The Extremely Low Activity Comet 209P/LINEAR During Its Extraordinary Close Approach in 2014

TL;DR

During its exceptionally close approach in 2014, Comet 209P/LINEAR exhibited the lowest water production rates and smallest active area ever recorded, indicating it is becoming an inert object.

Contribution

This study provides the first direct measurements of water production and active area for Comet 209P/LINEAR during its close approach, revealing its near-inert state.

Findings

Lowest water production rates of over 35 years

Smallest active area and active fraction for any comet

Detection of a double-peaked rotational lightcurve

Abstract

We present results from our observing campaign of Comet 209P/LINEAR during its exceptionally close approach to Earth during May of 2014, the third smallest perigee of any comet in two centuries. These circumstances permitted us to pursue several studies of this intrinsically faint object, including measurements of gas and dust production rates, searching for coma morphology, and direct detection of the nucleus to measure its properties. Indeed, we successfully measured the lowest water production rates of an intact comet in over 35 years and a corresponding smallest active area, ~0.007 km^2. When combined with the nucleus size found from radar (Howell et al. 2014), this also yields the smallest active fraction for any comet, ~0.024%. In all, this strongly suggests that 209P/LINEAR is on its way to becoming an inert object. The nucleus was detected but could not easily be disentangled from the inner coma due to seeing variations and changing spatial scales. Even so, we were able to measure a double-peaked lightcurve consistent with the shorter of two viable rotational periods found by Hergenrother (2014). Radial profiles of the dust coma are quite steep, similar to that observed for some other very anemic comets and suggestive that vaporizing icy grains are present.