Dolores Beasley/Don Savage

Headquarters, Washington, DC

(Phone: 202/358-1547)

Donna Weaver

Space Telescope Science Institute, Baltimore, MD

(Phone: 410/338-4493)

Michael Purdy

Johns Hopkins University, Baltimore, MD

(Phone: 410/516-7160)

Nancy Neal

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-0039)

Steve Roy

Marshall Space Flight Center, Huntsville, AL

(Phone: 256/544-6535)

Dr. Wallace Tucker

Chandra X-ray Observatory Center, Harvard-Smithsonian Center for
Astrophysics, Cambridge, MA

(Phone: 617/496-7998)

RELEASE: 00-117

When NASA’s two great observatories, the Hubble Space
Telescope and the Chandra
X-ray observatory, recently observed comet LINEAR (C/1999 S4)
astronomers received some abrupt surprises.

Using the Hubble Space Telescope, researchers were surprised
to catch the icy comet in a brief, violent outburst when it blew
off a piece of its crust, like a cork popping off a champagne

The eruption, the comet’s equivalent of a volcanic explosion
— though temperatures are far below freezing (about minus 100
degrees Fahrenheit or minus 40 degrees Celsius) in the icy regions
of the nucleus or core — spewed a great deal of dust into space.
This mist of dust reflected sunlight, dramatically increasing the
comet’s brightness over several hours. Hubble’s sharp vision
recorded the entire event and even snapped a picture of the chunk
of material jettisoned from the nucleus and floating away along
the comet’s tail.

“We lucked out completely,” said Hubble comet-watcher Harold
Weaver of the Johns Hopkins University, Baltimore, MD. “In one
surge of brilliance this under-performing comet showed us what it
could have been. Comet LINEAR generally has not been as bright as
we had hoped, but occasionally does something exciting.”

Though comet nuclei have been known to fragment, Hubble’s
sharp vision is revealing finer details of how they break apart.
This unexpected glimpse at a transitory event may indicate that
these types of “Mt. Saint Helens” outbursts occur frequently on
the comet, because it is unlikely that Hubble just happened to
catch one isolated event, Weaver said.

The orbiting observatory’s Space Telescope Imaging
Spectrograph tracked the streaking comet for two days, July 5 to
7, capturing the leap in brightness and discovering the castaway
chunk of material sailing along its tail. When the Hubble
telescope first spied the comet 74 million miles (120 million km)
from Earth, it watched the icy object’s brightness rise by about
50 percent in less than four hours. By the next day, the comet was
a third less luminous than it had been the previous day. On the
final day, the comet was back to normal.

During the outburst’s peak, the astronomers believe that the
comet jettisoned the piece of its crust seen days later in the
tail. The renegade fragment moved away from the core’s weak
gravitational grasp at an average speed of about six miles per
hour, which is a little more than a brisk walking pace.

A week later, on July 14, NASA’s Chandra X-ray Observatory
imaged the comet and detected X-rays from oxygen and nitrogen
ions. The details of the X-ray emission, as recorded on Chandra’s
Advanced CCD Imaging Spectrometer (ACIS), show that the X-rays are
produced by collisions of ions racing away from the Sun with gas
in the comet.

“This observation solves one mystery. It proves how comets
produce X-rays,” said Carey Lisse of the Space Telescope Science
Institute, Baltimore, MD, leader of a team of scientists from the
institute; NASA’s Goddard Space Flight Center, Greenbelt, MD;
Johns Hopkins; the University of California, Berkeley; and the
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA. “With
an instrument like Chandra, we can now study the chemistry of the
solar wind and observe the X-ray glow of the atmosphere of comets,
as well as other planets such as Venus.”

Comet LINEAR was named for the observatory that originally
discovered it in September 1999. LINEAR is the acronym for Lincoln
Near Earth Asteroid Research, a project operated by the
Massachusetts Institute of Technology’s Lincoln Laboratory,
Lexington, MA, to search for Earth-approaching objects.

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc., for
NASA, under contract with Goddard Space Flight Center. The Hubble
Space Telescope is a project of international cooperation between
NASA and the European Space Agency.

Chandra’s ACIS instrument was built for NASA by the
Massachusetts Institute of Technology, Cambridge, and Pennsylvania
State University, University Park. NASA’s Marshall Space Flight
Center, Huntsville, AL, manages the Chandra program. The
Smithsonian’s Chandra X-ray Observatory Center controls science
and flight operations from Cambridge, MA.

Images associated with this release are available on the
Internet at:
and via links in:


Chandra images are available at: