Brian G. Marsden, who as director of the International Astronomical Union's Minor Planet Center (MPC) was the official record keeper for most of the astronomical discoveries in the solar system in recent decades, died November 18, according to an obituary bulletin from the MPC. He was 73.

The MPC, housed since Marsden took it over in 1978 at the Smithsonian Astrophysical Observatory in Cambridge, Mass., serves as a clearinghouse of data collected by observers around the world, amateur and professional alike. The center catalogues the data and computes orbits for objects such as near-Earth asteroids, main-belt asteroids, comets, and distant bodies in a region near the orbit of Pluto known as the Kuiper Belt. It also notifies astronomers of which newfound objects are in need of follow-up observations and confirmation.

Marsden served as MPC director until 2006, when he began serving in an emeritus capacity. The center's mission was a natural fit for Marsden, who as an 11-year-old in England was already working on ways to calculate the motion of the planets, according to the MPC bulletin. He studied at the University of Oxford, working on the orbital mechanics of comets, and moved to Yale University for graduate school.

In 1965 he came to the Smithsonian Astrophysical Observatory, where he continued to work on the motion of small bodies in the solar system and eventually took the helm of the MPC. As director, he took some heat in 1998 for suggesting that a large, newly discovered asteroid called 1997 XF11 had a chance—however slight—of striking Earth in the future. The announcement created a public stir, but analysis of more data essentially nullified the probability of impact.

The field of solar system studies changed dramatically during Marsden's MPC tenure. When he took over the center, the solar system had been catalogued in comparably little detail. Virtually all near-Earth asteroids, of which some 7,000 are now known, have been discovered in just the past 15 years. And the presence of the Kuiper Belt had yet to be confirmed; until 1992 Pluto was a lonely planet in the outer reaches of the solar system. Before long the books were filling up with Kuiper Belt objects, some of them rather large, and Pluto was demoted by the International Astronomical Union (IAU) in 2006 to a dwarf planet. As minder of the records that rendered the erstwhile ninth planet rather ordinary-looking, it was fitting that Marsden relinquished his position just as Pluto did.

"It was also at the IAU meeting in Prague that Dr. Marsden stepped down as MPC director," the MPC bulletin states, "and he was quite entertained by the thought that both he and Pluto had been retired on the same day." The obituary did not provide a cause of death.

For more on Marsden, see Steve Nadis's profile of the astronomer and his work with near-Earth asteroids, which appeared in the August 2003 issue of Scientific American.