Lakhsof and • still counting Asteroids are being discovered at an ever-increasing rate. The first was discovered way back in 1801. The XuYi observatory in China, with its one metre telescope has discovered over 300 asteroids in the past one year, writes C Shivaram

n recent times, two asteroids have been appropriate name as several of them be­ named after Indian scientists, Bhat­ have as 'half.<:omet' and 'half-asteroid'. Dur­ Itacharya and Goku Menon. These innu- ing the 1990s, the first of the TNO was dis­ merably present minor members of the covered, 1992QB, called cubewanos and solar system are now being discovered at an plutinos which have a 3:2 ever increasing rate. resonance with Neptune. The very first asteroid was discovered by There are other categories also like the Piazzi on January 1,1801 and named Ceres. objects (SDO) first of which It is still the largest asteroid known and was found in 1996 and asteroid families now quaIifies as a along with like Hirayama Pluto. Soon three more of the larger aster­ oids were discovered, mainly by Olbers. Near Earth Objects They include Vesta (the only asteroid visi­ Another class of asteroids which are of ble to the naked eye), Pallas and Juno. much interest are ones which come close to In 1868, the hundredth asteroid, Hekate the earth, so called Near Earth Objects was discovered. pioneered the (NEO). The earliest example was 1937 UB photographic technique and in 1891, made Hermes, first sighted in October 1937. Al­ the first photographic discovery of asteroid, most one kilometre in diameter, it is the 323 Brucia and the first as­ largest object known to have passed with­ teroid, 588 Achilles. in twice the moon's distance from earth. It This technique rapidly increased the again passed by five years later. Given its number of newly discovered asteroids and size and high velocity (> 20kms) it packs the by the year 1921, the thousandth asteroid Pi­ potential to cause massive devastation if it azzia was discovered. In the next 60 years, strikes the earth. i.e., by 1981, the ten thousandth asteroid, On October 15, 2003, it was again iden­ Myriostis was identified. By the year 2000, tified by the LONEOS search and was re­ at the start of the new millennium, the vealed to be a binary asteroid, with two ob­ number of asteroids had increased to one jects (half a kilometre across each) lakh, the hundred thousandth one being separated by barely a kilometre with an called Astronautical In the past eight years, ofl4 . ie from 2000 to 2008, the number of dis­ In all, 68 objects are known to have covered asteroids has increased by a factor passed within the moon's orbit, many de­ of nearly five. By November 2007, a grand tected after reaching perigee. No object total of 387,205 new objects (nearly four has been found to be on a collision course lakhs!) had been designated. with the earth. Apophis is expected to come within 30,000 km of the earth in April ~thi;;-30~OOO -k; of the ~arth in April Hundred ana going strong - 2029, but it is only thirty metres across. We An individual astronomer, Peter Birtwhis­ may recall that in 1994, pieces of tie, from England has discovered more Shoemaker-Levy collided with Jupiter. than hundred asteroids. His hundredth Fragment G, which had a diameter of three discovery, that of the main belt asteroid, km impacted Jupiter at 60kmjs. The ener­ 2008GE3 was made on April 7, 2008 and gy released was equivalent to a Hiroshima two days earlier, he had discovered two bomb exploding every second continuous­ other asteroids. ly for ten years. Several asteroids (well over a hundred) One such asteroid which hit the earth in are now !mown to be binaries and some of the Cretaceous period, 65 million years them rotate rapidly with a period of just ago, wiped out the dinosaurs causing large three hours. Pluto, now reclassified as a scale extinction. The frequency of such a dwarf planet, has been assigned the num­ collision is once in a hundred million years. ber 134,34D. It is now only one of several al­ There are dedicated telescopes monitoring most equally large bodies beyond Neptune, the NEOs and more than a thousand, the so-called Trans Neptunain Objects greater than a kilometre across, is identi­ (TNO), which now numbers nearly a thou­ fied. There have been several suggestions sand and come in all sizes, colours and for methods to deflect such asteroids if shapes. found to be on a collision course. The Xu Yi For 57 years, 944 Hidalgo, was the only observatory in China, with its new one me­ known asteroid object to travel beyond tre telescope has discovered over three Jupiter's orbit, until in 1977, Charles Kow­ hundred asteroids in the past one year. al discovered Chiron, orbiting between Sat­ New large telescopes planned like the LSST urn and Uranus. Around 50 objects have are expected to find few more lakhs of as­ been found which orbit further from the teroids. sun than Jupiter, yet closer than Neptune. (The author is with the Indian Institute of These objects are classified as Centaurs, an Astrophysics)

HOW MANY MORE? Several asteroids are now known to be binaries and some of them rotate rapidly with a period of just three hours. PHOTO: NASA