The Great Adventure of Voyager 2

Voyager 2 spacecraft approaches Neptune, giving us the first significant data about this planet date: 25/08/1989. No other spacecraft has visited Neptune since. The essential information that we currently have for Neptune come from this landmark flyby of Voyager 2 with Neptune. Before encountering Neptune, Voyager 2 had also a close encounter with Jupiter, Saturn and Uranus.

One light year is a good estimate for the distance between the stars of a galaxy. 1 light year is equal to 63241.1 times the distance of the Earth from the Sun (AU). Voyager 2 is currently at a distance of 120 AU from Earth, that is 1/527 the distance of a light year. During the encounter with Neptune, the engineering team carefully changed the probe's direction and speed so that it could do a close flyby of the planet's largest moon, Triton. Triton was discovered in 10/10/1846, by English astronomer William Lassell, (just 17 days after the discovery of Neptune by Johann Gottfried Galle using calculations by Urbain Le Verrier and John Couch Adams). It has a retrograde orbit, and is thought to be an object of the Kuiper belt, that was captured by the strong gravity of Neptune. After many million years, Triton will be destroyed by Neptune as it has an unstable orbit, that brings him gradually closer to Neptune. Triton has 99.5% the mass of all matter in orbit around Neptune! Voyager 2 revealed that Triton is geologically active, with an atmosphere of nitrogen, and traces of methane.

Active geysers were sending material upwards at a height of 8 km, which was later travelling perpendicularly at a distance of 150 km. This indicated that Triton in its interior must be hotter than on the surface. Voyager 2 recorded the lowest surface temperature ever recorded (on a moon or planet), a temperature of -235 C! A further interesting fact about Triton is that one side of it is invisible as seen from Neptune (the same happens with the Moon), and this is due to the synchronous rotational periods of Triton: 1. around its axis and 2. around Neptune. Triton has a diameter of 1,680 miles. Voyager 2 also revealed a surface with some craters, and volcanoes, mounds and round pits formed by icy lava flows. Triton's outer layer is a crust of frozen nitrogen, underneath which lies a mantle which is possible that it might be in liquid form and accommodate primitive forms of life, and finally underneath the mantle there is a core of rock and metal. The core has 2/3 the mass of the moon. Triton has a high albedo (percentage of reflected sunlight) of 0.7, and is noticeably larger than Pluto. Half of Triton's surface is covered with frozen nitrogen, and the rest is frozen carbon dioxide and frozen water. Voyager 2 approached Triton at a distance of 40,000 km and mapped 40% of it surface. Triton's volcanic activity, is driven by seasonal heating by the Sun, but it is possible that the strong gravity of Neptune plays a role. Triton, Io, Venus and Earth are the only bodies known for their volcanic activity in our Solar System.

Below you can see an image from NASA of the most active moon in the solar system (of Jupiter) "Io":

and at distance of 108 million km from the Sun, the extremely active Venus:

Voyager 2 carries a "Golden Record" of Earth sounds, pictures and messages. In particular the Golden record features the following music:

Voyager 2 enriched our knowledge also for Neptune itself. About 30 times farther from the Sun than Earth is, the icy giant receives only about 0.001 times the amount of sunlight that Earth does. Below you can see two landmark pictures of Neptune, that Voyager 2 took on the 26th of August:

Neptune is very similar to Uranus. It has 4 times the diameter of Earth. It's interior contains water, ammonia, and methane at high pressure. In the center lies a solid core, with the size of Earth. Its atmosphere is made of hydrogen, helium, and methane. Methane gives Neptune the same blue colour as Uranus. Neptune has six rings, which were revealed to us by Voyager 2. Today Neptune is so well known that it has a strong influence on the popular culture, for example a Dr. Who episode - "Sleep No More" - takes place on a space station orbiting Neptune:

The title is in reference to the Shakespeare play, Macbeth: "Sleep no more! Macbeth does murder sleep". Mark Gatiss, the writer of the episode mentions:

