more difficult to achieve than conventional deuterium–tritium fusion. Jeffrey Van Cleve, Carl Grillmair, and Mark Hanna instead focus on Uranus, because the delta-v required to get helium-3 from the atmosphere into orbit is half that needed for Jupiter, and because Uranus' atmosphere is five times richer in helium than Saturn's. Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto) and Saturn's Titan are the only moons that have gravities comparable to Earth's Moon. The Moon has a 0.17g gravity; Io, 0.18g; Europa, 0.13g; Ganymede, 0.15g; Callisto, 0.13g; and Titan, 0.14g. Neptune's Triton has about half the Moon's gravity (0.08g); other round moons provide even less (starting from Uranus' Titania and Oberon at about 0.04g). Jovian moons Radiation levels on Io and Europa are extreme, enough to kill unshielded humans within an Earth day. Therefore, only Callisto and perhaps Ganymede could reasonably support a human colony. Callisto orbits outside Jupiter's radiation belt. Ganymede's low latitudes are partially shielded by the moon's magnetic field, though not enough to completely remove the need for radiation shielding. Both of them have available water, silicate rock, and metals that could be mined and used for construction. Although Io's volcanism and tidal heating constitute valuable resources, exploiting them is probably impractical. Europa is rich in water (its subsurface ocean is expected to contain over twice as much water as all Earth's oceans together) and likely oxygen, but metals and minerals would have to be imported. If alien microbial life exists on Europa, human immune systems may not protect against it. Sufficient radiation shielding might, however, make Europa an interesting location for a research base. The private Artemis Project drafted a plan in 1997 to colonize Europa, involving surface igloos as bases to drill down into the ice and explore the ocean underneath, and suggesting that humans could live in "air pockets" in the ice layer. Ganymede and Callisto are also expected to have internal oceans. In 2003, NASA performed a study called HOPE (Revolutionary Concepts for Human Outer Planet Exploration) regarding the future exploration of the Solar System. The target chosen was Callisto due to its distance from Jupiter, and thus the planet's harmful radiation. It could be possible to build a surface base that would produce fuel for further exploration of the Solar System. HOPE estimated a round trip time for a crewed mission of about 2–5 years, assuming significant progress in propulsion technologies. Saturnian moons