The four largest and most massive satellites of Jupiter are, in order out from the planet, Io, Europa, Ganymede, and Callisto. Io and Europa form a pair roughly the same size, as do Ganymede and Callisto. Io has a diameter of 3636 kilometers, while Europa, the smallest of the four, is 3138 kilometers in diameter. Ganymede, the largest moon in the Solar System, measures 5262 kilometers across. Callisto, Jupiter's outermost large moon, is 4810 kilometers in diameter.
The presence of four large, massive moons enabled the Galileo spacecraft (top image above) to carry out a complex tour of the Jupiter system between December 1995 and September 2003. In the course of 34 revolutions around the giant planet, Galileo used gravity-assist flybys of the four moons to change its orbit without using propellants.
By contrast, Saturn and Neptune each have only one large, massive moon. Saturn's moon Titan, the second-largest moon in the Solar System, measures 5152 kilometers in diameter, while Neptune's moon Triton is just 2706 kilometers in diameter. The Cassini spacecraft (bottom image above), currently exploring the Saturn system, must rely on Titan for most of its gravity assists, which means that it must rely more on its finite supply of rocket propellant to get around in the Saturn system. A Neptune orbiter, with only Triton available for significant gravity assists, would face a similar challenge.
The four largest and most massive moons of Uranus (top and middle images below) are puny compared with Io, Europa, Ganymede, Callisto, Titan, and Triton. Titania, the largest, measures just 1578 kilometers in diameter. The others are Ariel (1158 kilometers), innermost of the four moons; Umbriel (1169 kilometers); and Oberon (1522 kilometers), outermost of the four. Titania orbits between Umbriel and Oberon.
In a paper published in the Journal of Spacecraft and Rockets shortly before Galileo concluded its tour, Andrew Heaton of NASA's Marshall Space Flight Center and James Longuski of Purdue University demonstrated that the Uranus system could support a complex Galileo-style tour. This was, they acknowledged, "contrary to intuition. . . because the Uranian satellites are much less massive than those of Jupiter." A Galileo-style tour was possible, they explained, because "the key to a significant gravity assist is not the absolute size of the satellite, but the ratio of its mass to the primary, and the mass ratios of the Uranian satellites to Uranus are similar to those of the Jovian satellites to Jupiter." Titania and Oberon form a large outer pair equivalent to Ganymede and Callisto, they noted, while Ariel and Umbriel form a small inner pair equivalent to Io and Europa. The "Uranian satellite system is nearly a smaller replica of the Jovian system," they wrote.
They then described a three-phase, 811-day Uranus system tour. After launch from Earth in March 2008 and a gravity-assist flyby of Jupiter in September 2009, the Uranus tour spacecraft would fire its rocket motor to capture into an elliptical Uranus orbit on Valentine's Day in 2018. This would mark the start of the first Uranus tour phase, which would be devoted to matching the orbital inclination of the Uranian equator, ring system, and moons.
Uranus is tipped on its side relative to the other planets in the Solar System, and its moons have equatorial orbits. They wrote that the Uranian system would appear edge-on to the Sun in 2007, then would tilt gradually until the planet and its moons pointed their north poles at the Sun in 2028 (bottom image below). When the Uranus tour spacecraft reached the planet in 2018, the Uranus system would be tilted 13.6° relative to the Sun. The spacecraft would fly first past Titania in May 2019 at a distance of 316 kilometers, allowing that moon's gravity to "crank" its orbital plane. A total of nine similar Titania flybys over 261 days would place the spacecraft into the same plane as the Uranian equator and moons.
The second phase of the Uranus tour, the energy-reduction phase, would see the spacecraft reduce the size of its orbit, thus shortening its orbital period, while conducting a thorough exploration of the four largest Uranian moons. This would begin with a 414-kilometer flyby of Oberon 287 days after the spacecraft entered Uranus orbit and would proceed through eight Ariel flybys, five Umbriel flybys, three Titania flybys, and four additional Oberon flybys over the course of the next 395 days. The closest flyby of the tour would occur during this phase; the spacecraft would pass 54 kilometers over Umbriel's icy landscapes at the start of its 14th revolution about Uranus almost exactly a year (364.3 days) after arriving at the planet.
Heaton and Longuski did not include the intriguing moon Miranda in their list of close flybys because it orbits close to Uranus and, with a diameter of just 470 kilometers, is less than half the diameter of Ariel, the smallest moon used for gravity assists. Proximity to Uranus and low mass would mean that Miranda could contribute little to shaping the Uranus tour spacecraft's orbit. Presumably, the spacecraft would image Miranda whenever its tour route took it relatively close by.
The third phase of the tour would commence 691 days after Uranus arrival with a 151-kilometer Umbriel flyby. The somewhat arbitrary goal of the third phase would be to place the Uranus tour spacecraft into orbit around Ariel. Through three additional Umbriel flybys and four Titania flybys over 120 days, the spacecraft would nearly match Ariel's orbit about Uranus, reducing its velocity relative to its target to slightly less than one kilometer per second. The Uranus tour spacecraft would then use its rocket motor to place itself into orbit about Ariel.
"Feasibility of a Galileo-Style Tour of the Uranian Satellites," A. Heaton and J. Longuski, Journal of Spacecraft and Rockets, Vol. 40, No. 4, July-August 2003, pp. 591-596.
