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Exploring Neptune:

A Probe is Proposed for Flight in 2015

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California Institute of Technology artist's vision of the the Neptune orbiter
California Institute of Technology artist's vision of the nuclear powered Neptune orbiter dropping one of two probes to investigate the atmosphere of the blue planet Neptune, and a lander on the surface of the moon Triton in the foreground.
A major interplanetary exploration mission may head out from Earth in 2015 to arrive at the planet Neptune and its largest moon, Triton, in 2027.

A large spacecraft carrying two probes and a lander would be sent out to investigate the ice planet orbiting in the far reaches of the Solar System some 2.8 billion miles from the Sun.

Until now, the only spacecraft to visit Neptune was Voyager 2, which flew by the giant planet in 1989. Voyager 2, which has traveled to more planets than any other probe in history, probably still will be operating as the new probe arrives at Neptune. However, Voyager 2 will be much farther away, some 10.5 billion miles from the Sun near the edge of the Solar System.

A number of nations might join with the United States in the large-scale 30-year project. After launch, the flight time to Neptune would be about 12 years.

The journey. The proposed spacecraft would be so large it might have to be sent to Earth orbit in two launches in 2015. The two halves of the craft then would be joined together in orbit.

On its outbound journey, the probe would make observations of the planet Jupiter as it flies by around 2020. Jupiter's gravity would increase the speed of the spacecraft.

Neptune Mission Timetable

  • 2015 – Two halves of the probe launched and joined in orbit
  • 2020 – The probe observes Jupiter during flyby and gains speed
  • 2027 – The main spacecraft sends two atmospheric probes toward Neptune
  • 2031 – The lander sets down on Triton to photograph and study the surface
  • The main spacecraft would arrive in the vicinity iof Neptue around 2027. One of its first duties would be the release of two probes to fall down through the planet's atmosphere.

    Four years later, in 2031, the main spacecraft would release a lander, which would touch down on the surface of the large moon Triton to photograph and study the surface.

    Neptune. Neptune is the eighth planet out from the Sun and outermost of the giant planets. That puts it beyond the planet Uranus, but usually closer than the planet Pluto.

    Sometimes, Pluto's orbit takes the planet inside the orbit of Neptune. Pluto last traveled within the orbit of Neptune from 1979 to 1999. Pluto in 1989 was about a billion miles closer to Earth than when it was discovered back in 1930.

    Neptune is the fourth largest planet. Its diameter is 30,700 miles. Neptune orbits in the outer regions of the Solar System almost three billion miles from the Sun.

    As an ice giant, Neptune is composed mostly of water, methane, ammonia and other compounds that can form ices at the temperatures found on our Solar System planets.

    Neptune at a Glance

  • Discovered in 1846
  • 13 moons have been seen
  • 6 rings have been seen
  • 4th largest planet
  • 30,770 miles in diameter
  • 8th planet out from the Sun
  • 2.8 billion miles to the Sun
  • Outermost of the giant planets
  • Takes 165 Earth years to complete
        one orbit around the Sun
  • Voyager 2 sent back photos in 1989 of a giant blue ball with winds racing along above the planet surface at speeds of nearly 700 mph (300 m/sec).

    Voyager found rings around the planet and a great dark spot on the surface, which is a storm system like that seen on Jupiter.

    When it was discovered in 1846, Neptune was the first planet located through mathematical predictions rather than observations of the sky.

    The planets are named after ancient gods. Neptune was the Roman god of the deep seas.

    Moons. Neptune has 13 moons. The largest is Triton, which is much larger than any of the other moons.

    Triton was discovered in 1846, a month after the discovery of Neptune. The other moons are Nereid, discovered from Earth in 1949, and Despina, Galatea, Larissa, Naiad, Proteus and Thalassa found by Voyager 2 in 1989. Three more small moons were discovered in 2002 and two more in 2003. The names Galatea and Larissa were controversial since asteroids previously had been given those names. Names are assigned by the nomenclature committee of the International Astronomical Union (IAU). The 2002 and 2003 Neptune moon discoveries have yet to be named.

    Voyager 2 image of Triton
    Voyager 2 photographed Neptune's largest moon, Triton, on August 24, 1989, from a distance of 330,000 miles.
    Triton. Of Neptune's 13 moons, Triton is second farthest from the planet, standing off at distance just beyond 220,000 miles (354,800 km).

    Triton is a very cold place covered with ice. However, it has geysers that shoot ice five miles up into Triton's thin atmosphere. They make Earth's geysers at Yellowstone National Park seem small.

    Triton is colder than any other object in the Solar System with a surface temperature of -391°F (-235°C). Voyager 2 found an extremely thin atmosphere above a surface scarred with gigantic cracks. Voyager's pictures revealed geysers spewing nitrogen gas and dark dust particles several miles high.

    Could there be water under Triton's ice? Could there even be life in that water? Scientists suspect the interior of Triton may be geologically active.

    Voyager 2 also found a thin layer of methane and nitrogen above the water ice on Triton's surface.

    Rings. Neptune has six rings, but its rings are different from Saturn rings. Neptune's rings are hard to see from Earth because they are much darker. Saturn rings are bright.

    Neptune's rings may be rocks and dust, which don't reflect much light. Saturn's rings are ice, which reflects light.

    The spacecraft. The 40 ton spacecraft at the heart of the mission would be powered by a nuclear fission reactor and would be driven to Neptune by an ion propulsion system.

    At Neptune, the main spacecraft would send two probes into the planet's atmosphere to record scientific measurements before being crushed by its pressure.

    The main craft then would move on to place its lander down on the 1,680-mile-wide moon Triton.

    Landing an 1,100 lb. (500 kg) robot probe safely on Triton will be a major engineering feat. The moon's atmosphere is too thin for parachutes so a new landing scheme will have to be devised.

    Once down on Triton, the lander would collect and analyze samples of the chemical properties of surface materials and send the data back to Earth along with images of the moonscape.

    Voyager 2 image of Neptune
    Voyager 2 passed Neptune in the summer of 1989, the first spacecraft to observe that planet. Passing about 3,000 miles above Neptune's north pole, Voyager 2 made its closest approach to any planet since it left Earth 12 years earlier. Five hours later, Voyager 2 passed about 25,000 miles from Neptune's largest moon, Triton.
    New inventions. To accomplish their task, scientists and engineers will need to design the entirely new spacecraft and probes, plan a precise trajectory that will take the craft to Neptune, design the science instruments and high data rate communication gear.

    They will have to devise an aerocapture plan as a means of stopping the outbound flight in the Neptune system. Aerocapture uses friction, encountered in the planet's upper atmosphere as a spacecraft flies through at a high speed, to slow the speed of the craft so it can fall into orbit around the planet.

    A nuclear electrical power generator will have to designed for the mission along with equipment for high-resolution mapping spectroscopy.

    Science objectives. The major interplanetary exploration mission to Neptune would have several science goals: The proposal. The Neptune mission is being proposed as a NASA Space Science Vision Mission by a team of researchers led by a scientist at the University of Idaho and a a robotic systems expert from Boeing.

    They presented their concept to the American Geophysical Union in fall 2005. They hope NASA will adopt the plan as a flagship mission similar to the large Cassini mission currently touring the planet Saturn's moons and rings.

    If NASA goes forward with the proposal, it would meet the goals of the Office of Space Science (OSS) Strategic Plans. They are:
    Learn more:
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