Exploration of Neptune

{{Main|Voyager 2}}

Image:Triton moon mosaic Voyager 2 (large).jpg]]

After Voyager 2 visited Saturn successfully, it was decided to fund further missions to Uranus and Neptune. These missions were conducted by the Jet Propulsion Laboratory, and the Neptunian mission was dubbed "Voyager Neptune Interstellar Mission". Voyager 2 started taking navigation images of Neptune in May 1988.{{cite book |last1=Ulivi |first1=Paolo |last2=Harland |first2= David M |date=2007 |title=Robotic Exploration of the Solar System Part I: The Golden Age 1957–1982 |publisher=Springer |page=426|isbn=9780387493268 }} Voyager 2{{'s}} observation phase proper of Neptune began 5 June 1989, the spacecraft officially reached the Neptunian system on 25 August, and data collection ceased on 2 October.{{cite web|url=http://voyager.jpl.nasa.gov/news/factsheet.html |title=Fact Sheet |publisher=JPL |access-date=3 March 2016}} Initially it was planned to use a trajectory that resulted in Voyager 2 passing around {{convert|1,300|km|abbr=on}} from Neptune and {{convert|8,200|km|abbr=on}} from Triton. The need to avoid ring material detected by stellar occultations prompted this trajectory to be abandoned, and a trajectory that largely avoided the rings but resulted in more distant flybys of both targets was plotted.{{cite book |last1=Ulivi |first1=Paolo |last2=Harland |first2= David M |date=2007 |title=Robotic Exploration of the Solar System Part I: The Golden Age 1957–1982 |publisher=Springer |pages=424–425 |isbn=9780387493268 }}

Image:Voyager.jpg

On 25 August, in Voyager 2{{'s}} last planetary encounter, the spacecraft swooped only {{convert|4,950|km|abbr=on}} above Neptune's north pole, the closest approach it had made to any body since it left Earth in 1977. At that time, Neptune was the farthest known body in the Solar System. It would not be until 1999 that Pluto would move further from the Sun in its trajectory. Voyager 2 studied Neptune's atmosphere, Neptune's rings, its magnetosphere, and Neptune's moons.See the "Neptune" page from JPL. The Neptunian system had been studied scientifically for many years with telescopes and indirect methods, but the close inspection by the Voyager 2 probe settled many issues{{example needed|date=December 2020}} and revealed a plethora of information that could not have been obtained otherwise.{{example needed|date=December 2020}} The data from Voyager 2 are still the best data available on this planet in most cases.{{citation needed|date=December 2020}}

The exploration mission revealed that Neptune's atmosphere is very dynamic, even though it receives only three percent of the sunlight that Jupiter receives. Winds on Neptune were found to be the strongest in the Solar System, up to three times stronger than Jupiter's and nine times stronger than the strongest winds on Earth. Most winds blew westward, opposite the planet's rotation. Separate cloud decks were discovered, with cloud systems emerging and dissolving within hours and giant storms circling the entire planet within sixteen to eighteen hours in the upper layers. Voyager 2 discovered an anticyclone dubbed the Great Dark Spot, similar to Jupiter's Great Red Spot. However, images taken by the Hubble Space Telescope in 1994 revealed that the Great Dark Spot had disappeared.{{cite web|url=http://hubblesite.org/newscenter/archive/releases/1995/21/image/b/ |title=Hubble Discovers New Dark Spot on Neptune |website=Hubblesite.org|publisher=NASA |date=19 April 1995 |access-date=4 March 2016}} Also seen in Neptune's upper atmosphere was an almond-shaped spot designated D2 and a bright, quickly moving cloud high above the cloud decks dubbed "Scooter".See "Neptune:In Depth" from NASA.

