By Jonathan Amos
BBC News science reporter, San Francisco
Neptune and its largest moon, Triton, could be the targets of a major space mission in the decades ahead, if a group of US researchers gets its way.
The team has put together a concept for a "mothership" and probes that would investigate the ice giant which orbits some 4.5bn km from the Sun.
So far, only one spacecraft, Voyager 2, has visited Neptune - a flyby in 1989.
A mission like the one being proposed could cost $3-4bn dollars and would probably need international partners.
"It would also take up the careers of the mission team," said Bernie Bienstock, a robotic systems project manger with aerospace company Boeing.
"It's probably like an 18-year mission but then there's all the lead time - another 10 years to do all the selling to Congress and Nasa, and do all the detailed engineering design.
"You're looking at about 30 years from beginning to end."
Neptune is the eighth planet from the Sun, beyond Uranus but nearer than Pluto.
Voyager showed us a blue giant with an extremely dynamic atmosphere; its winds race around the planet at speeds of 300m/sec.
Voyager also saw rings - much more tenuous than Saturn's - and pictured its "great dark spot", a storm system akin to those familiar in our images of Jupiter.
But it is Neptune's largest moon, Triton, which may be the big pull for science.
NEPTUNE - THE EIGHTH PLANET
Planet discovered 1846
Sun distance: 4.5bn km
Orbit: 165 Earth years
Observed satellites: 8
It has a surface of fascinating contrasts and geysers of nitrogen. It is probably not a natural satellite but a captured object which came in from the furthest reaches of the Solar System.
"The moon is geologically active - we've seen that from Voyager 2's pictures of geysers," said David Atkinson, a University of Idaho professor.
"It's just so different to all the other moons of Neptune, and the moons of Uranus and Jupiter, and it would make an excellent comparison with Kuiper Belt objects such as Pluto and its moon, Charon.
"It would provide us with a wealth of information about the origin and evolution of the outer Solar System."
Bienstock and Atkinson presented their team's concept of a Neptune mission here at the American Geophysical Union Fall Meeting.
They propose it become one of the US space agency's big flagship missions, of the type Nasa can fly only once a decade because of the scale and cost involved.
The concept envisions a 36-tonne spacecraft that would be powered by a nuclear fission reactor and ion-propulsion system.
Only this configuration would give the mission the power and flexibility to reach across the Solar System and complete its science goals.
These would include sending two probes on a collision course with the planet, to take readings in the atmosphere before being crushed by its pressure.
POSSIBLE MISSION TIMETABLE
2016 - spacecraft launched in two halves and joined in orbit
2020 - flyby of Jupiter to make observations and pick up speed
2029 - two atmospheric probes released for Neptune collision
2033 - lander put on Triton to study and image the surface
The mothership would then attempt to put a lander on the surface of 2,700km-wide Triton.
Getting down safely would be a colossal engineering challenge, Bienstock concedes.
"The probe would have a mass of about 500kg - 65% of that is a propulsion system to slow you down so you don't crash," he explained.
"There is a very thin atmosphere on Triton but there's not enough for parachutes to slow you down. You've got a lot of engineering overhead just to deliver the science package."
Once down, the lander could sample the chemical properties of surface materials and send back images of the alien landscape.
The whole concept has been put together under a Nasa Vision Mission contract.
This does not automatically mean, however, that such a mission will get to fly. Its purpose is to investigate possibilities, to help the agency understand the requirements of complex projects as it maps out future plans.