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The Mysterious Disintegration of Comet C/1999 S4 LINEAR

YOWUSA.COM, August 6, 2000
Marshall Masters

S4 LINEAR DisintegrationThe disintegration of Comet C/1999 S4 LINEAR was a unique astronomical event and has been accompanied by the most violent rash of forest fires in the last fifty years!  At present, 62 wildfires are burning in 11 western states. With the recent  addition of 500 US Army soldiers, 20,000 people are now fighting fires that have already devastated more than 747,101 acres. As US Marines undergo fire  fighting training, the outlook for a downward trend is bleak unless weather  conditions change dramatically, and soon.

These fires have also impacted the efforts of independent astronomers to observe Comet LINEAR.  Most notable is Mark Kidger, an astronomer at the Instituto de Astrofísica de Canarias who works at the observatories in La Palma and Tenerife.

CometLinear.com, 7/29/00

MARK KIDGER: Here, things  got a bit unpleasant. A major forest fire started further down the mountain  about 5pm... We were warned that the fire - the worst in La Palma for some years - was out of control and could reach us in an hour...

CometLinear.com, 8/3/00

Mark Kidger: A whole series  of amateurs have pointed me at their own images from around break-up. The best that I have seen so far have been from Jim Vail in California. By a  curious and unpleasant symmetry, his observations have also been hampered by a major forest fire nearby.

This article will first look at  the present status of the comet, and then discuss the two most prominent theories  as to why this comet disintegrated, along with some no so prominent theories  as well.

Current  Status of Comet Linear

NASA Science News, 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.
International Astronomical
Union Circular No.7472, 3-Aug-200

M. Kidger, Instituto de Astrofisica  de Canarias, reports analysis of continued observations of C/1999 S4 at  La Palma: "A 100-s exposure in R, taken by R. Corradi and N. O'Mahoney (Isaac  Newton Group of Telescopes) on Aug. 1.9 UT with the Wide Field Camera on the 2.5-m Isaac Newton Telescope, shows no evidence of a nuclear condensation or subnuclei within the coma. The seeing (measured from a short exposure at the same airmass) was 1".5. It is thus highly improbable that any fragments of the nucleus of significant size exist within the coma. The coma shows  a similar structure to that reported previously, with a well-defined sunward  boundary to the coma similar to the point of a lance, although the brightest  part of the coma is now displaced > 1' tailwards. The tail can be traced  at least 20'."
SpaceWeather.com, 5-Aug-2000
Comet  Linear Blew Apart
Comet LINEAR blew apart so thoroughly last week that astronomers can't find any sizable pieces fromComet  LINEAR blew apart so thoroughly last week that astronomers can't find any sizable pieces from its fractured icy core. New groundbased images of the  comet obtained August 1st at the Isaac Newton Telescope revealed no fragments  brighter than 22nd magnitude. Dr. Mark Kidger, who communicated the observations in IAU Circular # 7472, says the new data "show no evidence of a nuclear  condensation or subnuclei within the coma." The core has seemingly dissolved  into an amorphous haze of gas and dust.
NASA Comet Observation Home  Page, 5-Aug-2000
C/1999 S4 (LINEAR) - The Fizzle of 2000
Charles S. Morris
I dislike the term "fizzle"...but what else can one call it. 99 S4 is now running  between 2 and 2.5 magnitudes fainter than its original predictions. Barring  a major outburst (which seems highly unlikely), the comet may make m1 ~  6.0. Assuming no heliocentric brightening (or fading!),the comet should  brighten another 1.9 magnitudes due to decreasing geocentric distance.
I am back...finally. I have  about 700 e-mail messages to go through and will be updating the page over the next several days. I will update the most recent observations first  and will work backward.

While Mr. Morris is clearing his e-mail, lets take a look at what might have happened.

Since the release of the July 5-7, 2000 images from the Hubble Space Telescope, the "official" debate at this point has split between two camps.

Mark Kidger and Brian Marsden on the right with the Dirty Snowball theory, and Zdenek Sekanina of JPL on the left with a Fragment Collision Theory.

