Astronomers from MIT and Williams College were fortunate enough to be watching Pluto's moon Charon at the moment that it passed in front of a very dim star. By measuring how the light from this star dimmed as it passed behind the tiny, distant moon, they were able to come up with a very accurate measurement of Charon's size (606 km or 377 miles). They also determined that the moon doesn't have any appreciable atmosphere, lending evidence that it was formed when something smashed into a proto-Pluto millions of years ago.