Neptune Orbiter was a proposed NASA unmanned planetary spacecraft to explore the planet Neptune. It was envisioned that it would be launched sometime around 2016 and take 8 to 12 years to reach the planet; however, NASA's website no longer lists any possible launch date. The Neptune Orbiter was designed to answer many questions that still surround the planet. Its main mission was to study Neptune's atmosphere and weather, its ring system, and its moons, particularly Triton. The California Institute of Technology proposed one mission plan, while the University of Idaho and Boeing proposed an alternative approach.
Mission Summary and Status 
Neptune Orbiter's primary mission was to go into orbit and perform scientific analysis on the planet. The mission was first picked up from NASA in 2005. It was expected to use a launch rocket similar to the Delta IV or Atlas V. The orbiter's trajectory was to use one Venus gravity assist, and a Jupiter gravity assist before arriving at Neptune. The length of time from launch until Neptune arrival was estimated to be 10.25 years. One early proposed idea for the mission included a Uranus fly-by, although the fly-by was deemed unnecessary and was scratched off the schedule.
Just prior to arriving, the orbiter would release its two atmospheric probes which will transmit data before aerocapture. Then, it would begin Neptune Orbit Insertion by aerocapture. After adjusting its orbit to its regular science orbit, the orbiter would carry out studies of Neptune, its rings, atmospheric weather & climate, and satellites. The main phase of the science operations would take as long as 3 to 5 years with an extended mission phase lasting 3 more years.
In 2008, the mission was removed from NASA's possible future missions-list. According to NASA's 2010-budget, funding on missions to the outer solar system is currently aimed at the future Europa Jupiter System Mission. The remains are allocated to ongoing undertakings like Cassini–Huygens, Juno and New Horizons with the Neptune system not being part of any official considerations.
In 2011, NASA's decadal survey considered a mission to the ice giants – either Uranus or Neptune but for feasibility reasons dropped Neptune in favor of a Uranus orbiter and probe.
Power Sources 
Radioisotope thermoelectric generators 
The Caltech mission would be similar in design to previous NASA outer solar system missions. It would use Radioisotope thermoelectric generators for electrical power. As of 2006[update], this power source is the most likely choice for the mission. It would also reduce the costs of building the spacecraft, which is important with NASA's limited current science budget. Based on this proposal, the spacecraft design may be similar to Galileo, Cassini–Huygens, Voyager, and New Horizons, with conventional thrusters for propulsion.
Solar Panels 
Another proposal calls for using solar panels to provide electrical power to the spacecraft. The panels would also be inflatable reducing their mass. Although advancements in solar panel technology means it should be possible to achieve sufficient power at the distance of Neptune, the high cost of providing the solar panels would possibly eliminate the idea of using solar cells. As of 2006[update], this proposed power source would probably require significant technological advances and may add risk to the development schedule. This design would presumably also use conventional thrusters, as the dim sunlight in the outer solar system would probably be insufficient to power an ion propulsion system.
Nuclear Electric Fission 
For propulsion it is proposed that the Orbiter use a nuclear electric propulsion system that was planned for the now cancelled Jupiter Icy Moons Orbiter. This would take up to 20 years to reach the planet but would leave the mission with a large supply of fuel and electrical power for the orbital phase of the mission, allowing the spacecraft to orbit multiple bodies and to carry a greater variety of instruments. But this method of propulsion may be too controversial with the public and the development time and cost needed may be too much for NASA's budget.
The orbiter is considered the main part of the mission which will carry out the main mission objectives. Instruments that the Orbiter is planned to carry include a multispectral imaging system, to image the planet from ultraviolet to infrared. A magnetometer will be carried to investigate why the Neptune's magnetic field is oriented so far from the planet's axis of rotation.
Triton lander(s) 
NASA scientists and engineers have planned to send one or two mini-landers to Triton's surface and analyze the composition of the surface, the interior, and the possible nitrogen atmosphere, as well as to search for any liquid water and microscopic life forms. Although detailed exploration of Triton is a high priority for mission planners, NASA's current budget may threaten the ability to launch the Triton landers and possibly cancel them from the mission. Suggested landing sites focus on the north and south poles of Triton where large amounts of frozen water ice may be discovered. The landers' normal operational lifetime is estimated to be from 2 weeks to a month with a possible extended mission phase on Triton.
Atmospheric probe(s) 
Along with Triton landers, at least two atmospheric probes have also been proposed to go into Neptune's atmosphere and study the climate and weather of the stormy planet. Similar to the Galileo probe that descended into Jupiter's atmosphere, the descend of a probe into the atmosphere of Neptune would take about 2-3 hours until the planet's atmospheric heat and pressure would destroy it – the data transmitted during the descent would allow a detailed analysis of Neptune's atmosphere.
See also 
- Cassini–Huygens-level science from NASA’s “Neptune orbiter with probes” vision mission (PDF)
- The Case For A Neptune Orbiter/Multi-probe Mission (PDF)
- Neptune Revisited
- Reaching Toward Neptune: Two Ways to Explore an Ice Giant