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_Neptune_

_In the long run men hit only what they aim at.
- Thoreau_

_Table of Contents_
Neptune Introduction
[10]Neptune Statistics
[11]Animations of Neptune
[12]Views of Neptune
[13]Rings of Neptune
[14]Neptune Moon Summary

_Satellites of Neptune_
[15]Despina, [16]Galatea, [17]Larissa,
[18]Naiad, [19]Nereid, [20]Proteus,
[21]Thalassa, [22]Triton.

_Neptune Science_
[23]Neptune Science Summary
[24]Neptune Exploration Chronology
[25]Neptune Image/Animation Gallery

_Other Resources_
[26]Neptune's Ring System

[27][LINK] 

Neptune is the outermost planet of the gas giants. It has an
equatorial diameter of 49,500 kilometers (30,760 miles). If Neptune
were hollow, it could contain nearly 60 Earths. Neptune orbits the Sun
every 165 years. It has eight moons, six of which were found by
[28]Voyager. A day on Neptune is 16 hours and 6.7 minutes. Neptune was
discovered on September 23, 1846 by [29]Johann Gottfried Galle, of the
Berlin Observatory, and Louis d'Arrest, an astronomy student, through
mathematical predictions made by Urbain Jean Joseph Le Verrier.

The first two thirds of Neptune is composed of a mixture of molten
rock, water, liquid ammonia and methane. The outer third is a mixture
of heated gases comprised of hydrogen, helium, water and methane.
Methane gives Neptune its blue cloud color.

Neptune is a dynamic planet with several large, dark spots reminiscent
of [30]Jupiter's hurricane-like storms. The largest spot, known as the
[31]Great Dark Spot, is about the size of the earth and is similar to
the [32]Great Red Spot on Jupiter. Voyager revealed a small,
irregularly shaped, eastward-moving cloud _scooting_ around Neptune
every 16 hours or so. This _scooter_ as it has been dubbed could be a
plume rising above a deeper cloud deck.

Long bright clouds, similar to cirrus clouds on Earth, were seen high
in Neptune's atmosphere. At low northern latitudes, Voyager captured
images of cloud streaks casting their shadows on cloud decks below.

The strongest winds on any planet were measured on Neptune. Most of
the winds there blow westward, opposite to the rotation of the planet.
Near the Great Dark Spot, winds blow up to 2,000 kilometers (1,200
miles) an hour.

Neptune has a set of four rings which are narrow and very faint. The
rings are made up of dust particles thought to have been made by tiny
meteorites smashing into Neptune's moons. From ground based telescopes
the rings appear to be arcs but from Voyager 2 the arcs turned out to
be bright spots or clumps in the ring system. The exact cause of the
bright clumps is unknown.

The magnetic field of Neptune, like that of [33]Uranus, is highly
tilted at 47 degrees from the rotation axis and offset at least 0.55
radii (about 13,500 kilometers or 8,500 miles) from the physical
center. Comparing the magnetic fields of the two planets, scientists
think the extreme orientation may be characteristic of flows in the
interior of the planet and not the result of that planet's sideways
orientation or of any possible field reversals at either planet.

Neptune Statistics
Discovered by Johann Gotfried Galle
Date of discovery September 23, 1846
Mass (kg) 1.024e+26
Mass (Earth = 1) 1.7135e+01
Equatorial radius (km) 24,746
Equatorial radius (Earth = 1) 3.8799e+00
Mean density (gm/cm^3) 1.64
Mean distance from the Sun (km) 4,504,300,000
Mean distance from the Sun (Earth = 1) 30.0611
Rotational period (hours) 16.11
Orbital period (years) 164.79
Mean orbital velocity (km/sec) 5.45
Orbital eccentricity 0.0097
Tilt of axis (degrees) 28.31
Orbital inclination (degrees) 1.774
Equatorial surface gravity (m/sec^2) 11.0
Equatorial escape velocity (km/sec) 23.50
Visual geometric albedo 0.41
Magnitude (Vo) 7.84
Mean cloud temperature -193 to -153°C
Atmospheric pressure (bars) 1-3
Atmospheric composition
Hydrogen
Helium
Methane
85%
13%
2%

Animations of Neptune

* [34]Neptune in Rotation.
* [35]Neptune's Dark Spot.
* [36]Hubble Neptune Rotation Movie.

