Yesterday (March 17, 2011), the MESSENGER spacecraft became the first to orbit Mercury. Based on what I’ve seen, it was a nearly perfect orbit insertion. With so many things that can go wrong—and absolutely no way to intervene from so great a distance—this is a remarkable feat. I’m unabashedly a fan of robotic exploration, and so even though planets are not my area I follow missions like this with a lot of excitement. I don’t know anyone personally who has worked on MESSENGER, but I’ve known scientists who have worked on (or at least personally involved with) other space-based missions, and I know second-hand how much effort is involved in these endeavors. Congratulations to the team!

Mercury is the smallest planet (yes yes Pluto blah blah Pluto blah), and although it’s closer to Earth than the outer planets, in many ways it’s a lot harder to get to than (for example) Saturn. It’s certainly harder to get good pictures of Mercury from Earth than it is planets like Saturn, for a very similar reason: Mercury is fairly close to the Sun, so pictures tend to get washed out and the effect of the Sun’s gravity makes maneuvering just that much more complex. Up until the MESSENGER mission, we didn’t even have a complete picture of one side of Mercury. By the end of this mission, we’ll have not only a complete surface map but a better idea of the structure of the smallest planet.

Mercury is a weird world compared to the other three terrestrial planets (Earth, Venus, and Mars) and the Moon: it’s much denser, indicating that its core is a larger fraction of the interior compared with Earth. It experiences huge temperature variations due to its proximity to the Sun and its lack of a real atmosphere. It actually does have a transient atmosphere that’s much much much thinner than Earth’s or even Mars’, and MESSENGER will study that as well. Stay tuned—we have a lot to learn, and whatever we learn is bound to be interesting.