SPACEGUARD SURVEY PROGRESS: AUGUST 2005 David Morrison Next month, the number of known near-Earth asteroids (NEAs) larger than 1 km diameter (or, more precisely, brighter than absolute magnitude 18) should pass the 800 mark. As of August 8 the NASA NEO Program Office website [http://neo.jpl.nasa.gov] listed 793. The total number of NEAs discovered as of that date is 3496. If the population of NEAs larger than 1 km is 1100, 800 represents 73 percent completeness. It is interesting to note that 800 is already more than the total number of NEAs larger than 1 km predicted in some analyses carried out just a few years ago. For the metric of the Spaceguard Survey, which aims to find 90 percent of NEAs larger than 1 km, we are now more than 80% of the way to that goal. The recent pattern of discovery also shows an expected drop in the discovery rate for the larger NEAs. As the survey becomes more complete, there are fewer large NEAs to be found and a larger fraction of those detected are rediscoveries of asteroids already catalogued. The total number of NEAs larger than 1 km found each year has declined since 2000, with annual totals through 2004 of 131, 91, 101, 69, and 57. Although the number of large NEAs discovered each year is smaller, the total NAE discovery rate has continued to show a gradual increase. From 2000 through 2004 the annual totals are 362, 439, 485, 439, and 532. This shows that the discovery systems of the Spaceguard Survey have improved over time. However, the fraction of the new NEAs that are larger than 1 km has declined from about 25 percent of discoveries to near 10 percent. The reason we are finding fewer large NEAs is not a failure of the search systems but rather a real depletion in the population of undiscovered large NEAs. It is, thus, a measure of the success of the program. ================================ APOPHIS AND US New York Times Editorial
August 4, 2005 (Note: For background on the following editorial, see postings in the News Archive for July 22 and April 25, 2005). While the shuttle is in space and our attention is tilted upward, it's worth thinking about what else is up there, especially an asteroid named 99942 Apophis. When this object - a little more than a thousand feet across - was first discovered last year, astronomers estimated that it had a distant chance of striking Earth in 2029. After closer observation, it seems likely that this asteroid will still pass very near our planet, but without striking it. There is still a possibility, however, that as it swings by it may hit a gravitational "keyhole," shifting its orbit far enough to make it strike Earth in 2036. Beyond the question of whether Apophis deserves to be added to our regular list of doomsday worries, there's the practical matter of whether we can do anything about it. Hollywood heroics - trying to blast it out of the sky - might do more harm than good. NASA scientists think they have the technology to safely give it a nudge that might shift the orbit a few thousand feet. The trick is figuring out exactly when the asteroid will be in the exact place where such a modest bump would be effective. One possibility is to put a radio beacon on the asteroid, as if it were a member of a wolf pack in Yellowstone . That would let astronomers refine their predictions of the asteroid's potential orbit when it next approaches Earth, in 2012-2013. Given NASA's recent success in firing a probe at the comet Temple I, planting a beacon on an asteroid seems doable. You don't need to be a science-fiction writer to see a curious convergence here - the approach of a possibly Earthbound asteroid and the emergence of the scientific and technological capacity to cope with it. That, of course, is merely coincidence. Time will tell whether the risk from 99942 Apophis increases or whether it diminishes naturally, its orbit deforming away from harm, in ways that are well within the realm of probability. ================================== HOW TO SAVE THE PLANET: AN ASTEROID NAMED APOPHIS MAY PUT US TO THE TASK Leon Jaroff, Time Magazine on-line Posted Saturday, Aug. 13, 2005 For the vigilant and largely unsung astronomers who scan the skies for asteroids that could threaten the Earth, there is some good news and some bad news about 2004MN4. This was the initial designation for an asteroid that caused a flurry of alarm among scientists when it was discovered last December apparently heading for a frightening rendezvous with our planet on April 13, 2029. Astronomers figured that there was a one in 50 chance that MN4 would actually strike the Earth. Such an impact by an asteroid estimated to be as large as 1300 feet across could devastate a large region and perhaps, depending on where it hit, cause millions of casualties and untold billions in property damage. No wonder then that MN4 has been named Apophis, the Greek name for the Egyptian god of evil, destruction and darkness. But days after the initial discovery of the asteroid's trajectory, when astronomers found earlier, overlooked photos of the intruder in their archives and used them to refine estimates of its orbit, they were able to issue an all-clear. Apophis, it turns out, will come within as little as 15,000 miles from of Earth and will be visible to the naked eye in Europe and Africa on the evening of that April date, but will zoom safely past. Good news indeed. But there are still some reasons for concern. As it passes so close the asteroid, tugged by Earth's gravity, will change its orbital path. That could be very bad news. If the altered orbit results in Apophis passing through any of several "keyholes," specific regions of space only about 2,000 feet across, the asteroid would then return periodically to dangerously close encounters with Earth. Passage through the keyhole that astronomers think most likely to be the asteroid's target in 2029, for example, would bring it back to the near vicinity of Earth every seven years, beginning in 2036, posing a serious threat each time. This news was grist for the mills of the B612 Foundation (named after the fictional asteroid home of "The Little Prince," in Saint-Exupery's novel). The astronomers and scientists who founded B612 did so to alert Congress and the public to the menace of an asteroid strike and to lobby for a demonstration mission by 2015 that could show the feasibility of a controlled deflection of an object threatening to strike the Earth. In a letter sent last month to NASA Administrator Michael Griffin, former astronaut Rusty Schweickart, B612's chairman, called attention to the Apophis dilemma. He urged that a radio transponder, similar to those on commercial airliners, be landed on the asteroid so that astronomers might track its orbit precisely to determine if it will pass through a keyhole, and he requested that NASA quickly estimate the time required for both landing the transponder and a subsequent deflection mission that could alter the asteroid's orbit. Why the rush? The Apophis deflection, should it become necessary, must take place before the 2029 close approach. Earlier than that, just a simple nudge, accomplished, say, by firing a heavy object at the asteroid, could change its course enough to miss the crucial but small keyhole. Any time after that approach, should Apophis pass through the keyhole, we could be in trouble. NASA scientist David Morrison explains: "After 2029, the deflection would have to be vigorous enough to miss not just a tiny keyhole but the much larger target of the Earth itself. And such a deflection is far beyond present technology for an asteroid this large." Given that deadline, some 24 years from now, there's seemingly plenty of time to take action. But Schweikart, who admits he is not expert in mission planning, speculates that a transponder mission, from initial planning to implantation might take, say, eight years. And he thinks that a following deflection attempt, if it proves necessary, could require as long as 15 years to implement. That's cutting it a little close, and, says Schweikart, all the more reason that NASA quickly calculate some realistic mission times. "It may turn out," he says, "that we have to begin planning those missions right now." |
