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Long period comet C/2014 B1 (Schwartz) exhibits a remarkable opticalappearance, like that of a discus or bi-convex lens viewed edgewise. Ourmeasurements in the four years since discovery reveal a unique elongated dustcoma whose orientation is stable with respect to the projected anti-solar andorbital directions. With no tail and no trail, the limited influence ofradiation pressure on the dust coma sets a lower limit to the effectiveparticle size of 0.1 mm, while the photometry reveals a peak coma scatteringcross-section 27,000 sq km (geometric albedo 0.1 assumed). From the rate ofbrightening of the comet we infer a dust production rate of order 10 kg/s at 10AU heliocentric distance, presumably due to the sublimation of supervolatileices, and perhaps triggered by the crystallization of amorphous water ice. Weconsider several models for the origin of the peculiar morphology. Thedisk-like shape is best explained by equatorial ejection of particles from anucleus whose spin vector lies near the plane of the sky. In thisinterpretation, the unique appearance of C/2014 B1 is a result of a nearequality between the rotation-assisted nucleus escape speed (1 to 10 m/s for a2 to 20 kilometer-scale nucleus) and the particle ejection velocity, combinedwith a near-equatorial viewing perspective. To date, most other comets havebeen studied at heliocentric distances less than half that of C/2014 B1, wheretheir nucleus temperatures, gas fluxes and dust ejection speeds are muchhigher. The throttling role of nucleus gravity is correspondingly diminished,so that the disk morphology has not before been observed.

The Discus Comet: C/2014 B1 (Schwartz)