[meteorite-list] Fred Whipple and Comet 17P/Holmes - Cannibalizing one's sibling(s)

From: bernd.pauli at paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: 27 Oct 2007 09:50:25 UT
Message-ID: <DIIE.000000C1000021D3_at_paulinet.de>

WHIPPLE F.L. (1985) The Mystery of Comets ( Smithsonian Library of the Solar System, pp. 175-178):

Even more interesting is the strange, perhaps unique, activity of a comet discovered
by the British astronomer, E. Holmes, on November 6, 1892. It was a member of Jupiter's
family with an orbital period of 6.9 years and perihelion distance of 2.1 AU.

The great comet observer, Barnard, was then at Lick Observatory. He had already
discovered fifteen comets and observed many more. Thus his description of P/Holmes
on November 9 is highly significant:

"Its appearance was absolutely different from any comet I have ever seen - a perfectly
 circular and clean cut disk of dense light, almost planetary in outline with a faint,
 hazy nucleus ... (brightness = Andromeda Nebula)."

By the next night, it brightened perceptibly and he saw an outer faint diffuse envelope
some 80,000 kilometers in diameter. The comet must have brightened about a hundred times
within a very few days before discovery. It was ideally placed for observation in the
northern sky, not far from the frequently observed Andromeda nebula, and should have
been discovered earlier unless it had been much fainter.

What distinguished P/Holmes besides its unique appearance was its rare variation in brightness.
It faded very little for nearly a month, its coma growing larger all the time. Then it plummeted
in brightness by perhaps 200 times. By January 15, 1893, it looked like a faint globular cluster.
On January 16, observers in Europe were astonished to find that the comet had almost regained
its original naked-eye brilliance. It then faded quickly and was last seen in 1893 during April.

When P/Holmes was recovered in 1899 and 1906 it was inactive, intrinsically nearly 10,000 times
fainter than in its moment of glory in 1892. No one detected it again until 1964, but it was
photographed in 1972 and 1979, having lost another factor of ten or so in brightness.

In 1981, after Thomas C. Van Flandern had revived the question of possible comet twins, I looked
into the theory of such orbital pairs and collected the many observations of P/Holmes in 1892-93.
I found, theoretically, that if the nongravitational jet action is greater for one than for the other,
their mutual orbit can twist around so that it becomes more and more elongated. The pair can finally
collide.

In the case of P/Holmes, the first collision could have been a grazing encounter as the two nuclei
spiraled together. On the next encounter, seventy-three days later, the collision could have been
more central and final. From measures of expanding shells near the nucleus, I found that P/Holmes
was rotating with a period of 16.3 hours, unchanged from the middle of November 1892 until well
past its second outburst in 1893. After the first outburst, only one area on the nucleus was active.

A second one, however, appeared after the second outburst. The phases of the active areas fitted the
theory of a collision scenario if the unobservable geometry of the orbit was correct. In this scenario,
the grazing collision set off a very active region on the major nucleus, and the final impact created a
second active area while reactivating the first one.

Hence, P/Holmes may well represent the collisional demise of a small satellite comet in orbit about
a larger nucleus. But we may never know for certain. Only one other comet has acted like P/Holmes.
This is the faint comet P/Tuttle-Giacobini-Kresak, a member of the Jupiter family with a period of 5.6
years. In 1973, it exhibited two gross outbursts of some 4,000 times in brightness, separated by about
forty days. Since then, it has been very faint and inactive. It, too, may have been a double comet in
which the larger of the twins cannibalized its sibling.

Whatever the true cause of comet splitting, and whether or not some comets are double, evidence
abounds that our dirty snowballs may have shapes other than round spheres. Their surfaces are clearly
not uniform in composition, even over small distances. Some areas seem to be very active and some
very dusty but still active; still others are probably covered with rocky meteoric material. As the ices
sublimate from more active icy regions, they may leave behind large grotesque formations. The Space
Age gives us hope that someday we may see televised pictures of comet nuclei and thus be saved from
further speculation, at least about comet landscapes. This hope is bolstered by the flood of new
knowledge about comets that the Space Age has already given us.
Received on Sat 27 Oct 2007 05:50:25 AM PDT