Three photographs taken with NASA's Hubble Space Telescope chronicle a violent outburst in the life of comet LINEAR. HST tracked the comet from July 5 to 7, capturing a dramatic leap in its brightness [top image]; followed by seeing a wave of newly created dust from the outburst flowing into the coma, a shell of dust surrounding the core [middle image]; and culminating in the discovery of a castoff chunk of material from the nucleus sailing along its tail [the bright dot trailing behind the comet in the picture at bottom]. The white region represents the brightest part of the coma. The nucleus cannot be seen in these images because it is about a mile or so across, which is too small for the telescope to see. Photos: NASA, H. Weaver and P. Feldman (Johns Hopkins University), M. A'Hearn (University of Maryland), C. Arpigny (Liege University), M. Combi (University of Michigan), M. Festou (Observatoire Midi-Pyrenees), and G.-P. Tozzi (Arcetri Observatory)

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 bottle.

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.

The Chandra observations showed variations in the amount of X-rays produced by the comet. These were probably due to a solar flare that occurred on July 12 which would have increased the intensity of the solar wind. Photo: NASA/SAO/CXC/STScI/Lisse et al.

"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.