NASA's Mars Reconnaissance Orbiter reached the red planet in March 2006, and entered into a highly elliptical orbit-- the type of orbit easiest to achieve in that it requires the least amount of fuel to enter. MRO is pretty big, about the size of a bus, so fuel was at a premium.
The probe's orbit had to be lowered to get it into position to be able to map mars. To do this, the low point in the orbit was manipulated using on-board rockets to dip the probe into the upper part of the martian atmosphere, a maneuver called aerobraking. This idea was used in the movie "2010" to get the Leonov into a Jupiter orbit. In the movie, the effect was very dramatic and scary. In real life, MRO only dipped its toes into the air, so to speak, slowing itself gently every time it reached its closest approach to Mars. This changed the shape of the orbit, lowering it and making it more circular.
And now, finally, on September 11, MRO's orbit is where it needs to be to start the science! Initially, the high point of the orbit was 45,000 km over Mars' surface, but now it's at 486 km. Imagine-- at first, the orbits were very long, and the aerobraking maneuvers took place over a day apart. But as the orbit changed, and got smaller and shorter in period, the maneuvers got closer and closer together in time. Eventually they were every two hours! it's hard to imagine the nightmare it must have been to watch over that, and how exhausting it would have been... and so once again we see that the heroic efforts of the JPL teams have paid off.
Now the detailed work begins. The delicate instruments on-board the probe need to be switched on, tested, and unleashed. The most exciting one is arguably HiRISE, a very high-resolution camera that will be able to see objects as small as a meter across on the surface of Mars! The images will be incredible. I can't wait-- but I'll have to, since it'll be the end of the month before we start to see those images. Stay tuned here as usual, and I'll post 'em as I get 'em.
Image credit: NASA/JPL