The Missing Pieces of Comet LINEAR

Astronomers using the Hubble Space Telescope have spotted at least a half-dozen mini-comets with tails inside the fading coma of Comet LINEAR.

August 7, 2000 -- To the surprise and delight of astronomers, NASA's Hubble Space Telescope has discovered a small armada of "mini-comets" left behind by what seemed to be a total disintegration of the explosive comet LINEAR.

Hubble's powerful vision has settled the fate of the mysteriously-vanished solid nucleus of the comet, which disappeared from the view of Earth-bound telescopes following its passage around the Sun on July 26

Above: (lower right) This Hubble picture of comet LINEAR's core shows that the icy nucleus has been reduced to a shower of glowing "mini-comets." (upper left) A ground-based telescopic view (2.2-meter telescope) of Comet LINEAR taken at nearly the same time as the Hubble observations shows a diffuse elongated cloud of debris without any visible nucleus. The inset box corresponds to the HST field of view. [more information]

On July 27, ground-based observers lost sight of the bright core of the comet and suggested that the nucleus disintegrated into a pile of dust. Astronomers at the Space Telescope Science Institute (STScI) in Baltimore, MD, quickly reprogrammed Hubble to search for the missing nucleus. Johns Hopkins University astronomer Hal Weaver said he was stunned when the Hubble image popped up on his computer screen. "My first thought was Hubble Space Telescope does it again! We caught the fish! This is amazing, very exciting, very neat."

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Though comets have been known to break apart before, this is the first time astronomers have a close-up view of the dismantling of a comet's nucleus due to the Sun's heat. Since the 1950s, researchers assumed comet nuclei were loose clusters of ice and dust, called cometesimals, held together by gravity. Solar heat causes the ices to sublimate and violently release gas as explosions and garden hose-style jets. The pressure of the solar radiation blows away particles like debris caught in a gale.

Some astronomers think that the fragments now being seen in LINEAR may be the primordial building blocks of the original nucleus, the so-called cometesimals, which theory predicts should be several tens of feet across. The breakup of a comet tells scientists how it was put together in the first place. The cometesimals were built up from micron-sized grains of dust as it collected in the early solar system, roughly 4.6 billion years ago.

On Weaver's screen were at least a half dozen "mini-comets" with tails, resembling the shower of glowing fireballs from fireworks. They were clustered in the lance-head tip of an elongated stream of dust. An isolated brighter piece in front of the cluster may be the parent nucleus for the smaller fragments. Hubble's exceptional resolution and sensitivity allowed it to reveal the nuclei as separated bodies at a level of detail never before seen in a disintegrating comet.

Some astronomers find it hard to imagine how an object the size of a mountain could totally disintegrate in only two weeks. "Actually, I would have been more amazed if Hubble saw no pieces," adds co-investigator Carey Lisse, of STScI. "The comet's breakup was too violent and fast for it to completely vaporize. How do you pulverize something the size of a mountain?"

In hindsight Comet LINEAR began falling apart in June when the comet unexpectedly brightened, indicating an outburst of dust. Powerful gas jets nudged the comet along a chaotic path, another indication of a very volatile activity. Hubble fortuitously caught a piece blowing off on July 5 - which may have been the first of the cometesimals or a piece of crust. Another comet brightening happened on July 20. When the comet went around the Sun on July 26, the Sun's heat made the comet come unglued.

Above: This 3-frame sequence of Hubble Space Telescope images spanning July 5th through 7th shows the brightness of comet LINEAR increasing by 50% and then subsiding again as it blows off a piece of its crust, like a cork popping off a champagne bottle. [more information]

Weaver says it will be important for the largest ground-based telescopes to try and see the mini-comets as they spread apart. This may yield further clues on the structure of the original nucleus and the sizes of the remaining fragments.

Some astronomers believe this was Comet LINEAR's first visit to the inner solar system, after traveling for nearly the distance of one light-year (six trillion miles) from the vast comet storehouse called the Oort cloud. Other astronomers suggest that LINEAR may have been a fragile piece that broke off of a larger comet that visited our solar system more than 10 million years ago.

Scientists think that 20-30 percent of comets are so fragile they completely disintegrate when they pass the Sun.

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

Orbital Elements of C/1999 LINEAR S4 - from the Harvard Center for Astrophysics

Daily Ephemeris for C/1999 LINEAR S4 - from the Harvard Center for Astrophysics

C/1999 S4 (LINEAR) -- images and historical highlights from Gary Kronk's popular Comets & Meteors web site.

CometLinear.com -- images, updates and a discussion forum -- all about Comet LINEAR!

Recent Science@NASA Stories about Comet LINEAR

Here Comes Comet LINEAR -- July 5, 2000

Comet LINEAR Misbehaves -- July 28, 2000

Meltdown! -- July 31, 2000