I joined the professional staff of JPL in 1974, even slightly before the completion of my graduate education (B.S. Caltech, 1966; M.S. UCLA, 1967; PhD, UCLA, 1975). In 2002 I took early retirement from JPL, but continue my research under affiliation with the Space Science Institute of Boulder, Colorado, where I am a Senior Research Scientist. My formal training is as a theorist, specifically in orbital dynamics, but I quickly came to appreciate the need to tie theory to observations, and likewise to direct the course of observations by theoretical considerations. That is, observations need to be motivated not just by what is possible to observe, but as well by what is useful to observe: what observations are needed to critically test a theory or hypothesis, or to usefully improve the determination of a physically meaningful quantity?
My first application of this philosophy came early in my career at JPL, when I developed a theoretical model for the collisional evolution of asteroid spins (Harris 1979). In a companion paper (Harris and Burns 1979), we evaluated all data then available on asteroid rotation rates, which consisted of rotation periods for 182 asteroids. While I was able to demonstrate some differences in mean spin rate among taxonomic classes (suggesting for example that "M" asteroids are more dense than other classes and thus indeed rich in reduced metals), it was clear that the existing data base was inadequate to test some of the details of the theory and that much more could be learned. I therefore began an observing program to increase the available data, and especially to avoid biases in the sample. At Table Mountain Observatory, we developed observing strategies that increased productivity of lightcurve observations by a factor of several over techniques previously employed. As a result the total data set of rotation rates has grown to more than one thousand objects, many of them obtained at Table Mountain, and our methods of data taking and reduction (Harris et al. 1989a, Harris and Lupishko 1989) have become standard techniques at most of the other observatories doing this kind of observation. The scientific return from these observations has been substantial. We were the first to observe an "opposition spike" in the brightness of high albedo asteroids at very low phase angle (Harris et al. 1989b), which has led to a new explanation of the opposition effect in terms of coherent backscatter (constructive interference in twice-reflected light) rather than shadow hiding. We first identified a population of very slowly spinning asteroids with periods of rotation exceeding 2 days (compared to a mean value of ~10 hours), and have demonstrated that it is not a statistical fluke but a real sub-population with very anomalous rotation properties. The discovery of this population was largely due to the superior observing techniques that avoided biases against discovering such long periods by previous methods. The record slowest rotating asteroid (or any solar system body, for that matter) known is 288 Glauke, with a period of 2 months, which was first measured by us at Table Mountain (Harris et al. 1999). A remarkable corollary phenomenon to very slow rotation is that such slowly rotating asteroids might be tumbling, that is in a state of non-principal axis rotation, since the damping time scale of rotational wobble becomes longer than the age of the solar system at such low frequencies of rotation (Harris 1994). The first well-documented example of tumbling rotation is of the near-Earth asteroid 4179 Toutatis. The rotation state was discovered more or less simultaneously by radar and by optical observations, some of them from Table Mountain (Spencer et al. 1995). Since that time, about half a dozen other examples of tumbling motion have been extracted from lightcurve observations, many of them our observations from Table Mountain, including the asteroid 253 Mathilde (Mottola et al. 1995), recently imaged by the NEAR spacecraft on its way to Eros. At the other end of the rotation frequency spectrum we have recently documented a "barrier" to rapid rotation among small asteroids at a period of ~2.25 hours, which implies that even such small asteroids are mostly "rubble piles," rather than monolithic bodies, and hence incapable of spinning more rapidly without flying apart (Harris 1996a, Pravec and Harris 2000). Very recent studies suggest that both the fast and slow excesses in rotation rates may be due to radiation pressure effects on small asteroids (Pravec et al. 2005).
While the above studies of asteroid rotation have been the central focus of my research activities, I have continued to pursue other topics as well. The paper by Colombo, Goldreich and Harris (1976) was the first one to recognize the importance of gravitational density waves in the structure of Saturn's rings, which was spectacularly revealed by the Voyager flybys a few years later. A very short paper (Harris 1975) on the collisional breakup of particles in a planetary ring corrected an important oversight in a 1947 paper by Harold Jeffreys and re-established breakup of pre-existing satellites as a plausible mode of forming planetary rings. The discovery of substantial satellites imbedded within the rings of the giant planets strongly supports this mode of formation (Harris 1984). In addition to the application to planetary rings, the formalism of "impact strength" has led to an increased understanding of the processes of breakup and erosion of the asteroid belt, for example I led the organization of the fifth workshop devoted solely to this topic, in July 1998. Another theoretical study I published (Harris 1977) was the first attempt to derive an analytical theory for the way planets acquire spin as they accrete from the protoplanetary nebula. At least half a dozen subsequent studies, both analytical and numerical, have been carried out to verify, extend and/or improve upon my result.
