SP-404 Skylab's Astronomy and Space Sciences

 

4. Observations of Comet Kohoutek.

 


[
42]

Figure 4-1. Comet Kohoutek photographed with the far-ultraviolet electrographic camera during a Skylab spacewalk on December 25, 1973. It is shown here in a false-color reproduction of a black and white photograph.

Figure 4-1. Comet Kohoutek photographed with the far-ultraviolet electrographic camera during a Skylab spacewalk on December 25, 1973. It is shown here in a false-color reproduction of a black and white photograph.

 

[43] Comet Kohoutek (1973 XII) was discovered by Lubos Kohoutek during a search for asteroid images on photographic plates taken in early March1973 at the Hamburg Observatory, in the Federal Re-public of Germany. Calculations of its size and orbit showed it to be a large comet that would pass close to the Sun, reaching perihelion at the end of 1973.

This early discovery of a large comet in an orbit that would carry it close to the Sun prompted the National Aeronautics and Space Administration to initiate "Operation Kohoutek," a program to coordinate widespread observations of the comet from ground observatories, aircraft, balloons, rockets, unmanned satellites, and Skylab. This program was headed by Stephen P. Maran of the Goddard Space Flight Center. The third Skylab mission was rescheduled so as to make the best use of this opportunity-specifically to permit observations from Skylab during a period centered on perihelion. It is during this period that the most interesting and dramatic changes happen to comets, and it is also during this period that observations from the Earth's surface are hardest to make or even impossible because light from the nearby Sun is scattered by the Earth's atmosphere into instruments aimed at the comet.

Another factor making Comet Kohoutek an attractive subject for study was the fact that orbital calculations suggested it was a new comet-one that had never before passed close to the Sun and would therefore be expected to differ from comets that had periodically returned.

Figure 4-1 is a false-color contrast-enhanced picture of Kohoutek taken in Lyman-alpha light. It shows a hydrogen halo that is three times the diameter of the Sun. The brightest area (the center) is yellow, and the succession of colors outward indicate decreasing brightness.

Figure 4-2 shows Comet Kohoutek photographed on April 28, 1973, at the Kitt Peak National Observatory, Tucson, Arizona. The photograph was made with NASA's integrating vidicon system by astronomers Stephen P. Maran, Hong-Yee Chiu, and Robert W. .....

 


Figure 4-2. Comet Kohoutek (arrow) photographed at the Kitt Peak National Observatory on April 28, 1973, a few weeks after its discovery. North is toward the left; west is toward the top.

Figure 4-2. Comet Kohoutek (arrow) photographed at the Kitt Peak National Observatory on April 28, 1973, a few weeks after its discovery. North is toward the left; west is toward the top.

 

[44] .....Hobbs of the Goddard Space Flight Center. At that time, a few weeks after its discovery, the comet was still 4.2 AU from the Sun, the magnitude of the nucleus was 17.2, and the estimated total magnitude was about 15.

 

The Comet as Seen from Earth

 

When a comet is first observed, sometimes at great distances from the Sun, it appears as a fuzzy object among the stars, as in figure 4-2. As the comet comes closer to the Sun, its appearance changes, and it is seen as having a bright center surrounded by a fuzzy cloud. The bright center is called the central condensation and surrounds the nucleus. The fuzzy cloud is called the coma. The change is real and is caused by the Sun's heat. Some of the surface material is vaporized and, together with dust that is liberated, forms a cloud. The remains of the original mass and the cloud both reflect or scatter light and are seen as the nucleus and coma, respectively.

As a comet approaches within a couple of astronomical units of the Sun, its most spectacular feature, the tail, gradually appears. Figures 4-3 and 4-4 show Comet Kohoutek photographed from the Joint Observatory for Cometary Research, South Baldy Mountain, New Mexico. The photograph in figure 4-3, taken on December 7, 1973, shows the comet about 3 weeks before its closest approach to the Sun (perihelion).

When it gets close to the Sun, the comet disappears from view against the bright background. Emerging on the other side of the Sun, it is again visible. Some of the visual differences observed among comets are attributable to relative distances and angular positions between comet, Sun, and Earth. Other differences are probably caused by the size and constituents of the comet.

