Uranus

Uranus
This picture of Uranus was compiled from images recorded by Voyager 2 on January 10, 1986, when the NASA spacecraft was 18 million kilometers (11 million miles) from the planet.
Symbol
Date of discovery	March 13, 1781
Name of discoverer	William Herschel
Name origin	Roman (and Greek) name for heaven and father of Saturn.
Orbital characteristics
Primary	Sun
Order from primary	8
Perihelion	18.286 AU
Aphelion	20.096 AU
Semi-major axis	19.191 AU
Titius-Bode prediction	19.6 AU
Circumference	120.515 AU
Orbital eccentricity	0.046
Sidereal year	84.0016846 a
Synodic year	369.65 da
Avg. orbital speed	6.8352 km/s
Inclination	0.744° to the ecliptic
Rotational characteristics
Sidereal day	-17.24 h
Axial tilt	97.86°
Physical characteristics
Mass	8.6832 * 1025 kg (14.53 * earth)
Density	1,318 kg/m³
Mean radius	25,362 km
Equatorial radius	25,559 km
Polar radius	24,973 km
Surface gravity	8.69 m/s²
Escape speed	21.30 km/s
Surface area	8,084,000,000 km²
Mean temperature	76 K
Number of moons	27
Composition	83% hydrogen, 15% helium and 2% methane
Color	blue-green
Albedo	0.51

Uranus is the seventh planet from the sun, and the third largest. It is named after the father of Greek (and Roman) gods, who also happens to be the god of the sky. Uranus is a gas giant, but despite its great size compared to the terrestrial planets, it was not discovered until 1781 by William Herschel, who used a telescope.

Orbital characteristics

Uranus' average distance from the sun (semi-major axis) is 19.1 AU, which is close to the 19.6 AU predicted by the Titius-Bode law. Its sidereal year is 84.02 Earth years. The orbital excentricity is 0.046, which is nearly circular (though the orbit of Earth is more circular still). Uranus' orbit is very slightly inclined to the plane of the ecliptic; in fact its orbital inclination with respect to the ecliptic is the least of those of all the planets.

Rotational characteristics

Uranus' sidereal day is 17.24 Earth hours. Uranus' rotation is retrograde with respect to its orbit around the Sun and the orbits of most of the other planets. Moreover its axis is tilted 97.86° with respect to its orbital plane, so that in a sense the planet is rolling on its side.

The planet has a magnetic field, whose axis is oriented 60° away from its rotational axis and, furthermore, does not intercept the planet's center.

Physical characteristics

The planet’s radius is 25,559 km (15,882 miles). Uranus’s atmosphere is about 83 % hydrogen, 15% helium, and 2% methane. The methane, more than any other constituent, gives Uranus its color. Uranus' atmosphere is relatively featureless; even Voyager 2 did not resolve many more features than can be seen through an earthbound telescope.

Difficulties for naturalistic theories

Formation of the planet

The nebular hypothesis would not even have allowed Uranus and its twin, Neptune, to form at its tremendous distance from the Sun in the time generally allotted. As the journal Astronomy stated the problem:

“

Pssst … astronomers who model the formation of the solar system have kept a dirty little secret: Uranus and Neptune don’t exist. Or at least computer simulations have never explained how planets as big as the two gas giants could form so far from the sun. Bodies orbited so slowly in the outer parts of the sun’s protoplanetary disk that the slow process of gravitational accretion would need more time than the age of the solar system to form bodies with 14.5 and 17.1 times the mass of Earth.

”

Of course, the time allotted for the formation of the solar system is generally taken to be the alloted age of the Earth. In theory, the solar system could be older than the earth itself, but not much older. The solar system has other bodies in it that are much "younger." Nor does the above account for the body of evidence that the solar system is in fact quite young.

Orientation of the Uranian system

According to evolutionary theories of how the Solar System formed (i.e. the nebular hypothesis), Uranus' axis should not be tilted 98 degrees with respect to its orbit. Moreover its moons orbit Uranus around its equator, like the hands of a clock around a clock face. Uranus' ten rings also are around its equator, so they look vertical. The nebular hypothesis predicts none of these observations.

Naturalistic astronomers theorize that Uranus originally formed upright, but suffered a collision with another object, producing its severe tilt. This, however, raises more questions:

1. Why is its orbit less eccentric than those of all the other planets except only those of Venus, Neptune, and Earth?
2. Why is Uranus' orbit less inclined to the ecliptic than is that of any planet except Earth?
3. Why are the orbits of Uranus' moons so slightly inclined with respect to Uranus' present equator? Indeed, the vertical orientation of Uranus' moons and of some of its rings was already well-known to astronomers before the Voyager 2 flyby. If the moons formed before the collision, then another series of precisely timed collisions would have been required to change the azimuths of all the orbits, and the rings—an absurd violation of Occam's razor. And if they formed after the collision, then in the absence of any condensing gas cloud, they ought to be inclined at random.

Naturalists try to explain that last by suggesting that Uranus' moons are leftover debris from the collision that tilted its axis. However, Uranus' moons make up only one hundredth of one percent of the mass of the planet, and an impact violent enough to knock the giant over would have produced far more debris than that. (Earth's Moon, also supposed to be the result of a giant collision involving its primary, has a mass of 1.23% that of earth.) Moreover, not only are the moons of Uranus oriented well to Uranus' equator, but many of them orbit retrograde to Uranus' own rotation.

The magnetic field

As mentioned, Uranus' magnetic axis is 60 degrees displaced from Uranus' axis of rotation, and the axis also does not intercept the planet's center. Moreover the strength of Uranus' magnetic field presented another problem for the evolutionary model when Voyager 2 flew by the planet in 1986; however, it did not surprise creationists, as Dr. Russell Humphreys predicted its strength based on the creation model two years before Voyager flew by.

Radiation of energy

The other three gas giants (Jupiter, Saturn, and Neptune) radiate more energy than they absorb from the Sun. Uranus does not. Yet Uranus and Neptune are of comparable size, mass, atmospheric composition, and length of the sidereal day. No naturalistic theory can account for this salient difference. Uranus' peculiar lack of an internal heat source was known before Voyager and confirmed by Voyager.

Satellites

Uranus has twenty-seven named satellites and a number of concentric rings. The orbits of its satellites are slightly inclined to Uranus' own equator. Remarkably, many of the revolutions of Uranus' moons are retrograde. Uranus' satellites include Miranda, easily the most remarkable natural satellite in all the solar system.

Missions to Uranus

The only deep-space probe to visit Uranus thus far has been Voyager 2, which flew by Uranus on January 24, 1986. Voyager 2 discovered ten of Uranus' twenty-seven satellites, and two previously unknown rings. Voyager 2 then continued onward to Neptune before heading out of the solar system.