Views of Neptune

_Neptune_
This picture of Neptune was taken by [37]Voyager 2 on August 20, 1989.
One of the great cloud features, dubbed the _Great Dark Spot_ by
Voyager scientists, can be seen toward the center of the image. It is
at a latitude of 22 degrees south and circuits Neptune every 18.3
hours. The bright clouds to the south and east of the Great Dark Spot
constantly change their appearances in periods as short as four hours.
_(Copyright Calvin J. Hamilton)_
_The Interior of Neptune_
Our knowledge of the internal structure of Neptune is inferred from
the planet's radius, mass, period of rotation, the shape of its
gravitational field and the behavior of hydrogen, helium, and water at
high pressure. This cut-away view shows Neptune composed of an outer
envelope of molecular hydrogen, helium and methane roughly the mass of
one to two Earths. Below this region Neptune appears to be composed of
a mantle rich in water, methane, ammonia, and other elements. These
elements are under high temperatures and pressures deep within the
planet. The mantle is equivalent to 10 to 15 earth masses. Neptune's
core is composed of rock and ice, and is likely no more than one Earth
mass. _(Copyright Calvin J. Hamilton)_
HST Observes Triton and Neptune
This mosaic combines an almost true-color picture of Neptune taken by
the Hubble Space Telescope's (HST) Wide Field Planetary Camera (WFPC),
with a picture of Triton taken with the HST's Faint Object Camera.
Although faint, the image of Triton vaguely shows a brighter
equatorial region. The south pole is to the lower left.

The picture of Neptune shows a bright cloud feature at the south pole,
near the bottom of the image. Bright cloud bands can be seen at 30S
and 60S latitude. The northern hemisphere also includes a bright cloud
band centered near 30° N latitude. _(Courtesy of Ted Stryk)_
HST Observations of Neptune
These almost true-color pictures of Neptune were constructed from
HST/WFPC2 images taken in blue (467-nm), green (588-nm), and red
(673-nm) spectral filters. There is a bright cloud feature at the
south pole, near the bottom right of the image. Bright cloud bands can
be seen at 30S and 60S latitude. The northern hemisphere also includes
a bright cloud band centered near 30° N latitude. The second picture
was compiled from images taken after the planet had rotated about 180
degrees of longitude (about 9 hours later) to show the opposite
hemisphere.