Knowledge of the structure and composition of comets comes from spectral study and dynamic analysis. According to the most widely accepted theory of comet structure, the nucleus consists of a "dirty snowball," whose principal constituents are various frozen gases and solid particles of dust. The particles of minerals, or stony dust, range from perhaps a few millimeters in diameter to microscopic size and are held in place by the matrix of frozen gases.

As a comet approaches the Sun, its surface gradually heats up and the frozen gases begin to vaporize. Because of the low pressure in space, these materials will turn immediately from solid into gas and begin to form the comet's growing coma. The dust particles interspersed in the volatile matrix are released and initially mix with the gases in the coma.

 


Figure 4.3. Comet Kohoutek photographed from the Joint Observatory for Cometary Research, South Baldy Mountain, New Mexico, on December 7, 1973, about 3 weeks before perihelion.

Figure 4.3. Comet Kohoutek photographed from the Joint Observatory for Cometary Research, South Baldy Mountain, New Mexico, on December 7, 1973, about 3 weeks before perihelion.


[
45]

Figure 4-4. Changes in the appearance of Comet Kohoutek from January 11 to January 20, 1974. Photographs taken at the Joint Observatory for Cometary Research. Link to a larger picture.

Figure 4-4. Changes in the appearance of Comet Kohoutek from January 11 to January 20, 1974. Photographs taken at the Joint Observatory for Cometary Research.

 

[46] The neutral gases continue to expand into an ever larger cloud about the comet. While this happens, sunlight dissociates some of the more complex molecules into simpler chemical species. One dissociation product is atomic hydrogen, which gives rise to the very large hydrogen halo observed around Comet Kohoutek (fig. 4-1). Dissociated water vapor is probably the principal source of this hydrogen.

Some gases released from the comet become ionized and interact strongly with the solar wind-the flow of ions streaming from the Sun. This ionized gas forms a plasma tail. Such a tail (type I) points away from the Sun. It is usually not appreciably curved, although some twisting and irregular motion within the tail may be present, as is the case in figure 4-3.

Sunlight pushes the microscopic dust particles released from the comet away from the Sun, and away from the comet, to form a tail. This tail of dust, known as a type II tail, points generally away from the Sun, regardless of the direction in which the comet is moving. However, the different motion of the dust particles released at different times will cause the tail to appear somewhat curved. This type of tail is generally white because it unselectively reflects or scatters visible light from the Sun.

When the Earth is near the orbital plane of the comet, part of the dust tail occasionally seems, in projection, to be pointing toward the Sun. This type III "antitail," which consists of large dust particles, is discussed later. Such an "antitail" on Comet Kohoutek was visible from Skylab shortly after perihelion, before it was possible to view it from ground-based observatories.

Pictures of Comet Kohoutek shown in figure 4-4 were taken at the Joint Observatory for Cometary Research.....

 


Figure 4-5.  Typical comet spectrum (box) and the spectral capabilities of Skylab instruments (identified by experiment number at left). Link to a larger picture.

Figure 4-5. Typical comet spectrum (box) and the spectral capabilities of Skylab instruments (identified by experiment number at left).


[
47]

Figure 4-6. Comet Kohoutek's path around the Sun. Link to a larger picture.

Figure 4-6. Comet Kohoutek's path around the Sun.

 

....and show the changes in appearance from day to day, and in some cases from hour to hour, of an active comet. The curved dust tail reflects and scatters the light of the Sun and therefore appears white to yellow on a color photograph. The plasma tail shows many branches and numerous waves and whorls, indicating the irregular fluctuations of the solar wind. Studies of such plasma tails gave rise to the first ideas of the existence of the solar wind. In color photographs, the plasma tail appears bluish. In order to bring out details of the tail structures, these photographs were taken at exposures that overexposed the head, and therefore no details are visible in the central region of the comet.

 

Instrument Selection

 

During the preparations to observe Comet Kohoutek, it was determined that some of the instruments already on board Skylab for other experiments could be used to study the comet (fig. 4-5). Some of these instruments were designed to view the bright Sun, and their use to observe the comet was considered to be exploratory in....

 


[
48]

Figure 4-7. The geometry of observation of Comet Kohoutek- preperihelion.

Figure 4-8. The geometry of observation of Comet Kohoutek- perihelion.

Figure 4-7. The geometry of observation of Comet Kohoutek- preperihelion.

