☄ Wirtanen 46P/Wirtanen

How to follow comet Wirtanen live

The panel above recomputes the position of Wirtanen 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 Wirtanen 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 Wirtanen with no math at all.

Comet fact sheet

About Wirtanen

46P/Wirtanen is a short-period comet classified as hyperactive: it generates far more gas than expected for the size of its nucleus, estimated at just 1.2 to 1.4 km across. That disproportionate activity is attributed to an unusually high fraction of active surface area and sublimation of superheated ice. It was the original target of ESA's Rosetta mission before the December 2002 Ariane 5 failure forced a switch to 67P/Churyumov-Gerasimenko.

The 2018 apparition placed Wirtanen in an unusual position: very close to Earth, relatively bright, and positioned in Taurus near the Pleiades at peak visibility, making it an easy target even for inexperienced observers. The greenish coma spanning several degrees in diameter was visible in binoculars even under moderately light-polluted skies.

History and discovery

Carl Alvar Wirtanen, a Swedish astronomer working at Lick Observatory in California, discovered the comet on 17 January 1948 during a routine photographic survey. Its magnitude at discovery was about 16, invisible to the naked eye and detectable only on long-exposure photographic plates. The initial orbit had a period of 6.7 years and perihelion at 1.61 AU from the Sun, out of reach for any viable space mission of the era.

Over the following decades, calculations of Jupiter's gravitational influence revealed that a close approach to the giant planet in 1972 had profoundly reshaped the comet's orbit, shrinking perihelion from 1.61 AU to 1.06 AU and shortening the period to about 5.44 years. That orbital change transformed Wirtanen into an attractive spacecraft target and was directly responsible for the exceptional 2018 close approach.

ESA's 1993 decision to select Wirtanen as the Rosetta target raised the comet's scientific profile decades before the 2018 passage. When the spacecraft had to be redirected to 67P, scientific interest in Wirtanen did not disappear: it remained a priority observational target because of its hyperactive nature.

Orbit and returns

46P/Wirtanen orbits with eccentricity 0.6597 and inclination 11.7 degrees. Current perihelion is 1.055 AU from the Sun, just beyond Earth's orbit, and aphelion sits at 5.13 AU near Jupiter. The orbital period is about 5.44 years.

The December 2018 passage was exceptional: the comet closed to 0.0775 AU (roughly 11.6 million km) from Earth on 16 December, the closest Wirtanen had come to our planet in 70 years of observations. Perihelion fell on 12 December, four days before closest Earth approach, a favourable geometry that maximised apparent brightness. The 2023 return brought no comparable proximity. The next perihelion with good geometry is expected in late 2028.

Nucleus, coma and tail

Wirtanen's nucleus is remarkably small: estimated at 1.2 to 1.4 km across, it would fit entirely within a large city's boundaries with room to spare. Despite its size, the comet is classified as hyperactive: studies of the 2018 apparition estimated that between 3% and 40% of the nucleus surface is active (estimates vary by method, but in any case far above the 1% to 5% typical for most comets). The sublimation rate per unit of active area exceeds that of most similarly sized comets.

The coma in 2018 reached an angular diameter of several degrees, the largest recorded for any comet in recent years in terms of absolute sky extent. The enormous angular size resulted from the combination of an active nucleus and very small Earth distance. Conversely, the dilution of brightness over such a large angular area gave the coma a low surface brightness, making it easy in binoculars but diffuse at high telescope magnification.

The greenish colour of the coma was intense and noticeable even to casual observers. The green comes from emission by C2 (diatomic carbon) molecules excited by solar ultraviolet radiation, the same mechanism responsible for the characteristic colour of most active comets. The tail was discreet compared with comets like Hale-Bopp and Hyakutake, because low relative velocity with respect to Earth reduced the ion drag effect.

The sky spectacle

In December 2018, 46P/Wirtanen reached magnitude 3 to 4 in Taurus, near the Pleiades, and was visible to the naked eye under dark skies as a diffuse greenish patch. Its position near the Pleiades was a visual gift: observers could frame both the Pleiades cluster and the comet in the same wide-angle binocular field of view simultaneously.

For northern hemisphere observers, Wirtanen was visible all night in December 2018, high in the winter sky. The absence of a prominent tail clearly distinguished it from spectacular comets like Hale-Bopp: the spectacle was the enormous diffuse coma, not a linear tail. Long-exposure telephoto photographs showed the well-defined greenish coma and a nascent ion tail pointing away from the Sun.

Science and observations

The 2018 apparition was the most intensive observation campaign ever dedicated to 46P/Wirtanen. Earth proximity provided spatial resolution of roughly 50 km per arcsecond, allowing inner coma structure to be studied in unusual detail.

• Ethanol: a 2020 study based on radio astronomy data from the 2018 apparition found that Wirtanen releases ethyl alcohol (ethanol, C2H5OH) at a rate 20 times higher than other comets of similar activity, for reasons not yet fully understood. That discovery raised questions about the variability of organic composition among cometary nuclei of similar dynamic origin.
• Active surface fraction: water production estimates suggest between 3% and 40% of the nucleus surface is active (estimates vary by method); in any case, the fraction is unusual for such a small nucleus.
• Deuterium: deuterium-to-hydrogen ratio measurements in Wirtanen's water were published as part of radio astronomy campaigns, contributing to the database comparing D/H ratios across comets of different dynamical families.
• Coma structure: high-resolution imaging revealed jets of ejected material in specific directions, similar to those observed in Hyakutake and Hale-Bopp, indicating localised active regions on the nucleus.
• Rosetta connection: the 2018 data were compared directly with Rosetta mission data from 67P, enabling cross-calibrations between the two comets and broadening understanding of nucleus diversity within the Jupiter family.

Facts worth knowing

• 46P/Wirtanen was the original Rosetta mission target: the spacecraft was designed and partly built with Wirtanen in mind. The December 2002 Ariane 5 failure delayed launch and made the Wirtanen window unreachable, forcing the switch to 67P/Churyumov-Gerasimenko.
• The 2018 approach of 0.0775 AU was the fifth closest cometary pass of Earth since 1900.
• The conspicuous green colour of the coma in 2018 came from emission by C2 (diatomic carbon) molecules excited by sunlight, the same process that colours most active comets.
• Wirtanen's nucleus is so small that, placed over a large city, it would fit entirely within a single administrative district with room to spare.
• A 2020 study using 2018 apparition data found that Wirtanen releases ethanol at a rate 20 times higher than other comets of similar activity, for reasons not yet fully understood.
• Wirtanen's coma in 2018 had an angular diameter of several degrees, larger than the Full Moon, making it paradoxically easier to see in binoculars than in a telescope: high magnification dilutes the already low surface brightness of the extended coma.
• Jupiter's 1972 gravitational perturbation shrank Wirtanen's perihelion from 1.61 AU to 1.06 AU in a single pass. Without that event, the comet would never have been a viable spacecraft target or produced the spectacular 2018 apparition.

Other comets

See the full comet catalogue.