"I’ve had it in mind for a long time. I’ve never done one set in the future. And I’m very interested in insomnia. I’m slightly insomniac. Actually, I’m a lot better than I used to be. I used to be terrible, terribly bad. Those moments spent looking up at the ceiling… and I read this article about how essentially the pressure we’re all under these days to be permanently working is only going to get worse. So I thought if in the far future, what if they found a way of eliminating sleep altogether. They will. And something Doctor Who hasn’t done for a really long time is satire, really. And the best possible vehicle for satire is science fiction because you can imagine a world based on concept that you can have fun with, so that was the essential idea. It was going to be a two-parter. Essentially what happened is that all the backstory that was going to be throughout the two-parter got crunched down, so it’s there. But I feel like it’s good because it’s the best possible way of doing it. I did all of the work, but it’s only there very lightly. So in Neptune, around Neptune, they are actually from Triton, which is the chief moon where there is a colony where there are all of these cities, and they’ve developed a thing called the Morpheus process, which reduces sleep to five-minute bursts, and you can work a week without sleeping. There are people who go for it who are known as the Wide-Awakes, and they’re like the thrusting executives. They are the people who’d take cocaine. And then there are sort of refuseniks, who they would refer to as the Rips, the Rip Van Winkles. They don’t want to give up their sleep. It’s literally all we’ve got left. So that was the whole idea the sort of two factions. And it’s all there in the existing script, but instead they sort of throw away references rather than a fully worked-out thing. And then I had this idea. Two things I thought, at Steven’s encouragement, I said I’ll write this as a horror film, as it were going to be a movie. And then I’d rein back, and I kind of didn’t. And then I had the found footage idea, and that’s why it became a single because I don’t think it could sustain it over two. That was the reason behind that. It’s amazingly handy because essentially we had all of these conversations. Well, we can’t really have any music in it because it’s not like that. It’s a P.O.V. thing. That was the twist. But we had to make a decision. It’s still Saturday night Doctor Who. You can’t make it like a pitch-black horror film. And Steven kept saying, quite rightly, that let’s remind people all of the time that they are seeing different viewpoints. Otherwise, it’ll look like a very badly shot episode of Doctor Who. But the great get-out is that Rassmussen is ultimately making a film. He is making a film, and I was very particular about this. The first thing he says against the black is ‘You must not watch this,’ which, of course, is an invitation to watch. But it turns out that he is putting all of this footage together deliberately to put that glitch in people’s eyes and spread the Sandman. In the end, it kind of gets you out of all kinds of holes because there is a bit of music in there. There is a bit of fiddling about. It all makes sense because ultimately he is pulling the strings. It’s not an intentional metaphor because it is like sitting there and authoring your own episode. There’s a particular thing Clara suddenly says, ‘Because of the Sandmen.’ And [The Doctor’s] like, ‘What?’ And she says, ‘The Sandmen after the song.’ And he says, ‘I do the naming around here. It’s like the Silurians all over again.’ That’s because it’s a thing over the years like the Silurians are called the Silurians even though it’s the wrong time [they’re actually from an era between the Silurian and Eocene Periods]. And Ice Warriors aren’t really the Ice Warriors; they’re Martians, but they are the Ice Warriors. So I thought I’m just going to get in early. Let’s give them a name now because otherwise they’d be called the Rassmussen mutations or something. They’re called the Sandmen! That’s what they’re called. The reason for ‘Mr. Sandman’ is I’ve always been in love with that song. It’s very creepy, I think. Particularly, Russell T Davies talked to me about using it, and it was in the early drafts of ‘The Idiot’s Lantern,’ my second episode of Doctor Who all those years ago. And Russell said, ‘There’s something so weird about this song. It’s so chirpy but it’s so creepy.’ And it is, isn’t it? It’s a bit like Santa Claus. He sees you when you’re sleeping. There’s something a bit odd about it, and I’ve always loved that, and I thought, when things were coming together, I thought for this episode, that would be perfect, and again, the chirpiness of it. There’s a lovely bit at the end where Rassmussen is shot, and you hear the music carrying on in the background. It’s really eerie."

Neptune's day has a duration of 16 hours, and one year in Neptune is equal to 165 Earth years! Neptune has 13 moons, which are named after sea gods and nymphs in Greek mythology. Voyager 2 discovered 6 new moons: Despina, Galatea, Larissa, Proteus, Naiad and Thalassa. The Great Dark Spot of Neptune, a huge anticyclonic storm was also first discovered by Voyager 2.

Since this was a very dynamic feature of the Planet's outer layers, it has since disappeared. The Great Dark Spot, reminds us of Jupiter's Red Giant Spot. The Great Dark Spot had the size of the Earth, and it was the fastest wind ever recorded in the solar system, with speeds of 2,200 km/h. However, in 2016 a similar new spot appeared in Neptune's northern hemisphere the Northern Great Dark Spot (NGDS). Voyager 2 also revealed the existence of a tilted magnetosphere and auroras close to the poles. Before Voyager 2, scientists believed that Uranus's tilted magnetosphere was the result of its inclination with respect to the ecliptic. However today, comparative studies of the 2 planets (with the data of Voyager 2) point to the predominant theory that the magnetosphere is affected by the flow of material in the interior of the planets. Furthermore, although Neptune is more distant from the Sun than Uranus, and receives only 40% of the sunlight, they have the same surface temperature due to the internal heating of Neptune, which must be powered by gravitational forces and its inner intrinsic structure peculiarities. Even today, very little is known about the specifics of the internal structure details of Neptune, and unfortunately there are no immediate plans for a more evolved mission that will precede Voyager 2.