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Planet Profile
Neptune, which is usually the eighth planet from the Sun, is a very cold place. Sometimes, the 9th planet Pluto crosses over into Neptune's orbit and becomes the "eighth planet". Neptune's bluish color comes from its atmosphere of methane gas. The planet has eight moons and a very narrow, faint ring system. Composition and Chemistry Most of the mass in the planetary system is found in the outer solar system. Jupiter alone exceeds the mass of all the other planets combined. The chemistry of the outer solar system is also different, with hydrogen rather than oxygen dominating. When the planets and the Sun were forming, it was cooler in the parts of the solar nebula farther from the Sun. This allowed water ice and other volatile compounds to condense, whereas these materials remained as gas in the inner solar system. As a result, the forming stages of the outer planets, which includes Neptune, included a rather bigger mass of solid material to begin with. This means that the core bodies for the giant planets had more gravity to capture the gases hydrogen and helium, which were very abundant. So, the outer planets were able to grow significantly bigger than the ones closer to the Sun.
Planet Profile
Exploration of Neptune Only six spacecraft, five NASA and one European, have reached beyond the asteroid belt. The challenges of traveling so far away from Earth are great. Flights to the outer planets take years to decades, rather than the few months to reach Venus or Mars. The spacecraft must be very reliable because, even at the speed of light, messages between Earth and the spacecraft take several hours to arrive. If a problem develops near Jupiter, for example, the spacecraft computer must deal with it right then. It would be a disaster if the message had to be sent back to Earth and then be routed back to the spacecraft. These spacecraft also must carry their own energy sources, since there is not enough sunlight to supply energy to the solar cells. Heaters are required to keep instruments at the right temperature, and spacecraft must have powerful radio transmitters and large antennas if their data are to be transmitted back to Earth a billion kilometers away. Launched in 1977, the Voyagers 1 and 2 were the most successful missions to the outer solar system. The Voyagers carried 11 scientific instruments, including cameras and spectrometers, as well as instruments to measure the characteristics of planetary magnetospheres. Voyager 2 arrived at Jupiter four months later and then followed a different path to complete giant circle around the outer planets, reaching Neptune in 1989. The trajectory of the Voyagers was made possible by the approximate alignment of the four giant planets on the same side of the Sun. About once every 175 years, these planets are in a position that allows a single spacecraft to visit them all by using gravity-assisted flybys to adjust its course for each next encounter. We are fortunate that this happened so soon. Basic Characteristics Basic properties of Neptune compared to the other Jovian Planets
In contrast with Jupiter and Saturn, Neptune is small and differs in composition and structure. You can see this in the table above. Neptune has a mass of about 15 times that of Earth and hence only 5 percent as great as Jupiter. Their densities of 1.2 g/cm3 and 1.6 g/cm3, respectively are much higher than that of Saturn, in spite of their smaller mass and weaker gravity. This tells us that their composition must be fundamentally different, consisting for the most part of heavier materials than hydrogen and helium, the primary constituents of Jupiter and Saturn. We will return to the question of their composition shortly. Appearance and Rotation When we look at the giant planets and Neptune, we see only their atmospheres, composed primarily of hydrogen and helium gas. On Neptune the upper cloud deck is made of methane. Neptune has a slightly longer rotation period than the other giant planets(except for Uranus). It comes out to be 17 hours, this is determined from the rotation of its magnetic field. Ingenious, isn't it? Remember that the Earth and Mars have seasons because their axes of spin, instead of "standing up straight," are tilted relative to the Sun's orbital plane. This means that, as the Earth revolves around the Sun, sometimes one hemisphere and sometimes the other "leans into" the Sun. Neptune has about the same tilt as Saturn (29o); therefore, it experiences similar seasons as Saturn does only more slowly. Internal Heat Sources Neptune, different from Jupiter and Saturn, has a small internal energy source, while Uranus does not emit a noticeable amount of internal heat. As a result, these two planets have almost the same temperature, in spite of Neptune's greater distance from the Sun. No one knows why these two planets differ in their internal heat, though. |