If I'm not mistaken, the Mars Rover had a 1 megapixel camera. :)
My friend worked on that project.
[Edit]
It has 12μm x 12μm pixels whereas the 5D has 8μm and the 5DmkII has 6μm pixels. However each pixel on the Canons contain three (RGB) photosites to the rover's one.
To clarify:
In the Bayer filtered Canon CCD each individual photosensor is covered in a colored filter [1] in a pattern such as:
B G G R
So when capturing an image 1/4 of the sensors are capturing blue light, 1/4 capturing red, and 2/4 capturing green.
In contrast the CCD in the rover's cameras do not have filters in front of individual photosensors. Instead, there is a set of filters on a disc which are mechanically positioned in front of the sensor. Using this method three images are taken. On each with a red, green, and blue filter covering 4/4 of those same sensor elements giving you 2x-4x the number of elements capturing each color and thus much higher image quality at a given resolution.
B B B B then G G G G then R R R R
Similar multi-shot technology was used in the early days of terrestrial digital photography to provide better color, exposure latitude and resolution. [2] for a bit of discussion on multishot vs. single shot medium format digital camera backs.
[1] http://en.wikipedia.org/wiki/Bayer_filter
[2] http://www.luminous-landscape.com/reviews/cameras/h3d50ii.sh...