The greatest voyage of space exploration ever undertaken experienced its final highlight two decades ago, on 25 August 1989, when the intrepid spacecraft Voyager 2 flew within 4,950 kilometres of the beautiful blue ice giant Neptune. Twelve years after it had departed Earth, swinging past Jupiter, Saturn and Uranus during its epic journey, Neptune and its moons were last on the list before heading into the void of deep space.

This was truly a voyage into the unknown. Prior to 1989, Neptune had existed as just a tiny pale blue disc in telescope eyepieces, accompanied by two points of light, its moons Triton and Nereid. We knew, given its size and mass, that it was an ice giant like Uranus, 50,538 kilometres across, and not much else. As the planet loomed large in the viewfinders of Voyager 2’s cameras (its Imaging Science Subsystems were composed of two television-style video cameras – primitive by today’s digital imaging standards), unexpected features began to be discerned within the azure blue–green of its atmosphere, much to the excitement of those watching back on Earth, four light hours away.

Uranus, which Voyager 2 had flown past three years earlier, had turned out to be, dare we say it, a bit dull. There wasn’t much activity visible in its atmosphere, and most of the excitement was saved for its weird moons. As it receives a mere three percent of the sunlight that Jupiter does, and 900 times less than Earth, Neptune’s atmosphere was anticipated to be similarly sluggish, but as those first close-up pictures trickled in we found that it was anything but dull.

Embedded in that cool blue atmosphere (surely the prettiest colour of any planet in the Solar System) was a giant spot, larger than Earth itself, a titanic storm like Jupiter’s Great Red Spot. This ‘Great Dark Spot’ dominated Neptune’s southern hemisphere, with white, fluffy high-altitude clouds of frozen methane crystals around the border of this giant anticyclone. Meanwhile, streaky white clouds ‘scooted’ all the way around the planet in just 16 hours, and a smaller dark spot, christened ‘D2’, was being buffeted by winds blowing up to 2,400 kilometres, the most blustery in the entire Solar System. But time and time again mission scientists kept coming back to that Great Dark Spot. Where did it get its energy from? As Voyager 2 steadily approached Neptune throughout 1989, it had seen the spot drift up and down, from 27 degrees south to 17 degrees south. A few years later, the Hubble Space Telescope saw that it had not only stopped drifting, but had vanished totally from existence, to be replaced a little while later by another huge spot in the northern hemisphere. Whereas Jupiter’s writhing, churning atmosphere can keep its giant storm active for at least 400 years, Neptune is comparatively peaceful and hence its giant storms are shorter-lived. Over the last twenty years, we’ve seen Neptune with a Great Dark Spot of some kind at least fifty percent of the time, powered most likely by internal thermal energy.

As previously stated the blue colour of Neptune’s atmosphere is stunning, the result of atmospheric methane absorbing redder light but reflecting shorter wavelength blue light, in much the same way that Earth’s sky appears blue.

The invisible Neptune
Voyager 2 found that there was more to Neptune than met the eye. It had a magnetic field that the spacecraft stumbled upon, first detecting radio emissions from it and then crossing the bow shock (the outer boundary of the magnetosphere) the day before closest approach. Unusually, Neptune’s magnetic field is tilted by 47 degrees to the axis of rotation, and is offset from the dead centre of the planet by around 10,000 kilometres. Consequently Voyager 2 approached Neptune through the southern cusp of its magnetic field, which is at a tropical rather than polar latitude; indeed the southern magnetic pole was pointed towards that distant point of light that is the Sun when Voyager 2 arrived.

Perhaps one of the biggest surprises were Neptune’s rings. From Earth, astronomers had only detected the merest hint of ‘ring arcs’, rather than full rings, and speculation was rife as to how ring arcs could form. All that debate went out of the window when Voyager 2 discovered that the rings were actually complete, encircling the planet, but were so dark and diffuse that they were impossible to see in their fullest extent from Earth.

The main rings include the Inside Diffuse Ring (also called the Galle ring), which is 17,000 kilometres above the cloud tops and 42,000 kilometres from the centre of Neptune; the Inner Ring (also called the Le Verrier ring), 28,400 kilometres above the cloud tops and 53,000 kilometres from the centre of Neptune; the ‘Plateau’, which is a broad and diffuse sheet of smoke-particle sized dust 56,000 kilometres from the centre of Neptune; and the Main, or Adams, ring 63,000 kilometres from the centre of Neptune and towering 38,000 kilometres above the cloud tops. Voyager 2 was able to measure the size and distribution of the rings, and their constituents, by monitoring occultations of stars in ultraviolet light as the rings blocked the starlight, causing the star to flicker.

Voyager 2 passed over the north pole of Neptune to head away from the planet at an angle of 48 degrees to the plane of the Solar System. Deep space beckoned, but there was one more body for it to investigate: Triton. Moving around Neptune backwards, Triton is rockier and denser than any of the icy moons of Saturn and Uranus. It’s believed to be a captured Kuiper Belt object, 2,720 kilometres across, that wandered too close to the ice giant at some point in the distant past. Voyager 2 flew within 40,000 kilometres, photographing a surface that was frozen but certainly not inactive. Giant dark plumes of dust and nitrogen were seen spewing up to eight kilometres high, before drifting back to the ground. It’s the coldest place in the Solar System that we have measured the temperature of, just 38 degrees above absolute zero. It is quite possible that if Triton had never been captured by Neptune, we would now be calling it a dwarf planet like Pluto and Eris. Indeed, it is considered to be a fine analogue to Pluto, and the records of what Voyager 2 saw and measured on this frigid moon will undoubtedly be poured over in the coming years in anticipation of the arrival of NASA’s New Horizon’s spacecraft at Pluto in 2015, when we finally return to the outer Solar System.

Neptune's moons
               Size         Mean distance from Neptune
Triton     2,720km  353,000km
Proteus   400km     92,000km
Larissa   190km      48,800km
Galatea   180km     37,200km
Nereid    169km     5,560,000km
Despina  150km     27,700km
Thalassa  80km      25,200km
Naiad      54km      23,200km

New discovery
Although this famous fly-by was 20 years ago, new discoveries are still amazingly being made with the data and images that Voyager 2 returned to Earth. During its visit to the Neptunian system, Voyager 2 also discovered six new moons (listed above) including the 150-kilometre wide Despina. Ted Stryk, who is actually a professor of philosophy from Tennessee, USA, has achieved considerable success utilising modern image-processing techniques to reveal extra data in the images sent back by the Voyager probes, such as surface details in the dark hemisphere of Uranus’ moon Ariel. Recently, after realising that Neptune was the only world for which we don’t have a photograph of one of its moons transiting across its disc, he found one such image in the raw data files from Voyager. Cleaning up a sequence of four images, he found both the transit and shadow transit of Despina, an event that had been inexplicably missed by the Voyager imaging team 20 years ago. It is a breathtaking sequence of images that resonate even more when one considers that there are no current plans to return to Neptune, nor are we likely to go back for at least another 20 or 30 years.

In a matter of hours Voyager increased our knowledge of the outermost planet a hundredfold over what we had learnt in the previous 143 years since Neptune’s discovery in 1846. Since this final encounter Voyager 2 has been ploughing a lone furrow towards interstellar space. We still keep in touch, like distant penpals, and it has enough power to stay alive for another fifteen years. It has now passed through the termination shock – the outermost layer of the Sun’s magnetic domain, where the solar wind slams into the radiation field of interstellar space, and in about 296,000 years it will pass within 4.3 light years of Sirius, before continuing into the vast darkness, possibly long after we are gone.