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NASA Searches for a Snowball in Hell
By MARK GAFFNEY
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Open any astronomy book and you will read that comets are dirty snowballsconglomerates of ancient rock and ice left over from the creation of the solar system. And it must be true, right? After all, it says so in the textbooks, and surely the university professors can't be wrong. The problem is that over the five decades since Fred Whipple first proposed the snowball model in 1950, neither NASA nor anyone has proved that comets are actually made of ice. Every time NASA scientists focus their instruments on the surface of comets, they see only rocky stuff. Comets look like asteroids. So, where's the ice? After failing repeatedly to find it, NASA has concluded that the ice must be hidden by surface dust, or is buried out of sight. Deep Impact will attempt to resolve this question by looking below the surface.
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On May 1, 1996 the Ulysses spacecraft documented another previously unknown feature of comets, when it crossed the tail of Comet Hyakutake at a point more than 350 million miles from the comet's nucleus. The ephemeral tail, in other words, stretched across the equivalent of three and a half times Earth's distance from the suna number that is astonishing. The discovery was accidental and wholly unexpected. Scientists had never guessed that comet tails were so long. Ulysses had been studying the solar wind, and so, had the necessary equipment on board to detect the ions typically associated with comets. The satellite also recorded the magnetic field directional changes that are associated with comet tails. Detailed analysis showed that both kinds of data were in agreement. For most scientists, this was enough to confirm the discovery. Notice, the remarkable tail length means that when Comet Hyakutake moved around the sun toward its minimum point (perihelion), the invisible portion of its tail arced across a vast reach of the solar system. The fact that the tail maintained its integrity at such extreme distance is incompatible with the standard view that the tail is composed of materials blown away from the nucleus. Something more is going on, here. The question is: What?
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But the big event, also in 1996, was the discovery of X-rays coming from the head of Hyakutake. This discovery set the scientific world on its ear, because naturally occurring X-rays are associated with extreme temperatures: in the range of millions of degrees Kelvin. Yet, here they were coming from a supposed ball of ice. There was no immediate word from NASA about how or why an icy cold comet could produce X-rays. The discovery was the work of the German ROSAT satellite, and no mistake about it. During the next few years X-radiation was detected in half a dozen other cases, including the Comet Hale-Bopp.
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Not everyone was surprised by the discovery of X-rays. One astronomer named Jim McCanney actually predicted them. He did so as early as 1981 in a scientific paper first published in the journal Kronos. McCanney even urged NASA officials to look for X-rays when the agency was preparing a fly-by of Comet Giacobini-Zinner in 1985. At the time, NASA's ISEE-3 satellite had already completed its original mission, and was being reprogrammed for comet study. The spacecraft had X-ray equipment on board, and McCanney urged NASA to use it. Instead, NASA shut down the equipment to conserve power. NASA's experts concluded that there was no point in leaving the X-ray detector on, since there couldn't possibly be X-rays coming from a cube of ice.
www.counterpunch.org/gaffney01292005.html
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