Some comets may break apart over and over again in the
farthest reaches of the solar system, challenging a theory
that comets break up only occasionally and not too far from
the Sun, says a researcher from NASA’s Jet Propulsion
Laboratory (JPL), Pasadena, Calif.

A system of comets called “sungrazers,” named for their orbit
that closely brushes the Sun, reveals important clues about
how these bodies break up. Most sungrazing comets are tiny —
the smallest could be less than 10 meters (30 feet) across —
and move in a highway-like formation of comets that pass near
the Sun and disintegrate.

Dr. Zdenek Sekanina, senior research scientist at JPL,
reports in the September 10 issue of the Astrophysical
Journal that many sungrazer comets arrive at the Sun in
clusters and on parallel paths. He emphasizes that such tiny
fragments would have disintegrated if they had come so close
to the Sun on an earlier trip. Therefore, the parents of
these tiny sungrazers must have broken up after their
previous encounter with the Sun and continued to break up far
from the Sun on their journey through the solar system.

Sekanina’s sungrazer studies challenge an earlier theory that
the only place these comets break up is very close to the
Sun, as the strong pull of its gravity cracked their loosely
piled chunks of dust and ice. The gradual, continuing
fragmentation gives birth to all the sungrazers, the most
outstanding examples of splitting comets.

“Astronomers never before realized that there could be a
fairly orderly pattern in breaking up, so that one comet
cascades into large families of smaller comets, and that this
process could be an important part of a comet’s natural life
cycle,” Sekanina said.

Sungrazers are not the only comets that can break up far from
the Sun. Sekanina points to new observations of comet 57P/du
Toit-Neujmin-Delporte, whose fragmentation has led to the
formation of a similar, though less prominent, highway of
tiny comets. All fragments separated from the comet beyond
the orbit of Mars.

Images taken by the European Space Agency’s and NASA’s Solar
and Heliospheric Observatory have shown the many tiny
sungrazing comets. A movie showing two of these comets can be
found at:

Nearly seven years’ worth of images from the solar
observatory revealed more than 400 sungrazers in the Sun’s
immediate neighborhood. Sekanina estimates that currently
there may be as many as 200,000 sungrazer comets the size of
the ones the observatory detected.

The Solar and Heliospheric Observatory is a project of
international cooperation between the European Space Agency
and NASA. The spacecraft is part of the International Solar-
Terrestrial Physics program, a global effort to study the
interaction of Sun and Earth. The instrument that observed
the comets was developed at the U.S. Naval Research
Laboratory, Washington, with collaborators in the United
Kingdom, France and Germany. JPL, a division of the
California Institute of Technology in Pasadena, manages the
Planetary Atmospheres program for NASA’s Office of Space
Science, Washington.