Uranus Orbiter, The other proposed ice-giant mission Options

[Rob Pinnegar]

Since the Neptune Orbiter thread has started to veer into talking about a Uranus orbiter as well, it seemed like a good idea to start a topic for Uranus.

[mchan]

To a first order, there can be Jupiter gravity assist trajectories every dozen or so years to Saturn, Uranus, and Neptune. Practically, a Jupiter gravity assist to planets further out is possible once a year for several years in a row.

The Jupiter flyby distance will vary from year to year depending on how much the trajectory needs to be "bent" and the energy of the orbit before the Jupiter flyby (which dictates the v-infinity of the hyperbolic trajectory around Jupiter). E.g., for "bend", compare that in the Saturn trajectories of Pioneer 11 vs the Voyagers and Cassini. E.g., for energy of orbit before Jupiter flyby, compare the "bend" in Jupiter flybys of Cassini (lower energy) vs. New Horizon (higher energy). Outer planet orbiters tend more to lower energy orbits to reduce orbit insertion delta-V, but must trade this off against longer flight times.

For an idealized calculation, Jupiter gravity assist opportunities are roughly spaced at the synodic period between Jupiter and the target planet.

For Saturn, 1 / (1/11 - 1/29) ~= 17.7 years
For Uranus, 1 / (1/11 - 1/84) ~= 12.6 years
For Neptune, 1 / (1/11 - 1/165) ~= 11.8 years

Time between Voyager (1979) and Cassini (2000) Jupiter flybys is 21 years which is within "several years" of the idealized calculated opportunities. Time between Pioneer 11 (1974) and Voyager (1979) was only 5 years, but look at how much Pioneer's trajectory got "bent".