Over at the Planetary Society Weblog, Emily Lakdawalla wrote a wonderfully educational post the other day on this amazing picture of the Phoenix spacecraft as it descended under its parachute, taken by the Mars Reconnaissance Orbiter (MRO) HiRISE camera .
As Emily explains with the help of HiRISE optical expert Timothy Reed, there was much more to this well-known image than meets the eye. You should read her excellent explanation, but let me summarize the key points (it's OK, I'm an optical engineer - don't try this at home!).
Basically the HiRISE camera is like a digital camera with one long, skinny row of pixels (it's really more than one row, and it's not exactly straight, but read Emily's post for those details). Instead of 2816 x 2112 pixels like one of my cameras, it's like 20,000 x 1. So how does it take full rectangular pictures? It takes advantage of the spacecraft's known motion along its orbit. It grabs one row of 20,000 pixels, moves a little along the orbit, then takes the next row, and so on (this is also simplified, see Emily again). This "pushbroom" method only works well if the row of pixels is perpendicular to the ground track, but the spacecraft is designed to hold itself in the right orientation for this to work. As it zooms high over the surface in its almost circular polar orbit, it records long, skinny strips of images 20,000 pixels wide of the fixed ground passing below. Cool.
But what about the Phoenix? It was moving in a different direction, not parallel to MRO's orbital ground track. So the clever MRO engineers set it up so the spacecraft would temporarily rotate ("slew") along the direction and at the rate of the predicted track of the descending Phoenix spacecraft (at just the right time!), thus allowing a sharp image of Phoenix to be captured. Of course this meant that the ground in the background was not "moving" perpendicular to the HiRISE row of pixels, so there was some blurring and other stuff there. But that is still some juggling act! Optics, orbital mechanics, and spacecraft control systems combine to get an amazing snapshot from Mars (not to mention terabytes of high resolution surface images constantly streaming back to Earth, which is MRO's real job).
Whew, my partial explanation is almost as long as Emily's full explanation, but I hope it helps. Now go read her post - it has additional complexities (but also additional pictures).