Uranus

Knowledge about Uranus came slowly because of its distance. Even when it is closest, Uranus shows a disk of only 4 in (10 cm) in apparent diameter through a telescope and is 5.7^m apparent magnitude (barely visible to the unaided eye even in the best observing conditions). Herschel discovered Oberon and Titania, the outermost and largest, respectively, satellites of Uranus in 1787. Determination of their orbits around Uranus from observations gave their periods of revolution P and mean distances A from Uranus. This allowed one to determine Uranus' mass from the general form of Kepler's third law; it turned out to be 14.5 Earth masses. Using its radius of 15,873 mi (25,560 km), one calculated Uranus' mean density (its mass divided by its volume), which is 1.27 grams/cm³. This indicated that Uranus is a smaller type of Jovian planet similar to Jupiter and Saturn; they are characterized by large masses and sizes and low mean densities (compared to Earth), and are inferred to consist largely of gases.

The planes of the orbits of Oberon and Titania were expected to lie in or near the plane of Uranus' equator, since most other planetary satellites have orbital planes that are in or near the equatorial planes of their planets. When the orbital planes of Oberon and Titania were determined, however, they indicated that the plane of Uranus' equator is almost perpendicular to the plane of its orbit around the sun (and also to the ecliptic). This is unlike the other planets, whose equatorial planes are tilted by at most 30° to the planes of their orbits around the sun. This implied that Uranus' axis (and poles) of rotation lie almost in its orbital plane. This conclusion has been confirmed by observations of Uranus' rotation from the Voyager 2 spacecraft in 1986. This is the first of several interesting characteristics we have discovered for Uranus, and gives Uranus interesting seasons during its year (period of revolution around the sun), which is 84.1 Earth years long. The Uranian seasons will be discussed in more detail below. The cause of this unusual orientation of Uranus' rotation axis is still unknown and is now the subject of considerable speculation and theoretical research. One theory is that the orientation of its rotation axis was produced by the collision of an Earth-sized body with Uranus near the end of its formation.

Unexplained perturbations of Uranus' orbit in the early nineteenth century led to the prediction of the existence of a still more distant large planet, resulting in the discovery of Neptune in 1846. Neptune is the most distant (30.06 a.u. mean distance from the sun) Jovian planet, and it has several properties (mass, size, rotation period, rings, and magnetic field) like those of Uranus.

Three more satellites of Uranus, closer to it than Titania, were discovered during the 105 years after Neptune's discovery. They are, in order of closeness to Uranus, Umbriel and Ariel, discovered in 1851 by Lassell (1799–1880), and Miranda, discovered in 1948 by G. P. Kuiper (1905–1973). These discoveries showed that Uranus has a satellite system comparable to those of Jupiter and Saturn, although Titania, its largest (980 mi [1,580 km] diameter) and most massive satellite, and the slightly smaller Oberon, are comparable in size and mass to Saturn's satellites Iapetus and Rhea rather than to its much larger satellite Titan and Jupiter's four Galilean satellites. All other satellites of Uranus are smaller and less massive.

The best telescopic observations of Uranus from Earth's surface show a small, featureless, bluish green disk. Spectroscopic observations show that this color is produced by the absorption of sunlight by methane gas in its atmosphere; this gas is also present in the atmospheres of Jupiter and Saturn. Observations of occultations (similar to eclipses) or stars by Uranus indicated that Uranus' atmosphere is mostly composed of molecular hydrogen and helium, which are also the main components of the atmospheres of Jupiter and Saturn.

Observations at infrared wavelengths (that are longer than those of red light), where planets radiate away most of their heat energy, show that Uranus radiates at most only slightly more infrared radiative energy than its atmosphere absorbs from sunlight. Any excess energy originating from Uranus' interior can be attributed to the decay of radioactive elements, which also produces much of the heating in Earth's interior. This is not true for the other Jovian planets, which all emit as much as twice as much infrared energy as their atmospheres absorb from sunlight; this requires another internal energy source for them, which is possibly continuing gravitational contraction.

One of the last major discoveries about the Uranus system from Earth-based observations was made on March 10, 1977, during observations of Uranus' occultation of the star SAO 158657, when J. L. Elliot's (1943–) group and other observers noticed unexpected dimming of the star's light before the occultation and again after it. These dimmings were correctly identified with the existence of several faint, thin rings orbiting Uranus well inside Miranda's orbit which were hitherto undetected; unlike Saturn's rings, they are too faint to be directly observed from Earth's surface by ordinary methods. Uranus' rings have been observed several times since then during stellar occultations and by the Voyager 2 spacecraft. The rings are very dark; their albedos (the fraction of the light that falls on them which they reflect) are only about 0.05.

A second major discovery made by Earth-based infrared observations was the detection of water ice on the surfaces of some of Uranus' satellites.

Let us now return to the seasons of Uranus. The fact that Uranus' rotation axis lies almost in the plane of its orbit around the sun means that at some season its south pole will be pointed nearly at the sun, and nearly all of its southern hemisphere will be in continuous sunlight (early southern hemisphere summer), while nearly all of its northern hemisphere will be in continuous night (early northern hemisphere winter). These seasons occurred in 1901 and in late 1985, and will occur next in 2069. The sun was last above Uranus' equator in December 1965, when it rose at Uranus' south pole and set at the north pole; the sun will then shine continuously on Uranus' south pole for the next 41.6 years until mid-2007, when it will again be above Uranus' equator and will set at the south pole and rise at the north pole, which will be in continuous sunlight for the next 42.5 years while the south pole will be in continuous night. The north pole will point closest to the sun in early 2030 (early northern hemisphere summer), and the sun will set there and rise at the south pole in 2050. Calculations show that a horizontal unit surface area, say a square meter, at either pole will receive over a Uranian year about 1.5 times the sunlight that the same surface would receive at Uranus' equator over a Uranian year (84.1 Earth years).

Figure 1. Trajectory of Voyager 2 through the Uranus system in January 1986. Illustration by Hans & Cassidy. Courtesy of Gale Group.