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.”

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

“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.

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.”

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

“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!”

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

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.