Dirty  Snowball Theory

Favoried by Mark Kidger of the Las Palmas observatory and Dr. Brian G. Marsden, an astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. (Marsden was one of the original founders  of the now-defunct Space Guard effort to track near Earth objects) is the dirty snowball theory.

In simple terms, the essence of  this theory is that the comet was collection of rock glued together by ice. As the ice melted, it disintegrated. Simple!

  • Mark Kidger and Brian Marsden: Here is what might have happened: As perihelion approaches, most of the  ice on the sunward face of the nucleus has sublimed away and we see a tailing-off  of activity as the comet runs out of fresh ice to sublime.
  • At perihelion, fresh ice is  exposed on the face of the nucleus previously in shadow. This provokes a  large outburst of activity. The outburst tails-off as the fresh ice is rapidly consumed by the heat of perihelion. After a period of strong activity, the comet exhausts most of its ice.
  • There is now nothing holding together the solids. The nucleus begins to fragment. As it does so, a small amount of fresh ice from the core of the nucleus is exposed which acts as a detonator, subliming violently and pushing apart the fragments, causing the final, total disruption of the comet. The result is an expanding dust  cloud with very little ice within it, which fades rapidly.
  • This sequence of events corresponds  closely to what was seen.

While Kidger presents a very plausable theory based on his obersvations, he also tells us that he may not have the final answer to what remains a mystery.

  • Mark Kidger: Unlike Comet  Ikeya-Seki, or Comet Maccholz 2, or Comet Shoemaker-Levy 9 there are no  discrete (Comet LINEAR) fragments that have survived the breakup Unlike  Comet Ichimura, Comet LINEAR was very highly condensed and apparently highly active until the moment that it broke-up - comets that simply disappear  are almost always ill defined and highly diffuse objects, without a prominent  central condensation.
  • Why did Comet LINEAR break up? There are a whole series of theories about the breakup In fact, no one really knows why comets breakup at all.

Kidger's answer is refreshing because  it states a simple truth - we do not know why! However, his comment about the"curious and unpleasant symmetry" between forest fires and our interest Comet LINEAR begs the  question: "Do comets cause forest fires?"

In other words, could the same forces that caused the disintegration of Comet LINEAR also have something to do with incredible number of fires now burning out of control across the  globe?

But before we examine that possibility,  we must give our attention to an equally plausible theory that explains the disintegration of Comet LINEAR because the reason the comet disintegrated is the key to understanding everything else.

Fragment Collision Theory

Zdenek Sekanina of the Jet Propulsion  Laboratory in Pasadena, CA theorizes that S4 LINEAR collided with the trailing  fragments of a much large comet.

International Astronomical Union Circular No. 7471, July 30, 2000
Z. Sekanina, Jet Propulsion  Laboratory, reports: "The unusually large nongravitational forces found by B. G. Marsden (MPEC 2000-O07) suggest that comet C/1999 S4 was a trailing fragment of a more massive comet that has been moving in the same orbit,  arrived at perihelion long (centuries?) ago but (not surprisingly) was missed.  Trailing fragments of known comet pairs have a tendency to sudden disintegration (e.g., Sekanina 1997, A.Ap. 318, L5).

Sekanina has also studied the fragmentation  and breakup of other comets to include Comet  Shoemaker-Levy 9 and Machholz 2 (P/1994 P1) but when it comes theories about comets and space, the NASA Dirty Snowball theory stil rules the roost.

While Kidger favors the Dirty Snowball theory, he expresses the opinion that proving Sekanina's fragment collision theory would be highly unlikely.

KIDGER: One theory, proposed on a recent IAU Circular by Zdenek Sekanina is that Comet LINEAR is the  trailing half of a comet that broke-up previously. This is an ingenious theory, although it will be horribly difficult to prove. Given that Comet  LINEAR has fallen in from about 1 light year from the Sun one has to understand  why a comet falling in from so far out would break up.

However, this is not the first time that Sekanina has followed the beat of a different drummer and disagreed. Sekanina theory that Hale-Bopp had a satellite was assailed by Harold Weaver  at the Johns Hopkins University. When it comes to the Hubble Space Telescope, Weaver is the head honcho. If you see an HST image on the web, it means that  Weaver gave the OK.

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