But in just a week or so a human crew could have done everything that the rovers didMaybe but the MER mission allin cost about a billion dollars . A manned mission to Mars is estimated to cost a hundred times that much . So it ' s not whether a person can do everything Spirit and Opportunity did it ' s whether a person can do everything Spirit and Opportunity and two hundred cousins can do spread all over Mars . And the answer is that it ' s not possible , because in a lot of cases what you need is to be lucky to set down next to just the right interesting rock . Time doesn ' t matter . It takes months to get to Mars anyway , and it ' ll still be there years from now , so what does it matter whether it takes two weeks or two years to collect the information in the long run ? What matters is the cost if we can send twenty robotic missions for every one manned mission , we can achieve far greater coverage than with the manned mission . Manned exploration is romantic but it is fundamentally not costeffective .