With even consumer SLRs sporting sensors with 24 megapixel resolution these days, a natural question is why we're relying on cameras with 1.4 to 1.9 megapixels on a once-in-a-lifetime Mars exploration. We're hoping to bring you more details from mission scientists once the initial frenzy dies down a little, but can make some educated guesses as to the reasons for using such seemingly limited sensor chips:
- This was a long time coming. Space missions involve years of planning and development, so components are usually selected years before the final flight date. In the case of the Mars Science Landing mission, the first call for proposals for Curiosity's scientific instruments was in April 2004, and eight proposals were selected on December 14th of that year. Most hardware development was completed by November of 2008, so the designs were certainly frozen at that time, and probably much earlier. So we're not talking about 2012 technology, but tech from 2004. Back then, 2 megapixels wasn't all that shabby.
- Size matters. Even with a nearly 2,000-pound (899kg) rover, you still have to watch every gram of added weight. In the photography arena, this boils down to sensor size: To an order of approximation, the smaller the sensor, the smaller the optics can be, which means the smaller the mounting gimbal, etc, etc. At 13.38 x 9.52mm, Curiosity's main sensor is close to the size of the chips in Nikon's 1 series cameras (13.2 x 8.8mm), considerably smaller than even crop-frame DSLRs (typically 23.6 x 1.57mm or so). So there's less space to pack the pixels into.
- Megapixels aren't everything. It's not too hard to stitch larger images together from smaller ones, so fewer megapixels combined with a more telephoto lens makes a lot of sense, particularly when you want the best images possible from the limited sensor area you have to work with. So nice, big 7.2 micron pixels are much to be desired for their improved signal/noise ratio, dynamic range, etc.
- Don't bite off more than you can chew. A 36-megapixel sensor and 50-megabyte RAW files are dandy when your computer is right at the other end of a USB cable, but when you're "downloading" across a couple of hundred million kilometers, file size is a big concern. While Curiosity has a higher-bandwidth uplink back to Earth than earlier rovers did, it's still pretty limited. With a grand total of 31-32 megabytes per day of transmission capacity, the ability to grab image data in smaller chunks is pretty key.