Saturn Back
Neptune
orbit: 2,870,990,000 km (19.218 AU) from Sun
diameter: 51,118 km (equatorial)
mass: 8.683e25 kg
Careful pronounciation may be necessary to avoid embarassment; say "YOOR
a nus" , not "your anus" or
"urine us".
Uranus is the ancient Greek deity of the Heavens, the earliest supreme
god, who was the father of Cronus
(Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).
Uranus, the first planet discovered in modern times, was discovered by
accident by William Herschel while
searching the sky with a telescope on March 13, 1781; he first thought
that it was a comet. It had actually
been seen many times before but ignored as simply another star (the earliest
recorded sighting was in 1690
when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium
Sidus" (the Georgian
Planet) in honor of his patron, the infamous (to Americans) King George
III of England; others called it
"Herschel". The name "Uranus" was first proposed by Bode in conformity
with the other planetary names
from classical mythology but didn't come into common use until 1850.
Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.
Most of the planets spin on an axis nearly perpendicular to the plane of
the ecliptic but Uranus' axis is
almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus'
south pole was pointed almost
directly at the Sun. This results in the odd fact that Uranus' polar regions
receive more energy input from the
Sun than do its equatorial regions. Uranus is nevertheless hotter at its
equator than at its poles. The
mechanism underlying this is unknown.
Actually, there's an ongoing battle over which of Uranus' poles is its
north pole! Either its axial inclination is
a bit over 90 degrees and its rotation is direct, or it's a bit less than
90 degrees and the rotation is
retrograde. The problem is that you need to draw a dividing line *somewhere*,
because in a case like Venus
there is little dispute that the rotation is indeed retrograde (not a direct
rotation with an inclination of nearly
180).
Uranus is composed primarily of rock and various ices, with only about
15% hydrogen and a little helium (in
contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and
Neptune) are in many ways similar to
the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen
envelope. It appears that Uranus
does not have a rocky core like Jupiter and Saturn but rather that its
material is more or less uniformly
distributed.
Uranus' atmosphere is abut 83% hydrogen, 15% helium and 2% methane.
Like the other gas planets, Uranus has bands of clouds that blow around
rapidly. But they are extremely faint,
visible only with radical image enhancement of the Voyager 2 pictures.
Recent observations with HST show
larger and more pronounced streaks. The speculation is that the difference
is due to seasonal effects (the
Sun is now at a somewhat lower Uranian latitude which may cause more pronounced
day/night effects).
Uranus' blue color is the result of absorption of red light by methane
in the upper atmosphere. There may be
colored bands like Jupiter's but they are hidden from view by the overlaying
methane layer.
Like the other gas planets, Uranus has rings. Like Jupiter's, they are
very dark but like Saturn's composed of
fairly large particles ranging up to 10 meters in diameter in addition
to fine dust. There are 11 known rings,
all very faint; the brightest is known as the Epsilon ring. The Uranian
rings were the first after Saturn's to be
discovered. This was of considerable importance since we now know that
rings are a common feature of
planets, not a peculiarity of Saturn alone.
Voyager 2 discovered 10 small moons in addition to the 5 large ones already
known. It is likely that there are
several more tiny satellites within the rings.
Uranus' magnetic field is odd in that it is not centered on the center
of the planet and is tilted almost 60
degrees with respect to the axis of rotation. It is probably generated
by motion at relatively shallow depths
within Uranus.
Uranus is sometimes just barely visible with the naked eye on a very clear
night; it is fairly easy to spot with
binoculars (if you know exactly where to look). A small astronomical telescope
will show a small disk.
