Neptune

## Exploration

Main article: Exploration of Neptune
*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* also 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 also 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.

Since 2018, the China National Space Administration has been studying a concept for a pair of *Voyager*-like interstellar probes tentatively known as *Interstellar Express* or *Interstellar Heliosphere Probe*. Both probes will be launched at the same time in 2024 and take differing paths to explore opposing ends of the heliosphere; the second probe, *IHP-2*, will 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 on its mission throughout the Kuiper belt toward the tail of the heliosphere, 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. Such a hypothetical mission is envisioned to be possible in the late 2020s or early 2030s. However, there have been discussions to launch Neptune missions sooner. In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Orbiter with Probes" mission that does *Cassini*-level science. Another, more recent proposal 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 be on Neptune and its largest moon Triton to be investigated around 2029. The proposed *New Horizons 2* mission (which was later scrapped) might also have done a close flyby of the Neptunian system. Currently a pending proposal for the Discovery program, *Trident* would conduct a flyby of Neptune and Triton; however, the mission was not selected for Discovery 15 or 16. *Neptune Odyssey* is the current mission concept for a Neptune orbiter and atmospheric probe being studied as a possible large strategic science mission by NASA that would launch between 2031 and 2033, and arrive at Neptune by 2049.