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- The suggestion that comet Hale-Bopp might have a satellite
orbiting its nucleus, based on Hubble Space Telescope images, has resurfaced
amid doubts.
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- Reporting in the current issue of Earth, Moon, and Planets,
an international journal, Zdenek Sekanina of the Jet Propulsion Laboratory
writes about a Hale-Bopp satellite detected by applying theoretical modeling
to images taken with Hubble's Wide Field Planetary Camera 2 on five days
during 1996.
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- If further research confirms the satellite, it would
be the first known cometary satellite discovered in a stable orbit, Sekanina
said in a telephone interview.
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- While satellites have been spotted around other comets,
in each case they are drifting away from their host, destined to eventually
break free from the relatively minor gravitational tug. Hale-Bopp -- which
made its closest approach to the sun in 1997 -- is a huge comet, and Sekanina
said it is therefore a prime candidate for being capable of binding a satellite
into a stable orbit. The satellite might have broken off the main nucleus
during a previous pass through the solar system, he said, possibly because
of the gravitational effects of Jupiter.
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- Sekanina says the satellite is roughly 30 kilometers
in diameter (18 miles), compared with a main nucleus estimated to be 70
kilometers across (43 miles). The two objects appear to be separated by
about 160 to 210 kilometers. The satellite appears to take two or three
days to complete an orbit, Sekanina said.
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- Skepticism
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- Harold Weaver, a research scientist in the Department
of Physics and Astronomy at the Johns Hopkins University, has doubts about
the report, which is based on Hubble Space Telescope (HST) images Weaver
has studied (and actually provided to Sekanina). In a separate paper due
to appear in an upcoming issue of Earth, Moon, and Planets, Weaver and
a colleague, Philippe Lamy, make their case against the possible satellite.
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- "We argue that Sekanina's conclusions about multiple
nuclei cannot be trusted because he is trying to extract more information
from the HST images than they contain," Weaver told space.com. "In
particular, the (detectors) on HST are not perfect devices, and one has
to be very careful about interpreting every 'bump and wiggle' in the data
as evidence for companion nuclei. In other words, we feel that Sekanina
may be overinterpreting the data."
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- Sekanina agrees with Weaver that the satellite (or second
nucleus) does not appear on the Hubble image. "I don't see it either,"
Sekanina said. But while Weaver is an observer, Sekanina is a theorist.
He starts with the assumption that there is a satellite, then he tries
to find it in the data using a complicated modeling process that filters
the light reflected by dust surrounding the comet.
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- "The only way to extract the nuclei (from the images)
is to do a digital modeling of the central cloud," Sekanina said.
He applied two different techniques to five separate images. "I'm
reasonably certain that something is there," Sekanina said, "because
I see it on each of the five images using two different models of extraction
of the dust cloud."
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- Weaver is familiar with the modeling method, and respects
Sekanina's work in general, but still he doubts that there is anything
unusual in the images. He noted the new paper is similar to previous papers
by Sekanina that used similar methods to reach a similar conclusion.
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- "There are some interesting new results in the (new)
paper but, in my opinion, they do not strengthen the case for multiple
nuclei," Weaver said. "Of course, we could be wrong."
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- Another group of researchers, writing in the September
issue of Astronomy & Astrophysics, claims evidence from ground-based
observations that indicates a second nucleus in Hale-Bopp. This apparent
second nucleus is a different one than that proposed by Sekanina. It was
derived by applying a technique called adaptive optics, which attempts
to filter effects of Earth's atmosphere and resolve objects that otherwise
are not apparent in an image.
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- Weaver and Lamy also dispute this detection, based on
Hubble Telescope images taken at nearly the same time but which do not
show companions of the type claimed by the authors, "even though they
should have been easily detected in the Hubble Space Telescope data."
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- Why does it matter?
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- Weaver said there are more than two dozen confirmed examples
of comets splitting into fragments, the most famous recent example being
Comet Shoemaker-Levy 9, which plowed into Jupiter in several stages. It
is therefore conceivable that relatively large fragments could have broken
away from Comet Hale-Bopp.
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- "However, no one has ever detected a bound companion
to a cometary nucleus, as Sekanina is claiming, so that would be very exciting,"
Weaver said. "There are now several known examples of binary asteroids,
and maybe comets sometimes also have stable companions."
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- If a comet had two nuclei, Weaver said there is a chance
the two could collide, "which would temporarily produce a large outburst
in activity by the comet."
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- Meanwhile, Sekanina plans to get to work figuring out
the orbital details of his suspected satellite. One result of that effort
could be a better understanding of comets in general. "If we can calculate
the orbit, we could calculate the first mass of a comet ever," he
said.
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