Comet Kohoutek (pronounced "" , or "" ), formally designated as C/1973 E1, 1973 XII and 1973f, is a comet that passed close to the Sun towards the end of 1973. Early predictions of the comet's peak brightness suggested that it had the potential to become one of the brightest comets of the 20th century, capturing the attention of the wider public and the press and earning the comet the moniker of "Comet of the Century". Although Kohoutek became rather bright, the comet was ultimately far dimmer than the optimistic projections: its apparent magnitude peaked at only −3 (as opposed to predictions of roughly magnitude −10), and it was visible for only a short period, quickly dimming below naked-eye visibility by the end of January 1974.
The comet was discovered by and named after Luboš Kohoutek at the Hamburg Observatory on 18 March 1973; Kohoutek had been searching for Biela's Comet and had serendipitously discovered his eponymous comet while reviewing photographic plates for a different object. The comet was discovered farther away from the Sun than any previous comet. Conventional practices for predicting comet brightness led to generous projections of Comet Kohoutek's luminosity towards the end of 1973 and the beginning of 1974, leading to great anticipation within both scientific circles and the general public. Comet Kohoutek reached perihelion on 28 December 1973. Though the comet was then at its brightest, it could only be observed by scientific instrumentation and astronauts on Skylab. For most ground observers, Kohoutek only reached as bright as magnitude 0 when it emerged from the Sun's glare in January 1974. It quickly faded beyond naked-eye visibility later that month and was last observed in November 1974. Due to its underwhelming brightness after intense publicity, Kohoutek became synonymous with spectacular disappointment.
Because of its early detection and unique characteristics, numerous scientific assets were dedicated to observing Kohoutek during its 1973–1974 traversal of the inner Solar System, making Kohoutek the most well-studied comet at the time; the resulting findings significantly advanced the understanding of comets. The identification of larger and more complex molecules emanating from Kohoutek alongside related but simpler chemical species confirmed the hypothesis that comets were composed of larger molecules that dissociated into simpler products. The significant presence of gasses and plasma expelled from Kohoutek supported the longstanding "dirty snowball" hypothesis concerning the composition of comet nuclei. The detection of water, methyl cyanide, hydrogen cyanide, and silicon in Kohoutek were the first time such chemical species were observed in any comet. Its underwhelming display challenged longstanding assumptions regarding the light curve of similar comets entering the inner Solar System.
Kohoutek's highly eccentric orbit preceding its 1973 perihelion suggests that it may have been formed early in the formation of the Solar System or originated from a different planetary system. Its orbital period may have been initially in the order of several million years, or its 1973 apparition may have been its first trek into the inner Solar System. Its nucleus has an estimated average radius of .
Discovery
The comet was discovered on 18 March 1973 by Czech astronomer Luboš Kohoutek after reviewing twice-exposed photographic plates taken by the aperture Schmidt camera at the Hamburg Observatory in Bergedorf on 7 and 9 March. The object remained evident and was displaced slightly towards the west-northwest in the latter plate, confirming that it was moving against the background stars and not a transient or erroneous feature. Upon discovery, the comet had an apparent magnitude between 15.5 and 16 and lay in the constellation of Hydra; Kouhoutek described the comet as initially "diffuse with central condensation". The comet was located near the orbit of Jupiter around the time of discovery, away from the Sun and from the Earth. As a result, the comet was moving very slowly towards the west-northwest at around 0.2° per day. Based on subsequent orbital calculations, the discovery of the comet occurred roughly seven months before perihelion. The discovery of a comet at such large distances and long leadtimes before their perihelia was unprecedented; at the time comets were rarely discovered at such large distances from the Sun, as most were too faint to be detected. Kohoutek transmitted his findings to the Central Bureau for Astronomical Telegrams on 19 March, leading to the announcement of his discovery in circular 2511 of the International Astronomical Union. It was the sixth comet discovered in 1973 and thus given the designation 1973f. Due to increasing public attention towards the comet, the comet was also named Comet Kohoutek in honor of its discoverer Luboš Kohoutek.
The comet's discovery was serendipitous: beginning in 1971, Kohoutek had been searching for Biela's Comet, which had not been observed since 1852. Using the Hamburg Observatory's Schmidt camera, the initial search in October and November 1971 found 52 minor planets in a roughly 180-square-degree region of the sky. Preliminary orbits were determined for 35 of these newly-found objects, of which 15 were targeted for observation between January and April 1973. The 7 March photographic plate was intended to feature one of the objects, main-belt asteroid (8606) 1971 UG, but instead captured Comet Kohoutek. Comet Kohoutek was later precovered on a photographic plate taken on 28 January, exhibiting a similar brightness as during its discovery and a heliocentric distance of . The plate was also taken in pursuit of (8606) 1971 UG. The comet was the second discovered by Kohoutek in 1973; the first had been discovered on 28 February during the same search for Biela's Comet and was also found on a photographic plate that was intended to contain another of the newly discovered minor planets.
