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Credit: Left, NASA/SAO/CXC/STScI/Lisse et al.
Right, NASA, Harold Weaver (the Johns Hopkins University), and the HST Comet LINEAR Investigation Team


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May 20, 2005
The Explosive Demise of Comet Linear

When comet Linear blew apart in the summer of 2000, the event underscored the failure of popular comet theory to anticipate the actual attributes and behavior of comets. Linear was not the “dirty snowball” of modern comet lore.

In September 1999, the LINEAR telescope in New Mexico detected a comet out beyond the orbit of Jupiter, speeding toward the Sun. Because it was the first instrument to see it, the comet received its name from the telescope.

Linear was estimated to be about a mile wide. As it approached its perihelion in July 2000, many telescopes—including the Hubble Space Telescope—had the comet in clear view. Then strange things began to happen. On July 5, Linear brightened by more than 50 percent in just four hours. It was throwing off large quantities of dust—much more dust than the expected water or other volatiles.

Next, a chunk of the nucleus tore away and “blew” back into the tail where it continued to disintegrate, as can be seen in the Hubble Space Telescope images here.

Then, on July 14, the orbiting Chandra X-ray Observatory discovered that the “dirty snowball” was generating X-rays! (Photo above left).

The mystery of comet X-rays had begun only four years earlier. It had always been supposed that these “frozen” objects would exhibit none of the high-energy reactions necessary to produce X-rays. But then on March 27, 1996, the ROSAT satellite recorded X-rays on the sunlit side of Comet Hyakutake. A NASA report on Hyakutake notes that astronomers “were shocked by what they saw. ROSAT images revealed a crescent-shaped region of X-ray emission around the comet 1000 times more intense than anyone had predicted!” For four years the source of the X-rays remained a mystery, as the ROSAT, EUVE and BeppoSAX satellites detected X-rays and extreme ultraviolet radiation from more than half-a-dozen comets, including Hale-Bopp.

But now, Linear was giving astronomers some telling clues, and the implications were electrical. Chandra viewed the comet Linear repeatedly over a two-hour period. The Observatory’s press release reported that the X-rays were being produced “by collisions of ions racing away from the sun (solar wind) with gas in the comet. In the collision the solar ion captures an electron from a cometary atom into a high-energy state. The solar ion then kicks out an X-ray as the electron drops to a lower energy state”. The authors of the news release do not appear to have known that, in the electric model of comets, this was a predictable reaction between the negatively charged plasma of the comet’s coma and the positively charged ions in the solar wind—nature’s efficient means of X-ray production.

As seen in the X-ray image of Linear above, and as the electric model would anticipate, the X-ray production occurred at the interface of the negatively charged cometary plasma with the positively charged particles of the solar wind.

A NASA Science News story on Linear thus reports, “When ions from the Sun blow past a comet, their strong positive charge attracts negatively-charged electrons from cometary atoms and molecules. In effect, the ions try to neutralize their own unbalanced charge by stealing electrons from the comet”. The report states that electrons contributed by the comet, in uniting with the positive ions from the solar wind, “emit X-rays as they cascade from high-energy to low-energy ionic orbits. This process, called a ‘charge exchange reaction’, was first proposed in 1997 as a possible reason for cometary X-rays”.

But the NASA report assumes, in contradiction of evidence gathered for almost twenty years, that it is neutral atoms in the coma that contribute the electrons. More reasonable is the contention of the electric theorists that comets are the cathodes, or negatively charged objects, in an electrical exchange with the Sun. In this view, excess electrons will combine preferentially with the positive ions in the solar wind. In fact, the excess of electrons in a cometary coma was first noted in 1986, when the Giotto spacecraft detected an abundance of negatively charged atoms in the inner coma of Comet Halley.

Also, as a matter of historical record, the NASA statement that “charge exchange reaction” was first proposed in 1997 misses the mark by a century. The electric comet hypothesis has been around since the nineteenth century. Though it virtually disappeared from official scientific discussion by 1930, the concept received its greatest clarity from the contributions of engineer Ralph Juergens beginning in 1972. Juergens proposed an electric Sun model, along with the corollary that cometary comas and tails are produced by an electrical exchange between the Sun and the comet. Later, in the early 80’s, physicist James McCanney set forth his own version of the electric comet. He predicted that comets would be found to emit X-rays.

