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Physical Properties of Comet 28P/Neujmin 1: Nucleus Rotation, Shape, and Phase Function

Presentation #309.03 in the session Physical Characteristics of Cometary Nuclei.

Published onOct 20, 2022
Physical Properties of Comet 28P/Neujmin 1: Nucleus Rotation, Shape, and Phase Function

We performed imaging of 28P/Neujmin 1’s nucleus during a 5-month interval beginning in late 2021 through April 2022, with the comet’s heliocentric distance increasing from 3.0 to 4.1 AU following its unobservable perihelion passage early in 2021. Since Neujmin 1 has an unusually long orbital period of 18.2 yrs for a Jupiter family object, few opportunities for study have occurred in the modern era. The first studies for this very anemic object were in 1984, when visible and thermal photometry were obtained less than two months prior to perihelion. The lack of a dust coma (though gas was detected) permitted the first comet nucleus albedo measurements to be made and revealed a large nucleus and a very low albedo prior to the Halley mission findings. Observations in following years revealed a double-peaked rotational lighcurve and a period near 12.7 hr.

Preliminary results from our recent observational campaign of Neujmin 1 reveal a slowly varying lightcurve amplitude ranging from 0.5 to 0.7 mag, significantly greater than complete lightcurves obtained during prior decades, strongly implying that our viewing geometry of the nucleus was from much closer to the equator. The measured (synodic) period of 12.715±0.003 hr is the most precise value ever determined and, given the quite large nucleus size (effective radius of ~10.7 km) and the extremely low active fraction (<0.1%), we assume the sidereal period is unchanged since the 1980s. Although the range of phase angles was limited, ranging from 14.7 to 6.0 and back to 14.6 deg, the ensemble of data during 20 nights of observations yielded a tight constraint on the nucleus linear phase function coefficient of ~0.05 mag/deg, somewhat steeper than the average value of 0.04 mag/deg for comet nuclei.

Details of these and other results will be presented. This research was supported by NASA’s Solar System Observations Program grant 80NSSC18K0856.

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