Experts in the effects of space travel on the human body told a presidential commission on Wednesday that there were challenges but no “showstoppers” in building a permanent moon base, then sending astronauts to Mars. Aerospace medical experts Stanley Mohler and Mary Ann Frey, both longtime researchers in the field, identified a number of health risks future astronauts could face, from radiation poisoning to meteoroid collisions, but said NASA was developing plans for every known contingency.