The Discus Comet: C/2014 B1 (Schwartz)

Long period comet C/2014 B1 (Schwartz) exhibits a remarkable optical appearance, like that of a discus or bi-convex lens viewed edgewise. Our measurements in the four years since discovery reveal a unique elongated dust coma whose orientation is stable with respect to the projected anti-solar and orbital directions. With no tail and no trail, the limited influence of radiation pressure on the dust coma sets a lower limit to the effective particle size of 0.1 mm, while the photometry reveals a peak coma scattering cross-section 27,000 sq km (geometric albedo 0.1 assumed). From the rate of brightening of the comet we infer a dust production rate of order 10 kg/s at 10 AU heliocentric distance, presumably due to the sublimation of supervolatile ices, and perhaps triggered by the crystallization of amorphous water ice. We consider several models for the origin of the peculiar morphology. The disk-like shape is best explained by equatorial ejection of particles from a nucleus whose spin vector lies near the plane of the sky. In this interpretation, the unique appearance of C/2014 B1 is a result of a near equality between the rotation-assisted nucleus escape speed (1 to 10 m/s for a 2 to 20 kilometer-scale nucleus) and the particle ejection velocity, combined with a near-equatorial viewing perspective. To date, most other comets have been studied at heliocentric distances less than half that of C/2014 B1, where their nucleus temperatures, gas fluxes and dust ejection speeds are much higher. The throttling role of nucleus gravity is correspondingly diminished, so that the disk morphology has not before been observed.Comment: 36 Pages, 10 Figure

Light Curves and Colors of the Ejecta from Dimorphos after the DART Impact

On 26 September 2022 the Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, a satellite of the asteroid 65803 Didymos. Because it is a binary system, it is possible to determine how much the orbit of the satellite changed, as part of a test of what is necessary to deflect an asteroid that might threaten Earth with an impact. In nominal cases, pre-impact predictions of the orbital period reduction ranged from ~8.8 - 17.2 minutes. Here we report optical observations of Dimorphos before, during and after the impact, from a network of citizen science telescopes across the world. We find a maximum brightening of 2.29 $\pm$ 0.14 mag upon impact. Didymos fades back to its pre-impact brightness over the course of 23.7 $\pm$ 0.7 days. We estimate lower limits on the mass contained in the ejecta, which was 0.3 - 0.5% Dimorphos' mass depending on the dust size. We also observe a reddening of the ejecta upon impact.Comment: Accepted by Natur

Properties of Irregular Satellites and Fragmenting Comets

In this thesis, I investigate the nature of two small body populations; the irregular satellite populations of the giant planets and the properties of fragmented nuclei of comets. In both cases the objective is to understand evolutionary processes acting on primitive solar system objects. An optical color survey of 43 irregular satellites enabled color comparisons with other small body populations that may reflect upon the origin of the irregular satellites. Ultrared matter (color index B-R ≥ 1.6), while abundant in the excited Kuiper belt and Centaur populations, is depleted from the irregular satellites. Also, the color distributions of the irregular satellites at each giant planet are statistically similar to each other, consistent with a common source region and/or evolutionary mechanism. Separately, the current observed supply of comets allows for estimates on the masses of their outer solar system source regions, however, comet fragmentation may occur more often than previously thought, which will lead to shorter estimates of comet lifetimes than predicted. As a case study, I analyzed archival Hubble Space telescope images of comet 73P/Schwassmann-Wachmann 3 (73P). The measured rotation period of the nucleus is much longer than the critical period for rotational instability for any reasonable nucleus density and shape, even in the absence of tensile strength. The data also show hundreds of fragments within 73P-B and 73P-G on which photometry was used to measure the brightness distribution of the fragments. I also measure the motion of these fragments and find the relative speeds of the fragments within 73P-B are a few m/s, implying an impulsive breakup about 7 days prior to the observations. Both the irregular satellites and comets are small bodies comprised of primitive material. The origin and evolution of the small bodies describe the early formation and evolution of the solar system itself