The search for evidence of past water and past life on Mars.
Gusev crater seemed to be one of the most promising places on Mars to look for evidence of past water and for fossils, but Opportunity's landing site turned out to be even more promising than that.
The most challenging part of development was that the design of the whole surface operations was in flux, and we weren't sure exactly how our automated planner would fit in, or even when it would be used, until just a few months before landing.
The most challenging part of uplink operations is coming in for the Mars night shift and trying to read reports fast enough catch up with what happened the previous sol. My day starts when the science team is having their last meeting of the day, and they're working out the details of what they'll request of us. Almost everyone else in the room has been working on the request all day and has seen most of the images that have just come down.
I was in elementary school when Apollo 11 landed on the moon. At that time, there were optimisitic predictions that by the 21st century there would be orbiting space colonies where thousands of people would live and work. I wanted to be one of those.
Although I was always interested in space exploration, it didn't occur to me to work in a space-related job on Earth until I bought a tape (now out of print) called Minus Ten and Counting, with songs about space. An underlying theme running through many of the songs is that the success of astronauts in space rests on the efforts of many people working on Earth to support them.
I believe a civilization needs a frontier to keep it healthy; otherwise it will turn inward and decay. Also, in the long term, keeping all of our eggs in one basket -- having humans and all other known forms of life confined to one fragile planet -- is risky.
I was originally hired for some non-space-related research, but I've been lucky enough to be involved in a variety of projects, including a little Space Shuttle related work, a good bit of Mars related work, and a few other space projects.