Image:Proteus (Voyager 2).jpg]]

The fly-by of the Neptunian system provided the first accurate measurement of Neptune's mass, which was found to be 0.5 percent less than previously calculated. The new figure disproved the hypothesis that an undiscovered Planet X acted upon the orbits of Neptune and Uranus.Tom Standage (2000). The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. New York: Walker. p. 188. {{ISBN|978-0-8027-1363-6}}.{{cite web|url=http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/|title=Thirty-four years after launch, Voyager 2 continues to explore|date=August 20, 2011 |author1=Chris Gebhardt |author2=Jeff Goldader |work=NASASpaceflight}}

Neptune's magnetic field was found to be highly tilted and largely offset from the planet's centre. The probe discovered auroras much weaker than those on Earth or other planets. The radio instruments on board found that Neptune's day lasts 16 hours and 6.7 minutes. Neptune's rings had been observed from Earth many years prior to Voyager 2{{'s}} visit, but the close inspection revealed that the ring systems were full circle and intact, and a total of four rings were counted.

Voyager 2 discovered six new small moons orbiting Neptune's equatorial plane, dubbed Naiad, Thalassa, Despina, Galatea, Larissa and Proteus. Three of Neptune's moons—Proteus, Nereid, and Triton—were photographed in detail, of which only the last two had been known prior to the visit. Proteus proved to be an ellipsoid, as large as gravity allows an ellipsoid body to become without rounding into a sphere, and appeared almost as dark as soot in color. Triton was revealed as having a remarkably active past, with active geysers, polar caps, and a very thin atmosphere characterized by clouds of what is thought to be nitrogen ice particles. At just {{convert|38|K|C|abbr=on}}, it is the coldest known planetary body in the Solar System. The closest approach to Triton, the last solid world Voyager 2 would explore close by, was about {{convert|25000|mi|abbr=on|order=flip}}.

A list of previous and upcoming missions to the outer Solar System can be found at the List of missions to the outer planets article.

{{As of|February 2024}}, there are no approved future missions to visit the Neptunian system. NASA, ESA and independent academic groups have proposed and developed concept missions to visit Neptune.

After the Voyager flyby, NASA's next step in scientific exploration of the Neptune system was considered to be a flagship orbiter mission.{{cite news |last=Clark |first=Stephen |url=http://spaceflightnow.com/2015/08/25/uranus-neptune-in-nasas-sights-for-new-robotic-mission/ |title=Uranus, Neptune in NASA's sights for new robotic mission |work=Spaceflight Now |date=25 August 2015 |access-date=7 September 2015}} Such a hypothetical mission was envisioned to be possible in the late 2020s or early 2030s. Another concept mission proposed for the 2040s is called the Neptune-Triton Explorer (NTE).{{cite web|url=http://www.lpi.usra.edu/vexag/road_map_final.pdf| title=Solar System Exploration| publisher=Science Mission Directorate (NASA)| date=September 2006| access-date=5 August 2015}} NASA has researched several other project options for both flyby and orbiter missions (of similar design as the Cassini–Huygens mission to Saturn). These missions are often collectively called "RMA Neptune-Triton-KBO" missions, which also includes orbital missions that would not visit Kuiper belt objects (KBOs). Because of budgetary constraints, technological considerations, scientific priorities and other factors, none of these have been approved.{{cite web| url=http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_059324.pdf| title=Planetary Science Decadal Survey, JPL Rapid Mission Architecture, Neptune-Triton-KBO Study Final Report| publisher=NASA| date=February 2010| access-date=5 August 2015}}

Several mission concepts have been developed to visit the Neptune system, including:

{{Gallery

|File:Triton Ocean World Surveyor.jpg

|Triton Ocean World Surveyor

|alt1=A gray hexagonal prism-shaped space probe with a large communications antenna, two radioisotope thermoelectric generators marked in green and long magnetometer boom, intended for orbit around Neptune

|File:Nautilus Spacecraft.tif

|Nautilus

| alt2= A large hexagonal prism-shaped space probe in gold-colored foil, with two magnetometer booms and a prominent cup-shaped antenna, bound for Neptune

