In Rob Manning's book [0] : He explained how hard it is to land on Mars. It's nicely sum up in those two sentences :
“There’s too much atmosphere on Mars to land heavy vehicles like we do on the moon, using propulsive technology completely,” said Manning, “and there’s too little atmosphere to land like we do on Earth. So, it’s in this ugly, grey zone.” [1]
The recent success of Curiosity, Spirit and Opportunity must not shadow the fact that most Mars missions have failed.
[0] "Mars Rover Curiosity: An Inside Account from Curiosity's Chief Engineer
“There’s too much atmosphere on Mars to land heavy vehicles like we do on the moon, using propulsive technology completely,” said Manning, “and there’s too little atmosphere to land like we do on Earth. So, it’s in this ugly, grey zone.” [1]
The recent success of Curiosity, Spirit and Opportunity must not shadow the fact that most Mars missions have failed.
[0] "Mars Rover Curiosity: An Inside Account from Curiosity's Chief Engineer
[1] http://www.universetoday.com/7024/the-mars-landing-approach-...