Neptune Orbiter
Neptune Orbiter was a proposed NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 unmanned planetary spacecraft to explore the planet Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. 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
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

. The California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...

 proposed one mission plan, while the University of Idaho
University of Idaho
The University of Idaho is the State of Idaho's flagship and oldest public university, located in the rural city of Moscow in Latah County in the northern portion of the state...

 and Boeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...

 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 that of the Delta IV
Delta IV rocket
Delta IV is an active expendable launch system in the Delta rocket family. Delta IV uses rockets designed by Boeing's Integrated Defense Systems division and built in the United Launch Alliance facility in Decatur, Alabama. Final assembly is completed at the launch site by ULA...

 or Atlas V
Atlas V
Atlas V is an active expendable launch system in the Atlas rocket family. Atlas V was formerly operated by Lockheed Martin, and is now operated by the Lockheed Martin-Boeing joint venture United Launch Alliance...

 rocket. 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
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 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
Aerocapture is a technique used to reduce velocity of a spacecraft, arriving at a celestial body with a hyperbolic trajectory, in order to bring it in an orbit with an eccentricity of less than 1. It uses the drag created by the atmosphere of the celestial body to decelerate. Only one pass in the...

. 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
Europa Jupiter System Mission
The Europa Jupiter System Mission – Laplace was a proposed joint NASA/ESA unmanned space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere...

. The remains are allocated to ongoing undertakings like Cassini–Huygens, Juno
Juno (spacecraft)
Juno is a NASA New Frontiers mission to the planet Jupiter. Juno was launched from Cape Canaveral Air Force Station on August 5, 2011. The spacecraft is to be placed in a polar orbit to study the planet's composition, gravity field, magnetic field, and polar magnetosphere...

 and New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

 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
Uranus orbiter and probe
A Uranus orbiter and probe mission to explore the planet Uranus, its atmosphere, rings, and moons, was recommended to NASA in 2011 by its . A mission study was conducted which also considered Neptune; however, for feasibility reasons Neptune was dropped in favor of Uranus. Two white papers on...


Radioisotope thermoelectric generators

The Caltech mission would be similar in design to previous NASA outer solar system missions. It would use Radioisotope thermoelectric generator
Radioisotope thermoelectric generator
A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...

s for electrical power. , 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
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...

, Cassini–Huygens, Voyager
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

, and New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

, with conventional thrusters for propulsion.

Solar Panels

Another proposal calls for using solar panel
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...

s 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. , 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
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 system that was planned for the now cancelled Jupiter Icy Moons Orbiter
Jupiter Icy Moons Orbiter
The Jupiter Icy Moons Orbiter was a proposed spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, the suspected ocean of which is one of the places where simple alien life is a possibility in our solar system...

. 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.

Radioisotope electric propulsion (REP), powered by Radioisotope thermoelectric generators or Radioisotope Stirling Converters, has recently been studied as a means of propelling a Neptune mission.


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
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 to infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

. A magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...

 will be carried to investigate why the Neptune's magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 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
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 atmosphere, as well as to search for any liquid water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 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, it would take about 30 minutes to an hour of detailed analysis of the atmosphere. Then it would be planned to continue its descent until the planet's atmospheric pressure would crush and destroy them.
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