Figure 4-8. The geometry of observation of Comet Kohoutek- perihelion.

 

....nature. To design, build, test, and launch new instruments was not possible since such a process generally takes 2 to 3 years. However, one instrument, the backup far-ultraviolet electrographic camera (S201) used on Apollo 16, was available. After minor modifications, it was taken up to Skylab by the third crew.

The far-left column of figure 4-5 lists the instruments on Skylab, identified by experiment numbers. As individual instruments are discussed, these numbers will be given for reference. To the right of each number is shown the portions of the electromagnetic spectrum in which the instrument makes observations. The wavelength scale for this spectral response is given in angstroms or micrometers along the bottom of the blue box. Within the box a typical comet spectrum is shown, with indications of several of the molecules responsible for individual features. Thus figure 4-5 illustrates which Skylab instruments could be expected to measure the various spectral features of a comet.

 

Comet Orbits

 

The path of a comet is essentially an ellipse, sometimes of such great eccentricity that it is very nearly a parabola. An ellipse is a closed curve, and a comet with an elliptical path may be expected to have a finite period of revolution about the Sun. Early predictions for Comet Kohoutek indicated it had an elliptical orbit with a period 50000 to 200000 years for one such journey. A part of this uncertainty was associated with the difficulty of making an early determination of the comet's position in space. Also significant are the perturbations experienced by the comet in its long journey, especially those caused by the gravitational force of Jupiter.

Kohoutek's closest approach to the Sun (fig 4-6), at a distance of 0.142 AU, was made on December 28, 1973, although to an Earth observer the comet appeared closer on December 27. The orbit was inclined to the ecliptic plane by 14°, and the Earth passed through this plane on December 9, 1973. Another interesting circumstance occurred on January 15, 1974, when the dust tail's position relative to the Earth gave an observer on Earth the best view of its entire length.

 

Skylab's Location

 

Because Skylab was above the Earth's atmosphere, its instruments were in a favorable position to photograph Comet Kohoutek as it swung around the Sun. Because Skylab orbited the Earth approximately 15 times daily, the viewing opportunities were shorter, but more frequent, for the astronauts than for observers on Earth.

Observations by instruments in the airlock module were made during the brief periods that the comet was visible but the Sun was not (because Skylab was in the Earth's shadow). The observations also required maneuvering Skylab. Before perihelion, Kohoutek was viewed between "cometrise" and sunrise (fig. 4-7). This observation period varied from 0 to 26 min, as determined by the positions of the comet, the Sun, and the space station.

 


[
49]

Figure 4-9. The geometry of observation of Comet Kohoutek- postperihelion.

Figure 4-9. The geometry of observation of Comet Kohoutek- postperihelion.

 

When the comet was nearest the Sun, the astronauts focused the solar observatory instruments on the comet (fig. 4-8). This was done almost continuously during 21 consecutive orbits centered around the closest angular approach, as seen from the Earth and Skylab, on December 27, 1973.

Like the preperihelion observations, the postperihelion measurements required shadowing and had to occur after sunset but before "cometset" (fig. 4-9). The maximum postperihelion time available for viewing Kohoutek was approximately 17 min during each orbit.

 

Hydrogen Halo

 

Photographs of the halo about the comet were taken in hydrogen Lyman-alpha light with the electrographic camera (S201) aboard Skylab and are shown in figure 4-10. The size and brightness of the halo are plotted in....

 


Figure 4-10. Comet Kohoutek's halo photographed in hydrogen Lyman-alpha light with the electrographic camera aboard Skylab. In all photographs the Sun is toward the bottom. All times are GMT. A.11-26-74, 23:29. B.12-5-74, 22:08. C. 12-12-74,  01:46 (near horizon); D. 12-16-74, 17:41 (near (Greek letter) pi,  (Greek letter) delta Scorpio); E. 12-23-74, 16:03 (near  (Greek letter) theta Ophiuchus); F. 12-25-74, 21:33. Link to a larger picture.

