University of Hawaii
Institute for Astronomy

David J. Tholen

Ph.D., Planetary Sciences, University of Arizona, 1984

Physical and Dynamical Studies of Small Bodies
(Asteroids, Comets, Planetary Satellites, Pluto)

Research Description

Pluto has long been a research interest of mine. From 1984 to 1991, I obtained an extensive set of photometric observations of Pluto while Charon's orbit was sufficiently close to being seen edge-on from Earth such that the bodies occulted and eclipsed each other. Those observations led to the first reliable diameter measurements and density determination for Pluto and Charon. In the early 1990s, I served on NASA's Outer Planets Science Working Group that recommended a spacecraft mission to Pluto as a high priority for outer Solar System science. That committee's efforts finally paid off with the 2006 January launch of NASA's New Horizons spacecraft bound for Pluto, scheduled to arrive in 2015. In 1992 and 1993, shortly after the Hubble Space Telescope began operations, Marc Buie (Lowell Observatory) and I acquired a set of 60 images of Pluto and Charon that we used to refine the orbit of Charon (see figure below). An unexpected result was the non-zero eccentricity of Charon's orbit around Pluto. Following the discovery of two more satellites of Pluto in 2005, Buie and I, along with Will Grundy (also at Lowell Observatory) have been awarded time on the Hubble Space Telescope to acquired additional astrometric and photometric data of the Pluto system. These observations are scheduled to commence in early 2007, so we expect to have exciting new information about the new satellites soon.

Other dynamical work involves the Jovian and Saturnian satellites. For the purpose of improving the satellite ephemerides in advance of the Cassini spacecraft encounter, I observed satellite-satellite mutual events for the latter during the ring-plane crossing of 1995. I also observed similar events involving Jupiter's Galilean satellites in 1997 to measure Io's secular acceleration and compare the energy dissipation with heat flow measurements.

During graduate school, I worked on a colorimetric survey of the asteroids, and my doctoral dissertation presented a new asteroid taxonomy, which continues in wide use to this day. During that survey, we took every opportunity to observe what were then rather rare appearances by near-Earth asteroids. Following the realization that these objects represent an impact hazard to the Earth, in 1991 I was asked to serve on two NASA committees to deal with the questions of how to detect near-Earth asteroids and how to deflect any asteroid that appeared to be on a collision course with the Earth. Since that time, NASA has funded several sky surveys designed to find near-Earth asteroids, though they have concentrated on finding asteroids beyond the orbit of the Earth.

Recently, I have started tackling the difficult problem of finding near-Earth asteroids with orbits whose aphelion (greatest distance from the Sun) coincides with the Earth's orbit. Such objects could impact the Earth, but they would never be found by the other near-Earth object search efforts, which examine near-Earth space at distances greater than the Earth's distance from the Sun. The difficulty is caused by the extremely high phase angle of the objects being sought (a crescent Moon is considerably fainter than a nearly full Moon, to use an appropriate analogy) and the brief amount of time the required portion of the sky can be imaged shortly after sunset and before sunrise. The first such object to be discovered is 1998 DK36, which my thesis student, Robert Whiteley, and I found on February 23. Other near-Earth asteroid discoveries credited to this project include the Apollo-type asteroids 1997 QK1 and 1998 DV9, both of which are considered potentially hazardous objects. Our largest discovery is 1999 OW3, estimated to be nearly 5 km in diameter. Our most famous discovery is 2004 MN4, now known as (99942) Apophis, which will make an extremely close approach to the Earth on 2029 April 13.

I am on the science team for the Japanese Hayabusa spacecraft mission to the near-Earth asteroid (25143) Itokawa, which was intended to be the world's first sample return mission to an asteroid. The rendezvous occurred between 2005 September and November. The timeline called for the spacecraft delivering its samples to Earth in mid-2007; however, a thruster anomaly delayed the journey home. Fortunately, the orbital geometry repeats every three years, so we're now hopeful for the sample return occurring in mid-2010.

I also work on the Small Bodies Node of the Planetary Data System, which archives planetary data related to asteroids, comets, and dust. In connection with that activity, I supplied software to the 2MASS infrared sky-survey project to predict which asteroids, comets, and planets might appear in each of the 2MASS scans. Over the course of the multi-year project, we extracted a homogeneous set of infrared JHK magnitudes for thousands of asteroids. That same software will soon be adapted for use with the upcoming WISE spacecraft mission.

The orbit of Charon. The dots represent the observations, and the solid curve represents the fitted orbit. To minimize the scatter produced by the varying sub-Earth latitude and topocentric distance during the fifteen months spanned by the observations, the data have been corrected to the single orbit interval centered on the epoch 1993 February 22. The orbital angular momentum vector points roughly toward the west, and the eastern portion of the orbit passes on the near side of Pluto.

Collaborative Work

Pluto-Charon research is being carried out in collaboration with Marc Buie, Lowell Observatory. Satellite mutual event analysis is being done with Kaare Aksnes, Institute of Theoretical Astrophysics, University of Oslo, Norway.

Honors and Awards

You can also email David J. Tholen.

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