https://phys.org/news/2018-12-december-comet-rosetta-memories.html

"When we began to study Rosetta, one of the most important tasks was to create a list of comets that could be reached by a spacecraft launched on an Ariane 5 and carrying sufficient payload to study the comet," says Gerhard Schwehm, who was the ESA Rosetta project scientist at the time.

"It turned out that a mission to Comet 46P/Wirtanen with launch in early 2003 would be one of the best opportunities. This initiated an intensive observation campaign from the ground to prepare for the mission."