This morning when I was walking the dog, I paused to appreciate the sight of the early sunlight shining through the dew-drenched grass and sparkling on the dripping ponderosa pine branches. It was a nice peaceful moment, and it made me think of what a different world we’re sending Curiosity to. The amount of liquid water that I saw glimmering on the grass this morning probably hasn’t been seen anywhere on Mars in billions of years.

In terms of the longevity of our rovers, that’s certainly a good thing. Think of it: Opportunity has been on the surface of Mars for eight years and not a drop of water has fallen on her. Curiosity’s primary mission is about two Earth years, and who knows how much longer it will last, all while navigating a bone-dry desert.

A lot of times people will ask whether our rovers ever come home after exploring Mars. It was even the subject of an xkcd comic, but it really misses the point. The rovers are built on Earth, but Mars is their home. On Earth the gravity is too high and the environment is too wet. MSL’s massive arm can’t extend all the way under Earth’s gravity: the instruments at the end of the arm weigh too much and put too much stress on the joints. But once Curiosity is on the surface of Mars, that arm will finally be able to stretch to its fullest extent. When the mast raises and takes in that first panorama, it will be our first glimpse of a new landing site on an alien world. For Curiosity, it will be the welcome sight of home.

On Earth, MSL can't extend its arm all the way, but under the lower gravity of Mars it will be able to extend all the way.