Voyager 2 at Neptune

The Grand Tour's final stop — after 12 years it revealed Neptune and Triton's nitrogen geysers.

The moment of Voyager 2 at Neptune: https://nowsky.org/e/voyager2-neptune — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

On 25 August 1989 Voyager 2 skimmed 4,950 km above Neptune's north pole — the last stop of a twelve-year Grand Tour and a feat of navigation unimaginable at launch: after 4.3 billion km it arrived within tens of kilometres of its aim point.

Neptune was not a quiet blue ball. An Earth-sized storm, the Great Dark Spot, circled the southern hemisphere, the strongest winds in the solar system blew at 2,100 km/h, and a bright cloud nicknamed "Scooter" raced across the disk. The planet radiates 2.6 times the heat it receives from the Sun.

Five hours later the probe passed the moon Triton at 40,000 km. Nitrogen geysers rose 8 km from a surface at −235 °C, and its terrain resembled the skin of a cantaloupe. Triton orbits backwards relative to Neptune's spin and is thought to be a captured Kuiper-belt object.

After the encounter Voyager 2 turned toward the planetless southern sky. That single day remains the only close look humanity has had at Neptune. The orbit replay shows the final arrival at the end of the twelve-year trajectory.

Why it matters

The encounter was also a feat of communication. The signal from 4.3 billion km was weaker than a 20-watt bulb, received only by arraying several radio telescopes on Earth. Bringing in the Parkes dish in Australia set the standard for later Deep Space Network operations.

The Neptune Voyager 2 saw established the "ice giant" class — worlds made mostly of water, ammonia and methane ices, unlike Jupiter and Saturn. It is the most common size of exoplanet, yet in our own solar system that one day is all we have.

Triton's geysers were the third confirmed site of current volcanic activity in the solar system, after Earth and Io. Similar in size and composition to Pluto, Triton became the rehearsal for New Horizons' Pluto encounter in 2015.

How to watch

At magnitude 7.8 Neptune is invisible to the naked eye but star-like in binoculars and a tiny blue-green disk in a small telescope. It currently lies near Pisces and reaches opposition in September, when it is up all night.

Select Neptune in the sky view for its altitude, bearing and rise time from your location; follow it in the orbit view and move the time to 25 August 1989 to replay Voyager 2 skimming over the north pole.

In the 3-D interstellar view Neptune's orbit (30 AU) is drawn as the inner boundary of the outer solar system, with the Kuiper belt and Voyager 2's current position (about 142 AU) beyond it.

See this for yourself — replay the moment in the orbit view

Background

Before the flyby Voyager 2 discovered six new moons and four ring fragments, and resolved the puzzle of ground-based "ring arcs": the rings are complete but clumpy, with dense sections that had shown up as arcs.

The Great Dark Spot was gone when Hubble looked in 1994, and new dark spots have since appeared and vanished in the northern hemisphere. Unlike Jupiter's Great Red Spot, Neptune's storms seem to live only a few years.

The 2023 US planetary decadal survey ranked a Uranus orbiter first and a Neptune–Triton mission as the next candidate. No Neptune mission is yet approved, so Voyager 2's single day remains the only close data for now.

The trajectory for the Neptune encounter carried one sacrifice. Seeing Triton close required skimming over Neptune’s north pole and bending the orbit southward, which sent Voyager 2 48° below the ecliptic, beyond any further planetary encounter. In exchange came the Triton flyby and an occultation of Neptune’s atmosphere. In the months before the encounter the ring positions were updated from ground-based observations and the course corrected three times, and the closest approach came within one second of the plan.

FAQ

How close did it pass?

4,950 km above Neptune's north pole on 25 August 1989, then 40,000 km from Triton five hours later.

Is the Great Dark Spot still there?

No. It had vanished by Hubble's 1994 observations; other dark spots have come and gone since.

Why is Triton special?

It orbits backwards, so it is thought to be a captured Kuiper-belt object, and it has active nitrogen geysers.

When is the next Neptune mission?

None is approved; a Uranus orbiter has priority and Neptune is the next candidate.

→ More sky news

Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.