There are still many medical issues that need to be resolved, and unlike the technological ones, these issues may make prolonged exposure to weightless conditions a lethal hazard. Astronauts in orbit for more than 100 days return to Earth in terrible physical condition. A 1-way trip to Mars takes over 250 days. The following is my table for putting a person on Mars in terns of the probability of this event up to the indicated year. It is a gut reaction based on where I gather the medical and engineering technology is. I give a probability for the actual landing, and for when we have ironed out the medical and engineering problems, and assumes the Space Station accelerates the solution of medical problems:

                  Landing            Technology
Year            Probability          Problems solved
1999               0.0                   10.0
2005               1.0                   20.0
2010              10.0                   40.0
2015              25.0                  100.0
2020              40.0                 
2025              60.0                  
2030              80.0
2035              90.0
2040             100.0

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