#6. Bad weather - Scientists on Earth have a hard enough time predicting Earth weather, and it's all the more difficult to predict weather on a planet millions of miles away. Mars is notorious for long, cold winters and dust storms that can blanket the entire planet. While the mission is scheduled to land during a good weather period, anything can happen. Previous landers have suffered reduction in capability due to dust coating their solar panels. The already feeble sunlight cannot penetrate a very thick layer of dust, which is the eventual doom of all solar-powered Mars rovers. 

However, Curiosity has its own internal power supply so it will not need to worry about dust storms cutting off its electricity. Still, static charges, extra-cold weather, and fine dust particles can still wreak havoc on the craft, especially over the long term. Unfortunately, bad weather is the least of Curiosity's worries.

Read More: Mars Rover lifts off without a hitch.

#5. Getting stuck - The surface of Mars is sandy and rocky which makes it difficult for rovers to move on the surface. The most famous example is the Mars Spirit rover, which despite its amazing success, eventually got stuck on the planet and engineers were unable to free the craft. Scientists have a good idea how to drive on Mars and choose their routes meticulously. In addition, Curiosity has much larger wheels than its predecessors, making successful maneuvers more likely. However, the possibility remains that the lander could get stuck. And getting stuck could put an early end to a very expensive mission. 

#4. Instrument failure - Should Curiosity safely land, the mission won't be out of danger. The possibility of hardware or software failure remains a concern. Several missions to Mars have suffered instrument failures. While the most common instrument failure seems to be failures of communication equipment, scientific instruments can fail too. A failed boom or probe or a broken instrument can kill at least one key component of a scientific mission. The more instruments there are, the more complications, and the more possibility of breakage, and Curiosity is packed with instruments.

Other famous examples include the failure of Skylab's solar panels to deploy properly and the mirror for the Hubble Space Telescope. Luckily in those cases, astronauts were able to intervene directly and conduct repairs. That won't be possible for Curiosity. 

#3. Communications failure - The distance between Earth and Mars varies greatly (between 36 and 250 million miles!) Earth regularly catches up to, then overtakes and leaves behind the red planet as both orbit the Sun at different periods. The Martian year is almost 687 Earth-days long. As the separation between the two planets changes, so does the amount of time needed to communicate with the orbiters and landers there. Messages can arrive in as little as three minutes or take as long as 22.

In addition to distance, the both planets are rotating with craft frequently passing behind Mars (from our perspective) and being unable to communicate. In fact, any number of dynamic factors can interrupt communications. 

Communications failures claim space missions from time to time. At least seven of the 49 attempted Mars missions have been lost due to communication difficulties that couldn't be overcome. A number of other satellites sent into space have also been lost because of communication problems. 

A number of things cause these failures, but the extreme nature of space is certainly a factor. Although satellites are built to withstand the harsh space environment, a number of things can go wrong. The extreme forces of launch, the deep freeze of space, or blasts of solar radiation can break, freeze, or fry electronics. The impact of landing can be worse than expected and something can come lose. Or get stuck. The most famous example of this was the Galileo mission to Jupiter. The orbiter's high-gain antenna became stuck and never fully deployed. Luckily, engineers were able to come up with a solution by repurposing the satellite's low-gain antenna to transmit scientific data. While speculation abounds, nobody will likely know the real reason why the antenna became stuck. 

Until Curiosity is on the ground transmitting data, communications failure is a real possibility. 

#2. Surface crash - Not counting the probes that failed to even attempt landings because of various problems, five of the 49 previous missions have ended in a man-made impact crater on the surface of Mars. The precise nature of each crash remains unknown but all can ...