Originally scheduled to launch in the fall of 2009, Mars Science Laboratory (image: NASA) took a two-year hit in schedule, and a big hit in cost, when it had to delay to the 2011 launch window. Now, a common question I get from my friends and family is why the delay was a full 2 years. Well, the answer can be somewhat complicated, and I am no orbital mechanics expert, but I can give you a pretty good answer.
Basically, it costs a BUNCH of money to send each and every pound into space (you can find a lot of different stats on this, but anywhere between about $1600 – $10,000 per pound is not out of the question, depending of course on the launch vehicle and destination). Fuel, of course, is pretty dense and therefore expensive. The rocket itself (in this case it will be an Atlas V), uses tons, literally TONS, of fuel to launch it's payload (in this case MSL) into space, and the more the spacecraft weighs (including it's own fuel), the more fuel the rocket needs! Whew that is hard to follow, but as a result, it is advantageous for a mission to use as little fuel as possible. So, pretty much all missions outside of low-earth orbit use what's called a minimum energy transfer (if you're feeling like geek-ing out, check out this site for a tutorial). Basically a minimum energy transfer will have the spacecraft leave Earth's orbit on a trajectory that is tangent to its path around Earth, and will enter orbit around Mars in a similar fashion. In order to do this, the spacecraft has to leave Earth's orbit at a precise time so that the position of Mars relative to Earth will allow this trajectory to work. What that boils down to is an approximately 2-3 week window every two years or so.
So, what's the point of MSL? NASA's Jet Propulsion Laboratory (JPL) is overseeing the project and reports that MSL will support a "long-term effort of robotic exploration of the red planet." The "Curiosity" rover will try to gather data to determine if Mars is or was able to support microbial life.
From my "system safety" perspective, MSL is going to be very interesting and challenging mission. First of all, I just took over from a coworker a few months ago. Second, MSL has many safety-critical systems and challenges during ground processing through launch. For example, it has two propulsion systems and a Radioisotope Thermal Generator (RTG). The RTG has to be cooled once installed in the spacecraft (which is done just a few days before launch), and so the various cooling systems on the ground and within the spacecraft itself are also critical. Basically, this mission is going to keep me on my toes!