Many astronomers, geologists and biologists believe that exploration of the Solar System provides knowledge that could not be gained by observations from Earth's surface or from orbit around Earth. But they disagree about whether human-crewed missions make a useful scientific contribution—some think robotic probes are cheaper and safer, while others argue that either astronauts advised by Earth-based scientists, or spacefaring scientists advised by Earth-based scientists, can respond more flexibly and intelligently to new or unexpected features of the region they are exploring.