"best known for his prediction of the return of SwiftTuttle, the comet associated with the spectacular annual shower of the Perseid meteors"
"conventional wisdom was that it would return around 1981. Marsden strongly suspected, 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, 11 years later than previously thought. His prediction proved to be correct."
"While still a schoolboy, he became an expert in calculating the positions of the planets and the orbits of newly discovered comets. At that time, only a few other people could make these laborious calculations... by the time he graduated, he had already developed something of an international reputation for the computation of orbits of comets."
"In 1998 Marsden became involved in controversy when he suggested to a New York Times journalist that a newly discovered asteroid, two kilometers across, might collide with the Earth in 2028. The asteroid, called 1997 XF11, was, however, soon found not to be a threat and Marsden later explained that he had said there would have been a collision “only under highly unusual circumstances”. The episode initiated a lively debate about the way astronomers should best inform the public about possible threats of such collisions.
He was again in the news when he played a key role in the demotion of Pluto from the status of major to minor planet in 2006."
Quotes from Times obit. I don't know whether this link works or works permanently.