The Planet of Doubt Jupiter and Saturn and on the surface of Uranus itself. At the time "The Planet of Doubt" takes place at the turn of the 22nd century, the limited range of the spaceships ensures that Uranus can only be reached from the American base on Titan when Saturn reaches conjunction with Uranus, an event that occurs once every forty years. The Young expedition explored the planet's south pole in 2060; now Hammond takes his ship, the "Gaea", to the north pole. Finding an ocean at the north pole, Hammond sends the "Gaea" spiraling southeast until they reach land. They find the
Atmosphere of Uranus part of stratosphere forming haze layers with an optical depth of about 0.01 in visible light. Condensation occurs at approximately 14, 2.5 and 0.1 mbar for ethane, acetylene and diacetylene, respectively. The concentration of hydrocarbons in the Uranian stratosphere is significantly lower than in the stratospheres of the other giant planets—the upper atmosphere of Uranus is very clean and transparent above the haze layers. This depletion is caused by weak vertical mixing, and makes Uranus's stratosphere less opaque and, as a result, colder than those of other giant planets. The hazes, like their parent hydrocarbons, are distributed unevenly across Uranus;