Figure 4-10. Comet Kohoutek's halo photographed in hydrogen Lyman-alpha light with the electrographic camera aboard Skylab. In all photographs the Sun is toward the bottom. All times are GMT. A.11-26-74, 23:29. B.12-5-74, 22:08. C. 12-12-74, 01:46 (near horizon); D. 12-16-74, 17:41 (near (Greek letter) pi, (Greek letter) delta Scorpio); E. 12-23-74, 16:03 (near (Greek letter) theta Ophiuchus); F. 12-25-74, 21:33.

 

[50]....figure 4-11 and are shown to have reached a preperihelion maximum on December 12, 1973. On the basis of a simplified comet model, and allowing for changes in camera sensitivity and Earth-to-comet distances, Thornton Page, principal investigator for these observations, suggests that a layer of volatile material in the comet nucleus boiled off just before December 12.

The general behavior of the hydrogen cloud surrounding the comet confirms earlier findings and gives support to the current "dirty-snowball" model. Data on the growth and morphology of the halo gathered by Skylab represent the first extended observation performed on any comet.

The curves in figure 4-12 illustrate how the comet's calculated hydrogen production rate varied with the comet-to-Sun distance. They are based on the S201 Skylab data and results of a similar experiment conducted from an Aerobee rocket. The outflow velocity of hydrogen atoms from the nucleus of the comet was calculated to be approximately 8 km/sec.

 


Figure 4-11. Size and relative brightness of Comet Kohoutek's hydrogen halo as a function of comet-to-Sun distance. Link to a larger picture.

Figure 4-12. Calculated rate of hydrogen production by Comet Kohoutek as a function of comet-to-Sun distance. Link to a larger picture.

Figure 4-11. Size and relative brightness of Comet Kohoutek's hydrogen halo as a function of comet-to-Sun distance.

Figure 4-12. Calculated rate of hydrogen production by Comet Kohoutek as a function of comet-to-Sun distance.

 

[51] Figures 4-13 through 4-15 illustrate the use of sophisticated image-processing techniques to reveal information not readily obtained from simple images. Figure 4-13 is a conventional image of the sky obtained on December 25, 1973, with the electrographic camera at wavelengths from 125 to 160 nm. Figure 4-14 was obtained from an enlargement of part of figure 4-13 by a photographic technique in which the varying optical density of the film (the "brightness" of the sky) is translated into various colors. The original image, of course, has a continuous gradation of density, but the processed image is divided into a few zones of color corresponding to ranges of density. Despite this loss, the false-color image reveals more detail to the human visual system than does the original.

In figure 4- 15, the same result was obtained by a much more elaborate method. The original image was scanned with a microdensitometer, and the density of the film was measured digitally at millions of points on a regular grid. From those millions of numbers, recorded on magnetic tape, a computer produced a "contour map" in which the lines are lines of equal intensity in the original image.

Figures 4-13 to 4-15 show a longer tail than do the other photographs at these wavelengths. The source of the light, however, is still an unsolved puzzle. The first conjecture was that it came from atomic oxygen radiating at 130.4 and 135.6 nm. Atomic oxygen, however, has too short a half-life under these conditions to give a tail of this length. It would not last long enough to travel that far at the calculated velocity. Another suggestion is that an unidentified molecule is absorbing and reemitting the far-ultraviolet light. Thornton Page suggests that the visibility of the extended tail in the far ultraviolet may result from scattering by dust particles.

 

The Ultraviolet Spectra of Kohoutek

 

Skylab's ultraviolet objective-prism spectrograph (ultraviolet stellar astronomy experiment, S019) obtained nine images of Kohoutek; five of these are shown in figure 4-16. Data analysis was performed at the University of Texas. The objective-prism spectrograph was designed to produce the ultraviolet spectra of stars at wavelengths of 130 to 500 nm. The second photograph shows several dispersed stellar images in which visible light lies at the right and far-ultraviolet light lies at the left of each highly elongated image. The bright core of the comet's image shows only a slight elongation because there is no significant ultraviolet radiation from the comet. The comet's images were formed by light with wavelengths from 300 to 500 nm and indicate a surprisingly sharp cutoff of ultraviolet radiation below 300 nm.

The comet nucleus in figure 4-16 appears double on the first three dates. This double structure is confirmed in figure 4-17, which is an isophote map of the comet's appearance on December 13. The two knots in the nucleus represent the wavelength separation of the two strongest emission bands in the 300- to 500-nm region of the spectrum-namely, the emission from OH at 309 nm and the emission from CN at 388 nm. The fact that the double structure is not so clearly seen in the last two photographs in figure 4-16 is evidence that the OH emission was considerably weaker on those dates.

