Jul 30, 2000





Comet LINEAR continues to disintegrate and could disappear completely within a few days.


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July 31, 2000 -- Astronomers around the world continue to monitor the unexpected disintegration of comet C/1999 S4 (LINEAR). Intense solar heating apparently triggered a massive disruption of the comet's fragile icy core when it passed close to the Sun last week. It is still bright enough to see through small telescopes so even amateur astronomers can watch the comet as it dissolves. If you do plan to look, don't wait. Experts think that comet LINEAR might disappear completely in a few days.

Above: This R-filtered image of comet LINEAR was captured on July 28, 2000, by M. Kidger at the Jacobus Kapteyn Telescope, Roque de los Muchachos Observatory, La Palma, Canary Islands. The innermost coma is elongated and rapidly fading.

The break up of a bright comet is unusual but not unprecedented. For example, comet Shoemaker-Levy 9 (SL-9) broke up before it struck Jupiter in 1994. SL-9 was discovered after it fragmented, so there is no record of what happened as it came to pieces. With comet LINEAR, astronomers have a ringside seat for the entire show.

"We have observed a few comets in the process of breaking up -- comet West in 1976, comet Ikeya-Seki in 1965 and others -- but never with so much detail as we're seeing in comet LINEAR," says Mark Kidger, an astronomer at the Instituto de Astrofisica de Canarias. Comet LINEAR's demise seems to be a bit unusual. "Cometary splittings rarely ever lead to the rapid disappearance of a comet like this - in fact, I don't know of another case"



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Kidger was the first to notice comet LINEAR disintegrating as he monitored a cloud of gas (called the "coma") surrounding the comet's core using the 1-meter Jacobus Kapteyn Telescope. Comet LINEAR, which has been falling toward the Sun since it was discovered in September 1999, made its closest approach to our star (perihelion) on July 26, 2000. Perihelion is a critical time for any comet. It's when solar heating of the icy core is most intense and when the comet swings around for its long return trip to the outer solar system.

"At perihelion there are very rapid aspect changes as regions of the nucleus previously in shadow are suddenly subjected to intense heating," continued Kidger. "This causes strong thermal stresses" that may have been a primary cause of LINEAR's breakup.

Something was already amiss the day before Comet LINEAR reached perihelion at a distance of 114 million km (0.74 AU) from the Sun.

"The very first images on July 25th were enough to show me that something odd was going on," recounts Kidger. "The comet's inner coma was no longer teardrop-shaped (the solar wind flowing around the comet's head causes this shape). It had a shape like a short, fat cigar. My first thought was 'Shoemaker-Levy.' It looked just like those first images of Comet Shoemaker-Levy 9 after it was discovered."

Kidger's images on subsequent nights confirmed that something dramatic was happening and he announced his findings in an International Astronomical Union (IAU) Circular (IAUC #7467) on July 27, 2000. As news of the breakup spread, astronomers around the world trained their telescopes on the comet. In another IAU Circular (IAUC # 7468) published July 28th, three teams of observers reported that they too saw evidence of a major event in the comet's nucleus.

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Left: The breakup of Comet LINEAR. Contours represent lines of constant brightness in Jacobus Kapteyn Telescope R-band images of the comet (credit: Mark Kidger). This 5-frame sequence spanning the interval from July 23rd to 27th shows the progressive elongation and disruption of the comet's core. Each contour map is 40 arcseconds on a side centered approximately on the core of the comet. (Mark Kidger notes that "the unusual aspect of the innermost contours on the July 23rd frame is because this region was so bright that it saturated completely in the images in a 5 second exposure. On subsequent nights the comet was nowhere near saturation."

Unlike comet Shoemaker-Levy 9, which broke into many well-defined bright fragments, comet LINEAR seems to be dissolving into an amorphous haze of gas and dust.

"There is some similarity of appearance to the two comets," says Brian Marsden of Harvard's Minor Planet Center. "An observation by Ian Griffin in New Zealand on July 29th shows the nucleus of C/1999 S4 (LINEAR) extended into a long, bright string. However, it does not seem to show discrete nuclei in that string, as D/1993 F2 (SL-9) did."

The differences between comets SL-9 and LINEAR result from their different sizes and distances from the Sun.

Comet Shoemaker-Levy 9 was larger than comet LINEAR, and it broke apart as the result of tidal stresses it experienced when it passed less than 100 thousand kilometers from Jupiter (within 1.4 Jupiter radii from the planet's center). SL-9 was far from the Sun (812 million km) when it fragmented and solar heating was not the primary cause of the break up. In fact, SL-9 wasn't even orbiting the Sun. The comet had been captured by the gravitational pull of Jupiter and was orbiting the giant planet instead.


