The comet with the shortest period
Comet Encke
Comet Encke circles the Sun in just 3.3 years: no other bright comet has so short a period. Observed at more than sixty returns, it is by now a worn-out object, with little ice left on its surface, and the source of one of the meteor showers richest in fireballs, the Taurids.
Perihelion passage of 22 October 2023, from the 3D simulation
- Nucleus
- 4.8 km
- Orbital period
- 3.3 years
- Perihelion
- 0.339 AU
- Aphelion
- 4.1 AU
- Discovered
- 1786
- Next perihelion
- 10 February 2027
Nucleus and tail
The nucleus of Encke is about 4.8 km across and extremely dark. It has never been seen up close: in the model on this page it is a sphere of that diameter. It rotates in about 11 hours. After thousands of perihelion passages most of its surface ice has been lost, and the comet is covered by a crust of dust from which gas escapes in only a few places.
That is why Encke is a faint comet: its coma is mostly gas and its dust tail is short, visible for a few weeks around perihelion. On the other hand it leaves unusually large grains behind. In 2007 the STEREO solar probe caught it as an eruption from the Sun tore its ion tail away; the tail then grew back.
Orbit
Encke takes 3.3 years to travel an orbit lying between 0.34 and 4.1 AU from the Sun: at perihelion it dips inside the orbit of Mercury, at aphelion it stays well short of Jupiter's. It is the only one of the best-known comets never to come near the giant planet, and astronomers still cannot explain with certainty how it ended up on such a small orbit. Its inclination is 11.4°.
Studying its returns, Johann Encke noticed that the comet arrived about two and a half hours early each time. He imagined a resisting medium filling space; the true explanation, found in the twentieth century, is that its gas jets act like small rockets. It was the first time forces other than gravity had been measured on a comet.
Discovery and exploration
It was first seen by Pierre Méchain, from Paris, on 17 January 1786; it was then found again by Caroline Herschel in 1795 and by Jean-Louis Pons in 1805 and 1818, each time mistaken for a new comet. In 1819 Johann Franz Encke showed, after weeks of calculation, that it was always the same one, and predicted its return for 1822. It was the second periodic comet to be recognised, after Halley's: hence the designation 2P.
No spacecraft has ever visited it: NASA's CONTOUR mission was due to do so in 2003, but the craft was lost shortly after launch. From Earth, Encke rarely reaches naked-eye visibility; binoculars are usually needed, and much depends on where it lies in the sky relative to the Sun. At its February 2027 perihelion it will be only 10 degrees from the Sun: hard to observe.
Frequently asked questions
Why is it named after someone who did not discover it?
Because it was Johann Encke who worked out what it was. The comet had been seen four times by three different astronomers without anyone recognising it; in 1819 Encke computed its orbit and showed that it was a single short-period object. It is one of very few comets, along with Halley's, named after the person who computed its orbit rather than the one who discovered it.
What are the Taurids?
A meteor shower active between October and November, produced by dust left by Encke and perhaps by a larger body of which the comet is a fragment. They are few, slow and often very bright: the grains are larger than usual and produce fireballs. Earth crosses the same stream in June too, in daylight.
Can comet Encke be seen with the naked eye?
Rarely. On its best passes it reaches magnitude 6 or a little better, at the limit of visibility under very dark skies, and always low on the horizon at twilight. With binoculars it is within reach of anyone who knows where to look, but only on returns when its position relative to the Sun is favourable.
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