☄ d'Arrest 6P/d'Arrest

How to follow comet d'Arrest live

The panel above recomputes the position of d'Arrest 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 d'Arrest 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 d'Arrest with no math at all.

Comet fact sheet

About d'Arrest

Heinrich Louis d'Arrest was 24 years old and working at the Leipzig Observatory when, in the early hours of June 28, 1851, he located a cometary object no one had catalogued before. The comet took his name and the designation 6P, becoming the sixth entry on the official list of short-period comets. Unremarkable in brightness but stable in orbit, 6P/d'Arrest is a textbook Jupiter-family member: a small body gravitationally captured by the giant planet and set on a relatively steady path between Mars and Jupiter for astronomical ages.

What makes 6P scientifically interesting is a subtle phenomenon: the comet consistently arrives at perihelion several days later than purely gravitational calculations predict. That delay is produced by the rocket-like effect of gas emission from the nucleus, and studying it has allowed precise determination of the nucleus spin-axis orientation and the size of the active surface regions.

History and discovery

Heinrich Louis d'Arrest discovered the comet on June 28, 1851 at the Leipzig Observatory during a systematic sky sweep. Subsequent calculations showed an orbital period of about 6.7 years, confirming it as a short-period comet. It was the third periodic comet confirmed as such by the astronomy of the time, after Halley and Encke.

There is a historical footnote: some older sources raised the possibility of an unrecognized earlier observation in 1678, but that identification was never conclusively established. What is certain is that d'Arrest was the first to record the object unambiguously. The same d'Arrest had already achieved fame by a different route: in 1846, five years before the comet, he helped Johann Galle locate Neptune in the sky for the first time, one of the most dramatic moments in the history of astronomy.

Orbit and returns

6P/d'Arrest is a typical Jupiter-family comet with a period of 6.57 years. Its perihelion sits at about 1.36 AU from the Sun, closer than 4P/Faye, while aphelion reaches roughly 5.7 AU within the Jovian influence zone. This relatively Sun-grazing path generates visible cometary activity, but the small nucleus caps peak brightness at around magnitude 9 during the best approaches.

A noted orbital quirk: 6P/d'Arrest consistently arrives at perihelion a few days later than purely gravitational calculations predict. This systematic delay is attributed to the non-gravitational force produced by outgassing from the nucleus during ice sublimation. Researchers determined from this delay that the nucleus bulk density is approximately 400 kg/m3 and that the spin axis is inclined roughly 60 degrees to the orbital plane.

Nucleus and family

The nucleus of 6P/d'Arrest is small, with a diameter estimated at roughly 1.6 to 2 km based on photometry of the bare nucleus far from perihelion. The albedo is very low, typical of the Jovian family, at around 0.03 to 0.04. Non-gravitational force measurements identified two active regions on the surface: one in the northern hemisphere of the nucleus at a latitude of roughly 84 degrees, and one in the southern hemisphere at about -42 degrees.

As a Jupiter-family member, 6P shares the typical history of gravitational capture by the giant planet from a more distant orbit, most likely originating in the Kuiper Belt. Jovian perturbations continue to reshape the orbit with each passage, and the comet's activity coefficient has been monitored for decades to detect possible changes in the sublimation level.

• Estimated nucleus diameter: 1.6 to 2 km
• Albedo: ~0.03 to 0.04
• Estimated density: ~400 kg/m3
• Active regions: two identified (N ~84 deg, S ~-42 deg)
• Group: Jupiter family (JFC)

How to observe

At its best, 6P/d'Arrest can reach magnitude 9 to 10, making it accessible to large binoculars or a 4-inch telescope under dark skies. In less favorable years it stays below magnitude 12 and demands larger instruments. The comet shows no spectacular structure: the coma is small and the tail, when present, is short.

The next perihelion is expected in the early 2030s. Experienced observers value 6P precisely for its orbital predictability and its recurring role as a target for photometric measurements that help refine models of cometary nucleus activity. For planning, JPL Horizons provides daily ephemerides with position, estimated magnitude, and heliocentric and geocentric distances.

• Minimum instrument: 10x50 binoculars or a 4-inch telescope
• Typical perihelion magnitude: 9 to 10 (favorable years), 11 to 12 (unfavorable years)
• Typical appearance: compact coma, no prominent tail
• Best position source: JPL Horizons or In-The-Sky.org

Science and historical observations

6P/d'Arrest has been a target for non-gravitational force studies for decades. Modeling published by Szutowicz and collaborators (2006, Icarus) showed that the systematic perihelion delay can be explained by an asymmetric sublimation model that accounts for the geometry of the spin axis and the distribution of active regions. That work is among the most detailed ever published on non-gravitational forces in a short-period comet.

Studying non-gravitational forces in comets like 6P has practical implications: small bodies that might eventually approach Earth can have their trajectories altered by these effects, and understanding them improves the accuracy of long-range impact forecasts. In the case of 6P/d'Arrest the current orbit poses no risk, but the physical mechanism is exactly the same that operates in potentially hazardous asteroids and comets.

Facts

• Heinrich d'Arrest helped Johann Galle locate Neptune on September 23, 1846, five years before discovering the comet that bears his name, making him a central figure in two of the greatest moments in nineteenth-century astronomy.
• 6P is the third numbered periodic comet in history, a reflection of how young rigorous orbital calculation was in the 1800s.
• The non-gravitational delay at perihelion was used to determine the nucleus bulk density at ~400 kg/m3, placing it among the least dense cometary nuclei known, comparable to compacted snow.
• No confirmed meteor shower is associated with 6P/d'Arrest; its debris trail does not cross Earth's orbit at a sufficient angle and density to produce detectable activity.
• The name d'Arrest is French: the final t is silent, though in anglophone astronomical texts it is often rendered phonetically as "duh-Ah-REST".
• The Leipzig Observatory where d'Arrest worked was founded in 1722 and was one of the most important centers of orbital calculation in the nineteenth century.

Other comets

See the full comet catalogue.