APL Colloquium

November 18, 2005

Colloquium Topic: Deep Impact and Comet 9P/Tempel 1: From Evolved Surface to Interior Primeval Dust

Until July 2005, the best understanding we had of a comet's nucleus was found by studying the material emitted from the surface into the comet's extended, gravitationally unbound atmosphere (i.e., the coma). This understanding of the solar system's most primitive bodies, however, assumed that the information obtained from the coma can be extrapolated back to the surface and sub-surface regions of the nucleus. There is now increasing evidence that comets evolve from their initial state due to the effects of solar insolation until their surfaces become devolatilized. In time, the depleted crust may become thick enough to stop activity altogether, making the comets appear as dormant "asteroids", hiding the pristine solar system material deep inside their interiors. For the first time, this scenario was tested on July 4, 2005 as NASA's discovery mission Deep Impact sent a 375 kg impactor into the nucleus of comet 9P/Tempel 1 at 10.2 km/s relative velocity in order to deliver 19 GJoules of energy and create a crater revealing sub-surface material of the nucleus. Despite obscuration of the crater by the initially thick cloud of dust, the Deep Impact flyby spacecraft obtained imagery and spectroscopy that showed the comet to be a porous, fragile, and dusty body with complex geology, containing the simple compounds and minerals involved in the formation of the solar system. Interestingly, although approximately 107 kg of material was released by the impact, there was no obvious long term change in the comet's behaviour. In this talk, I will report on some of the most exciting preliminary highlights from the Deep Impact mission, and include some observations taken from remote ground and space-based telescopes taken during the encounter (over 80 telescopes and 250 astronomers worldwide watched the event).

Colloquium Speaker: Carey M. Lisse

Dr. Carey M. Lisse is a Senior Research Scientist at APL and the University of Maryland. He is a member of the Science Team for the Deep Impact Discovery Mission. Previously he spent 1985-1995 as an Astrophysicist at NASA/Goddard Spaceflight Center, 1995-1997 as a Research Associate and 1997-1999 as an Associate Research Scientist at the University of Maryland. During 1990-2001, he was a Hubble Space Telescope Instrument Scientist at the Space Telescope Science Institute (STScI). He was a member of the NASA/IRTF D/Shoemaker-Levy 9 Observing Team, 1994; the NASA/IRTF C/Hyakutake Science Team, 1996; and the Constellation-X Facility Science Team, 1998-present. Dr. Lisse received a STScI Science Merit Award in 2000 and a Space Foundation Space Achievement Award from STScI in 2001. Asteroid 12226 Caseylisse was named in his honor in 2001. Dr. Lisse holds a B.A. in Chemistry from Princeton University, an M.S. in Chemistry from the University of California at Berkeley, and an M.S. and Ph.D. in Physics from the University of Maryland. His current research interests include small bodies, planetisimals, and dust in solar systems. His work involves the measurement of the physical properties of cometary nuclei using radio, IR, and optical observations, the measurement of the composition, particle size distribution, and emission history of cometary dust using dynamical modeling and optical/IR imaging photometry. In addition, Dr. Lisse is engaged in the study of the thermal behavior of asteroids and the interplanetary dust cloud, a phenomenological study of the comet-asteroid transition and fragmenting comets, the study of X-ray emission from comets, and the search for evidence of cometary gas and dust in exo-solar systems.