The nuclear-propelled Neptune orbiter (top) would carry two probes that it would despatch to investigate the atmosphere of the planet (middle). The mothership would then manoeuvre into a position to drop a lander on the surface of Triton (bottom).
Two different proposals were put forward in 2005 for unmanned spacecraft to explore Neptune and its moons, especially Triton. These studies are part of NASA's Vision Mission program to develop long-term space exploration goals.
A Cassini-like probe
One of the missions, proposed by a team led by Andrew Ingersoll of the California Institute of Technology, is for a Cassini-like spacecraft powered by RTGs (radioisotope thermoelectric generators). It would use conventional rocket propulsion and gravity assists to reach Neptune after a journey of about 12 years.
To enter orbit around the gas giant, Ingersoll's team is looking at the possibility of aerocapture – a technique that uses atmospheric friction to do all the braking without the need to expend any fuel. NASA has only previously employed aerobraking, which uses a combination of planetary atmosphere and rocket burns to achieve orbit.
Nuclear power to Neptune
The second proposal, led by David Atkinson of the University of Idaho, and Bernie Bienstock, of Boeing Satellite Systems, is for a giant, 36-ton spacecraft that would use nuclear electric propulsion. Its design owes much to NASA's plans for the Jupiter Icy Moons Orbiter (JIMO) mission under Project Prometheus – a vessel expected to use a nuclear fission reactor to power an ion engine. The trouble with this method is that it's slow: an ion engine propelled Neptune mission launched around 2016 would take time to build up enough thrust to reach the planet, entering orbit around 2035. It's great advantage, however, is that, upon arrival, the spacecraft would have plenty of remaining fuel and power to support a multi-pronged, long-duration study of the Neptune system.
Probes and landers
Both these schemes for exploring Neptune and its moons, envisage the use of a mother ship that carries a number of smaller, special-purpose craft. They envision sending three probes on a descent through Neptune's atmosphere at different latitudes. Ingersoll's team proposes a shotgun approach, releasing all three probes at once, whereas in the Bienstock and Atkinson mission, the probes would be dispatched sequentially.
Both studies also allow for a pair of Triton landers, though getting the spacecraft safely onto the surface of this enigmatic moon poses major challenges.