}}

The lowest-energy trajectory for a launch from Earth to Neptune uses a Jupiter gravity assist, opening an optimal launch window with a 12-year interval, when Jupiter is in a favourable position relative to the Earth and Neptune. An optimal launch window was open for such a Neptune mission from 2014 to 2019, with the next opportunity occurring from 2031.{{cite web | url=http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hansen_Argo_Neptune%20Mission%20Concept.pdf | title=Argo - A Voyage Through the Outer Solar System | author=Candice Hansen | website=SpacePolicyOnline.com | publisher=Space and Technology Policy Group, LLC | access-date=5 August 2015 | display-authors=etal | archive-date=24 September 2015 | archive-url=https://web.archive.org/web/20150924104222/http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hansen_Argo_Neptune%20Mission%20Concept.pdf | url-status=dead }} These constraints are based on the requirement of a gravity assist from Jupiter. With the new Space Launch System (SLS) technology in development at Boeing, deep space missions with heavier payloads could potentially be propelled at much greater speeds (200 AU in 15 years) and missions to the outer planets could be launched independently of gravity assistance.{{cite web | url=http://www.boeing.com/assets/pdf/defense-space/space/sls/docs/sls_mission_booklet_jan_2014.pdf | title=Space Launch Mission | publisher=The Boeing Company | date=2013 | access-date=6 August 2015 | url-status=dead | archive-url=https://web.archive.org/web/20150923193950/http://www.boeing.com/assets/pdf/defense-space/space/sls/docs/sls_mission_booklet_jan_2014.pdf | archive-date=23 September 2015 }}{{cite web| url=http://www.cbsnews.com/news/nasa-finalizes-2-8-billion-boeing-contract-for-sls-rocket-stage/ |title=NASA finalizes $2.8 billion Boeing contract for SLS rocket stage| author=William Harwood| work=CBS News| date=3 July 2014| access-date=6 August 2015}}

Space telescopes such as the Hubble Space Telescope have signified a new era of detailed observations of faint objects from afar, across the entire electromagnetic spectrum. This includes faint objects in the Solar system, such as Neptune. Since 1997, adaptive optics technology has also allowed for detailed scientific observations of Neptune and its atmosphere from ground-based telescopes. These image recordings now exceed the capability of HST by far and in some instances even the Voyager images, such as those of Uranus.{{cite book|author=Oddbjorn Engvold|title=Reports on Astronomy 2003–2005 (IAU XXVIA): IAU Transactions XXVIA|url=https://books.google.com/books?id=8s1JV-TrXacC&pg=PA147|year=2007|publisher=Cambridge University Press|isbn=978-0-521-85604-1|page=147}} Ground-based observations are however always limited in their registration of electromagnetic waves of certain wavelengths, due to the inevitable atmospheric absorption, in particular of high energy waves.{{Cite APOD |date=18 February 2000 |title=Neptune through Adaptive Optics}}[http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.66.7754&rep=rep1&type=pdf First Ground-Based Adaptive Optics Observations of Neptune and Proteus] Planetary & Space Science Vol. 45, No. 8, pp. 1031–1036, 1997

• Exploration of Mercury
• Exploration of Venus
• Exploration of Mars
• Exploration of Jupiter
• Exploration of Saturn
• Exploration of Uranus

{{notelist}}

{{Reflist|2}}

• [http://voyager.jpl.nasa.gov/science/neptune.html Neptune] Voyager 2 – The Interstellar Mission, Jet Propulsion Laboratory, California Institute of Technology
• [http://solarsystem.nasa.gov/planets/neptune/indepth Neptune: In Depth] Planets, NASA

• [http://voyager.jpl.nasa.gov/news/25_years_after_neptune.html 25 Years After Neptune: Reflections on Voyager] NASA Voyager website
• [http://photojournal.jpl.nasa.gov/targetFamily/Neptune Images of Neptune and All Available Satellites] Photojournal, JPL

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Category:Neptune

Category:Spaceflight

Category:Discovery and exploration of the Solar System

Category:Solar System