One feature that is conspicuous by its absence is the storm system
known as the [38]Great Dark Spot. The second [39]smaller dark spot,
DS2, that was seen during the Voyager-2 encounter was also missing.
The absence of these dark spots was one of the biggest surprises of
this program. These dramatic changes in the large-scale storm systems
and planet-encircling clouds bands on Neptune are not yet completely
understood, but they emphasize the dynamic nature of this planet's
atmosphere, and the need for further monitoring.
HST Observes High Altitude Clouds
These three images were taken on October 10, October 18 and November
2, 1994, when Neptune was 4.5 billion kilometers from Earth. Building
on Voyager's initial discoveries, Hubble has revealed that Neptune has
a remarkably dynamic atmosphere that changes over just a few days. The
temperature difference between Neptune's strong internal heat source
and its frigid cloud tops (-162° Celcius or -260° Fahrenheit) might
trigger instabilities in the atmosphere that drive these large-scale
weather changes. The pink features are high-altitude methane ice
crystal clouds.
_HST Finds New Dark Spot_
In June 1994, the Hubble telescope revealed that the [40]Great Dark
Spot found by Voyager 2 was missing. This new image taken on November
2, shows that a new spot near the limb of the planet has formed. Like
its predecessor, the new spot has high altitude clouds along its edge,
caused by gasses that have been pushed to higher altitudes where they
cool to form methane ice crystal clouds. The dark spot may be a zone
of clear gas that is a window to a cloud deck lower in the atmosphere.
_Cirrus-like Clouds_
This image shows bands of sunlit cirrus-like clouds in Neptune's
northern hemisphere. These clouds cast shadows on the blue cloud deck
35 miles below. The white streaky clouds are from 48 to 160 kilometers
(30 to 100 miles) wide and extend for thousands of miles. _(Copyright
Calvin J. Hamilton)_
_True-color Image_
This Voyager 2 image has been processed by computers so that both the
clouds' structure in the dark regions near the pole and the bright
clouds east of the Great Dark Spot are visible. Small trails of clouds
trending east to west and large-scale structure east of the Great Dark
Spot all suggest that waves are present in the atmosphere and play a
large role in the type of clouds that are visible. _(Courtesy
NASA/JPL)_
Great Dark Spot
Feathery white clouds fill the boundary between the dark and light
blue regions on the Great Dark Spot. The pinwheel shape of both the
dark boundary and the white cirrus suggests that the storm system
rotates counterclockwise. Periodic small scale patterns in the white
cloud, possibly waves, are short lived and do not persist from one
Neptunian rotation to the next. _(Courtesy NASA/JPL)_
Change In The Great Dark Spot
The bright cirrus-like clouds of Neptune change rapidly, often forming
and dissipating over periods of several to tens of hours. In this
sequence spanning two rotations of Neptune (about 36 hours) Voyager 2
observed cloud evolution in the region around the Great Dark Spot at
an effective resolution of about 100 kilometers (62 miles) per pixel.
The surprisingly rapid changes which occur over the 18 hours
separating each panel shows that in this region Neptune's weather is
perhaps as dynamic and variable as that of the Earth. However, the
scale is immense by our standards. The [41]Earth and the Great Dark
Spot are of similar size and in Neptune's frigid atmosphere, where
temperatures are as low as 55 degrees Kelvin (-360 F), the cirrus
clouds are composed of frozen methane rather than Earth's crystals of
water ice. _(Courtesy NASA/JPL)_
_High Resolution Crescent Image of Neptune_
This high resolution crescent image of Neptune was taken by the
[42]Voyager 2 spacecraft on on August 31, 1989. A circular storm
pattern can be seen on the limb of the planet. _(Copyright 2001 by
Calvin J. Hamilton)_
_Parting Look_
This Voyager 2 image shows a beautiful dual-crescent view of Neptune
and [43]Triton. The image, acquired on August 31, 1989, is a parting
tribute of the Voyager mission. _(Copyright Calvin J. Hamilton)_
Small Dark Spot
This image shows the _Small Dark Spot_, which is south of the _Great
Dark Spot_. The small spot is thought to be a storm in Neptune's
atmosphere, perhaps similar to Jupiter's Great Red Spot. _(Copyright
Calvin J. Hamilton)_
_Neptune Rings_
These two 591-second exposures of the rings of Neptune were taken by
Voyager 2 on August 26, 1989 from a distance of 280,000 kilometers
(174,000 miles). The two main rings are clearly visible and appear
complete over the region imaged. Also visible in this image is the
inner faint ring at about 42,000 kilometers (25,000 miles) from the
center of Neptune, and the faint band which extends smoothly from the
53,000 kilometer (33,000 miles) ring to roughly halfway between the
two bright rings. The bright glare in the center is due to
over-exposure of the crescent of Neptune. Numerous bright stars are
evident in the background. Both rings are continuous. _(Courtesy
NASA/JPL)_
_Twisted Rings_
This portion of one of Neptune's rings appears to be twisted.
Scientists believe it looks this way because the original material in
the rings was in clumps that formed streaks as the material orbited
Neptune. The motion of the spacecraft added to the twisted appearance
by causing a slight smearing in the image. _(Courtesy NASA/JPL)_

Rings of Neptune

The following table is a summary of the rings of Neptune.

Name Distance* Width Thickness Mass Albedo
1989N3R 41,900 km 15 km ? ? low
1989N2R 53,200 km 15 km ? ? low
1989N4R 53,200 km 5,800 km ? ? low
1989N1R 62,930 km < 50 km ? ? low

*The distance is measured from the planet center to the start of the
ring.

Neptune Moon Summary

The following table summarizes the radius, mass, distance from the
planet center, discoverer and the date of discovery of each of the
moons of Neptune:

Moon # Radius
(km) Mass
(kg) Distance
(km) Discoverer Date
[44]Naiad III 29 ? 48,000 [45]Voyager 2 1989
[46]Thalassa IV 40 ? 50,000 Voyager 2 1989
[47]Despina V 74 ? 52,500 Voyager 2 1989
[48]Galatea VI 79 ? 62,000 Voyager 2 1989
[49]Larissa VII 104x89 ? 73,600 Voyager 2 1989
[50]Proteus VIII 200 ? 117,600 Voyager 2 1989
[51]Triton I 1,350 2.14e+22 354,800 [52]W. Lassell 1846
[53]Nereid II 170 ? 5,513,400 [54]G. Kuiper 1949

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_Copyright © 1997-2001 by [58]Calvin J. Hamilton. All rights reserved._
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