Hubble images of Shoemaker-Levy 9

Above: Comet Shoemaker-Levy 9, pictured here in a Hubble Space telescope image, was broken into many pieces during a close encounter with the planet Jupiter in 1992. Two years later it came so close to the planet that the fragments actually plunged into Jupiter's atmosphere. [more information]

Comet LINEAR is a much smaller object that has been losing mass rapidly during its approach to the Sun. The Hubble Space Telescope recorded a house-sized fragment blowing away from the core on July 5th and powerful jets of gas vaporized by solar radiation have been pushing the comet to and fro. Solar heating is a more important factor in its breakup than gravitational effects. [more information]

"The small size of comet LINEAR and its exposure to solar radiation is causing a more complete and rapid dissolution than we saw in Shoemaker-Levy 9," continued Marsden. "The initial break-up of SL-9 was surely caused by tidal forces from Jupiter. If they had not later collided with Jupiter, several of those fragments would presumably still exist. C/1999 S4 (LINEAR), on the other hand, will probably have completely dispersed in a week or so."


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Comet LINEAR may still be bright enough for amateur astronomers to view in small telescopes, but it's fading fast. On July 27th, binocular observers in South America and Europe estimated the comet's visual magnitude to be +6.6 [ref]. That's almost bright enough to see with the unaided eye from dark-sky observing sites. Two days later, an experienced amateur in the Canary Islands reported a visual magnitude of +8.3, a factor of 6 decline in brightness.

"The surface brightness of the innermost coma is fading fast," says Kidger. "This should translate to a somewhat slower fade of the outer coma [that binocular and small telescope observers see] as the gas and dust in it disperses and is not replenished. Typically a comet may take several weeks for the coma to expand and fade down to the brightness of the sky background."

Above: On July 23rd, the Comet LINEAR's gaseous halo was bright and centrally condensed. Since then it has steadily faded as the core disrupts into an elongated train of debris. Credit: Mark Kidger, Jacobus Kapteyn Telescope.

Many well-known annual meteor showers, including the Perseids, Leonids and Geminids, are caused by dusty debris from comets burning up in the atmosphere of Earth. Such displays are harmless and beautiful. Unfortunately for meteor lovers, the orbit of comet LINEAR comes no closer to our planet than 28 million kilometers (0.18 AU). There will be no "Linearid" meteor shower. When comet LINEAR finally disappears from view in a few days or weeks, this memorable visitor from beyond the orbit of Neptune will be gone forever.




Stay tuned to Science@NASA for news and updates about comet LINEAR.



Anatomy of a Comet:


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The nucleus of a comet is an irregular ball of ice and dust typically 1 to 10 km in diameter. When the nucleus approaches the Sun, sunlight warms the surface and the solid ice sublimes (turns to vapor). The resulting cloud of water vapor and carbon dioxide surrounding the nucleus is called the coma. Most comets come from the most distant reaches of the solar system, far beyond the orbit of Pluto. By the time they are as close to the Sun as the Earth, the coma can be larger than Jupiter. Together the coma and the nucleus form the head of the comet.

Right: This image of Comet Halley's nucleus was taken by the European Space Agency Giotto spacecraft during a flyby on March 13, 1986. Scientists estimate that about 10% of the surface was boiling off into space. The stuff that boiled off Halley in 1986 may one day be seen again during an eta Aquarid meteor shower.
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When a comet is far from the Sun, only the head is visible as a smudge in photographs. As the comet nears perihelion (closest approach to the Sun) it sprouts two tails. The dust tail is composed of small (smoke-sized) dust particles carried off the nucleus by escaping gases. The dust tail shines by means of reflected sunlight and is the part of a comet that is usually easiest to see. A longer, blue-colored ion tail is made of charged gas that glows as electrons re-combine with ions to make uncharged molecules. The gaseous ion tail is pushed straight away from the Sun by the solar wind, while the brighter dust tail traces the comet's curved orbit. Usually the two tails point in slightly different directions. [more]

Above: This photograph of Comet Hyakutake highlights different components of the tail. The gold and red tail features are dust, made predominately of little bits of rock and carbon. The dust tail shines by reflecting sunlight. Extending past the dust tail is the comet's ion tail, shown here glowing in blue. The ion tail is composed mostly of ions of water, carbon monoxide, and cyanogen. The ion tail glows by emitting light when elections re-combine with electrically charged ions to make uncharged molecules.


Web Links


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 -pictures, updates, and a discussion board for observers of comet Linear.

Mark Kidger's Comet Home Page

Comet LINEAR's Summer Show - from Sky & Telescope, includes detailed finder charts and ephemerides.

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


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