section_path: Exploration
section_name: Exploration
paragraph_text: Exploration
  
  Voyager 2 is the only spacecraft that has visited Neptune. The spacecraft ' s closest approach to the planet occurred on 25 August 1989. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1 ' s encounter with Saturn and its moon Titan. The images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS all-night program, Neptune All Night.
  
  During the encounter, signals from the spacecraft required 246 minutes to reach Earth. Hence, for the most part, Voyager 2 ' s mission relied on preloaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon Nereid before it came within 4,400 km of Neptune's atmosphere on 25 August, then passed close to the planet's largest moon Triton later the same day.
  
  The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. Neptune's rotation period was determined using measurements of radio emissions and Voyager 2 showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring. The flyby provided the first accurate measurement of Neptune's mass which was found to be 0.5 per cent less than previously calculated. The new figure disproved the hypothesis that an undiscovered Planet X acted upon the orbits of Neptune and Uranus.
  
  Since 2018, the China National Space Administration has been studying a concept for a pair of Voyager -like interstellar probes tentatively known as Shensuo. Both probes would be launched in the 2020s and take differing paths to explore opposing ends of the heliosphere; the second probe, IHP-2, would fly by Neptune in January 2038, passing only 1,000 km above the cloud tops, and potentially carry an atmospheric impactor to be released during its approach. Afterward, it will continue its mission throughout the Kuiper belt toward the heliosphere tail, which is so far unexplored.
  
  After Voyager 2 and IHP-2 ' s flybys, the next step in scientific exploration of the Neptunian system is considered to be an orbital mission; most proposals have been by NASA, most often for a Flagship orbiter. In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Orbiter with Probes" mission that does Cassini -level science. A subsequent proposal, that was not selected, was for Argo, a flyby spacecraft to be launched in 2019, that would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would have been on Neptune and its largest moon Triton to be investigated around 2029. The proposed New Horizons 2 mission might have done a close flyby of the Neptunian system, but it was later scrapped. Currently a pending proposal for the Discovery Program, the Trident spacecraft would conduct a flyby of Neptune and Triton; however, the mission was not selected for Discovery 15 or 16. Neptune Odyssey is another concept for a Neptune orbiter and atmospheric probe that was studied as a possible large strategic science mission by NASA; it would have launched between 2031 and 2033, and arrive at Neptune by 2049. However, for logistical reasons the Uranus Orbiter and Probe mission was selected as the ice giant orbiter mission recommendation, with top priority ahead of the Enceladus Orbilander. Two notable proposals for a Triton-focused Neptune orbiter mission that would be costed right between the Trident and Odyssey missions (under the New Frontiers program) are Triton Ocean World Surveyor and Nautilus, with cruise stages taking place in the 2031–47 and 2041–56 time periods, respectively. Neptune is a potential target for China's Tianwen-5, which could arrive in 2058.