Neptune ... The Blue Giant

Introduction to Neptune

Neptune was the final object visited by Voyager 2 in 1989, and since that encounter, the Voyager 2 spacecraft has continued on its journey out of the Solar System. You cannot see this planet with your naked eye, and it is barely visible under the darkest sky with a pair of binoculars. Earth-bound telescopes reveal some details, but almost everything we know comes from that single satellite mission. Neptune may rival Saturn for beauty. Its blue color is unlike any other. Neptune is the windiest planet in the Solar System and holds the only major moon that orbits in a clockwise direction. Its discovery in 1849 is an interesting story, similar to the serendipitous discovery of Pluto.

Planetary Data

Mass (kg), and mass relative to Earth

10.30x1025 kg = 17.204 earths

Equatorial diameter (km)


Mean density (gm/cm3)


Acceleration of gravity (m/s2)


Velocity of escape (km/s)


Period of rotation

16.1 hours

Period of revolution

164.771 years

Aphelion (AU)


Aphelion (km)


Perihelion (AU)


Perihelion (km)


Mean orbital distance from the sun (AU)


Mean orbital distance from the sun (km)


Orbital velocity (km/s)






Inclination to the ecliptic

1.774 degrees

Inclination of the equator to the orbit

28.80 degrees

Number of natural satellites


Names of natural satellites

Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, Nereid, 5 new moons not yet given formal names

More Information on the Planet Neptune from the Nine Planets Website

Much of the information below is direct from the Nine Planets Website. Some material has been altered by me for this course, while other items and comments are directly copied. I hope to maintain a continuous update of this material to keep up with the findings from space satellites and telescopes.

Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus.
orbit: 4,504,000,000 km (30.06 AU) from Sun
diameter: 49,532 km (equatorial)
mass: 1.0247x1026 kg





In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea. During a recent family trip to Disney World in Orlando, Florida, we stood in line for 30 minutes waiting to visit princess Ariel and get a picture of the king's daughter with my daughters. As you can see in the image below, King Triton, also known as Neptune, is seen shooting water from his trident while my daughters stand in front of him.






After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25, 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal of in formation, too.

Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune, making Neptune the most distant planet from the Sun for a few years. This was actually the case between 1979 and 1999, but Pluto has since moved back out farther from the Sun than any other planet.

Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.





Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.

Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. This is hard for some students to grasp. So, consider the smell of methane gas (you know the source of this gas since your study of Uranus was completed earlier). Imagine the rotten smell of methane gas blowing up your nose at 2000 km/hr! This makes the cloudtops of Neptune a less-than-exciting place to visit.





Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun. So, although the temperatures are very cold (well below -200 C) the interior of the planet releases heat that causes large convection within the planet's interior, and as this heat wells up toward the upper layers of the gas giant, it makes Neptune warmer at the cloudtops than it would otherwise be.


At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular and almost triangular-shaped white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (below). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.





However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot had disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds. Then, in 1999, the HST took additional photos of Neptune and discovered that the Dark Spot had reappeared.



Neptune has banded weather, as do the other gas planets, not as pronounced as that of Jupiter, but more so than Uranus. The image to the left shows some of the clouds in a weather band, and what is so striking in this photograph is the vertical relief nature of these clouds. Earth's atmosphere has vertical clouds, but lacks the tremenously high velocity winds of Neptune that take such clouds and blow them into long bands instead of the pretty cumulus clouds of our home planet.





Neptune's Rings

Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure.

Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.

Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.



Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers. The orientation and location of the center of the magnetic field is a mystery, just as it is within the planet Uranus. The magnetic field of Neptune is offset from the center of the planet by almost two-thirds of its radius, and is tilted 47 degrees from the geographic pole.

Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program such as Starry Night.

Neptune's Satellites

Neptune has 13 known moons; 12 small ones and Triton. Most of the moons of Neptune are small and insignificant, but Triton is a fascinating object whose surface temperature makes it the coldest object in the Solar System, and whose orbit is retrograde. This means that it is orbiting clockwise instead of counter-clockwise, as all of the major moons of the solar system do. Please go to the Triton page to learn more about its secrets.