Planet Profile
Mass (kg)
86.83 x (10^24)
Volume (km3)
6,833 x (10^10)
Radius (1 bar level) (km)
Equatorial
25,559
Polar
24,973
Volumetric mean radius (km)
25,362
Ellipticity
0.023
Mean density (kg/m^3)
1,318
Gravity (eq., 1 bar) (m/s^2)
8.69
Escape velocity (m/s)
21,300
GM (km^3/s^2)
5.794 x (10^6)
Bond albedo
0.90
Visual geometric albedo
0.51
Visual magnitude V(1,0)
-7.19
Solar irradiance (W/m^2)
3.71
Black-body temperature (K)
35.9
Moment of inertia (I/MR^2)
0.225
J2
3343.43 x (10^-6)
Orbital
parameters
Semimajor axis (km)
2,869.6 x (10^6)
Sidereal orbit period (days)
30,685.4
Tropical orbit period (days)
30,588.740
Perihelion (km)
2,734.0 x (10^6)
Aphelion (km)
3,005.2 x (10^6)
Synodic period (days)
369.66
Mean orbital velocity (km/s)
5.48
Orbit inclination (deg)
0.773
Orbit eccentricity
0.04724
Sidereal rotation period (hours)
17.24*
Obliquity to orbit (deg)
97.86
* Magnetic coordinates (as determined by the Voyager 2 Radio Science experiment)
Uranian Magnetosphere
Dipole field strength: 0.228 gauss-Ru3
Dipole tilt to rotational axis: 58.6 degrees
Dipole offset (planet center to dipole center) distance: 0.3 Ru along the
rotation axis
Note: Ru denotes Uranian radii, here defined to be 25,600 km
Uranian Atmosphere
Surface Pressure: >>100 bars
Average temperature: ~58 K
Temperature at 1 bar: ~76 K
Density at 1 bar: ~0.42 kg/m3
Wind speeds: 0-200 m/s
Scale height: 27.7 km
Mean molecular weight: 2.64 g/mole
Atmospheric composition
Major: Molecular hydrogen (H2) - 89%;
Helium (He) - 11%
Minor (ppm): Methane (CH4)
Aerosols: Ammonia ice, water ice, ammonia hydrosulfide,
methane ice(?)
Uranus has 15 known moons.
Unlike the other bodies in the solar system which have names from classical
mythology, Uranus' moons take
their names from the writings of Shakespeare and Pope. They form two distinct
classes: the 10 small very
dark inner ones discovered by Voyager 2 and the 5 large outer ones. They
all have nearly circular orbits in
the plane of Uranus' equator (and hence at a large angle to the plane of
the ecliptic).
| Satellites | Distance
(km) |
Radius
(km) |
Mass
(kg) |
Discoverer | Date |
|---|---|---|---|---|---|
| Cordelia | 50,000 | 13 | N/A | Voyager 2 | 1986 |
| Ophelia | 54,000 | 16 | N/A | Voyager 2 | 1986 |
| Bianca | 58,000 | 22 | N/A | Voyager 2 | 1986 |
| Cressida | 62,000 | 33 | N/A | Voyager 2 | 1986 |
| Desdemona | 63,000 | 29 | N/A | Voyager 2 | 1986 |
| Juliet | 64,000 | 42 | N/A | Voyager 2 | 1986 |
| Portia | 66,000 | 55 | N/A | Voyager 2 | 1986 |
| Rosalind | 70,000 | 27 | N/A | Voyeger 2 | 1986 |
| Belinda | 75,000 | 34 | N/A | Voyager 2 | 1986 |
| Puck | 86,000 | 77 | N/A | Voyager 2 | 1986 |
| Miranda | 130,000 | 236 | 6.30e^19 | Kuiper | 1948 |
| Ariel | 191,000 | 579 | 1.27e^21 | Lassel | 1851 |
| Umbriel | 266,000 | 585 | 1.27e^21 | Lassel | 1851 |
| Titania | 436,000 | 789 | 3.49e^21 | Herschel | 1787 |
| Oberon | 583,000 | 761 | 3.03e^21 | Herschel | 1787 |
Uranus'
Rings
| Ring | Distance
(km) |
Width
(km) |
Albedo | Eccentricity |
|---|---|---|---|---|
| 1986U2R | 38,000 | 2,500 | N/A | N/A |
| 6 | 41,840 | 1-3 | ~15 x (10^-3) | 0.0010 |
| 5 | 42,230 | 2-3 | ~15 x (10^-3) | 0.0019 |
| 4 | 42,580 | 2-3 | ~15 x (10^-3) | 0.0010 |
| Alpha | 44,720 | 7-12 | ~15 x (10^-3) | 0.0008 |
| Beta | 45,670 | 7-12 | ~15 x (10^-3) | 0.004 |
| Eta | 47,190 | 0-2 | ~15 x (10^-3) | N/A |
| Gamma | 47,630 | 1-4 | ~15 x (10^-3) | 0.0001 |
| Delta | 48,290 | 3-9 | ~15 x (10^-3) | N/A |
| 1986U1R | 50,020 | 1-2 | N/A | N/A |
| Epsilon | 51,140 | 20-100 | ~15 x (10^-3) | 0.0079 |
Sun
Venus
Mars
Saturn
Neptune