That’s one huge rock. Would have been neat if they did it on Hally’s comet.
The Halley’s rendezvous thirty years ago was pretty good for its time, and resulted in good data about the composition of the tail. The biggest proof of concept for the 4 billion mile trip the Rosetta probe has taken was probably the Clementine mission to map the lunar surface; it took a low-energy, manana kind of trip to get there.