Comet Linear had more evidence to present. As the comet neared its perihelion or closest approach to the Sun—about 114 million kilometers (70 million miles) from the Sun, or three quarters of the distance from the Sun to Earth—astronomer Mark Kidger was observing Linear with the Jacobus Kapteyn Telescope at La Palma in the Canary Islands. He noted something strange. The normal teardrop shape of the coma was undergoing an unexpected metamorphosis. Over several nights he watched the comet elongate into a "cigar" shape. Kidger soon realized that the nucleus of Linear was breaking apart—and catastrophically. This was not merely a fragmentation of the comet into separate visible pieces. The comet was dissolving in front of his eyes.

“Comet LINEAR seems to be dissolving into an amorphous haze of gas and dust”, exclaimed a NASA Express Science News release. “The break-up of Comet Linear as it swept past the sun last week has shocked astronomers into rethinking theories of the origins of such rocky ice balls”, reported on August 4, 2000.

How did this happen? A NASA release of July 31, 2000, reports that, “Intense solar heating apparently triggered a massive disruption of the comet's fragile icy core when it passed close to the Sun”. Kidger suggested the same thing, invoking “intense heating” and “thermal stresses” on the comet. But it is not reasonable to assume that a mile-size ice chunk would explode in space under something as mild as solar radiation millions of miles from the Sun. As an icy body sublimates in the Sun, it cannot even convey heat a few inches into its interior. An explosion due to heating, involving extreme forces deep within a body, is unthinkable.

Many comet watchers began to consider seriously whether comets are actually loosely aggregated collections of "mini-comets", permitting them to fly apart when disturbed. Some began to speak of Linear as an aggregation of cosmic fluff—a “wimpy fluff ball”, as astronomer Donald Yeomans put it.

But prior picture of the comets Halley and Borrelly—and most recently of comet Wild 2—make clear that comet nuclei are solid objects. It was the Stardust mission to Wild 2 that produced the best pictures ever of a comet nucleus. It showed a well-defined and cratered surface with no indications of separate objects held in a flimsy aggregation.

More details on comet nuclei will be forthcoming soon, when the Deep Impact mission fires a 370 kilogram copper projectile into the nucleus of Comet Tempel 1. The event is scheduled for July 4.

In the electric model of comets, there is nothing unexpected in an explosive demise. As a comet moves through the radial electric field of the Sun, approaching perihelion, the nucleus suffers the maximum electrical stress. This usually results in an increase in brightness of the nucleus due to a larger number of cathode arcs operating simultaneously, explosively removing solid material from the nucleus and accelerating it into space to form the dust tail. Both of these conditions were noted in the case of Comet Linear, suggesting that the comet was progressing toward an internal discharge.

A comet nucleus can be compared to the insulating material in a capacitor. As charge is exchanged from the comet’s surface to the solar wind, electrical energy is stored in the nucleus in the form of charge polarization. This can easily build up intense mechanical stress in the comet nucleus, which may be released catastrophically, as in a capacitor when its insulation suffers rapid breakdown. The comet will explode!

As suggested by electrical theorist Wallace Thornhill, “comets break up not because they are chunks of ice ‘warming’ in the Sun, and not because they are aggregations of smaller bodies, but because of electrical discharge within the nucleus itself”.

There were more surprises. Perhaps the greatest shock came from analysis of the debris left by the comet’s dissolution. According to Hal Weaver, an astronomer at Johns Hopkins University in Baltimore (as reported in an AP story on May 18, 2001), researchers were “surprised at the ratio of ice to dust and rock in Linear”. Analysis showed that Linear “had about 100 times more solid rock and dust than ice”.

But the problem of missing water on the nucleus of comets is as old as the Giotto probe of Comet Halley, which could not find any definitive evidence of water but did find evidence against the presence of water. No water could be found on the nucleus of comet Borrelly. When comet Shoemaker-Levy 9 broke apart, astronomers reasoned that the fractured nucleus would expose fresh ices that would sublimate furiously. So several ground-based telescopes and the Hubble Space Telescope trained their spectroscopes on the tails of the fragments of SL-9, looking for traces of volatile gases. None of the gases was found.

Events and observations surrounding the breakup of Comet Linear thus offer many pointers to the true electrical nature of cometary intruders. Comets may or may not possess volatiles, and we can be confident that comets exhibit much more than sublimating ices. Only electric discharge will account for the full range of new data on comets.


David Talbott, Wallace Thornhill
Amy Acheson
  CONTRIBUTING EDITORS: Mel Acheson, Michael Armstrong, Dwardu Cardona,
Ev Cochrane, C.J. Ransom, Don Scott, Rens van der Sluijs, Ian Tresman
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