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August 28, 2000 -- Last week, the ESA/NASA Solar
and Heliospheric Observatory shattered its own record for comet
discoveries when astronomers announced that the orbiting spacecraft
had recorded its 200th sungrazing comet. Michael Oates, an amateur
astronomer in Britain, spotted SOHO-200 in an online image captured
by one of SOHO's coronagraphs. The picture showed the comet evaporating
as it plunged through the superheated solar corona.
"With 200 discoveries, SOHO is way ahead [of any
other comet hunter]," says Brian Marsden of Harvard University's
Minor Planet Center. The automated asteroid and comet search
program "LINEAR
is a distant second with 50 comets. Among individuals, Carolyn
Shoemaker is of course the leader, with 30-something discoveries."
Right: The SOHO C2 coronagraph captured this image of
a sungrazing comet 0.9 degrees from the Sun on April 29, 2000.
The solid brick-colored disk in the middle is the coronagraph's
occulting disk; the light red graphic circle shows the true size
of the Sun.
SOHO's primary mission is to investigate the solar interior,
the solar atmosphere, and the solar wind. Space weather forecasters
rely on the satellite, which enjoys an uninterrupted 24-hour-a-day
view
of the Sun, for advance warning of solar eruptions and geomagnetic
storms. The comets are a bonus.
"No one expected to find all these comets when we launched
SOHO nearly 5 years ago," says Doug Biesecker, a solar physicist
at the Goddard Space Flight Center. "They came as a huge
surprise, there's no doubt about it."
Only comets that pass perilously close
to the Sun catch SOHO's attention. The vast majority, like
SOHO-200, don't survive the encounter. They swoop so low over
the blazing solar photosphere that their icy cores vaporize completely.
Most of SOHO's 200 comets no longer exist -- they disintegrated
hours after they were discovered!
The key(s) to spotting comets so close to the Sun are SOHO's
extraordinary coronagraphs. A coronagraph is a device that blocks
out the Sun's blinding glare with an occulting disk so that the
faint corona is visible, as well as surrounding stars and planets.
Coronagraph images at the SOHO
web site are updated every 30 minutes or so. About once each
week the photos include a faint comet that anyone can discover
if they happen to be the first to look.
Above: In the left image Comet Hyakutake passes
grandly through the SOHO C3 coronagraph's field of view at the
beginning of May 1996. A more typical, sungrazing comet appears
in the right image. The faint fragment was discovered
by amateur astronomer Kazimieris Cernis of Vilnius, Lithuania,
on 4 February. It marked SOHO's 100th comet discovery. (Editor's
Note: Although SOHO-100 came close to the Sun, and thus qualifies
as a "sungrazer," it was not a Kreutz sungrazer
as described in the text. SOHO-100 did not share the same orbit
as that family of comets possibly resulting from the breakup
of a giant comet 2,000+ years ago.)
Remarkably, most sungrazing comets appear to be fragments of
a single giant comet that broke apart near perihelion (closest
approach to the Sun) long ago. Marsden speculates that the parent
might have been a bright comet seen by the Greek astronomer Ephorus
in 372 BC. Ephorus reported that the comet split in two. Splits
may have occurred again and again, producing the Kreutz sungrazer
family (named after the nineteenth-century German astronomer
who studied them in some detail). The sungrazers share an elliptical
orbit that brings some of the fragments less than 50,000 km from
the Sun.
"The association of the Ephorus comet with the 'original'
break-up is speculative," cautions Marsden. "Some may
argue that the breakup took place much longer ago, while others
might say that it happened in a somewhat different manner and
quite a bit more recently. We really don't know much about
what happened."
The nucleus of the original comet may have been as wide as
100 km, or 10 to 20 times larger than an ordinary comet. Most
of the short-lived fragments seen nowadays by SOHO are tiny --
probably no more than a few tens of meters across. Occasionally,
pieces come along that are large enough to survive the Sun's
searing heat, and these can emerge from their close encounter
as bright naked-eye comets. One such
fragment in 1965, Comet Ikeya-Seki, was visible in broad daylight.
