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A Monstrous Nucleus Hale-Bopp Observations with By studying Hubble Space images the have estimated that Hale-Bopp’s nucleus may be about Hubble and IUE Surprise 19-25 miles (30-40 km) in diameter. The average is thought to have a nucleus of about 3 miles (5 km) in diameter, even Astronomers smaller. The comet or that struck the 65 million years ago, possibly Hubble News, NASA causing the extinction of the dinosaurs, was probably about 6-9 miles (10-15 km) across. Completing an unprecedented year­ place, with “vents” being turned on and Because Hale-Bopp was unusually long study of Comet Hale-Bopp using two off as new patches of icy material are bright when it was still a great distance NASA , the Hubble Space rotated into sunlight for the first time. away, well outside the of , it Telescope and the International has given scientists their best view ever of Explorer (IUE), astronomers the changes in a comet’s nucleus as it gets report that they are surprised to find that A Complex, Mottled Nucleus closer to, and is progressively heated by, the different ices in the nucleus seen to be To their surprise, astronomers found the . Those changes, in turn, provide isolated from each other. They also report that water ice sublimates (turns directly information about the composition and seeing unexpectedly brief and intense from the frozen solid into a gas) at a structure of , which are believed to bursts of activity from the nucleus during different rate than the ices, implying be remnants from the formation of the the monitoring period. The Hubble that those components are not contained system, about 4.6 billion years ago. observations suggest that the nucleus is within the water on the comet. This conc­ Learning more about comets can provide huge, 19-25 miles (30-40 km) across. The lusion is further supported by Hubble data important information about the materi­ key results are described in the following. showing that the rate at which dust left the als and processes that formed the .

Fig-1 . Comet Hale-Bopp, , wide field plane­ tary caamera 2

Violent Eruptions on the Comet's nucleus was very different than the Surface sublimation rate of water. This result is The Space Telescope Science Institute is oper­ During the course of long-term contrary to previous models for a comet’s ated by the Association of Universities for observations, which began in August 1995, nucleus which suggest that the trace Research in , Inc. (AURA), for astronomers unexpectedly caught the components, such as carbon disulphide NASA, under contract with the Goddard Spa­ comet going through a sudden brief out­ ice, are contained inside the ce Flight Center, Greenbelt, MD. The Hubble burst, where, in little more than an hour abundant ice on the comet, frozen water. Space telescope is a project of international the amount of dust being spewed from the As water sublimates, the trace components nucleus increased at least eight-fold. It and dust should be released at similar cooperation between NASA and the Europe­ appears that the surface of Hale-Bopp’s rates, but this is not what Hubble an Space Agency (ESA).

Europhysics News 28 28 News 1997 Europhysics nucleus must be an incredibly dynamic observed.