 

Kohoutek Visually Observed

 

Figure 4-18 shows sketches of Comet Kohoutek made by Edward G. Gibson, scientist pilot of the third crew. These sketches are based on the crew's collective impressions of the comet's appearance on December 29, 1973, as observed through 10-power binoculars. The upper drawing was made just after perihelion and shows the long, yellow-white "antitail" pointed toward the Sun. The head of the comet is extremely bright, and the tail is pointing away from the Sun. The lower drawing...

 


Figure 4-13. Comet Kohoutek and stars photographed on December 25, 1973, by Skylab's electrographic camera filtered for wavelengths from 125 to 160 nm.

Figure 4-13. Comet Kohoutek and stars photographed on December 25, 1973, by Skylab's electrographic camera filtered for wavelengths from 125 to 160 nm.


[
52]

Figure 4-14. False-color enhancement of the photograph shown in figure 4-13.

Figure 4-14. False-color enhancement of the photograph shown in figure 4-13.


[
53]

Figure 4-15. Isophote plot of the photograph shown in figure 4-13.

Figure 4-15. Isophote plot of the photograph shown in figure 4-13.

 

....illustrates isophotes (lines of equal intensity) with the comet brightest at the nucleus and least at the outer edges. The length of the visible comet was determined from the angle it subtended, estimated from the amount of the field it filled in binoculars with a 7° field of view.

Pencil sketches and notes were made of the color, shape, size, and intensity of the comet on 10 different days. The color sketches shown in figure 4-19 were made by an artist after the mission, utilizing Gibson's notes and recollections.

 

Kohoutek Passing the Sun

 

The solar observatory's control and display panel is shown in figure 4-20. It was from this panel that the astronauts oriented Skylab to point the solar instruments at the comet as it passed near the Sun.

The white-light coronagraph (S052), designed for solar corona observations by R. MacQueen and associates at the High Altitude Observatory in Boulder, Colorado, was used to obtain measurements while the comet appeared closest to the Sun. This instrument made more than 1600 photographs from December 14, 1973, to January 6, 1974, at wavelengths of 350 to 700 nm. Analysis of the photographs provided data on dust-production rates and grain-size distribution in the comet.

A 27-sec exposure of Comet Kohoutek (fig. 4-21) was made at 2353 GMT on December 28, 1973. The instrument was not pointed directly at the Sun. The picture shows the "antitail," or "sunward spike," of the comet approximately 13 hr after perihelion passage. Instrumental vignetting has reduced the brightness of the tail so that it does not appear. Detailed photometric measurements of these images provide data that permit the size, distribution, and production rate of the dust particles that make up the spike to be estimated as a function of time. Images of distant stars as faint as visual magnitude 7.4 are visible in the original photographs.

Figure 4-22 shows a series of photographs taken by the white-light coronagraph on December 27 and 28, 1973, when the comet was passing the Sun. The first picture was taken at 0441 GMT on December 27 and the last at 0201 GMT on December 28. The coronagraph was designed to improve photographs of the corona by....

 


Figure 4-16. Ultraviolet images of Comet Kohoutek obtained with Skylab's objective-prism spectrograph. Dates and exposure times (from left to right): December 13, 200 sec; December 16, 270 sec; January 7, 400 sec; January 8, 500 sec; and January 12, 720 sec.

Figure 4-16. Ultraviolet images of Comet Kohoutek obtained with Skylab's objective-prism spectrograph. Dates and exposure times (from left to right): December 13, 200 sec; December 16, 270 sec; January 7, 400 sec; January 8, 500 sec; and January 12, 720 sec.



[
54]

Figure 4-17. Isophote plot of the ultraviolet photograph of Comet Kohoutek taken on December 13, 1973 (see figure 4-16).

Figure 4-17. Isophote plot of the ultraviolet photograph of Comet Kohoutek taken on December 13, 1973 (see figure 4-16).

 

.....flattening the intensity gradient through deliberate vignetting. Because of this designed characteristic of the instrument, the image of the comet shows reduced brightness when the comet is near the occulting disk. Photometric analysis of the white-light coronagraph data for this period provided information on Kohoutek's brightness variation with time and filled the gap in Earth-based observations between preperihelion and postperihelion.

