James E. Richardson

EPOXI at Comet Hartley 2

In situ observations show that comet Hartley 2 is an unusually hyperactive comet. Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.

Modeling impact ejecta plume evolution: A comparison to laboratory studies

This study demonstrates the efficacy of a recently developed, impact crater“excavation flow properties model” (EFPM) that accomplishes the following goals:(1) uses hydrodynamic streamline theory to develop the expressions that extend theclassic impact ejecta scaling relationships into regions near the crater rim, wheretarget strength and/or gravity bring crater excavation flow to a halt; (2) links this new,general ejecta position/velocity scaling relationship to the existing general cratersize/volume scaling relationship, including the transition region between gravity‐ andstrength‐dominated cratering; (3) provides a means for estimating impact ejecta plumemass‐density and ejecta blanket thickness, as a function of azimuth and distance from theimpact site, in conditions ranging from low to high target strength; and (4) brings inour experimentally derived knowledge of impact ejection angles and the effects of obliqueimpact to develop a useful 2‐D and 3‐D model of both leading‐edge and trailing‐edgeejecta plume behavior. In this work, this excavation flow properties model is used tosimulate the images and data produced by three laboratory impact studies which utilizedmodern, laser‐based, non‐intrusive means to investigate ejecta plume formation,expansion, and fallout from three different perspectives.