URANUS

NASA image of Uranus from Voyager 2     Uranus is the third "gas giant" planet of four in proximity to the Sun.  Overall, Uranus is the seventh planet in our solar system, residing at an average distance of 1.8 billion miles from the Sun (twice as far as Saturn).  Its rotation is quick, at about 18 hours, enough to flatten the planet at its poles (a characteristic of all gas giants).  It takes 84 years for Uranus to revolve around the Sun and the aqua-colored planet has 27 known satellites.  Six of these satellite discoveries have been since 1997.   

Atmosphere and Weather:   For many years, it was generally understood that Uranus was a bland planet.  However, recent analysis from the Hubble Space Telescope shows Uranus to be a weather dynamic planet, characterized by huge storms (some covering half or more the United States in size), banded cloud cover and the brightest cloud tops in the outer solar system.  In late 2006, the Hubble telescope discovered a dark cloud on Uranus.  This storm was two thirds the size of the United States.  Clouds enshroud the planet with a latitudinal banding like Jupiter and Saturn, although much more faint.  The average temperature of the clouds is -315 degrees F and ammonia and methane ice crystals make up the cloud composition.  It has been recently found that windHubble Space Telescope Image of Uranus speeds on Uranus range from 90 to 360 mph and the planet's average temperature is a frigid -353 degrees F.  The coldest temperature found in Uranus' lower atmosphere so far is -371 degrees F., which rivals Neptune's frigid temperatures.  Findings from Hubble reveal that clouds circle Uranus at over 300 mph.  Shown at right, this near infrared picture taken by the Hubble Telescope reveals the bright cloud tops on the left side of the disk.  Like the other gas giants, Uranus has an atmosphere composed of mostly hydrogen, followed by helium and a little methane.    

What is very interesting about Uranus is that the planet has an enormous 98 degree tilt on its axis.  Basically, it is laying on its side with the poles receiving the direct sunlight.  This makes for extreme seasons and when the Sun rises at one of the poles, that pole will receive direct sunlight for 42 years.  Therefore seasonal variations are immense, in that as the dark side of the planet comes out of its 40 plus year slumber, the frozen atmosphere heats up dramatically causing violent storms.  Curiously though, Uranus is still warmer at its equator than the poles, even though the poles receive the direct sunlight with a very low sun angle over the equatorial region.  It is not well understood why.  In addition, unlike the other gas giants, Uranus does not radiate more heat than it receives.  This suggests that the planet may have a cold interior, lacking an internal heat source. 

A side note:  Uranus has a very narrow, complex ring system that appears to be fragile in that it wobbles.  The other gas giants do not have "wobbly" rings.  Uranus is the last planet in our solar system which can be seen by the naked eye.  However one has to have an extremely dark sky and good eye sight to spot Uranus without the aid of binoculars.  Uranus shines at an average magnitude of +5.5 which is beginning to push the limits of what the human eye can detect.  Looking in the proper location, Uranus can be easily found through a pair of standard binoculars.

QUICK FACTS
(Data is from NASA Goddard)

Average distance from Sun 1.8 billion miles
Perihelion 1.7 billion miles 
Aphelion 1.86 billion miles
Sidereal Rotation 17.24 Earth hours
Length of Day 17.24 Earth hours
Sidereal Revolution 84 Earth years
Diameter at Equator 31,693 miles
Tilt of axis 97.77 degrees
Moons 27 known
Atmosphere Hydrogen (82.5%), Helium (15%), Methane (2.3%) 
Discoverer William Herschel
Discovery Date March 13, 1781

DEFINITIONS:

Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  Earth's sidereal rotation is 23 hours, 57 minutes.
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  Earth's length of day = 24 hours
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a body's orbital plane is perfectly horizontal, the axis tilt is the amount of tilt of the body's equator relative to the body's orbital plane.  Earth is tilted an average of 23.45 degrees on its axis.


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.