Universe Puzzle No. 4

As with last week’s Universe Puzzle, something that cannot be answered by five minutes spent googling, a puzzle that requires you to cudgel your brains a bit, and do some lateral thinking. This is a puzzle on a “Universal” topic – astronomy and astronomers; space, satellites, missions, and astronauts; planets, moons, telescopes, and so on.

What’s the next number in the sequence? 401, 172, 85.2

There are no prizes for the first correct answer – there may not even be just one correct answer – posted as a comment (the judge’s decision – mine! – will be final), but I do hope that you’ll have lots of fun.

Post your guesses in the comments section, and check back on Wednesday at this same post to find the answer. Good luck!

UPDATE: Answer has been posted below.

42.5 is the answer; it’s the mean orbital period of Io, in hours; the first three members of the sequence are the mean orbital periods of Callisto, Ganymede, and Europa (source)

Well done scibuff!

Check back next week for another Universe Puzzle.