if we go to the second planet from the sun the gravity would be more tense and would have different affects on humans. the gravity on venus would pull them down more than earth because it is closer to the sun by two planets away. but we also need to think about the air and the atmosphere because if we just go there then we might die. the air on venus is un supportive to humans and the water is not there or it might be a type of water that if you drink it it will kill you. the storms on venus are acid storms, the rain is acid. so i think the only reason why the author is saying that venus is a challenge is because not only the facts above. but also that the fact is that on the serface, temperetures average over 800 degrees fahrenheit. and the atmospheric pressure is 90 times greater than what we humans experience on earth. so he also might be saying that it might be a challenge because if we take a rocket, it might be torn apart if we dont have the right mesurements.