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Uranus
Uranus is
the seventh planet from the Sun and is the third
largest in the solar system. It was discovered by
William Herschel in 1781. It has an equatorial
diameter of 51,800 kilometers (32,190 miles) and
orbits the Sun once every 84.01 Earth years. It has a
mean distance from the Sun of 2.87 billion kilometers
(1.78 billion miles). The length of a day on Uranus
is 17 hours 14 minutes. Uranus has at least 15 moons.
The two largest moons, Titania and Oberon, were
discovered by William Herschel in 1787.
The atmosphere of Uranus
is composed of 83% hydrogen, 15% helium, 2% methane
and small amounts of acetylene and other
hydrocarbons. Methane in the upper atmosphere absorbs
red light, giving Uranus its blue-green color. The
atmosphere is arranged into clouds running at
constant latitudes, similar to the orientation of the
more vivid latitudinal bands seen on Jupiter and
Saturn. Winds at mid-latitudes on Uranus blow in the
direction of the planet's rotation. These winds blow
at velocities of 40 to 160 meters per second (90 to
360 miles per hour). Radio science experiments found
winds of about 100 meters per second blowing in the
opposite direction at the equator.
Uranus is distinguished by
the fact that it is tipped on its side. Its unusual
position is thought to be the result of a collision
with a planet-sized body early in the solar system's
history. Voyager 2 found that one of the most
striking influences of this sideways position is its
effect on the tail of the magnetic field, which is
itself tilted 60 degrees from the planet's axis of
rotation. The magnetotail was shown to be twisted by
the planet's rotation into a long corkscrew shape
behind the planet. The magnetic field source is
unknown; the electrically conductive,
super-pressurized ocean of water and ammonia once
thought to lie between the core and the atmosphere
now appears to be nonexistent. The magnetic fields of
Earth and other planets are believed to arise from
electrical currents produced in their molten
cores.
Uranus'
Rings In 1977, the first nine rings of Uranus were
discovered. During the Voyager encounters, these
rings were photographed and measured, as were two
other new rings and ringlets. Uranus' rings are
distinctly different from those at Jupiter and
Saturn. The outermost epsilon ring is composed
mostly of ice boulders several feet across. A very
tenuous distribution of fine dust also seems to be
spread throughout the ring system.
There may be a large
number of narrow rings, or possibly incomplete rings
or ring arcs, as small as 50 meters (160 feet) in
width. The individual ring particles were found to be
of low reflectivity. At least one ring, the epsilon,
was found to be gray in color. The moons Cordelia and
Ophelia act as shepherd satellites for the epsilon
ring.
| Uranus
Statistics |
| Discovered by |
William
Herschel |
| Date
of discovery |
1781 |
| Mass
(kg) |
8.686e+25 |
| Mass
(Earth = 1) |
1.4535e+01 |
| Equatorial radius
(km) |
25,559 |
| Equatorial radius (Earth =
1) |
4.0074 |
| Mean
density (gm/cm^3) |
1.29 |
| Mean
distance from the Sun (km) |
2,870,990,000 |
| Mean
distance from the Sun (Earth = 1) |
19.1914 |
| Rotational period
(hours) |
-17.9 |
| Orbital period
(years) |
84.01 |
| Mean
orbital velocity (km/sec) |
6.81 |
| Orbital eccentricity |
0.0461 |
| Tilt
of axis (degrees) |
97.86 |
| Orbital inclination
(degrees) |
0.774 |
| Equatorial surface gravity
(m/sec^2) |
7.77 |
| Equatorial escape velocity
(km/sec) |
21.30 |
| Visual geometric
albedo |
0.51 |
| Magnitude (Vo) |
5.52 |
| Mean
cloud temperature |
-193°C |
| Atmospheric pressure
(bars) |
1.2 |
Atmospheric
composition
- Hydrogen
- Helium
- Methane
|
83%
15%
2% |
Neptune
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
Voyager. A day on Neptune is 16 hours and 6.7
minutes. Neptune was discovered on September 23, 1846
by 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
Jupiter's hurricane-like storms. The largest spot,
known as the Great Dark Spot, is about the
size of the earth and is similar to the 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 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% |
Pluto
Although
Pluto was discovered in 1930, limited information on
the distant planet delayed a realistic understanding
of its characteristics. Today Pluto remains the only
planet that has not been visited by a spacecraft, yet
an increasing amount of information is unfolding
about this peculiar planet. The uniqueness of Pluto's
orbit, rotational relationship with its satellite,
spin axis, and light variations all give the planet a
certain appeal.
Pluto is usually farther
from the Sun than any of the nine planets; however,
due to the eccentricity of its orbit, it is closer
than Neptune for 20 years out of its 249 year orbit.
Pluto crossed Neptune's orbit January 21, 1979, made
its closest approach September 5, 1989, and will
remain within the orbit of Neptune until February 11,
1999. This will not occur again until September
2226.
As Pluto approaches
perihelion it reaches its maximum distance from the
ecliptic due to its 17-degree inclination. Thus, it
is far above or below the plane of Neptune's orbit.
Under these conditions, Pluto and Neptune will not
collide and do not approach closer than 18 A.U. to
one another.
Pluto's rotation period is
6.387 days, the same as its satellite Charon.
Although it is common for a satellite to travel in a
synchronous orbit with its planet, Pluto is the only
planet to rotate synchronously with the orbit of its
satellite. Thus being tidally locked, Pluto and
Charon continuously face each other as they travel
through space.
