Mercury in The Fiction of Leigh Brackett - Astronomical Characteristics

Astronomical Characteristics

Mercury in the Solar System stories of Leigh Brackett is the closest planet to the Sun. Its most notable characteristic is that it is tidally locked to the Sun, that is, it makes a full rotation once for each of its 88-day orbits. As a result, it keeps the same face directed towards the Sun at all times, just as the same side of the Moon always faces the Earth. The sun-facing side is known as the Sunside; the side facing away from the sun is called the Darkside.

A Mercurian sidereal day (the period of rotation) lasts about 87.97 Earth days, but since Mercury does not rotate with respect to the Sun, there is no such thing as a solar day or night in usual sense. However, although the times of rotation and revolution are identical, due to Mercury's eccentric orbit, the subsolar point varies in position by up to 20° east or west of the mean. An observer standing on the boundary between Sunside and Darkside at perihelion could observe the sun set below the horizon, only to rise again at aphelion, stay in the sky for 44 Earth days, and then set again at perihelion (an observer on the other side of the planet would see the opposite sequence). Observers closer to the mean antisolar point would observe a shorter day, those closer to the subsolar point would observe a longer one. As a result, there is a strip, over 500 km wide at the equator and narrowing toward the poles, where there is an 88-day long day-night cycle; this region is known as the "Twilight Belt".

Although Mercury possesses a breathable oxygen atmosphere close to its surface, most of the planet is uninhabitable, as surface temperatures on Mercury range from metal-melting temperatures in the immediate subsolar point in the center of the side of Mercury facing the Sun (the Sunside) to cold enough to liquefy oxygen on the side of Mercury facing away from the Sun (the Darkside). In the Twilight Belt, however, temperatures fluctuate between Sunside heat and Darkside cold; in a few places, there is a habitable zone where temperatures are not too extreme for life.

Mercury also has a highly eccentric orbit, which slowly slowly precesses around the Sun. Some scientists hypothesize that another planet may exist in an orbit even closer to the Sun to account for this perturbation. This hypothetical planet has been named Vulcan.