HOUSTON - In August, NASA's Curiosity rover is set to arrive on the surface of Mars, where, if all goes well, it will begin a wide range of scientific tests. Such missions provide a great deal of scientific data, but the general public is most excited about the prospect of sending humans to Mars, which NASA hopes to accomplish by 2030. As costs soar and budget battles intensify, however, some notable critics in the scientific community are questioning the whole idea of putting humans in space.