A substantial fraction of my time in the last several years has been devoted to a broad study of the hazard of impacts on the Earth by asteroids or comets, and what to do about it. Topics I have addressed include the feasibility of deflecting an asteroid from a collision course (Ahrens and Harris 1992, 1994), studies of the strategy for searching and cataloging Near-Earth Objects (Harris 1997), and philosophical/political pieces on what we should and should not do about the impact hazard (Harris 1996b, Harris et al. 1994). The latter paper, entitled "The Deflection Dilemma," has a most unusual cast of authors including Carl Sagan, and Greg Canavan, the inventor of "Brilliant Pebbles" in the Star Wars program. In the paper by D'Abramo et al. (2001), we present a new method of estimating the completion of current NEO surveys. Currently, I maintain a "scorecard" of the rate of current discoveries of NEOs, including updating the D'Abramo et al. completeness estimate, and provide assessments of survey strategies for various present or putative NEO search systems.
In the past few years I have indulged in several studies that I would characterize more as amusements than rigorous science, but some nevertheless are quite important, and indeed have generated more public interest than some more serious works. One "amusement" was a dynamical study (Harris 1993) refuting the claim that the lunar crater Giordano Bruno was formed by an impact in 1178, leaving a "Corvid" meteor stream similar to (but much weaker than) the Leonid meteor stream. A few years ago I became embroiled in the controversy over "mini-comets" proposed by Louis A. Frank. Shortly after Frank's latest claim at the 1997 AGU spring meeting, I did an evaluation of the relative detectability of these putative objects by optical vs. radar means (optical is much better, but they aren't seen), and published a couple papers in the Journal of Geophysical Research on the subject, at the urging of Alex Dessler, the former editor of GRL who accepted the original Lou Frank paper on mini-comets a decade ago, and now regrets it. Most recently, along with my colleague in impact hazard studies, Clark Chapman, we published an article (Chapman and Harris 2002) on risk perception and response relating to the 9/11 terrorist attacks.
Finally, I should mention activities that I regard as "civic duties" to the scientific community. In addition to the usual run of committees, science working groups and review panels, I have served as Chair of the Division on Dynamical Astronomy of the American Astronomical Society and the Secretary-Treasurer of the Division for Planetary Sciences of the AAS, which is an organization of 1,000 planetary scientists worldwide, from 1995 to 2001. I served as President of Commission 15 (Physical Studies of Comets, Minor Planets and Meteorites) of the International Astronomical Union from 1991 to 1994. I served as Chair of the Astronomy Division of the American Association for the Advancement of Science (AAAS) in 2002-2003. Over the last several years, I have served on seven thesis examination committees at the University of Paris, ranging from DEA (equivalent to MS) to Habilitation (equivalent to advancement to tenured rank on a University faculty). I consider this quite an honor, especially for someone who barely speaks a word of French.
Although I have not been a member of a spacecraft experiment team, I have been called upon to advise the Voyager project on imaging sequences for the Uranus and Neptune encounters, by the Galileo project to evaluate possible hazards of flying by an asteroid, and eventually for advice on targeting the Gaspra and Ida encounters. I served as the JPL Study Scientist for the original Near-Earth Asteroid Rendezvous (NEAR) mission when it was an "Observer" mission, and since its reincarnation as a Discovery mission I have been called upon by the science team for advice on the Mathilde encounter and more recently regarding the Eros rendezvous. I also served for a number of years on the Rosetta Science Definition Team, when it was still a comet sample return mission. During 2002, I served on the NASA NEO Science Definition team to re-evaluate the impact hazard from smaller NEOs and to consider possible deeper surveys to catalog smaller objects. At the same time, I have been involved with NSF sponsored studies of the "Large Aperture Synoptic Survey Telescope" (LSST), which would be partially devoted to deeper NEO surveys, and I have advised the University of Hawaii project Pan-STARRS, with similar goals.
Selected Bibliography (I list only a selection of what I consider to be my most important papers, plus a more complete selection of recent papers. A full listing would include more than 100 journal papers or book chapters, and more than 100 abstracts of papers presented.)
Last modified: January 3, 2008.