When the SOHO (Solar and Heliospheric Observatory) mission was launched
in late 1995, no one could have anticipated the phenomenal trove of comets
that its images would provide. Previous solar observatory spacecraft, SOLWIND
and Solar Max, had revealed a handful of comets on images taken by special
cameras called coronagraphs that mask the sun's disk to reveal the corona,
but SOHO has far outstripped them. On August 12, the 500th comet was found on
SOHO images.
Some 87% of SOHO's comets belong to the Kreutz group of sungrazing
comets, and are fragments of some of the brightest comets ever seen; the
Kreutz family includes the Great Comets of 1843, 1882 and 1965, all of which
were visible in daylight when near the Sun. The first SOHO comets were found
by professionals from NASA and the European Space Agency, joint sponsors of
the mission, but in August, 1999, shortly after SOHO images were made public
on the project's
website--http://sohowww.nascom.nasa.gov/data/realtime-images.html--Terry
Lovejoy of Australia became the first amateur to find a SOHO comet (actually,
two on the same day). Soon a trickle of amateur discoveries became a torrent,
and over the next three years amateurs found over 400.
Michael Oates, an British amateur astronomer, pioneered the technique of
using advanced photo-processing techniques to search SOHO's archives for
previously missed comets, particularly from the early years before the images
were adequately patrolled. Working both with archival and current (real-time)
images, Oates has found 136 comets, roughly a tenth of all comets in the
historical record.
It is perhaps fitting that Germany's Rainer Kracht was the one to find
SOHO-500. Kracht has been the most prolific SOHO comet hunter over the past
year, finding 63 since August 2001, many of them archival comets belonging to
three new families of sungrazing comets that were discovered earlier this
year.
Just as fitting, SOHO-500 belongs to one of these non-Kreutz groups. It
is the 31st member of the Meyer group, first identified by Maik Meyer of
Germany, in January. Meyer, who created the Catalogue of Comet Discoveries
website--http://www.comethunter.de/--which contains discovery information on
all known comets, noticed similarities in the orbits of three non-Kreutz SOHO
comets. He tried recalculating the orbits of other "stray" comets and found
three more with nearly identical orbital elements--thus the Meyer group.
Meyer says of his discovery, "The essential thing is that amateurs can
still make big contributions to science, even from the computer desk. In this
case it was possible because the SOHO team has a free distribution policy of
their data (as well as the Minor Planet Center with the astrometric data). I
don't want to promote myself. I was just lucky to find the first new group.
It could have been easily another person, but I am an amateur, and this is
the main point."
On the same International Astronomical Union circular in which the Meyer
group was announced, Brian Marsden of the Harvard/Smithsonian Center for
Astrophysics reported another comet group that he had discovered. Soon after,
Kracht identified a new comet group that now bears his name. The Marsden
group has 12 known members, the Kracht group 8. The Marsden and Kracht groups
may be loosely related. Unlike the Kreutz group, no members of the Meyer,
Marsden or Kracht groups have been seen other than on SOHO images.
The comets of all three new groups tend to appear stellar, tailless and
faint. However, there is evidence that the daytime Arietid meteor stream may
be related to the Marsden group. Comet 96P/Machholz and the Quadrantid meteor
shower also show some orbital elements similar to the Marsden group, so they
may also be loosely connected (probably originating from the same parent
body). Orbits of all three groups place the comets south of the Sun around
perihelion, which may help explain why no comets belonging to these groups
appear in the historical record.
Analysis of SOHO's trove of Kreutz comets has given astronomers a clearer
idea not only of the nature and history of the group, but also of the way
that comets split apart. Over the centuries, various comets have been
observed to split into two or more pieces. Usually this happens close to
perihelion, when the heat and tidal forces are at their greatest. This is
particularly true of Kreutz sungrazers, whose orbits can take them less than
100,000 miles from the Sun's seething disk. (Some are even believed to
collide with the Sun.)
Careful analysis of pairs of Kreutz fragments, however, has shown that
such fragmentation can take place at any part of the comet's orbit, even near
aphelion. None of SOHO's Kreutz comets have been observed to survive
perihelion; the fragmentation of the Kreutz group has been progressive and
swift.
Kreutz comets can be grouped into two subgroups, each with slightly
different orbital characteristics. The main representative of Subgroup 1 is
the Great Comet of 1843; of Subgroup 2 it is the Great Comets of 1882 and
1965. The latter two comets are believed to be the largest components of a
daylight comet that was observed to split in 1106 AD. It is possible that the
brilliant comet of 372 BC that was observed by the Greek astronomer Ephorus
was the parent of all the Kreutz comets we see today; its main split into the
two Kreutz subgroups may have taken place as recently as the 3rd Century AD,
when the comet was far from the Sun.
SOHO has another instrument called SWAN (Solar Wind Anisotropy) that can
detect comets. It produces a small image of the entire sky in which bright
points in ultraviolet light, some of them comets, can be seen. Although only
three comets have been discovered with SWAN, it was recently determined that
many of the comets found visually by amateurs could have been seen earlier on
SWAN images, which will undoubtedly get much more attention in the future. A
recent SWAN comet (2002 O6) is currently visible in the morning sky at about
sixth magnitude.
The advent of automated sky surveys such as LINEAR, designed to detect
near-Earth asteroids but which also find numerous comets, has seriously cut
into the number of visual comet discoveries by amateurs in recent years.
There has been a resurgence this year, with four visual discoveries by
amateurs since February, but with new and more powerful surveys planned, it
may be harder still for backyard astronomers to be the first to discover
comets in their telescopes.
But just as technology is closing one door, it has opened another in
allowing amateurs to detect some of the smallest comets ever seen, which
nonetheless are relatives of the most spectacular comets to grace our skies,
and to find relations between hitherto unconnected comets. The contributions
(and enthusiasm) of amateurs have been critical in establishing SOHO's place
in comet hunting, and amateurs will play a key role as the spacecraft's comet
totals continue to grow.
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