A team of American engineers is studying the feasibility of a nuclear-powered mission to Neptune and its icy moons, on the outer edges of the solar system.
A huge spacecraft driven by a compact nuclear reactor could drop a series of probes into Neptune's atmosphere and help answer questions about the birth of the solar system.
The team, led by Boeing and backed by Nasa, has begun the 12-month study of what one scientist calls "the ultimate in deep space exploration".
Neptune is a gas giant, the eighth - and, through an accident of planetary orbits sometimes the most distant - of the nine planets.
It has 11 known moons and the biggest, Triton, is bigger than the ninth planet, Pluto. No spacecraft has been near Neptune since Voyager 2 sped past in 1989.
The Neptune mission is one of 15 vying for commitment from the US space agency. Its proposers say the journey could answer profound questions: because it is one of the most distant members of the family, Neptune's fabric is more likely to match the original material of the solar system.
"Neptune is a rawer planet," Paul Steffes of the Georgia Institute of Technology, one of the team, said. "It is less influenced by near-sun materials and it has had fewer collisions with comets and asteroids. It is more representative of the primordial solar system than Jupiter or Saturn."
Neptune is a puzzle. It is roughly 30 times the Earth's distance from the sun. It has seasons, and winds whip across its gaseous surface at 900mph.
The Earth's seasons and weather are driven by the sun.
On Neptune the sun has only a nine-hundredth of the brilliance, so any violence in the atmosphere must be driven by something in the planet.
Triton, too, poses riddles. It is an icy ball with a coating of nitrogen, but between 1989 and 1998 astronomers watched it warm by 5%. Once again, no-one knows why.
It may not be a natural child of Neptune: it may have been a passing comet from a faraway region called the Kuiper belt captured and imprisoned by Neptune's powerful gravitational field.
"Triton was formed way out in space," said Prof Steffes, who will outline his vision to the American Geophysical Union in San Francisco next week.
"It is not even a close relative of Neptune. It's an adopted child. We believe Kuiper belt objects were key to the development of our solar system, so there's a lot of interest in visiting Triton."
The Neptune mission could build on Project Prometheus, Nasa's plan for a nuclear-powered ion drive spaceship that could tour Jupiter and its icy moons.
A chemical rocket would lift it into Earth orbit, then an electric propulsion system powered by a fission reactor would propel it into deep space with 10 times the thrust of a chemical rocket. Because ion drive space craft can make huge journeys with much less fuel there would be room for scientific instruments.
If it gets Nasa's final blessing, the Neptune orbiter would be launched around 2016 and arrive at Neptune around 2035. In addition to studying the planet's atmosphere it would put two landers on Triton.