HALLOWEEN ENCOUNTERS
4 1/2 years ago the world was startled with the news that an asteroid was due to make an extremely close approach to Earth three decades later. Calculations had indicated that the asteroid in question, known as 1997 XF11 and having been discovered in December 1997 by James Scotti with the Spacewatch program based in Arizona, would be passing only 29,000 miles (just over one-tenth the moon's distance) from Earth on October 26, 2028.
This announcement in mid-March 1998 created a wide flurry of excitement, and images of 1997 XF11 were soon found on photographs that had been taken at Palomar Observatory in California in March 1990 (although the asteroid's images hadn't been noticed at the time). With a much longer baseline to work from, revised calculations soon showed that the "miss distance" at the 2028 encounter has been moved out to a somewhat more comfortable 580,000 miles.
Although the threat of a potential impact with Earth from this particular asteroid was thereby greatly diminished, the incident reinforced the thought that such a threat is very real, and has helped spawn new search efforts and procedures for identifying new threats. Nowadays significant numbers of near-Earth asteroids are routinely being discovered, and each of these is examined to see how close they might come to Earth during coming decades, with this information constantly being revised as new data becomes available. At this writing the closest "definite" forthcoming approach by a known asteroid within the next two centuries is by an object known as 2000 WO107, which will pass 50,000 miles from Earth on December 1, 2140.
1997 XF11 itself, meanwhile, has been fairly well studied since those exciting days in early 1998, and earlier this year it was assigned the permanent "serial number" (35396). Furthermore, at the end of this month it makes the closest approach to Earth that it will make prior to its 2028 encounter. Shortly after 5:30 P.M. MST on Thursday, October 30, 1997 XF11 will pass 5.9 million miles from our planet. It is approaching us from the sunward side, and doesn't become detectable until about the end of this week. Over the course of the next couple of weeks it emerges rapidly into the evening sky, and it should be visible in larger backyard telescopes as it travels through the constellations of Sagittarius, Capricornus, and Aquarius.
The Halloween season has produced other notable asteroid encounters in the past. Perhaps the most dramatic of these took place several decades ago when, on October 28, 1937, Karl Reinmuth at the Konigstuhl Observatory in Heidelberg, Germany, discovered a very fast-moving asteroid on a photograph he had just taken. This object showed up on a handful of photographs taken elsewhere in the world -- the earliest of these being on the 25th -- and it was detected for the last time on the 29th. This handful of observations over a span of only four days could not produce an especially accurate orbital calculation, but these were enough to show that the asteroid had passed just over 450,000 miles from Earth -- slightly less than twice the moon's distance -- on the 30th, and then had moved into the daytime sky. Reinmuth christened the asteroid Hermes, after the fleet-footed messenger of the gods in Greek mythology.
This approach was by far the closest-known encounter of an asteroid with the Earth at the time, and this record stood for over fifty years. Hermes itself, however, has been lost ever since those four days in 1937, despite the fact that its orbital period is only about two years. The search for Hermes has been somewhat of a "holy grail" among those who study near-Earth asteroids, and there has been hope that, with the comprehensive survey programs currently operating, sooner or later it would be re-discovered.
There is some reasonable possibility that this has now happened. On September 30 of this year the LINEAR program based at White Sands Missile Range in New Mexico discovered an asteroid, dubbed 2002 SY50, that is traveling in an orbit remarkably similar to that of Hermes, and moreover seems to be about the same size as Hermes. But because the orbit can produce close approaches to Mercury, Venus, and Earth -- which create gravitational effects on the orbit that can't be calculated precisely until the orbit is better known -- it is not possible at this time to conclusively state whether or not the two objects are identical. By early 2003 the observations should be extensive enough to prove, or disprove, their identity.
Regardless whether or not it is the long-lost Hermes, 2002 SY50 is also currently making a fairly close approach to Earth; shortly before 6:30 A.M. MST on Saturday, November 2, it passes 7.8 million miles from us. It is presently located in the constellation of Aquarius, and over this next week should be detectable with larger backyard telescopes as it travels westward through that constellation and the adjoining constellation of Capricornus. By mid-November it will have moved into our daytime sky and will no longer be visible.
A curious event for sky-watchers takes place around 2:15 A.M. MST on Friday, November 1, when 1997 XF11 and 2002 SY50, traveling in opposite directions in our sky, pass just twenty arcminutes (about two-thirds of the moon's apparent diameter) from each other. The two asteroids will be 1.9 million miles apart, and from here on Earth 1997 XF11 will be the nearer, brighter, and more southerly of the two. This meeting takes place near the star Gamma Capricorni, and while this will be below the horizon as seen from southern New Mexico, it should be visible from locations to the west. In any event, those who successfully observe these two asteroids during the next week or two can have the satisfaction of knowing they have (probably) spotted two of the objects that have played pivotal roles in our growing understanding of the asteroids that pass near Earth.