Perihelion: 1973 December 28.43, q = 0.142 AU
This comet’s story starts about two years before its actual discovery when Brian Marsden (at the IAU’s Central Bureau for Astronomical Telegrams) performed some orbital calculations of the long-lost periodic Comet 3D/Biela (a previous “Comet of the Week”). The comet had not been seen since the mid-19th Century and had long been thought as having disintegrated, however, Marsden calculated that if there was any surviving fragment it might pass close to Earth in late 1971. Despite several searches, nothing was found, which reinforced the overall consensus that Comet Biela had indeed completely disintegrated.
One of the people who had conducted searches for Comet Biela in late 1971 was the Czech astronomer Lubos Kohoutek, at the Hamburg Observatory in Germany. While he found no trace of the comet, he did nevertheless discover several previously-unknown asteroids, some of which were in favorable locations for recovery in early 1973. While attempting to recover these, Kohoutek discovered two comets, the second of these being a 16th-magnitude object in western Hydra that first appeared on a photograph taken on March 7 – my 15th birthday, incidentally.
This particular comet was located at a heliocentric distance of 4.75 AU at the time of its discovery, but once orbital calculations revealed that it was over nine months away from perihelion passage which would take place at the very small heliocentric distance of 0.14 AU, excitement began to build rapidly within the astronomical community, as it appeared likely that a true “Great Comet,” perhaps even the “Comet of the Century,” was on the way in. Some of the initial forecasts, in fact, suggested a peak brightness as bright as magnitude -10, and even though these were later “downgraded” to about magnitude -3 or -4, this still nevertheless indicates a very bright comet, and the world waited . . .
Comet Kohoutek had brightened to about 14th magnitude by the time it disappeared into evening twilight in early May. When it emerged into the morning sky during the latter part of September it was close to 11th magnitude, somewhat fainter than expected, and while it brightened steadily after that, it did so more slowly than was expected. It became visible to the unaided eye during late November and was near 4th magnitude, with a tail a few degrees long, in early December, but was only about 3rd magnitude when it disappeared into the dawn a few days after mid-month.
Around the time of its perihelion passage, Comet Kohoutek was too close to the sun to be visible from the ground, but it was detected rather easily from space. It appears to have undergone an outburst in brightness right around perihelion, as images taken by coronagraphs aboard NASA’s Skylab space station and aboard the Orbiting Solar Observatory 7 (OSO-7) satellite suggest it may have been as bright as magnitude -3. Astronauts aboard Skylab and aboard the Soviet Union’s Soyuz 13 mission observed it, and according to the Skylab astronauts it was as bright as magnitude -2 the day after perihelion, with a distinct anti-tail, although it faded somewhat rapidly and the anti-tail all but disappeared over the subsequent days, even though the main tail grew longer.
The comet was perhaps near magnitude 0 when it first became visible in the evening sky right at the very end of December but had faded to 2nd magnitude by the time it became widely observed during the first week of January 1974. The tail reached a maximum length of about 15 degrees around mid-month – around the time of closest approach to Earth, 0.81 AU – but the comet itself dropped below naked-eye visibility by the end of January. It was followed telescopically until April, and after conjunction with the sun was photographed for one last time in early November, by which time its heliocentric distance had increased to 5 AU and it had faded to magnitude 22.
Although Comet Kohoutek did become a relatively decent naked-eye object when viewed from dark rural sites, it clearly was not the “Comet of the Century” that some of the initial forecasts suggested. A significant part of the reason for this is that Comet Kohoutek was a first-time visitor from the Oort Cloud, and as was discussed in the “Special Topics” presentation on “Great Comets,” such objects tend to be unusually bright and active when at larger heliocentric distances but this activity level will often slow down dramatically as they approach perihelion. Indeed, compared to some other first-time Oort Cloud comets that have appeared since then, Comet Kohoutek’s performance wasn’t all that poor.
While it might not have been a “Great Comet,” Comet Kohoutek was nevertheless a rewarding and productive comet from a scientific perspective. It was, for example, the first comet to be observed by an interplanetary spacecraft, as NASA’s Mariner 10 mission obtained ultraviolet observations of it while en route to Venus. It was also the first comet to be extensively observed in the radio part of the electromagnetic spectrum, and among other substances, organic molecules like methyl cyanide and hydrogen cyanide were detected inside of a comet for the first time.
The general public, certainly, which had been expecting the “Comet of the Century,” was to a large extent greatly disappointed, and even today Comet Kohoutek is often associated with the cometary equivalent of a “fizzle.” While part of this was due to the comet’s slower rate of brightening due to its being a first-time visitor from the Oort Cloud, a non-trivial part of this is also due to widespread over-hyping in the popular media, and the ignoring of the more conservative predictions once astronomers started making those. The comet ended up being featured quite a bit in the popular media of the time, including within published stories, television programs, music, and even within comic strips, and at least one religious cult in the U.S. incorporated its appearance into their beliefs. Now that it has been “broken in,” so to speak, perhaps it might put on a better show to anyone who is around to see it when it returns 75,000 to 80,000 years from now.