Once described by the New York Times as a "Cheery Herald of Fear," Marsden specialised in tracking asteroids and comets and computing their orbits. He was once described as the "sentinel protecting Earth", and his work was crucial in helping to track potentially Earth-threatening objects. Marsden was perhaps best known for his 1998 announcement that an asteroid, known as 1997 XF11, might strike the Earth in 2028, causing catastrophic damage. Charles Alcock, Director of the Harvard-Smithsonian Centre for Astrophysics, said that "Marsden was one of the most influential comet investigators of the 20th century and definitely one of the most colourful, with an equally pleasant demeanour."
Marsden's 1998 announcement that the object called 1997 XF11 could collide with Earth caused a public stir and gave him a degree of notoriety. He explained that he did this as a "last-ditch" effort to encourage further observations, including searches for data from several years earlier to refine calculations of the object's orbit; and that is what happened. Observations from 1990 emerged soon after, and new calculations demonstrated that the object was highly unlikely to collide with Earth in the foreseeable future. Critics, however, suspected that Marsden had 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.
The comet prediction of which Marsden was most proud, however, was that of the return of Comet Swift-Tuttle, the comet associated with the Perseid meteor shower each August. Swift-Tuttle had been discovered in 1862, and the conventional wisdom was that it would return around 1981. Marsden had a strong suspicion, however, that the 1862 comet was identical with one seen in 1737, and this assumption allowed him to predict that Swift-Tuttle would not return until late 1992. This prediction proved to be correct; this comet has the longest orbital period of all the comets whose returns have been successfully predicted.
Marsden also played a pivotal role in the "demotion" of Pluto from full-planet to dwarf-planet status. He was interested in the discovery of what he called "transneptunian objects", although colleagues referred to them as objects in the Kuiper Belt, the region extending from the orbit of Neptune to edge of the solar system.
When what seemed the first of these transneptunian objects was discovered in 1992, Marsden argued that they were not the first, because Pluto, discovered in 1930 and somewhat larger, had to be the first. More specifically, he was the first to suggest, correctly, that three further transneptunian objects discovered in 1993 were exactly like Pluto, in the sense that for every two of their solar orbits Neptune orbits the sun three times. So he became a firm advocate of "demoting" Pluto.
With the discovery of Eris, another object comparable to Pluto, in 2005, the 2006 International Astronomical Union (IAU) created a new category of "dwarf planets". This now includes both Pluto and Eris, together with two further transneptunian objects known as Makemake and Haumea, as well as the largest asteroid, Ceres. Pluto was designated minor planet 134340, though this decision remains controversial.
It was also at the IAU meeting in Prague that Marsden stepped down as MPC director after 28 years; he was entertained by the thought that both he and Pluto had been retired on the same day. He remained working at the MPC (and the CBAT, the Central Bureau for Astronomical Telegrams) in an emeritus capacity.
Born in 1937 in Cambridge, Brian Geoffrey Marsden was the son of Thomas and Eileen; his father was a maths teacher at the local high school and his mother ran the home. His interest in astronomy was sparked one day in 1942 when he came home from primary school and found his mother in the backyard watching an eclipse of the sun through a candle-smoked glass, a practice now highly discouraged. However, what most impressed him, and eventually led him to his career, was that the eclipse had been predicted.
At the age of 11, Marsden entered the Perse School, Cambridge, and it was while there that he began making calculations of the orbits of the planets and their moons, using tables of seven-place logarithms. He always maintained that making such computations by primitive means significantly increased one's understanding of the science involved. At the age of 16, he joined and began regularly attending the monthly London meetings of the British Astronomical Association (BAA).
He quickly became involved with the Association's Computing Section, which was known specifically for making astronomical predictions other than those that were routinely being prepared by professional astronomers for publication in almanacs around the world. As an undergraduate at New College, University of Oxford, Marsden convinced the BAA to lend him a mechanical calculating machine, which increased his computational productivity. By the time he received his undergraduate degree he had already developed somewhat of an international reputation for the computation of comets' orbits, including new discoveries.
On completing his mathematicsdegree in 1959, Marsden enrolledat Yale University, receiving his PhD on "The Motions of the Galilean Satellites of Jupiter" in 1965. Thereafter, he joined the staff of the Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, and in 1968 became head of the CBAT, which is responsible for disseminating information about the discoveries of comets, novae, supernovae and other objects of astronomical interest.
It is the CBAT that actually names the comets (generally for their discoverers), and it has also been a repository for the observations of comets to which orbit computations need to be fitted. He became the director of the MPC in 1978, and became the longest-serving Associate Director of the Harvard-Smithsonian Centre for Astrophysics, from 1987 to 2003.
Among the numerous awards he received, Marsden particularly appreciated the 1995 Dirk Brouwer Award from the American Astronomical Society's Division on Dynamical Astronomy and the 1989 George Van Biesbroeck Award, then presented by the University of Arizona (now by the AAS) for services to astronomy.
Martin Childs
Brian Geoffrey Marsden, astronomer: born Cambridge 5 August 1937; married 1964 Nancy Lou Zissell (one son, one daughter); died Burlington, Massachusetts 18 November 2010.
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