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Odd object blurs line between comets and asteroids

News Team



Publicat Duminică, 30 Septembrie 2007, ora 09:14

      An odd object seen repeatedly plunging close to the Sun is likely a comet masquerading as an asteroid, scientists say.
     
      The object, called P/2007 R5, was first spotted passing near the Sun in 1999 and was seen again in 2003 by the Solar and Heliospheric Observatory (SOHO) spacecraft.
     
      Assuming that these appearances were two passes of the same object, Sebastian Hoenig of the Max Planck Institute for Radio Astronomy in Bonn, Germany, worked out its orbit and correctly predicted its return in September 2007.
     
      It has been officially designated as a short-period comet, defined to be a comet that is seen passing near the Sun more than once and that takes less than 200 years to complete an orbit. SOHO has discovered many comets, but this is the first confirmed to be on a short period.
     
      At the time of its discovery, however, some astronomers suggested P/2007 R5 might be an asteroid rather than a comet, because it did not have the classic comet features such as a tail and a coma – the cloud of gas and dust that surrounds a comet's body, or nucleus.
     
      It still has not shown any of these features, but on its latest passage near the Sun, when it was just 15% of Mercury's distance from the star, it underwent a dramatic change, brightening by a factor of a million before fading again.
     
      These huge swings in brightness are common for comets, but not for asteroids. They occur when the Sun's heat vaporises frozen water and carbon dioxide on the comet, a process that blasts dust off its surface. Sunlight reflecting off the dust increases the comet's brightness.
     
      The object's orbit suggests it is a comet from the Kuiper belt, a reservoir of icy bodies out beyond the orbit of Neptune, says Scott Sheppard of the Carnegie Institution of Washington in DC, US.
     
      Most of its surface ices were probably baked off during previous passes near the Sun, so that it now shows relatively little activity when it feels the Sun's heat, he says.
     
      Karl Battams of the US Naval Research Laboratory in Washington, DC, who runs SOHO's comet discovery programme, agrees. "It is quite possibly an extinct comet nucleus of some kind," he says. The observations suggest the nucleus is quite small, just 100 to 200 metres across.
     
      The object responsible for the annual Geminid meteor shower is thought to be another such extinct comet, called 3200 Phaethon.
     
      Although P/2007 R5 has lost most of its comet-like flair, it should consider itself lucky. Many of the comets SOHO spots approaching the Sun get lost in its glare and never emerge again, presumably because the Sun's heat and gravity causes them to disintegrate, Sheppard says.
     
      P/2007 R5 is not the only object that has been difficult to classify as either asteroid or comet. In the past few years, objects in the asteroid belt between Mars and Jupiter have been found to sport comet-like tails.
     
      "We're finding now that it's not so cut and dry - objects in the outer main belt of asteroids could easily have ices [in them]," Sheppard told New Scientist. "Some people are calling them 'main belt' comets."
     
      "There's probably not a sharp cutoff," he adds. "Comets probably have a lot of rock in them and asteroids probably have a lot of ice in them as well."

© Copyright News Team
Sursa :   NewScientist.com
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