☄ Giacobini-Zinner 21P/Giacobini-Zinner

How to follow comet Giacobini-Zinner live

The panel above recomputes the position of Giacobini-Zinner 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 Giacobini-Zinner 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 Giacobini-Zinner with no math at all.

Comet fact sheet

About Giacobini-Zinner

Comet 21P/Giacobini-Zinner, discovered by Michel Giacobini on December 20, 1900, and recovered by Ernst Zinner in 1913, was the first comet ever visited by a spacecraft. On September 11, 1985, the American probe ICE (International Cometary Explorer) flew through the comet's tail just 7,800 km from the nucleus, collecting data on the plasma and magnetic environment of a comet in situ for the first time. The nucleus is estimated at about 2.0 km in diameter, and the comet is the parent body of the Draconids, an October shower famous for extreme variability between normal and storm years.

The comet passed its most recent perihelion on March 25, 2025, completing another 6.62-year orbit around the Sun. The October 2025 Draconids were closely monitored by researchers for any increase in activity associated with the recent return.

History and discovery

Michel Giacobini, a French astronomer working at the Nice Observatory (not Paris, as sometimes incorrectly cited), discovered the comet on December 20, 1900, in the constellation Aquarius. He specialized in systematic sky sweeps for comets. The object was not observed at the next 1907 apparition due to unfavorable geometry. Ernst Zinner, a German astronomer at the Bamberg Observatory, independently rediscovered the comet on October 23, 1913, not knowing it was the same object. The identity was established after the fact, and the comet received the hyphenated name honoring both observers.

In 1926 the Draconids were observed as an identifiable meteor shower for the first time, with rates of 15 to 20 per hour. In 1933 the Draconids produced an exceptional storm - estimates range from 10,000 to 30,000 per hour - with reports of meteors visible in daylight. The October 1946 storm reconfirmed the link, with estimated rates of 6,000 to 10,000 per hour. More recently, moderate outbursts were recorded in 2011 (about 300 per hour) and 2012.

Orbit and returns

21P/Giacobini-Zinner has an orbital period of approximately 6.62 years, with perihelion about 1.03 AU from the Sun (slightly beyond Earth) and aphelion roughly 6.0 AU, beyond Jupiter's orbit. Orbital inclination is about 31.8 degrees to the ecliptic. The comet belongs to the Jupiter-family comet (JFC) group, whose orbits are perturbed and shaped by the gravitational influence of the giant planet.

Well-documented modern returns include 1985, 1992, 1998 (when the comet passed about 0.36 AU from Earth), 2005, 2012, 2018 (a very favorable return, reaching magnitude 7) and the most recent on March 25, 2025. The 6.62-year period means roughly two returns per decade, making the comet regularly accessible to amateur astronomers. In favorable returns such as 2018, amateur telescope images captured a striking green coma tens of thousands of kilometers across.

Nucleus, coma and tail

21P/Giacobini-Zinner's nucleus is estimated at about 2.0 km in diameter - a modest-sized object, slightly smaller than Tempel-Tuttle. Estimated density is approximately 0.4 to 0.6 g/cm³, typical of Jupiter-family comets. The surface, like other active comets, is probably covered by a dark organic-rich crust, with active regions exposed by ice sublimation.

The most visually distinctive feature of Giacobini-Zinner near perihelion is its bright green coma. The green color is produced by emission from diatomic carbon (C2) molecules and cyanogen (CN) radicals, excited by solar ultraviolet radiation. In 2018, amateur photographs captured a vivid green coma that went widely viral on astronomy social media. The ion (plasma) tail points directly away from the Sun, while the less prominent dust tail forms a slightly different angle. Water production rate in favorable returns is estimated at roughly 10²⁸ to 10²⁹ molecules per second.

How to observe

21P/Giacobini-Zinner typically reaches magnitude 7 to 9 in favorable returns, making it a target for 10x50 binoculars or moderate-aperture amateur telescopes (80 to 150 mm). In less favorable returns it may fade below magnitude 10, requiring larger instruments. The green coma, when well developed, is visible in long-exposure photographs even with modest equipment.

The comet's main public legacy is the Draconid meteor shower (also called Giacobinids), active around October 8 to 9, with the radiant in the head of the constellation Draco. The Draconids have one of the lowest entry velocities among annual showers: approximately 20 km/s, less than a third of the Leonid speed. That low speed produces slow, often reddish or orange meteors with long burn times and persistent trains of several seconds.

In normal years the Draconids produce only 5 to 10 meteors per hour, but when Earth crosses fresh debris filaments - typically in comet return years or the years immediately following - rates can spike to hundreds or thousands per hour. For Draconid observation: best just after nightfall (the Draco radiant is circumpolar in the northern hemisphere but highest in early evening), with the observer facing north. The southern hemisphere has very limited visibility, as Draco is a high northern declination constellation.

Missions and scientific exploration

The ICE probe (International Cometary Explorer), originally called ISEE-3 (International Sun-Earth Explorer 3), was launched in August 1978 to study the solar wind between Earth and the Sun from the L1 Lagrange point. In 1982, NASA decided to redirect the probe to encounter Giacobini-Zinner as a precursor to the European fleet that would visit Halley in 1986. The redirection required a complex series of lunar gravity-assist maneuvers between 1983 and 1984 - five lunar flybys over 15 months - an unprecedented feat of orbital navigation.

On September 11, 1985, ICE flew through the comet's tail 7,800 km from the nucleus, measuring the magnetic field, ion tail plasma, solar wind modified by the comet's presence and charged particles. The probe carried no camera, but its scientific instruments collected data that were the first to map the "cometary bow shock" - the transition region between the undisturbed solar wind and the comet's inner magnetosphere - physically preparing the Giotto-Halley mission and defining instrumentation for subsequent cometary probes.

In 2014, citizen enthusiasts attempted to reactivate ICE as it returned near Earth after 30 years of travel. NASA no longer had the necessary ground transmitters, but a group of amateur engineers coordinated by Citizen Space Exploration used vintage radio antennas to establish partial communication with the probe, an unprecedented citizen-science project.

Trivia and records

• ICE was the first human-made object to encounter a comet up close. The 7,800-km distance to the nucleus during the September 1985 flyby was determined mainly from magnetic measurements, as the probe carried no camera.
• The Draconid storm of October 1933 was one of the most intense of the 20th century, with estimates of 10,000 to 30,000 meteors per hour and reports of meteors visible in daylight. Observers described the sky as "raining stars."
• Draconid meteors enter the atmosphere at just 20 km/s - less than a third of Leonid speed. That low velocity makes them relatively easy to capture on video, as the burn time is longer.
• Michel Giacobini discovered five comets during his career at the Nice Observatory, but Giacobini-Zinner is the only one bearing his name that remains in active circulation as a catalogued periodic object.
• The 2018 comet return generated iconic photographs of the green coma that circulated widely on astronomy social media, bringing the object to a far broader audience than professional astronomers alone.
• The Draconid radiant lies in Draco, a circumpolar northern constellation - making the Draconids practically invisible from latitudes below -30 degrees south, including most of Brazil.
• ICE made double history in 1985: it was both the first spacecraft encounter with a comet and the first use of repeated lunar gravity-assist maneuvers to radically redirect a probe already in mission.

Other comets

See the full comet catalogue.