Unlike most planets, but
similar to Uranus, Pluto rotates with its poles
almost in its orbital plane. Pluto's rotational axis
is tipped 122 degrees. When Pluto was first
discovered, its relatively bright south polar region
was the view seen from the Earth. Pluto appeared to
grow dim as our viewpoint gradually shifted from
nearly pole-on in 1954 to nearly equator-on in 1973.
Pluto's equator is now the view seen from Earth.
During the period from
1985 through 1990, Earth was aligned with the orbit
of Charon around Pluto such that an eclipse could be
observed every Pluto day. This provided opportunity
to collect significant data which led to albedo maps
defining surface reflectivity, and to the first
accurate determination of the sizes of Pluto and
Charon, including all the numbers that could be
calculated therefrom.
The first eclipses (mutual
events) began blocking the north polar region. Later
eclipses blocked the equatorial region, and final
eclipses blocked Pluto's south polar region. By
carefully measuring the brightness over time, it was
possible to determine surface features. It was found
that Pluto has a highly reflective south polar cap, a
dimmer north polar cap, and both bright and dark
features in the equatorial region. Pluto's geometric
albedo is 0.49 to 0.66, which is much brighter than
Charon. Charon's albedo ranges from 0.36 to 0.39.
The eclipses lasted as
much as four hours and by carefully timing their
beginning and ending, measurements for their
diameters were taken. The diameters can also be
measured directly to within about 1 percent by more
recent images provided by the Hubble Space Telescope.
These images resolve the objects to clearly show two
separate disks. The improved optics allow us to
measure Pluto's diameter as 2,274 kilometers (1413
miles) and Charon's diameter as 1,172 kilometers (728
miles), just over half the size of Pluto. Their
average separation is 19,640 km (12,200 miles).
That's roughly eight Pluto diameters.
Average separation and
orbital period are used to calculate Pluto and
Charon's masses. Pluto's mass is about 6.4 x
10-9 solar masses. This is close to 7 (was
12 x's) times the mass of Charon and approximately
0.0021 Earth mass, or a fifth of our moon.
Pluto's average density
lies between 1.8 and 2.1 grams per cubic centimeter.
It is concluded that Pluto is 50% to 75% rock mixed
with ices. Charon's density is 1.2 to 1.3
g/cm3, indicating it contains little rock.
The differences in density tell us that Pluto and
Charon formed independently, although Charon's
numbers derived from HST data are still being
challenged by ground based observations. Pluto and
Charon's origin remains in the realm of theory.
Pluto's icy surface is 98%
nitrogen (N2). Methane (CH4) and traces of carbon
monoxide (CO) are also present. The solid methane
indicates that Pluto is colder than 70 Kelvin.
Pluto's temperature varies widely during the course
of its orbit since Pluto can be as close to the sun
as 30 AU and as far away as 50 AU. There is a thin
atmosphere that freezes and falls to the surface as
the planet moves away from the Sun. NASA plans to
launch a spacecraft, the Pluto Express, in 2001 that
will allow scientists to study the planet before its
atmosphere freezes. The atmospheric pressure deduced
for Pluto's surface is 1/100,000 that of Earth's
surface pressure.
Pluto was officially
labeled the ninth planet by the International
Astronomical Union in 1930 and named for the Roman
god of the underworld. It was the first and only
planet to be discovered by an American, Clyde W.
Tombaugh.
The path toward its
discovery is credited to Percival Lowell who founded
the Lowell Observatory in Flagstaff, Arizona and
funded three separate searches for "Planet X." Lowell
made numerous unsuccessful calculations to find it,
believing it could be detected from the effect it
would have on Neptune's orbit. Dr. Vesto Slipher, the
observatory director, hired Clyde Tombaugh for the
third search and Clyde took sets of photographs of
the plane of the solar system (ecliptic) one to two
weeks apart and looked for anything that shifted
against the backdrop of stars. This systematic
approach was successful and Pluto was discovered by
this young (born 4 Feb 1906) 24 year old Kansas lab
assistant on February 18, 1930. Pluto is actually too
small to be the "Planet X" Percival Lowell had hoped
to find. Pluto's was a serendipitous discovery.
| Pluto Statistics |
| Discovered by |
Clyde W. Tombaugh |
| Date of discovery |
February 18, 1930 |
| Mass (kg) |
1.27e+22 |
| Mass (Earth = 1) |
2.125e-03 |
| Equatorial radius
(km) |
1,137 |
| Equatorial radius (Earth =
1) |
0.1783 |
| Mean density
(gm/cm^3) |
2.05 |
| Mean distance from the Sun
(km) |
5,913,520,000 |
| Mean distance from the Sun (Earth
= 1)) |
39.5294 |
| Rotational period
(days) |
-6.3872 |
| Orbital period
(years) |
248.54 |
| Mean orbital velocity
(km/sec) |
4.74 |
| Orbital eccentricity |
0.2482 |
| Tilt of axis
(degrees) |
122.52 |
| Orbital inclination
(degrees) |
17.148 |
| Equatorial surface gravity
(m/sec^2) |
0.4 |
| Equatorial escape velocity
(km/sec) |
1.22 |
| Visual geometric
albedo |
0.3 |
| Magnitude (Vo) |
15.12 |
Atmospheric
composition
- Methane
- Nitrogen
|
0.3 |
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