The actual icy core of the comet, called its nucleus, can be anywhere from a mile across to 50 miles across for the rare 'super comets' like Hale-Bopp. By the time they enter the inner solar system and their ices begin to turn to gas, they billow like clouds into objects with cores thousands of miles across and tails that can stretch 10s of MILLIONS of miles as the ices in the nucleus are heated and give off gas and dust clouds and streamers.
Here is JPL Press Release on April 6, 2000. This story was also carried by many major news papers too:
During an unplanned rendezvous, the Ulysses spacecraft found itself gliding though the immense tail of Comet Hyakutake, revealing that comet tails may be much, much longer than previously believed.
"The odds that Ulysses' flight path would intersect the comet tail were probably less likely than someone breaking the bank at Monte Carlo," said Dr. Edward Smith of NASA' s Jet Propulsion Laboratory, Pasadena, CA, the Ulysses project scientist and a co-investigator for its magnetometer instrument. Before the unexpected encounter, Ulysses was hundreds of millions of kilometers away
from Comet Hyakutake and far beyond the visible tail.
"This tail extends half a billion kilometers (more than 300 million miles). That's more than three times the distance from the Earth to the Sun," said Dr. Nathan Schwadron, of the University of Michigan in Ann Arbor, a member of one of two Ulysses teams that made the discovery independently of one another. Findings from both teams appear in the April 6 issue of the journal Nature.
Copyright 1997 Dr. Sten Odenwald
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