The experiments recently done on mice on the ISS have concluded that, at least for them, around 1/3 of Earth gravity is the minimum required to preserve health.

The experiments recently done on mice on the ISS have concluded that, at least for them, around 1/3 of Earth gravity is the minimum required to preserve health. So providing only 1/5 g for artificial gravity is likely to be insufficient. Unfortunately, in the experiments with mice 1/3 g, like on Mars, was the minimum required to prevent severe health problems. For really having no health problems, 2/3 g was needed, i.e. more than on Mars. There are chances that the exploration of the Solar System by humans will not really be possible otherwise than in very big spaceships with good radiation shielding and artificial gravity, which would allow humans to stay on them for an indefinite time. Then for the exploration of Mars or of other planets, satellites or asteroids, such a big spaceship will stay in an orbit around that body and exploration teams will live on the surface only for short times, short enough to not be affected by the insufficient gravity. It is likely that such big spaceships would be practical only when propelled by nuclear reactors, assembled in an Earth orbit, or on the Moon. Or else, for living on Mars one would have to use underground bases where the habitable part would be rotating, to enhance the gravity.