SOHO Spacecraft Bags 102 Comets

Dolores Beasley

Headquarters, Washington, DC

Phone: 202/358-1753

William Steigerwald

Goddard Space Flight Center, Greenbelt, MD

Phone: 301/286-5017

RELEASE: 00-23

The record is to comet-hunting what Mark McGwire’s home-run
streak is to baseball: In just four years of operation, the Solar
and Heliospheric Observatory (SOHO) spacecraft has found 102
comets, making it by far the most successful comet-hunter in
history.

Calculations have shown that the latest comets discovered
with SOHO are previously unknown (undiscovered) comets, with the
102nd comet observed by Dr. Douglas Biesecker, of SM&A, Vienna,
VA, and NASA’s Goddard Space Flight Center, Greenbelt, MD, a
member of the SOHO team personally responsible for 45 of the
discoveries.

A cooperative project between the European Space Agency and
NASA, SOHO has revolutionized solar science. It also revealed an
amazing number of suicidal comets plunging into the solar
atmosphere.

Like nearly all of SOHO’s comet discoveries, the latest comet
showed up in images from the Large Angle and Spectrometric
Coronagraph (LASCO) instrument. This is a set of coronagraphs that
view the space around the Sun out to 12.5 million miles, while
blotting out the bright solar disk with masks. LASCO watches for
ejections of electrically charged gas from the Sun that threaten
to disturb the Earth’s space environment. As a bonus of
unanticipated size, it also proved ideal for capturing objects
falling to the Sun. Still pictures and movies from LASCO are
freely available on the Internet, and even amateur astronomers
have used them to discover comets.

Ten comets discovered by SOHO, including SOHO # 100, 101 and
102, passed the Sun at a safe distance. However, the rest of the
SOHO comets vaporized in the solar atmosphere. Near misses are
well known, and 100 years ago Heinrich Kreutz in Kiel, Germany,
realized that several comets seen buzzing the Sun seemed to have a
common origin, because they came from the same direction among the
stars. These comets are now called the Kreutz sungrazers, and the
92 vanishing SOHO comets belong to that class.

“SOHO is seeing fragments from the gradual breakup of a great
comet, perhaps the one that the Greek astronomer Ephorus saw in
372 BC,” said Dr. Brian Marsden of the Center for Astrophysics in
Cambridge, MA. “Ephorus reported that the comet split in two.
This fits with my calculation that two comets on similar orbits
revisited the Sun around AD 1100. They split again and again,
producing the sungrazer family, all still coming from the same
direction.”

Their ancestor must have been enormous by cometary standards.
“The rate at which we’ve discovered comets with LASCO is beyond
anything we ever expected,” said Biesecker. “We’ve increased the
number of known sungrazing comets by a factor of four. This
implies that there could be as many as 20,000 fragments.”

Life is perilous for a sungrazer. The mixture of ice and dust
that makes up a comet’s nucleus is heated like the proverbial
snowball in hell, and it can survive its visit to the Sun only if
it is quite large. What’s more, the strong tidal effect of the
Sun’s gravity can tear the loosely glued nucleus apart. The
disruption that created the many SOHO sungrazers was similar to
the fate of Comet Shoemaker-Levy 9, which went too close to
Jupiter and broke up into many pieces that eventually fell into
the massive planet in 1994.

The history of splitting gives clues to the strength of
comets, which will be of practical importance if ever a comet
seems likely to hit the Earth. Also, the fragments seen as SOHO
comets reveal the internal composition of comets, freshly exposed,
in contrast to the much-altered surfaces of objects like Halley’s
Comet that have visited the Sun many times.

The count of SOHO’s comet discoveries would be one fewer
without a late bonus from SOHO’s Solar Wind Anisotropies (SWAN)
instrument, which looks away from the Sun to survey atomic
hydrogen in the Solar System. In December 1999, the International
Astronomical Union retrospectively credited SOHO with finding
Comet 1997 K2 (SOHO
# 93) in SWAN full-sky images from May to July 1997. It remained
outside the orbit of the Earth even at its closest approach to the
Sun, and thus did not vaporize entirely.

More information and images are on the Internet at: