Mars has company these days. The rover Curiosity touched down safely and has sent back an image to prove it. Many people thought it a strange choice for NASA to pick a 2 megapixel camera sensor for this mission. 40MP sensors exist and even normal sized ones are cheap to use and readily available. So why go with just 2MP?
Because Curiosity can only transmit roughly 31MB per day. That's less than 1 gigabyte per month. Per Month!