This image of the nucleus of Halley's comet comes from the Giotto spacecraft. Click on image for full size version (67K JPG) Image courtesy of JPL

Among the things we learned from a suite of spacecraft which visited Halley's comet in 1986 are

what a comet nucleus looks like up close (no one had ever seen the nucleus before.) that the nucleus had craters and other surface features that evaporation occured only from specific jets, or cracks in the surface how big the coma is, compared to the actual nucleus (100,000 miles vs. 15 miles) that instead of being bright like a surface made of ice, the nucleus was "dark" as if the ice were covered with something how the magnetic field of the Sun responded to the presence of the comet coma