Neptune, But Not Soon

Science requires much more patience than science fiction. NBC never let Star Trek finish its five-year mission, yet NASA has two teams developing missions to Neptune that would take 12 to 20 years just to reach the planet and begin collecting data.

Each team proposes to use a different propulsion system and its own distinctive schemes for exploring Neptune and its unique moon, Triton.

John Newman commented in a 1958 science article for Nebula Science Fiction, “Curiously enough, we know less about the conditions on the surfaces of most of the planets of the System than we do about the surfaces of many of the stars… Jupiter, Saturn, Uranus and Neptune have so highly compressed and deep atmospheres that their true surfaces are invisible.”

Recent missions have explored the first two planets mentioned. Neptune is the favored target between the last two because of the oddities about its moon, Triton — for example, it has a retrograde orbit, in the opposite direction of Neptune’s rotation, a fact that encourages speculation Triton may be a Kuiper Belt object captured by planetary gravity.

NASA’s first Neptune team envisions something like the Cassini Mars mission that could use conventional rocket propulsion and gravity assists to reach the planet in 12 years. Orbit would be achieved through aerocapture , which puts the vehicle in orbit after one pass, in contrast to the aerobraking technique used for Cassini Mars which requires multiple orbits and repeated burns by a spacecraft’s engine.

The second team’s plan would use a nuclear fission reactor and ion propulsion to arrive at Neptune in 20 years, because an ion engine takes time to build up enough thrust. However, an ion thruster is 10 times more efficient than a chemically powered engine. One has been powering the Deep Space 1 spacecraft since its launch on Oct. 24, 1998.

Both missions would land probes on Neptune, but deploy them in a different manner according to each team’s own ideas about probe survival and usefulness.

Neptune hasn’t attracted a lot of interest from sf writers. On the other hand, scientists have yet to learn enough about the place to disqualify the science used in what few stories are set there. Indeed, Sydney fans noticed in 1999 that the passage of time has actually been kind to one space opera, Edmund Hamilton’s The Universe Wreckers, in which Neptunians terrorize the solar system. When the book was written, Pluto hadn’t been discovered. But by the time the Sydney Futurian Society got around to discussing the story, in 1999, Neptune was further away than Pluto anyway.