Smaller Bodies of the Solar System

Comets, Asteroids, TNOs and What They Tell Us About How Planetary Systems are Formed

Michael F. A'Hearn

Distinguished University Professor of Astronomy Emeritus, University of Maryland

Videography by Nerine & Robert Clemenzi, Edited by Nerine Clemenzi
Copyright © Philosophical Society of Washington.  All rights reserved.


Much of the excitement about planetary exploration is focused on Mars, and a large fraction of NASA's planetary exploration budget and effort is directed to that planet. Nevertheless, the smaller bodies of the solar system - comets, asteroids, and trans-Neptunian objects ("TNOs") - may be even more important exploration targets, particularly for understanding how the solar system was formed.  Perhaps partly in recognition of their importance, NASA's human exploration program has evolved beyond a "first the moon, then Mars" approach to a more flexible and nuanced strategy that is likely to include exploration of a Near-Earth Asteroid.

This talk will consider recent developments in understanding the nature of these small bodies, and what the results imply for understanding how the solar system was formed.  Topics will include the results of recent dynamical studies, the Deep Impact experiment, and remote sensing of comets, asteroids and TNOs.  Comets are perhaps the most valuable of these targets for understanding planetary formation, but asteroids are likely a better target for future human exploration missions.  The dynamical studies of recent years have caused a complete rethinking of where the planets formed and how they migrated during and after their formation (the formation scenario). These results have profound implications for the distribution of asteroidal types in the asteroid belt, the size of Mars, and the separation of Oort-cloud comets from Jupiter-family comets.  Recent chemical abundance measurements independently require a scenario similar to that envisioned by the dynamicists.  Surprisingly, TNOs may have little to do with the comets that we observe.

Michael F. A'Hearn

About the Author:

Michael A'Hearn is a Distinguished University Professor Emeritus in the Department of Astronomy at the University of Maryland, where he has been a faculty member since completing his PhD at the University of Wisconsin more than 45 years ago.  Starting as a ground-based observer, he has studied comets and asteroids for most of his career with some early diversions to study interstellar dust.  He has made observations at all wavelengths from the far-ultraviolet to 18-cm, including developing optical instruments for ground-based telescopes.  He gradually made a transition to using space-based observatories, particularly the International Ultraviolet Explorer and subsequently the Hubble Space Telescope, among others. He has participated in nearly all robotic cometary missions, beginning with the ill-fated CRAF mission, which was cancelled.  He was the Principal Investigator for the Deep Impact mission and for the EPOXI mission.   He was a team member of the Stardust NExT mission, and he is a member of two instrument teams on ESA's Rosetta mission.  He also is the Principal Investigator for the Small Bodies Node of NASA's Planetary Data System, which archives all NASA-funded data relevant to comets, asteroids, and planetary dust.

His significant scientific achievements include the first major survey of abundances of volatiles in comets, discovery of unexpected molecules such as S2 in comets, direct determination of the densities of cometary nuclei both at the surface and in bulk (Deep Impact), strong evidence against differentiation of volatiles in the cometary nucleus at 10-20 meter depths, and the first physical and chemical evidence from comets supporting migration of the giant planets in the early stages of formation of our solar system.

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