Monday February 6, 2006
Mystery of Kohoutek
By D.J. BATZER
THERE are times when failure tells us more than success. We learn from failure, while too often success only makes us smug.
On the evening of March 7, 1973 while he was making photographic observations of minor planets at the Hamburg Observatory in West Germany, Lubes Kohoutek discovered a diffuse spot of light moving very slowly northwestward in the constellation of Hydra (the Water Serpent).
The story may be said to begin with an item which appeared in Sky and Telescope magazine in May 1973, which reported that the newly-discovered comet Kohoutek 1973f should become a conspicuous naked-eye object of first magnitude, or brighter, by year end.
When it was discovered, comet Kohoutek was 5 AU from the Sun (one astronomical unit, AU, is the average Earth/Sun distance). According to preliminary orbital elements calculated by Brian Marsden, of the Smithsonian Astrophysical Observatory, it will be only 0.14 AU from the Sun at perihelion (point of closest approach). Because of its brightness at 5 AU, it showed much promise of being a bright comet. Also, it would pass Earth at less than 95 million kilometres after perihelion, which was considered favourable.
In 1965 a total of 583 comets had plotted orbits. Of these, 99 were short-period (less than 200 years) comets. When an object has a maximum orbital distance 20 times its minimum, it has an eccentricity, “e”, greater than 0.9. Halley’s comet, for example, has a period of 76 years and an “e” of 0.967. If e is 0.999 it would have a period of 32,000 years.
If we are trying to measure long-period orbits from observation, this is made over a tiny fraction of the orbit and it is nearly impossible to determine e accurately. Of the 583 “measured” orbits, 284 are known to be fictitious!
When we see a comet coming in from the edge of the solar system we can presume its “e” will not be significantly less than 1.000. But it cannot be exactly 1.000 as that would be a parabola and not an ellipse.
When comet Kohoutek first showed up, it was inbound from a pretty good distance out, presumably from the so-called Oort cloud, a halo of perhaps 50 million comets, with distances ranging from 30,000 to 50,000 AU from the Sun.
A typical comet from the Oort cloud will have a period of three million years, which period was assumed for Kohoutek. There was an excellent chance that this would be its first pass through the inner solar system. If so, it would be rich in frozen gases and make a spectacular display, as tens of thousands of cubic metres of gas boiled away. This was the promise of Kohoutek, but it did not happen.
What went wrong
The first peculiarity began showing up as it was found increasingly possible to refine the elements of the comet’s orbit, as more of the orbit became available for analysis. With each successive approximation the “e” decreased. Ultimately it was found that the orbital period was about 75,000 years, far less than originally thought. The mean orbital distance was about 1,800 AU. This is too little for Kohoutek to have come from the Oort cloud.
A different sort of problem began showing up in October 1973 when brightness measurements showed that infrared luminosity was comparable to reflected sunlight. This implied that matter being shed was mainly dust particles and there was little indication of large quantities of gases.
Matters looked up briefly when Kohoutek passed perihelion. There were a few brief reports of “quite satisfactory” luminosity, but this burst of optimism faded by the first week of January 1974.
The peculiarity of the comet became increasingly pronounced. Why should the comet have been so bright at discovery yet so disappointing at perihelion? Why is the spectral data so different from other comets?
The difficulties can be resolved if Kohoutek is a Kuiper Belt Object (KBO) with a diameter of about 50km to 75km, rather than 15km to 30km, a stoney basalt body of irregular dimensions, deeply fissured. The surface overlain to a depth of a metre or more by a blanket of frozen gases, which sublimated at a much greater rate than is usual for a comet at the same distance from the Sun. This could account for the early observations and the initial promise.
The layer of frozen gas would burn off rather rapidly as Kohoutek approached the Sun. This left the brightness to occasional pockets of frozen gas, the larger size of the body (as discussed above) and a layer of dust or regolith, which coated the surface. It would develop a broad diffuse tail slightly curved and showing a spectrum typical of dust grains reflecting sunlight. At the time of Kohoutek’s discovery, the existence of KBO was not known.
There is still no sharp dividing line between asteroids and comets, as witness the body named Chiron orbiting beyond Saturn.