As for the first part, Cost, Radiation exposure to humans, No water or heat, and unfit soil. As for the second, Astronomers, Botanists, and obviously your fair share of rocket scientists and math matitions for the way up.
 
        
             
        
        
        
okay hold on, they wont let my answer go through so i'll post this and write it in the comments :)
 
        
             
        
        
        
"ir-" is the root of "irrelevant"
        
                    
             
        
        
        
Earth is the only planet out of the two that can support life. Mars can not. Both Earth and Mars are both planets. Mars is hotter than Earth. Earth has a moon. Both planets are made up of rock. They differ in colours.