Neptune Orbiter

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 concept would have answered many questions about the nature of the planet.

Its objective 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 in 2004, while the University of Idaho and Boeing proposed an alternative approach in 2005.

Mission summary and status
Neptune Orbiter's primary mission was to go into orbit and perform scientific studies of the planet. The mission concept was first proposed to NASA in 2005. It was proposed 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.

Just prior to arriving, the orbiter would release its two atmospheric probes, which would transmit data before aerocapture. Then, it would begin Neptune orbit insertion by aerocapture. After adjusting its orbit to its planned science orbit, the orbiter would carry out studies of Neptune, its rings, atmosphere, weather, and its natural satellites. The main phase of the science operations would have taken from 3 to 5 years with a possible extension lasting 3 more years.

In 2008, the mission was removed from NASA's possible future missions list. According to NASA's 2010 budget, funding to missions to the outer Solar System was aimed at the future Europa Jupiter System Mission. The remaining budget was allocated to ongoing undertakings such as 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 an ice giant—either Uranus or Neptune—but for feasibility reasons recommended a Uranus orbiter and probe.

Radioisotope thermoelectric generators
The Caltech mission concept would have been similar in design to previous NASA outer Solar System missions. It would use radioisotope thermoelectric generators for electrical power. Based on this proposal, the spacecraft design would be similar to Galileo, Cassini–Huygens, Voyager 2, and New Horizons, with conventional thrusters for propulsion.

Solar panels
Another proposal called for using solar panels to provide electrical power to the spacecraft. The panels would 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. This design would presumably also use conventional thrusters, because the dim sunlight in the outer Solar System would probably be insufficient to power an ion propulsion system.

Orbiter
The orbiter would have carried out the main mission objectives. Some proposed instruments were a multispectral imaging system to image the planet from ultraviolet to infrared, and a magnetometer, 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 considered 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. Suggested landing sites focused on the north and south poles of Triton where large amounts of frozen water ice may be discovered. The landers' normal operational lifetime was estimated to be from 2 weeks to a month with a possible extended mission.

Atmospheric probe(s)
Along with Triton landers, at least two atmospheric probes were proposed to descend through 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 descent 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.