Radius (km)

Mass (kg)




48,000 km



Voyager 2



50,000 km



Voyager 2



53,000 km



Voyager 2



62,000 km



Voyager 2



74,000 km



Voyager 2



118,000 km



Voyager 2



355,000 km






5,509,000 km





S/2002 N1

15,686,000 km

30 km




S/2002 N2

22,337,190 km

19 km




S/2002 N3

22,613,200 km

19 km




S/2003 N1

46,738,000 km

19 km




S/2002 N4

47,279,670 km

30 km




Below are three images of Neptunian moons, Triton, Proteus, and Nereid, from left to right as you look below. The images of Proteus and Nereid are really fuzzy, but at least you can get a peek at these tiny moons. The reason Proteus was discovered AFTER Nereid, even though it is larger is because Nereid orbits farther away and Proteus is too close to Neptune to be seen with a large, Earth-bound telescope.


During the latter part of 2002, a team of scientists led by Matthew Holman of the Smithsonian Center for Astrophysics and J.J. Kavelaars of Canada's National Research Council found three new moons orbiting Neptune. They are all very small and are the first moons to be discovered around Neptune since the Voyager 2 flyby in 1989, and they are the first to be observed from a ground-based telescope since 1949. What makes the discovery of these little moons so interesting is the realization that each giant planet may have an irregular population of moons caused by some ancient collision between a former moon and a passing comet or asteroid. The collision would produce chunks of the original moon and eject other parts.

Neptune's Rings

Please go to the webpage that describes the unusual rings of Neptune. Below is a chart that gives a statistical overview of the faint rings of Neptune. (distance is from Neptune's center to the ring's inner edge)


Distance (km)

Width (km)

Technical name

Common name















Lassell, Arago






For a more detailed summary of the planet, go to Voyager Science Summery, or go to the JPL Webpage devoted to Neptune.


After Voyager 2 flew by Triton, it continued in a southerly trajectory relative to the ecliptic, and is continuing to travel out of our solar system. There are two things which control the future of the Voyager spacecraft ... the amount of nuclear fuel as well as the amount of thruster engine fuel. The spacecraft uses the decay of Plutonium to generate heat, which in turn is converted into electricity, that can be used to send signals to the JPL facility in Pasadena, as well as receive signals from Earth. The thruster fuel is used to point the Voyager receiver/transmitter antenna at the Earth. Scientists believe that there is enough of both fuels to keep Voyager 2 operational until about 2015-2020. It is hoped by that time, that Voyager will have escaped the heliosphere (the bubble or gas blown out by the Sun), cross the heliopause (where the Sun's wind-blown bubble ends and interstellar space begins), and enter interstellar space. The mission has been renamed Voyager Interstellar Mission, and scientists are continuing to maintain contact with both Voyager 1 and 2.

Remember, you are responsible for the information on the Rings of Neptune as well as its Moons.

This information in the last paragraph of this page is found in several other places within my course because I believe it to be incredibly important, interesting, and relevant to a potential creative writing assignment of yours :)

1) Give a brief description of the physical appearance of Neptune as seen from a spacecraft whizzing by.

2) Why do scientists believe Neptune has a strikingly beautiful blue color?

3) Who discovered Neptune and when was this discovery made?

4) What is unusual about the magnetic field of Neptune?

5) What is the name of the only spacecraft ever to visit Neptune, and when did this spacecraft arrive there?

6) What is the most unusual activity found to be happening on Triton?

7) What is the surface temperature of Triton, and why do scientists believe it is so cold there?

8) How many rings surround Neptune, and what is unusual about the nature of the outer ring?

9) What is unusual about the orbit of Triton?

10) What might be a challenge to a manned mission to Neptune that is different from what you may have noted for any previous planet?

Now, it is time to put your reading and comprehension skills to the test. Please look over the 10 questions listed below, and when you feel you have correct answers, go to the Neptune Quiz page and submit them to me.

You have now concluded the Outer Gas Giant Planets Unit, and are ready to move on first to Voyager Mission Recount, then to the Voyager Quiz and Commentary, and then to the Ice Worlds.

You could always go to the Syllabus, but why would you do that when you are so curious about the Voyager project.

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