 

Other Solar Observatory Results

 

Five photographic plates exposed in the solar observatory's ultraviolet spectrograph (S082B), operating in the spectral range of 97 to 394 nm, resolved the line profile of 121.6-nm Lyman-alpha light reflected and scattered by Comet Kohoutek. However, any other emission lines in this waveband were not detected because of basic limitations of the instrument's sensitivity, Skylab's pointing capability, and the comet's brightness. Richard....

 

 


Figure 4-18. Sketches of Comet Kohoutek made by Edward G. Gibson, scientist pilot of the third Skylab manned mission, illustrating the crew's collective impressions of the comet's appearance on December 29, 1973. The scale is E-M units (Earth-Moon distance, ~380 000 km). Link to a larger picture.

Figure 4-18. Sketches of Comet Kohoutek made by Edward G. Gibson, scientist pilot of the third Skylab manned mission, illustrating the crew's collective impressions of the comet's appearance on December 29, 1973. The scale is E-M units (Earth-Moon distance, ~380 000 km).


[
55]

Figure 4-19. Artist's conception of a changing Comet Kohoutek, based on sketches and descriptions by scientist pilot Edward G. Gibson. Link to a larger picture.

Figure 4-19. Artist's conception of a changing Comet Kohoutek, based on sketches and descriptions by scientist pilot Edward G. Gibson.



Figure 4-20. Astronaut Gibson at control and display panel of the solar observatory aboard Skylab.

Figure 4-20. Astronaut Gibson at control and display panel of the solar observatory aboard Skylab.


[
56]

Figure 4-21. The << antitail>> (sunward spike ) of comet kohoutek seen in a 27-sec exposure made at 2353 gmt on december 28 , 1973 about 13 hr after perihelion passage. the instrument was not pointed directly sun. Link to a larger picture.

Figure 4-21. The "antitail" (sunward spike) of Comet Kohoutek seen in a 27-sec exposure made at 2353 GMT on December 28, 1973, about 13 hr after perihelion passage. The instrument was not pointed directly at the Sun.


[
57]

Figure 4-22. Series of photographs taken by the white-light coronagraph on December 27 and 28, 1973, when Comet Kohoutek was passing the Sun. Link to a larger picture.

Figure 4-22. Series of photographs taken by the white-light coronagraph on December 27 and 28, 1973, when Comet Kohoutek was passing the Sun.

 

[58] Tousey and David Bohlin of the U.S. Naval Research Laboratory reported that the full width at half maximum intensity of the Lyman-alpha line was 0.013 nm. H. Uwe Keller of the Laboratory for Atmospheric and Space Physics, University of Colorado, showed that this linewidth was consistent with a hydrogen outflow velocity of 8 to 10 km/see, which agrees with other observations of Kohoutek's hydrogen halo.

An ultraviolet photoelectric scanning instrument (S055) in the solar observatory operated in the spectral range of 29.6 to 135.0 nm. This instrument was used to detect emission by hydrogen in the coma for correlation with other instruments. Edmond Reeves of the Harvard College Observatory reported that the intensity of the Lyman-alpha line of atomic hydrogen was measured on a number of occasions during scans of the comet.

 

Search for Helium

 

An attempt was made with the extreme-ultraviolet spectroheliograph (S082A) to photograph the comet in the radiation from neutral and singly ionized helium at 58.4 and 30.4 nm, respectively. Although it was believed most unlikely that Comet Kohoutek would produce a helium halo, a long exposure with the extreme-ultraviolet spectroheliograph was made during the comet's closest approach to the Sun. The Naval Research Laboratory team reported that no image was detected. The instrument's sensitivity therefore sets an upper limit for the column density of such a helium halo.

 

Search for X-Rays

 

Attempts to measure the emission of X-rays were made with two solar observatory X-ray instruments. The exposure was considered exploratory in nature. The instruments were capable of measuring wavelengths of 0.2 to 6.0 nm. Kohoutek was not expected to give off any X-rays, but some possibility existed that more energetic radiation from the Sun might have caused the comet to fluoresce, yielding information on the nature of cometary material. However, Skylab's X-ray instruments recorded no such emission from Kohoutek.

