A minor quibble with the summary above. On January 7, 1610, Galileo only recorded 3 "fixed stars" next to Jupiter. Two of the Galilean moons, Io and Europa, were too close together for Galileo to separate with his 20x power telescope. He continued to observe three moons at most, either because one or more moons were too close to Jupiter and were lost in the glare of the planet, Callisto was too far from Jupiter and was thus out of his telescope's field-of-view, or two of the moons were too close together, during subsequent nights, until January 13, when he was able to see all four for the first time.
Wikipedia is wrong on one point. True, his first observation of all four moon at once didn't come until January 13 and he didn't realize that there were four and not three until that time, but that doesn't mean that one moon's discovery (in Wikipedia's case, Ganymede) should be attributed to that date. By that point, he had observed all four on multiple occasions, just not all four at once. And to that point he hadn't even come to the conclusion that they were in orbit around Jupiter with their own separate orbits, moving a different speeds, until two days later, let alone ascribe identities to each of the stars he saw, connecting one star he saw with another from a different day, beyond the one to the east, the one to the west, and the one in the middle.
Mars Polar Lander happened. If you actually want to perform comprehensive science at these targets, you actually need to spend money.
In other words, you can have two out of three of "faster, better, cheaper", but not all three at the same time.
Secondly, an orbiter is needed to determine the thickness of the ice shell, which is important if you want to access the ocean. Designing a mission that needs to dig down through 2-5 km of ice is quite a bit different than digging through 20-30km. Plus, an orbiter might be able to find areas where the shell is thinner, further helping later lander developers pick a landing site.
IF I HAD A MINE SHAFT, I don't think I would just abandon it. There's got to be a better way. -- Jack Handley, The New Mexican, 1988.