"The moon, obviously, has its advantages [over Mars when considering future human spaceflight missions]. It is several orders of magnitude closer to Earth, which makes it a superb training ground for missions to Mars. Decades from now, when the first astronauts headed to Mars finish firing their rocket engines, their ship will be on an inevitable course that will require months, if not years, to return to Earth, and they had better be prepared for every contingency. The moon is only a few days from our home planet. As every test pilot knows, you should always use a buildup approach when developing new aircraft. In the same vein, our future astronauts will be well served to use the ISS and moon as test beds for the first Mars missions."