Orbit
Orbital elements for Comet Kohoutek were calculated by British astronomer Brian G. Marsden soon after the comet's discovery, using photographic plates taken of the comet on 7 March, 9 March, and 21 March. These calculations suggested that Kohoutek had a nearly parabolic orbit lying close to the ecliptic with an eccentricity between 0.9999 and 1 and an inclination of 14.3°. Such an orbit meant that the comet would approach very close to the Sun, with perihelion occurring on 28 December 1973 at a distance of only . The close perihelion and the comet's brightness upon discovery were analogous to other comets that had become very bright. The calculated orbit also suggested that Kohoutek's close pass of the Sun could be its first traversal of the inner Solar System. Marsden's calculated orbit placed Kohoutek's initial semi-major axis at . Alternatively, Kohoutek may have had an orbital period of 4 million years before experiencing the gravitational perturbations from the Solar System's planets, which would make Kohoutek a long-period comet. This idea was supported by Stephen P. Maran, the head of NASA's efforts to study Kohoutek. Maran believed that the comet initially had a perihelion farther out than the orbit of Jupiter until it was gravitationally perturbed by a passing star, lowering its perihelion to within the orbit of Mercury and its orbital period to 4–5 million years; additional gravitational interactions between the comet and the planets would have shortened the comet's orbital period further to about 75,000 years. The closest approach of Kohoutek to Earth occurred on 15 January 1974 and be no nearer than , preventing the resolution of its nucleus via Earth-based instruments.
Both the Minor Planet Center and the JPL Small-Body Database list Kohoutek as having a hyperbolic trajectory when it was near perihelion, but the orbit became bound to the Sun by 1978. The comet is not expected to return for about 75,000 years. Some of the meteoroids ejected by Kohoutek during its initial approach, particularly those with diameters no smaller than , were placed into stable orbits around the Sun.
As seen from Earth between 1973 and 1974, the comet took a southeastward path across the sky similar to Comet Halley's path between 1985 and 1986. The visibility of the comet was most favorable to observers in the Southern Hemisphere and the tropics. It was in the evening sky from the time of its discovery to the end of September 1973, after which the comet became a morning sky object. After being positioned in Hydra upon the time of discovery, Comet Kohoutek moved across the constellations of Sextans, Leo, Crater, Corvus, Virgo, Libra, Scorpius, and Sagittarius by the end of 1973. Kohoutek was near the boundary of Sagittarius and Ophiuchus during perihelion when it was visually separated from the Sun by only 0.5°. In 1974, the comet moved across the constellations of Capricorn, Aquarius, Pisces, and Taurus. , comet Kohoutek is about from the Sun in the constellation of Gemini. It is currently receding from the Sun at .
Structure and composition
Kohoutek's highly eccentric orbit and possible lack of prior planetary or solar interactions suggest that the comet may have been a primordial body of either the Solar System or that it may have originated from another planetary system. The comet may have also originated from the Oort cloud. A 1976 analysis of photometry and water loss rates estimated that the nucleus had a radius of around and an albedo of around 0.67. A photometric analysis of Kohoutek, using Mercury as a reference, established an upper limit of for the diameter of Kohoutek's nucleus. An attempt to detect a radar echo from Kohoutek's nucleus using the Haystack Radio Telescope received no radar returns, constraining the nucleus's size to under . The comet has a total absolute magnitude (at 1 AU) of 5.8 and a nuclear absolute magnitude of 9.5. During Kohoutek's 1973–1974 apparition, its tail's width ranged from around near the coma to farther away. Detection of positive carbon monoxide ions showed that the tail was at least in length. A more yellow and orange appearance of the dust tail of Kohoutek during its perihelionas observed by astronauts on Skylabwas likely the result of light scattering by basaltic dust particles with sizes of around 0.5 μm. The tail lacked color closer to the coma near perihelion, indicating a large distribution of particle sizes and resulting in a white appearance. Observations from the Joint Observatory for Cometary Research in Socorro, New Mexico, were able to trace the blue ion tail of Kohoutekfeaturing more prominently than the comet's dust tailto a distance of away from the nucleus. The ion density within the tail several million miles away from the nucleus was about 10 cm−3, while the maximal electron density within the tail was around 20,000 cm−3. At a distance of around from the Sun, the plasma outflow in Kohoutek's tail generated a weak magnetic field with a strength comparable to the interplanetary magnetic field.