The End of an Era in Cometary Astronomy: The Deceleration of Comet Encke

TL;DR

This paper documents the recent transition of Comet Encke from a state of nongravitational acceleration to a deceleration, marking the end of a two-century period of its orbital anomaly.

Contribution

It provides the first precise observational evidence of the transition from acceleration to deceleration in Comet Encke's orbit, confirming a long-standing theoretical prediction.

Findings

The comet's nongravitational acceleration has ceased.

Two independent orbit determinations confirm the transition.

The era of persistent acceleration in Encke's orbit is over.

Abstract

It is noted that effective very recently, the orbital motion of Encke's comet has become affected by a very slight nongravitational deceleration. Soon after J. F. Encke established in the early 19th century that the comet was returning to perihelion every 3.3 years, he also discovered that the object was notorious for returning to perihelion a little earlier than predicted by the Newtonian theory. The acceleration persisted over a period of two centuries, but its rate was gradually decreasing, Generations of cometary astronomers were curious to know whether or not the comet would eventually move in purely gravitational orbit. A model based on the assumption of a precession of the comet's nucleus, which predicted that the acceleration would change into a deceleration, was not published until 1979. This transition has now been documented by two independent, highly-accurate orbit determinations. The era of the comet's persevering nongravitational acceleration is finally over.