Voyager 2 at Neptune
On 25 August 1989 Voyager 2 skimmed over Neptune's north pole. At the last stop of its twelve-year Grand Tour it revealed a giant dark storm and geysers on the moon Triton.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 25 August 1989 Voyager 2 passed about 4,950 km above Neptune's north pole. Neptune is about 50,000 km across, so compared with the planet this was a skim just above the clouds. It was the final stop of the Grand Tour, a twelve-year journey since the 1977 launch past Jupiter, Saturn and Uranus. Neptune was then roughly 4.5 billion km from Earth.
Neptune was not the calm blue ball it looked like from afar. An Earth-sized dark storm, the Great Dark Spot, was turning in its southern hemisphere. Winds reached about 2,100 km/h, the fastest yet measured on any planet in the solar system and more than twice the speed of an airliner. One white cloud raced around the planet so quickly that it was nicknamed the Scooter.
Neptune gives off about 2.6 times as much heat as it receives from the Sun. That means a lot of heat is still left inside the planet. Scientists think this inner heat helps drive Neptune's powerful winds and storms. It explains why the weather is so wild so far from the Sun.
About five hours after passing Neptune, Voyager 2 flew by its largest moon, Triton, at about 40,000 km. Triton's surface was extremely cold, at about −235 °C, yet geysers were seen sending nitrogen gas and dark dust up to about 8 km high. The orbit view reproduces the moment Voyager 2 arrives at Neptune at the end of its twelve-year path.
Mission goals
Visiting Neptune was also a struggle against distance. Voyager 2's transmitter had a power of only about 20 watts, similar to a single bright light bulb. After crossing about 4.5 billion km, the signal was extremely faint by the time it reached Earth. So NASA linked its own antennas with others, including the Very Large Array radio telescope in New Mexico and the Parkes radio telescope in Australia, to catch it.
What Voyager 2 saw at Uranus and Neptune made clear that they are a different kind of planet from Jupiter and Saturn. Jupiter and Saturn are mostly hydrogen and helium, while Uranus and Neptune contain a great deal of water, ammonia and methane, so they are called ice giants. Among planets found around other stars, worlds the size of Neptune or a little smaller are common. Yet the only close-up data we have on our own ice giants comes from Voyager 2.
Triton orbits in the opposite direction to Neptune's spin. Large moons that orbit backwards like this are rare in the solar system. So scientists think Triton once wandered in the Kuiper belt beyond Neptune and was later captured by the planet. Triton is similar to Pluto in size and make-up, so it was a useful comparison when New Horizons visited Pluto in 2015.
Mission chronology and background
As it approached Neptune, Voyager 2 discovered six moons that had never been seen before. It also solved a puzzle from observations on Earth, which had made the rings look like broken arcs. The rings in fact go all the way around the planet, but one of them has sections where particles are especially crowded together, and only those sections had shown up from Earth.
To see Triton up close, Voyager 2 had to skim over Neptune's north pole. The price was that its path was bent sharply southward, and it could never meet another planet afterwards. Scientists could make this choice because Neptune was the final destination anyway.
When the Hubble Space Telescope looked at Neptune in 1994, the Great Dark Spot had already vanished. Since then new dark storms have appeared and faded again and again. Unlike Jupiter's Great Red Spot, which has lasted for centuries, Neptune's storms seem to come and go within a few years. No new mission to Neptune has been approved, so Voyager 2's data will remain the only close-up observations for some time.
Other milestones of this mission
- Voyager 2 launch — the Grand Tour
- Voyager 1 launch
- Voyager 1 enters interstellar space
- Voyager 1 passes one light-day from Earth(planned)
Where to see it — sky and simulator
At about magnitude 7.8, Neptune is too faint for the naked eye. In binoculars it looks like a small star, and in a small telescope like a tiny bluish disc. It currently lies near Pisces and reaches opposition, opposite the Sun in the sky, around autumn, when it is up all night.
Select Neptune in the sky view to see its height, direction and rising time from where you are. Follow it in the orbit view and move the date to 25 August 1989 to reproduce the moment Voyager 2 skims over its north pole. In the 3-D interstellar view, the Kuiper belt and Voyager 2's current position lie beyond Neptune's orbit, about 30 AU from the Sun.
See this for yourself — replay the moment in the orbit view
FAQ
How close did it pass?
About 4,950 km above Neptune's north pole on 25 August 1989, and about five hours later it passed the moon Triton at about 40,000 km.
Is the Great Dark Spot still there?
No. It had vanished by the time Hubble looked in 1994, and other dark storms have come and gone since.
Why is Triton special?
It is a large moon that orbits against Neptune's spin, so it is thought to be a captured Kuiper-belt object. Despite the extreme cold, it has erupting geysers.
When is the next Neptune mission?
None has been approved yet. The US ten-year plan chose a Uranus orbiter as its top large mission.
Related
- Voyager 2 launch — the Grand Tour
- Voyager 1 launch
- New Horizons at Pluto
- Neptune — object index
- Voyager 2 — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.