Scientists estimate that Ikeya-Seki's icy nucleus was some kilometers
wide.
 "In the old days we thought these
comets were a rarity because there were only a few like Ikeya-Seki
visible from the ground," continued Marsden. "The smaller
fragments are tremendously faint except for a few days before
perihelion. By the time SOHO spots them they are in their death-throes."
Left: In August 2000 NASA's Hubble Space Telescope
spotted fragments from the breakup
of Comet LINEAR. A similar disruption of Ephorus's comet
2,000 years ago may have created the existing family of sungrazing
comets seen by SOHO today. [more information]
SOHO's impressive spate of comet finds can be attributed in large
part to the efforts of amateur astronomers. SOHO data are freely
available to anyone with an internet connection and a computer.
Both realtime
and archival images
are accessible at the SOHO web site, a popular destination for
comet hunters.
"In late February, just after we announced our 100th comet,
I mentioned in an email message to (amateur astronomer) Michael
Oates that there might be undiscovered comets in images from
our C2 coronagraph," says Biesecker. SOHO carries two coronagraphs.
The C2 instrument monitors the corona from 2.5 to 6 solar radii
from the center of the visible Sun. Its sister coronagraph, called
"C3," has a wider field of view. It monitors activity
at distances of 4 to 30 solar radii.
"Before this summer, most of SOHO's comets had been found
in C3 images," said Biesecker. "We really didn't expect
to find many in C2, because by the time the comets are close
enough to the Sun to fall within C2's narrow field of view they
are moving very quickly. We only record coronagraph images every
half hour, so it's easy to miss fast-moving comets in the C2
data."
Nevertheless, Biesecker's February communique to Oates was the
beginning of an avalanche. Nearly, two-thirds of the comets discovered
since then were located by amateurs examining archived C2 images.
SOHO-200 was found by Oates in a picture from April 1997.
 "It
turns out that C2 is a better instrument than C3 for finding
comets during the months of May through July each year,"
explained Biesecker. "Sungrazing comets seem to reach a
peak brightness at a certain distance from the Sun, between 10
and 12 solar radii. When we see a comet in a coronagraph image,
we're not seeing its true distance from the Sun. What we see
is its distance projected onto the flat plane of the image. In
the May to July time frame the projected distance corresponding
to 10-12 solar radii falls within the C3 instrument's vignetting.
But, they show up nicely in C2 images. At that time of the year
the comets appear to move more slowly, too, so we get a few additional
images of the comet." All these factors have combined to
make amateur searches through the archives so productive.
"Amateurs have even taken the lead on realtime discoveries,"
added Biesecker. "If a comet zooms through the coronagraph's
field of view at 2 a.m. here at Goddard, someone in Europe is
probably looking at the web site while we're asleep!"
Above: This 4-minute exposure of sungrazing comet Ikeya-Seki
was captured by Roger Lynds at Kitt Peak, Arizona, on the morning
of 1965 October 29. Unlike most of the sungrazing comets recorded
by SOHO, Ikeya-Seki survived its close encounter with the Sun
to become a bright naked-eye object.Copyright© Roger Lynds,
all rights reserved. [more
information]
By convention, comets discovered in SOHO data are named after
the spacecraft rather than the astronomer. Nevertheless, say
amateur comet hunters, being the first to spot a comet streaking
past the Sun can be a real thrill.
If you're interested in joining the hunt for sungrazing comets,
a good place to start is the Solar and Heliospheric Observatory's
realtime
images web page where coronagraph data are posted every 30
minutes, and sometimes even more frequently. Data from the satellite
are available to the general public at the same time as to the
scientific community. If you think you've found something, first
review the basic
criteria for a discovery before forwarding the details to
scientists at the Goddard Space Flight Center. Confirmed finds
are posted daily on the "What's
New" area of http://sungrazer.nascom.nasa.gov.
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SOHO (the Solar and Heliospheric Observatory) is a mission
of international cooperation between NASA and the European Space
Agency. It is managed by the Goddard Space Flight Center for
the NASA HQ office of Space Science. |
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