☄ Ikeya-Zhang 153P/Ikeya-Zhang

How to follow comet Ikeya-Zhang live

The panel above recomputes the position of Ikeya-Zhang every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination), and a live countdown to the next perihelion. 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 Ikeya-Zhang 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 Ikeya-Zhang with no math at all.

Comet fact sheet

About Ikeya-Zhang

Comet 153P/Ikeya-Zhang combines three unique distinctions in the history of cometary astronomy: independent simultaneous discovery by two observers on different continents, identification with a historical comet of 1661, and the longest confirmed orbital period of any periodic comet at 366.51 years. The formalisation of that period as an official record was especially remarkable: for the first time in history, a comet with a period greater than 156 years was confirmed by observation at two distinct apparitions.

Brightening to magnitude 2.9 in March 2002, 153P was naked-eye visible for weeks and captured in photographs by amateurs and professionals worldwide. Its perihelion at 0.507 AU plunged the nucleus well inside Venus's orbit, producing a compact, bright coma and a tail several degrees in angular extent.

History and discovery

On 1 February 2002, Chinese astronomer Zhang Daqing from Kaifeng (Henan Province) photographed an uncatalogued fuzzy object in Cetus using a 20 cm reflector and reported the discovery to the IAU's Central Bureau for Astronomical Telegrams (CBAT). A few hours earlier, Japanese veteran observer Kaoru Ikeya, who already had five comets to his name, had independently recorded the same object with a 25 cm reflector, but Zhang's report reached the bureau first owing to the time-zone difference. The object received the designation C/2002 C1 (Ikeya-Zhang), preserving both names as standard co-discovery practice.

Shortly after the announcement, Dan Green of the IAU CBAT compared the new comet's orbital elements with historical records and found a precise match with a comet observed in 1661 by Polish astronomer Johannes Hevelius, corroborated by Chinese astronomers that same year. The confirmation was published in an IAU Circular and made 153P the first comet to receive a periodic designation based on a historical identification rather than on instrumentally observed successive returns.

Later research by Ichiro Hasegawa and Shuichi Nakano, published in the Annals of the Astronomical Society of Japan, suggested that comets recorded in AD 877 and AD 1273 were likely earlier apparitions of the same object, extending 153P's documented human observation history to more than eleven centuries.

Orbit and historical returns

153P/Ikeya-Zhang holds the record for the longest confirmed orbital period of any periodic comet at 366.51 years. For comparison, Halley's Comet has a period of about 75 years, and the second-longest confirmed periodic period belongs to 109P/Swift-Tuttle at around 130 years. With perihelion at 0.507 AU from the Sun, the comet dips inside Venus's orbit at each return.

The precise orbital determination of 153P was published in 2003 in the Monthly Notices of the Royal Astronomical Society by Hasegawa and Nakano, with subsequent refinement by the MPC observer network. The confirmation that the orbital elements fit within the uncertainty window of Hevelius's 1661 observations was the decisive argument for formalising the longest period among periodic comets.

Nucleus, coma and tail

The 2002 observations with professional telescopes and radio telescopes allowed the coma of 153P to be characterised in detail. The water production rate at perihelion was estimated at about 2 to 4 tonnes per second, consistent with a nucleus diameter of 5 to 8 km. OH, CN, C2 and CS emissions were detected in optical spectra, with abundance ratios typical of long-period Oort Cloud comets.

The ion tail developed clearly during the weeks around perihelion, reaching several degrees in angular extent. The dust tail, while present and visible in photographs, was less spectacular than the ion tail, suggesting the comet was relatively rich in volatile components compared to its solid dust load, a characteristic of objects that have spent little time in the warm inner Solar System.

The spectacle in the sky

During the 2002 apparition, the comet was naked-eye accessible for several weeks before and after perihelion on 18 March. Northern-hemisphere observers had the best geometric conditions in the first half of 2002, with the comet moving through Cetus, Aries and Perseus between February and April.

At perihelion on 18 March 2002, magnitude 2.9 placed the comet on a par with the brightest stars in Orion, visible without any optical aid under skies with modest light pollution. The coma appeared as a diffuse nebulosity with a condensed central core, and the tail stretched 3 to 5 degrees in good wide-field photographs. At dark-sky sites, 7x50 binoculars revealed the coma texture and tail direction with great clarity.

Naked-eye visibility lasted until the end of April, when magnitude dropped below 5. In May the comet was still an easy binocular object and a small-telescope target through mid-June.

Science and observations

The identification of 153P with Hevelius's 1661 comet opened a unique window for testing long-period orbital evolution models. With combined data from the 1661 and 2002 apparitions, and the probable medieval apparitions of 877 and 1273, it became possible to fit models including centuries of accumulated gravitational perturbations, especially from Jupiter and Saturn.

Johannes Hevelius, who observed the comet in 1661 from his observatory in Danzig (modern Gdansk, Poland), published the comet's positions in his Cometographia of 1668, a fundamental work of seventeenth-century astronomy. The quality of his angular measurements, made with sextant instruments without a telescope, was precise enough to allow retroactive identification 341 years later. IAU researchers calculated that the accumulated orbital position uncertainty after 341 years is only a few tens of AU at aphelion, within the error margin of Hevelius's historical observations.

Spectral observations in 2002 with the Nancay radio telescope (France) and IRAM detected HCN, CS and H2CO emissions in the coma, providing data on the volatile composition of an ultra-long-period comet at its most recent inner Solar System passage.

Key facts

• At 366.51 years, 153P holds the record for the longest confirmed orbital period among all periodic comets, surpassing 109P/Swift-Tuttle (about 130 years) and the famous Halley (about 75 years).
• The discovery was simultaneous and independent on two continents: Kaoru Ikeya in Japan and Zhang Daqing in China, separated by a few hours and a time zone, both using high-quality amateur equipment.
• Johannes Hevelius, who observed the comet in 1661, is the same astronomer who mapped the lunar surface with remarkable precision for his era, introducing lunar region names still in use today, such as Mare Tranquillitatis and Oceanus Procellarum.
• Chinese records from 1661 describe the comet as a hui xing (hairy star), traditional Chinese terminology for comets with a developed tail, consistent with the expected appearance of a magnitude-3-to-4 object.
• Kaoru Ikeya had previously co-discovered the celebrated Ikeya-Seki comet (C/1965 S1), a Kreutz-family sungrazer that passed just 0.0078 AU from the Sun in October 1965 and reached an estimated magnitude near -10.
• The next perihelion of 153P is expected around the year 2369, placing the next visible apparition beyond the horizon of any generation alive today.
• The historical confirmation of 153P was the first time a comet with a period greater than 156 years was observed at two distinct returns, setting a new milestone in cometary astrophysics.

Other comets

See the full comet catalogue.