p>One problem, says Giorgini, is that our calculations do not include effects arising from the fact that Earth is not a perfect sphere. This slightly alters its gravitational field and could make a difference to the asteroid's path when it swings close to Earth.</p><p>Yet the most powerful steer could come from the way the sunlit asteroid radiates heat, says Giorgini. Radiation gives rise to a small thrust, and since warmer areas of the asteroid radiate more than cooler ones, there is a net force on the asteroid. This phenomenon - the Yarkovsky effect - means our calculations of Apophis's path could be out by millions of kilometres, according to Giorgini, who will present his results at the Asteroid, Comets, and Meteors conference in Baltimore, Maryland, on 17 July.</p><p>Unfortunately, we can include this effect only if we know how Apophis spins, which has so far been impossible to measure. <strong><font color="#ff0000">There will be a chance to do so with the 305-metre Arecibo radio telescope in Puerto Rico when Apophis comes within a few million kilometres of Earth in 2013.</font></strong> The observations made then may even lengthen the odds of an impact and show that the effects highlighted by Giorgini won't matter.</p><p>Yet the US National Science Foundation is considering cancelling the funding for Arecibo. Ironically, shutting down the telescope might require sending an expensive space mission to orbit the asteroid so it can be tracked. The US House of Representatives has directed NASA to plan such a mission, in a bill now awaiting Senate approval.</p><p>Alan Harris of the Space Science Institute in La Canada, California, says watching Apophis will reveal much about the limitations of our calculations of asteroid motion. He adds that the vast majority of asteroids that could present a risk to Earth have not been seen, and that one of these is much more likely to strike us before 2036.</p></div> <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>