and Mars. Some have suggested that Ceres could act as a main base or hub for asteroid mining. However, Geoffrey A. Landis has pointed out that the asteroid belt is a poor place for an asteroid-mining base if more than one asteroid is to be exploited: the asteroids are not close to each other, and two asteroids chosen at random are quite likely to be on opposite sides from the Sun from each other. He suggests that it would be better to construct such a base on an inner planet, such as Venus: inner planets have higher orbital velocities, making the transfer time to any specific asteroid shorter, and orbit the Sun faster, so that the launch windows to the asteroid are more frequent (a lower synodic period). Thus Venus is closer to the asteroids than Earth or Mars in terms of flight time. Transfer times for the journeys Venus–Ceres and Venus–Vesta are only 1.15 and 0.95 years respectively along minimum-energy trajectories, which is shorter even than Earth–Ceres and Earth–Vesta at 1.29 and 1.08 years respectively. Moons of outer planets Human missions to the outer planets would need to arrive quickly due to the effects of space radiation and microgravity along the journey. In 2012, Thomas B. Kerwick wrote that the distance to the outer planets made their human exploration impractical for now, noting that travel times for round trips to Mars were estimated at two years, and that the closest approach of Jupiter to Earth is over ten times farther than the closest approach of Mars to Earth. However, he noted that this could change with "significant advancement on spacecraft design". Nuclear-thermal or nuclear-electric engines have been suggested as a way to make the journey to Jupiter in a reasonable amount of time. The cold would also be a factor, necessitating a robust source of heat energy for spacesuits and bases. Robert Zubrin has suggested Saturn, Uranus, and Neptune as advantageous locations for colonization because their atmospheres are good sources of fusion fuels, such as deuterium and helium-3. Zubrin suggested that Saturn would be the most important and valuable as it is the closest and has an excellent satellite system. Jupiter's high gravity makes it difficult to extract gases from its atmosphere, and its strong radiation belt makes developing its system difficult. On the other hand, fusion power has yet to be achieved, and fusion power from helium-3 is