 

Kohoutek's Antitail

 

As mentioned earlier, Comet Kohoutek displayed an "antitail" (sunward spike) just after perihelion. The astronauts' sketches and the white-light coronagraphs provided data that helped to clarify the antitail's development and the distribution of particles in the comet.

A theoretical analysis was made by H. Uwe Keller of the paths of dust particles leaving the comet under the influence of a constant radiation pressure (zero initial velocity relative to the comet). The calculations were made for dust particles of four sizes. The smaller particles are driven relatively large distances away from the comet; larger particles move smaller distances away from the comet. As the comet nears perihelion, the trail....

 


Figure 4-23. Theoretical trajectories (syndynes) at 2400 GMT on December 29, 1973, of dust particles leaving the comet under the influence of a constant radiation pressure (zero initial velocity relative to the comet). The calcultations were made for four sizes of particles. Top: calculations for the comet's orbital plane. Bottom: calculations to the skyplane  (i.e., an idealized observer's view from the Earth). The term 1-µ is the acceleration exerted on a particle by solar radiation pressure, expressed as a fraction of solar gravity on the particle. The term t is the time in seconds since the particles left the nucleus. Link to a larger picture.

Figure 4-23. Theoretical trajectories (syndynes) at 2400 GMT on December 29, 1973, of dust particles leaving the comet under the influence of a constant radiation pressure (zero initial velocity relative to the comet). The calcultations were made for four sizes of particles. Top: calculations for the comet's orbital plane. Bottom: calculations to the skyplane (i.e., an idealized observer's view from the Earth). The term 1-µ is the acceleration exerted on a particle by solar radiation pressure, expressed as a fraction of solar gravity on the particle. The term t is the time in seconds since the particles left the nucleus.


[
59]

Figure 4-24. A 120-sec exposure made on December 5, 1973, showing the comet with a slight tail. (A solar panel is seen in the foreground).

Figure 4-25. A 60-sec exposure made immediately after the photograph in figure 4-25, with the lens purposely focused at 15 ft.

Figure 4-24. A 120-sec exposure made on December 5, 1973, showing the comet with a slight tail. (A solar panel is seen in the foreground).

Figure 4-25. A 60-sec exposure made immediately after the photograph in figure 4-25, with the lens purposely focused at 15 ft.

 

.....of larger particles emitted earlier becomes visible by scattered and reflected sunlight. Its position is such that it appears as a "sunward spike." Figure 4-23 shows the loci of particles as calculated for 2400 GMT on December 29, 1973. This time happens to be close to those of the white-light photographs shown in figure 4-22. The curves in the upper diagram are in the comet's orbital plane; those in the lower diagram are in the skyplane (i.e., an idealized observer's view from the Earth).

Before the mission, Z. Sekanina of the Smithsonian Astrophysical Observatory predicted that a "sunward spike" would probably develop just after perihelion. The astronauts' observations were consistent with his prediction, indicating that Kohoutek's preperihelion dust emission was very high. From the deviation in the direction of the sharp leading edge of the spike from the sunward direction (some 5° to 7°), he estimated that the part of the spike near the comet's head contained dust particles of submillimeter or even millimeter size and that the material in the spike was produced by ejections beginning at least some 2 months earlier.

G. A. Gary and C. R. O'Dell of the Marshall Space Flight Center described the appearance of the sunward spike as evidence of large particles being ejected near the perihelion. They showed that the basic features of the antitail are explained by the detailed theory of particle trajectories developed by Finson and Probstein. They also related the finite apparent spike length to the mass of the heaviest particles that overcame the gravitational attraction of the nucleus. Their analysis of the spike length made use of data from the Skylab astronaut observations.

 

Photometric Studies

 

When Comet Kohoutek became visible through Skylab's windows, a program to photograph it twice a day was begun, using a 35-mm camera equipped with a 55-mm, f/1.2 lens. The purpose of this activity was to determine the brightness of the comet as often as possible. Information about changes in its nucleus and coma resulting from its close passage by the Sun could be deduced from such a brightness history. The experiment (S233) was conducted for Charles A. Lundquist and associates at the Marshall Space Flight Center.

