Jul 27, 2000

Comet LINEAR Misbehaves




Comet LINEAR did not become a spectacular naked-eye object as many stargazers had hoped, but it is intriguing astronomers with its peculiar dynamic behavior.


animation of Hubble images of comet LINEAR
July 28, 2000 -- Dashing the hopes of many sky watchers around the world, comet C/1999 S4 (LINEAR) was too faint to see with the unaided eye as it made its closest approach to Earth on July 22nd. Its visual magnitude appears to have peaked between magnitude +6.0 and +6.5, just below the threshold for naked-eye observations. Nevertheless, the comet is still a good target for binoculars or small telescopes. [See Sky & Telescope for finder charts].

LINEAR may have disappointed casual stargazers, but it is intriguing professional astronomers with unpredictable behavior including jets that are perturbing the comet's orbit and an outburst in July that may have sent a fragment hurtling away from the comet's core.

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]




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Comet LINEAR surprised astronomers using the Hubble Space Telescope (HST) with a brief, violent outburst on July 5, 2000. The comet's brightness soared by a factor of 1.5 during a four hour period. Two days later astronomers spotted at least one house-sized fragment trailing the nucleus by more than 450 km.

"We lucked out completely," said Hubble comet-watcher Harold Weaver of the Johns Hopkins University, Baltimore, MD in a press release. "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."

"The July 5th flare was probably associated with the separation of the fragment," says Brian Marsden of the Minor Planet Center at Harvard University. "Fragmentation releases fresh icy material and exposes it to solar radiation, causing the comet momentarily to brighten."

If such a flare had occurred this week as LINEAR was approaching the Sun (it was closest to the Sun on July 26th) the comet might have become visible without a telescope.

Something similar happened to the already-brilliant comet C/1996 B2 (Hyakutake) in 1996. Ground based telescopes recorded transitory flares while the Hubble Space Telescope and others captured pictures of fragments flying away from the nucleus (see below).


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Like comet Hyakutake, comet LINEAR is a "dirty snowball" from the outer reaches of the solar system. Its nucleus is laced with volatile gasses that vaporize furiously as the comet falls toward the Sun. Marsden explains that this is probably LINEAR's first visit to the inner solar system, and it has a greater proportion of vaporizable material than comets that have passed by the Sun many times before (e.g., Halley's Comet). C/1999 S4 is losing so much of its mass to solar vaporization that it's being pushed and shoved by the reaction force of its own gaseous jets. Just as a jet airplane under its own power does not follow a ballistic trajectory, LINEAR's orbit is not a perfect gravitational ellipse.

Right: This 1996 Hubble Space Telescope picture shows three small pieces that broke away from the core of comet Hyakutake. The fragments were forming their own tails when Hubble captured this image. [more information]

"Basically, I think C/1999 S4 is simply a rather small comet," says Marsden. "If comets of similar composition lose material by means of a constant shrinking of the radius, the relative mass loss will be largest for the smallest comets. So that's why the non-gravitational terms in the equations describing LINEAR's orbit are so large for this first-pass comet, as opposed to C/1956 R1 (Arend-Roland) in 1957, for example."


LINEAR S4's erratic jets make predicting the long term fate of the comet tricky.

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"The large nongravitational effect complicates the calculation of long-term motion," continued Marsden. "Without the nongravitational effect, comet LINEAR would be back in some 30,000 years. With it, I don't know."

Left: Is comet LINEAR breaking up? Mark Kidger, an astronomer at the Instituto de Astrofisica de Canarias, captured this picture of comet LINEAR's coma (the cloud of gas surrounding the icy nucleus) on July 25, 2000, using the one meter Jacobus Kapteyn Telescope in the Canary Islands. The contours show lines of constant brightness in the inset image. Kidger reports that the coma has developed a peculiar elongated shape unlike its appearance on previous nights. He speculates that the rapidly-changing comet could be fragmenting into smaller pieces following its recent close pass by the Sun. Click for a side-by-side comparison of contour plots from July 23rd and July 25th.

Comet LINEAR reached its maximum northern declination on July 18, 2000, and now it's heading for the southern sky. Amateur astronomers in the northern hemisphere can track the comet through early August as it moves from Ursa Major, through Leo and into Virgo. After the beginning of August the fading comet can be seen best from south of the equator. Southern observers with access to a telescope are in for a treat on August 20th when the comet passes close to the Sombrero galaxy (M104). From southern Africa LINEAR S4 will pass right in front of M104, affording an opportunity for truly unique astrophotos. [more information from Sky & Telescope]



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Above: Science@NASA reader Larry Koehn contributed this illustration of comet LINEAR's motion through the Solar System. The comet passed 56 million km from Earth on July 22nd and 114 million km from the Sun on July 26, 2000. It appears to be a first-time visitor to the inner solar system traveling in an orbit that will eventually return it beyond Neptune in 2013.

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.

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.

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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