Astronomer Brian G. Marsden, a comet and asteroid tracker who stood sentinel to protect the Earth from collisions with interplanetary rocks and other remnants of the solar system's creation, died Thursday of cancer at Lahey Clinic Medical Center in Burlington, Mass. He was 73.
Director emeritus of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., Marsden was perhaps best known for his 1998 announcement that an asteroid known as 1997 XF11 might strike the Earth in 2028, causing untold damage. The announcement sparked additional studies which quickly showed that such an impact was unlikely.
Marsden, who was once called "a cheery herald of fear" by the New York Times, also played a key role in the demotion of Pluto from major to minor planetary status, which also gained him a certain amount of infamy.
"Brian was one of the most influential comet investigators of the 20th century, and definitely one of the most colorful," astronomer Charles Alcock, director of the Harvard-Smithsonian center, said in a statement.
Marsden said he made the announcement about 1997 XF11 as a "last-ditch effort" to encourage the acquisition of further observations to refine calculations of the object's orbit, and that is indeed what transpired. Photos from 1990 emerged the next day and new calculations showed that the object was highly unlikely to strike our planet.
Critics, however, suspected that Marsden made the announcement in an effort to secure more funding for the search for interplanetary objects that could potentially strike Earth and that, too, has happened as such objects have grown in the public consciousness.
Marsden was also interested in the discovery of what he called "transneptunian" objects and his colleagues called "objects in the Kuiper Belt," the region extending from the orbit of Neptune to the edge of the solar system. When the first of these objects was discovered in 1992, Marsden countered that they were not the first, because Pluto — albeit somewhat larger — had to be considered among them. Pluto had been discovered in 1930. He became a firm advocate of "demoting" Pluto.
The 2005 discovery of Eris, a dwarf planet similar in size to Pluto, led to the inevitable demotion of Pluto to dwarf planet status in 2006. Marsden was entertained by the fact that Pluto was "retired" as a planet on the same day that he retired as director of the Minor Planet Center.
The feat Marsden was most proud of was his prediction of the return of Comet Swift-Tuttle, which is associated with the Perseid meteor shower that occurs each August. Most astronomers thought that the comet, which had been discovered in 1862, would make its reappearance in 1981. Marsden, however, suspected that it was identical with one seen in 1737. That meant it would not reappear until late 1992. He was correct.
Brian Geoffrey Marsden was born Aug. 5, 1937, in Cambridge, England. His interest in astronomy was sparked one day in 1942 when he came home from primary school and found his mother, Eileen, in the backyard watching an eclipse of the sun through a candle-smoked glass — a practice now highly discouraged. What most impressed him was that the eclipse had been predicted.
While still in high school, Marsden began making calculations of the orbits of the planets and their moons — using tables of seven-place logarithms. As an undergraduate at New College at the University of Oxford, he convinced the British Astronomical Assn. to loan him a mechanical calculating machine, which increased his productivity.
After receiving his degree in mathematics, he enrolled at Yale University, receiving his doctorate in astronomy in 1965. He joined what was then the Smithsonian Astrophysical Observatory upon graduation and in 1968 became head of Central Bureau for Astronomical Telegrams.
Amateur and professional astronomers who observed anything new in the skies would send a telegram of their findings to the center, which would then notify other astronomers by telegram or postcard. Eventually that system was replaced by electronic notifications. The last time the bureau received a telegram was in 1995 when Thomas Bopp sent word of his discovery of the object now known as Comet Hale-Bopp. ( Alan Hale almost received sole credit because he sent an e-mail.)
In 1978, the Minor Planet Center was moved to the Smithsonian from the Cincinnati Observatory; Marsden also became its head.
Marsden is survived by his wife of 45 years, the former Nancy Lou Zissell; a daughter, Cynthia Louise Marsden-Williams of Arlington, Mass.; a son, Jonathan Brian Marsden, of San Mateo; three grandchildren; and a sister, Sylvia Custerson, of Cambridge, England.