Encounter with Neptune

Neptune is one of the class of planets -- all of them beyond the asteroid belt -- known as gas giants; the others in this class are Jupiter, Saturn and Uranus. These planets are about 4 to 12 times greater in diameter than Earth. They have no solid surfaces but possess massive atmospheres that contain substantial amounts of hydrogen and helium with traces of other gases. Voyager 2 is one of twin spacecraft launched more than a decade ago to explore the outer solar system. Between them, these spacecraft have explored four giant planets, 48 of their moons, and their unique systems of rings and magnetic fields. Voyager 1, launched September 5, 1977, visited Jupiter in 1979 and Saturn in 1980. It is now leaving the solar system, rising above the ecliptic plane at an angle of about 35°, at a rate of about 520 million kilometers a year. Voyager 2, launched August 20, 1977, visited Jupiter in 1979, Saturn in 1981 and Uranus in 1986 before making its closest approach to Neptune on August 25, 1989. Voyager 2 traveled 12 years at an average velocity of 19 kilometers per second (about 42,000 miles an hour) to reach Neptune, which is 30 times farther from the Sun than Earth. Voyager observed Neptune almost continuously from June to October 1989. Now Voyager 2 is also headed out of the solar system, diving below the ecliptic plane at an angle of about 48° and a rate of about 470 million kilometers a year. Both spacecraft will continue to study ultraviolet sources among the stars, and their fields and particles detectors will continue to search for the boundary between the Sun's influence and interstellar space. If all goes well, we will be able to communicate with the two spacecraft for another 25 to 30 years, until their nuclear power sources can no longer supply enough electrical energy to power critical subsystems.

Nearly 4.5 billion kilometers (3 billion miles) from the Sun, Neptune orbits the Sun once in 165 years, and therefore has made not quite a full circle around the Sun since it was discovered. With an equatorial diameter of 49,528 kilometers (30,775 miles), Neptune is the smallest of our solar system's gas giants. Even so, its volume could hold nearly 60 Earths. Neptune is the densest of the four giant planets, about 64 percent heavier than if it were composed entirely of water. The most obvious feature of the planet in Voyager pictures is its blue color, the result of methane in the atmosphere. Methane preferentially absorbs the longer wavelengths of sunlight (those near the red end of the spectrum). What are left to be reflected are colors at the blue end of the spectrum.

Neptune is a dynamic planet, even though it receives only 3 percent as much sunlight as Jupiter does. Several large, dark spots are reminiscent of Jupiter's hurricane-like storms. The largest spot is big enough for Earth to fit neatly inside it. Designated the Great Dark Spot by its discoverers, the feature appears to be an anticyclone similar to Jupiter's Great Red Spot. Neptune's Great Dark Spot is comparable in size, relative to the planet, and at the same latitude (the Great Dark Spot is at 22° south latitude) as Jupiter's Great Red Spot. However, Neptune's Great Dark Spot is far more variable in size and shape than its Jupiter counterpart. Bright, wispy "cirrus-type" clouds overlaying the Great Dark Spot at its southern and northeastern boundaries may be analogous to lenticular clouds that form over mountains on Earth. At about 42° south, a bright, irregularly shaped, eastward-moving cloud circles much faster than does the Great Dark Spot, "scooting" around Neptune in about 16 hours. This "scooter" may be a cloud plume rising between cloud decks.

Another spot, designated "D2" by Voyager's scientists, is located far to the south of the Great Dark Spot, at 55° south. It is almond-shaped, with a bright central core, and moves eastward around the planet in about 16 hours. Voyager also measured heat radiated by Neptune's atmosphere. The atmosphere above the clouds is hotter near the equator, cooler in the mid-latitudes and warm again at the south pole. Temperatures in the stratosphere were measured to be 750 kelvins (482°C/900°F), while at the 100 millibar pressure level, they were measured to be 55 K (-218°C/-360°F). Heat appears to be caused, at least in part, by convection in theatmosphere that results in compressional heating: Gases rise in the mid-latitudes where they cool, then drift toward the equator and the pole, where they sink and are warmed.

Long, bright clouds, reminiscent of cirrus clouds on Earth, can be seen high in Neptune's atmosphere. They appear to form above most of the methane, and consequently are not blue. At northern low latitudes (27° north), Voyager captured images of cloud streaks casting their shadows on cloud decks estimated to be about 50 to 100 kilometers (30 to 60 miles) below. The widths of these cloud streaks range from 50 to 200 kilometers (30 to 125 miles), and the widths of the shadows range from 30 to 50 kilometers (20 to 30 miles). Cloud streaks were also seen in the southern polar regions (71° south), where the cloud heights were about 50 kilometers (30 miles). Most of the winds on Neptune blow in a westward direction, which is retrograde, or opposite to the rotation of the planet. Near the Great Dark Spot, there are retrograde winds blowing up to 2,400 kilometers per hour (1,500 miles an hour) -- the strongest winds measured on any planet, including windy Saturn.

This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera. The images were taken at a range of 4.4 million miles from the planet, 4 days and 20 hours before closest approach. The picture shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager's cameras could resolve them. North of these, a bright cloud band similar to the south polar streak may be seen.

Image credit: NASA.

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