Introducing Neptune
Neptune taken by Voyager 2 Image Credit: NASA/JPL

Introducing Neptune

29/03/2021Written by Alex Thompson

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Voyager 2's Neptune. Credit: NASA/JPL

At an almost unimaginable distance, we could travel to the Sun and back 14 times before we could arrive at Neptune. This dense gas giant is the most difficult planet to study, with only the Voyager 2 spacecraft visiting as part of its tour of the outer planets.

The first recorded observation of Neptune was by Italian astronomer Galileo Galilei in 1612. Galileo used one of the first telescopes to observe an object in the position we now know is occupied by Neptune.

However, he mistook the object for a fixed star and, unfortunately, is now not credited with discovering the planet. It would be over 200 years before Neptune was officially discovered.

Discovering Neptune

Discovering Neptune
Urbain le Verrier

In 1843 British mathematician John Couch Adams, using the data he had and was able to source from the then Royal Astronomer, Sir George Airy, began work on the orbit of Uranus. Over a three year period he provided several different estimates of a new planet.

Around the same time, and independently of Adams, French astronomer Urbain Le Verrier began to develop his own theories, although he was not supported by his compatriots.

Le Verrier wrote to Johann Gottfried Galle at the Berlin Observatory instructing him to use the Observatory’s large refractor telescope to search for the planet. Using a recently drawn sky chart and with the help of an observatory student named Heinrich d’Arrest, Galle spotted the planet in the sky around 12° from where Adams had predicted.

Although Adams and Le Vernier were happy to share the credit for the discovery, an argument broke out between Britain and France. In recent years Adams’ claim to have co-founded the planet has come under scrutiny and more often Le Verrier is seen as Neptune’s sole discoverer.

Triumph of Neptune - mid-third century AD. Musée archéologique de Sousse.

In fact, following the recovery of crucial documents, often called the “Neptune papers”, that had been stolen from the Royal Greenwich Observatory, astronomers say they can prove that when the shock of the French discovery began to sink in, Airy tried to rectify the situation, and claimed that the young mathematician Adams was poised to pinpoint the planet.

Several names were suggested for the new planet including Janus, Oceanus and even Le Verrier, but eventually the internationally accepted name suggested by Le Verrier himself, Neptune, was adopted.

Neptune is the Roman god of the sea and so was a good fit with the other planets also named after Roman deities. Subsequently the first moon of Neptune, spotted just 17 days after the planet’s discovery, was named Triton after the son of Poseidon (the Greek god corresponding to the Roman Neptune). The moons found since have been named after lesser sea gods and nymphs from Greek mythology.

Neptune Factfile

Neptune Factfile
Natural-colour view of Neptune with Proteus, Larissa and Despina from the Hubble Space Telescope
Neptune Factfile
Voyager 2's Neptune. Credit: NASA/JPL

Size: Neptune is 24,622km in diameter, four times wider than Earth, but still the smallest of the four gas giants. To scale, if the Sun was the size of a large yoga ball, the Earth would be a small pea and Neptune an average sized marble.

Gravity: The gravity on Neptune is 11.15 m/s2, 1.14 times that of the Earth’s meaning we would all weigh a little more on Neptune.

Age: Like our Sun and the rest of the Solar System, Neptune is over 4.5 billion years old and was formed by gas and dust forced together by gravity.

Distance from Earth/Sun: Neptune is an incredible 4.62 billion km from our planet, a little less than 30 AU (astronomical unit, with 1 AU being the distance between Earth and the Sun.) This makes Neptune nearly 31 AU from the Sun with light from our star taking over four hours to reach the planet.

Atmosphere: The atmosphere of Neptune comprises of 80% hydrogen, 19% helium and a trace amount of methane. It is actually this small amount of methane that gives Neptune its blue colour as it absorbs red wavelengths and reflects blue light outwards.

Temperature: With an average temperature of -201°C Neptune has the coldest average temperature in the Solar System. Its largest moon, Triton, is the coldest known object in the solar system with an average temperature of -235°C, less than 40°C above ‘absolute zero’.

Climate: The winds of Neptune can reach a staggering 1300mph, more than five times the strongest wind ever recorded on Earth and equal to the top speed of a fighter plane. More typically speeds vary between 22mph in an easterly direction and nearly 700mph in a westerly direction.

Rotation: The time taken for the planet to rotate once is 15 hours and 58 minutes, meaning for every two Earth days there are three days on Neptune.

Speed: Neptune travels at over 12,000mph; however because of the huge orbit it takes around the Sun a year on the planet is 165 earth-years long. In 2011 Neptune completed its first orbit around the Sun since its discovery in 1846.

Moons: There are currently 14 known moons around Neptune with Triton accounting for over 99.5% of the mass around the planet.

Neptune Missions

Neptune Missions
Global Color Mosaic of Triton, taken by Voyager 2 in 1989
Neptune Missions
Proposed Trident Mission

In 1989 NASA’s Voyager 2 mission became the first (and as of writing, only) mission to visit Neptune, coming within 5000km of its North Pole. The mission gave scientists previously unknown information regarding the planet’s atmosphere, magnetic fields and climate, in addition to discovering six new Neptunian moons. Since this mission Neptune has been studied using both ground-based and space telescopes, such as Hubble.

Whilst major space agencies, such as NASA, ESA and JAXA, have expressed interest in sending missions to Neptune, the reality is a little more complicated.

The planet’s distance from Earth is a major hurdle, due to the time it would take to get there, the long gaps between launch windows (once every thirteen years) and the mechanics of making such a long distance mission possible.

The budget allocated for a mission to Neptune could be used to fund multiple missions closer to home and we could get the results in a fraction of the time. Any mission to Neptune that includes a orbital insertion would require a significantly longer journey, using the gravities of several other planets to slow to the appropriate speed to make landing achievable. A “fly-by” mission would be quicker and cheaper, but potentially at the expense of scientific data and discovery.

One proposed spacecraft that could head to the Neptune system is NASA’s Trident mission. This craft is mooted to explore Triton, a body that is of significant interest to scientists for several reasons. Triton is believed to have liquid water beneath its icy surface, which excites scientists as where you find water on Earth, you find life. Triton is geologically active with a very young surface, in fact Voyager 2 saw active geysers erupting sublimated nitrogen gas. Finally, its ionosphere (the ionized part of Earth’s upper atmosphere) is ten times more than any other moon in the Solar System, particularly perplexing as this is usually driven by the Sun, which Triton is nowhere near.

It’s all these unknowns that make Triton a great place for exploration. The next possible launch window is in 2025 and the Trident team will hope they get the go ahead to become the second mission to visit the system of the furthest planet in our planetary neighbourhood.