Comet West

Comet West

Comet West to Yield Clues To Early Solar System

During the first two weeks of March 1976, comet West (1975n) put a show that rewarded the early riser with a glimpse of a special phenomenon that provided astronomers with important clues to the early solar system. Results of many scientific experiments will not be available for months, but armed with interpretations of past comets, we can look at a few of the comet`s characteristic features that anyone with an ordinary camera and small telescope could record.

Although comets share basic similarities, they also appear quite individual to the observer on Earth due to differences in orbital elements and viewing geometry, as well as intrinsic physical differences. According to early calculations by Brian Marsden of the Smithsonian Astrophysical Observatory, comet West is a long period comet traveling in a nearly parabolic orbit inclined about 43 degrees to the ecliptic. It approached from the south and had a perehilion distance of about 18.5 million miles.

The comet was bright enough shortly after perihelion (which occurred on February 25) to be seen with a six inch telescope in broad daylight. The reason was apparent a few days later: After moving away from the Sun’s glare, a highly structured dust tail was seen stretching more than 20 degrees behind the nucleus. The high, variable production of dust at perihelion produced synchrones whose change in position reflected the change of the comet’s position as it moved around the Sun.

Comet West. Photograph: J. Linder/SEO

A generalized scenario of what is thought to happen to a comet can help us interpret what was seen in comet West. First, the nucleus, believed to be somewhere between one and six miles in diameter, as an icy amalgamation of dust grains; some grains are silicates ranging in diameter from 0.025 inch to less than .000025 inch. The distribution of dust in the nucleus is unknown, but comet West appeared to display in inhomogeneous mixture. As the comet approached the Sun, it heated up and the ices (probably water, methane and ammonia) vaporized. The outgassing pressure drove the dust away from the nucleus. When the dust no longer collided with gas molecules, solar radiation pressure drove the dust particles away from the Sun to form the dust (or type II) tail. At the same time, some of the gases went through photodissociation and florescence processes. The produced gaseous ions were accelerated by the solar wind and associated magnetic field to velocities over 9,000 m.p.h.

The gas (type I) tail was viewed distinct from the type II tail because of its different positional angle. This “double tail” became more prominent as the type I component became more pronounced and was particularly striking in color photographs, since the type I tail was blue. A look at an objective prism spectrum shows why this was true.

When the light of the comet was broken into a spectrum, a series of emission features were present. The most common was due to carbon and cyanogen molecules (including the somewhat rare C3 molecule) in the coma and ionized carbon monoxide (CO+) in the type I tail. Overlying this is a solar continuum from solid dust particles reflecting sunlight. The relative intensities of the emissions vary from comet to comet. Whether this variation is due to intrinsic composition differences or age (how many times the comet has come close to the Sun and expended some of its volatiles) is unknown. The spectrum of comet West’s tail indicated well developed CO+ emission showed up as monochromatic straight tail images in the blue end of the spectrum. The type II tail simply shows a continuous spectrum with peak intensity around 5,500 angstroms. Comet Kohoutek did know show a good separation of its tails; color film was used to identify the comet’s tail. Comet West did not have this problem, since color differences were obvious, giving the comet an especially asthetic appeal.

The tails grew fainter as they diffused into space and the production rate of new material decreased as the comet moved away from the sun.

Small instruments showed daily changes in the come close to the nucleus. They were subtle changes, hinting at some kind of activity in the nucleus. Detail in the coma was nothing like the striking spiral structure observed in comet Bennett in 1970. The objective prism spectrum had emission features of carbon (both C2 and C3), cyanogen, sodium and methylidyne associated with the coma. Slit spectra obtained with larger telescopes showed much more. Perhaps the most spectacular phenomenon of comet West was the splitting of the nucleus into several pieces during and after perihelion passage. Several observers noticed a secondary nucleus which parted slowly from the primary.

By mid-March observers at New Mexico State University Observatory photographed four distinct nuclei slowly spreading apart. Photographs with the Mt. Lemmon Observatory 61 inch telescope showed their relative brightness changed by as much as 20 percent. Each nucleus developed its own tail, probably a gaseous one of CO+. Tracing the motions of the four bright, separate nuclei back in time, it appears that the nucleus first split around February 25. On March 9 or 10, the secondary piece broke again, leaving behind a piece almost motionless with respect to the first (all these motions in the plane of the sky, since there is no available radial velocity data).

This sort of observation is consistent with an icy conglomerate nucleus with an inhomogeneous mixture of volatiles that may produce fractures. Another consequence of inhomogeneous mixture might be jets of gazes that may be large enough to impart more spin and cause break-up. Observations argue against a loose sandbank model of particles rotating about a center of mass, since such a configuration could not break up into several well defined groups of nearly the same visual magnitude.

It appears that we witnessed a clump of primitive solar system material, kept intact in the deep freeze far from the disruptive forces of the sun, which was recently perturbed. There is still a great deal to learn from satellite observations made in the ultraviolet and application of new ground based techniques on the spatial and temporal distribution of the comet’s components. But, our understanding and appreciation of the “hairy” members of the solar system has deepened.

(Astro-News, Latest News from the World of Astronomy. Astronomy, August 1976)