☄ Kohoutek C/1973 E1

How to follow comet Kohoutek live

The panel above recomputes the position of Kohoutek every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination). It runs on the same kind of engine observatories use, a Kepler solver applied to the JPL osculating orbital elements, so the numbers are not a static snapshot, they keep ticking.

Just below, the top-down map of the Solar System shows exactly where Kohoutek is right now among the planets. You can fast-forward time with the day slider, zoom and pan, compare its distance to another body with a click, and press "Next event" to jump straight to perihelion. It is the most direct way to grasp the orbit of Kohoutek with no math at all.

Comet fact sheet

About Kohoutek

C/1973 E1 was discovered by Lubos Kohoutek in March 1973 when it was still more than 4.7 astronomical units from the Sun, an exceptional lead time for an Oort Cloud comet. The discovery distance fed optimistic brightness forecasts; some initial calculations suggested magnitude -4 to -10, daylight-visible. What arrived at perihelion on 28 December 1973 reached only magnitude -3 briefly while passing near the Sun, and remained below naked-eye visibility at night, well short of the expectations broadcast to the general public by television and newspapers. The disappointment was real; the science was not.

Kohoutek was the best-studied object of its era: Operation Kohoutek, coordinated by NASA, mobilised observatories worldwide, high-altitude aircraft instruments, and the three Skylab 4 astronauts, who dedicated part of their time aboard the space station to the comet. The result was the discovery of organic molecules in comets for the first time in history, an achievement that shaped all subsequent cometary research.

History and discovery

Lubos Kohoutek, a Czech astronomer working at the Hamburg Observatory in Germany, discovered the comet on 7 March 1973 while reviewing photographic plates from the observatory's telescope. It was an accidental discovery during an asteroid survey. The comet was 4.77 AU from the Sun at the time, making it one of the earliest pre-perihelion cometary discoveries in the history of astronomy up to that point.

That lead time was mistakenly interpreted as a guarantee of exceptional brightness. The reasoning was simple: if a comet was already so active so far from the Sun, it would be extraordinary close up. What the calculations failed to adequately account for is the mechanism of dynamically new comets on their first visit to the inner Solar System: they release the most volatile gases from the nuclear surface layer during the distant approach. By perihelion, much of the most active surface material has been depleted. This phenomenon, now well documented, was poorly understood in 1973.

The world press ran enthusiastic headlines. National Geographic published a preview expecting a celestial spectacle. The New York Times projected a comet brighter than the full Moon. When the disappointment arrived, it was equally widely reported, and the name Kohoutek entered informal English vocabulary as a synonym for something hyped that underdelivered.

Orbit and physical nature

C/1973 E1 has a near-parabolic orbit with an original orbital period estimated at several million years, classifying it as a dynamically new Oort Cloud visitor. This means it was likely on its first pass through the inner Solar System since the Solar System formed. Perihelion occurred on 28 December 1973 at 0.142 AU from the Sun, inside Mercury's orbit.

The nucleus was estimated at about 5 to 10 km in diameter and survived perihelion without fragmenting, notable given the intense heat at that solar distance. Activity was high at large distances because the nucleus surface was rich in carbon monoxide (CO) and other hyper-volatile molecules that sublimate at low temperatures, far from the Sun. By the time the nucleus heated further, the surface layer of these volatiles had been partially depleted, reducing gas production and the coma.

What makes it unique

Kohoutek is unique for two reasons that survived the brightness disappointment. First, it was the first comet to be detected and systematically studied by radio astronomy, revealing complex organic molecules never previously found in a comet. Second, it was the first comet studied in detail by astronauts from a space station, above the terrestrial atmosphere that absorbs wavelengths crucial for chemistry.

The detection of hydrogen cyanide (HCN) was the first of a parent molecule in a comet by radio astronomy. Next came the detection of methyl cyanide (CH3CN), the first time a molecule with more than three atoms had been found in a comet. Before Kohoutek, it was unknown whether comets contained complex organic molecules. After Kohoutek, it became clear that comets are reservoirs of organic chemistry, a result with profound implications for the origin of life on Earth.

NASA extended the Skylab 4 mission, originally planned for 56 days, to a record 84 days motivated in part by the opportunity to observe Kohoutek. Astronauts Gerald Carr, William Pogue and Edward Gibson made photographic and spectrographic observations of the comet above the atmosphere, gathering data that complemented ground-based measurements at wavelengths blocked by ozone and water vapour.

Observations and science produced

Operation Kohoutek was the most comprehensive coordinated cometary observation campaign ever conducted to that point. It involved observatories on all continents, instruments in high-altitude aircraft (including NASA's Kuiper Airborne Observatory), space probes, and the Skylab 4 astronauts.

From the ground, the comet was marginally naked-eye visible in the northern hemisphere in December 1973 and January 1974, but conditions were poor: perihelion fell in December when the comet was close to the Sun in the sky. After perihelion the tail became more photographable in the January 1974 morning sky. Southern hemisphere observers had better geometric conditions.

The scientific results were, by the standards of the era, revolutionary for cometary chemistry:

• First radio-astronomy detection of HCN (hydrogen cyanide) in a comet.
• First detection of CH3CN (methyl cyanide) in a comet: the first molecule with more than three atoms ever found in one.
• First detection of a comet at 18 cm wavelength (OH radical).
• Coma spatial distribution data obtained from Skylab at ultraviolet wavelengths.
• Detailed study of cometary activity evolution from 4.7 AU to perihelion and beyond.

Debates and legacy

Kohoutek's disappointment generated a lasting professional debate about communicating uncertainty to the public. Because brightness calculations were based on measurements from comets with more previous Solar System passes, the atypical behaviour of a dynamically new comet, active early but "depleted" by perihelion, was not predicted. The astronomical community learned to explicitly distinguish between dynamically new and returning comets when making brightness forecasts.

Today, comet brightness forecasts include wide uncertainty ranges and explicit warnings that comets are inherently unpredictable. Phrases such as "possibly naked-eye visible" replaced "certainly daylight-visible." This communication standard was born in large part from the Kohoutek episode.

Operation Kohoutek was the template for the International Halley Watch (IHW) that studied Comet Halley in 1986, and for similar campaigns devoted to subsequent comets including Hyakutake (1996) and Hale-Bopp (1997). Kohoutek, despite its popular disappointment, was the unwitting architect of modern coordinated cometary astronomy.

Trivia

• The word "Kohoutek" became an informal adjective in English to describe something overhyped that disappointed: a product, a film, a political promise. Merriam-Webster did not formally record it, but media usage persisted for decades.
• Musician Sun Ra released an album called "Kohoutek" in 1974, inspired by the comet. Kraftwerk made an instrumental composition with the same name. Todd Rundgren released "Inta Kohoutek." The comet had a cultural impact disproportionately larger than its brightness warranted.
• NASA extended Skylab 4 from 56 to 84 days partly because of the comet, making it the longest Skylab mission. The three crew members were the last humans to occupy the station: Skylab was deactivated in February 1974 and fell from orbit in 1979.
• The first radio-astronomy detection of OH in a comet, in 1973, using the Nancay radio telescope in France, was motivated by Kohoutek and opened the entire field of cometary radio astronomy.
• Lubos Kohoutek discovered two other comets besides C/1973 E1: C/1969 O1 and C/1973 D1. C/1973 E1 is by far the most famous, paradoxically better known for disappointing than for the scientific results it produced.
• The perihelion of 0.142 AU places Kohoutek in the category of moderate "sungrazer" comets, inside Mercury's orbit, though not as close as the Kreutz-family comets, which can come within 0.01 AU of the centre of the Sun.

Other comets

See the full comet catalogue.