Figure 4-24 is a 120-sec exposure made on December 5, 1973, and shows the comet with a slight tail. A solar panel, slightly out of focus, is in the foreground. Photographs, with the comet in focus, were used to locate the comet exactly and to record a sharp image of its tail.

Figure 4-25, a 60-sec exposure, was made immediately after the picture in figure 4-24; however, the lens of the camera was purposely focused at 15 ft. Defocusing the lens facilitated measuring the total exposure recorded by each image, by ensuring that most of the images were not overexposed. The brightness of the comet was found by comparing the density of its image with those of stars of known brightness.

The photograph in figure 4-26 was taken on December 22, 1973, and was the last 35-mm photograph made....

 

 


[
60]

Figure 4-26. Comet Kohoutek on December 22, 1973, before perihelion, with approximately 5° of its tail visible.

Figure 4-26. Comet Kohoutek on December 22, 1973, before perihelion, with approximately 5° of its tail visible.

 

....before perihelion. Approximately 5° of Kohoutek's tail is visible. Later, the comet was too close to the Sun for this type of photography. The picture in figure 4-27 was obtained from figure 4-26 by the false-color enhancement process discussed earlier. In it, colors are assigned different levels of brightness, from brightest to least bright: red orange, dark red, blue, and black. The extremely bright area at the top of the photograph, making it difficult to see where the tail actually ends, is caused by reflections from a strut of the Skylab.

Figure 4-28 is a 120-sec exposure made on January 10, 1974, 13 days after perihelion. Approximately 7° of the comet's tail can be seen. Streaking of the star images was caused by motion of the space station during the exposure.

 

Kohoutek's Visual Magnitude

 

William A. Deutschman of the Center for Astrophysics, Harvard College Observatory, and the Smithsonian Astrophysical Observatory studied the changing visual magnitude of Comet Kohoutek as it approached and passed the Sun. He concluded that the postperihelion magnitude was dimmer and decreased more rapidly with....

 


[
61]

Figure 4-27. False-color enhancement of the photograph shown in figure 4-26.

Figure 4-27. False-color enhancement of the photograph shown in figure 4-26.

 

.....distance from the Sun than did the magnitude before perihelion. His result is confirmed by the photographs from Skylab. This behavior is opposite to that of some comets, which are brighter after perihelion than before, for the same distances from the Sun. Deutschman's curves (figs. 4-29 and 4-30) are based on 476 observations by 62 observers on Earth. The data are divided into 231 preperihelion and 245 postperihelion observations. The magnitude points have been corrected for the variation in distance between the Earth and the comet, and by a standard aperture correction for individual observers. The quantity R is the distance between the Sun and Kohoutek in astronomical units. Observation dates are plotted, superimposed with values of log R. The curves represent a best fit to two theoretical treatments of comet magnitudes.

 

Best Observed Comet

 

By good fortune, the Skylab operation overlapped the passage of Comet Kohoutek through the inner solar system. This circumstance and the extensive observational capability of Skylab instrumentation were catalysts for a worldwide effort to measure all aspects of the comet.

 


[
62]

Figure 4-28. Comet Kohoutek photographed in a 120-sec exposure on January 10, 1974. 13 days after perihelion.

Figure 4-28. Comet Kohoutek photographed in a 120-sec exposure on January 10, 1974. 13 days after perihelion.

 

This effort was indeed successful to the degree that Comet Kohoutek became the best observed and studied comet in history. Directly, through unique measurements made on board, and indirectly, through stimulus to others, Skylab can be credited with this success.

 


Figure 4-29. Comet Kohoutek's visual magnitude before perihelion based on 231 ground-based observations from September 1 through December 28, 1973. The +, x, and o symbols are, respectively, visual, photographic, and image tube observations. Link to a larger picture.

Figure 4-29. Comet Kohoutek's visual magnitude before perihelion based on 231 ground-based observations from September 1 through December 28, 1973. The +, x, and o symbols are, respectively, visual, photographic, and image tube observations.


[
63]

Figure 4-30. Comet Kohoutek's visual magnitude after perihelion (based on 245 ground-based observations between December 28, 1973 and March 29, 1974. Link to a larger picture.

Figure 4-30. Comet Kohoutek's visual magnitude after perihelion (based on 245 ground-based observations between December 28, 1973 and March 29, 1974.


previousindexnext