Mercury is the planet nearest the Sun, and hence is difficult to observe. Until the late nineteenth century it was believed to rotate on its axis every 24 hours or so, but this opinion was displaced by that of Giovanni Schiaparelli (1835-1910) and Percival Lowell, who contended that it kept the same face permanently towards the Sun. Twentieth-century sf writers thus pictured it as having an extremely hot "dayside", a cold "nightside" and a narrow "twilight zone". This image persisted until the 1960s, when it was discovered that Mercury rotates on its axis rapidly enough to have a day somewhat shorter than its year.
The earliest visit to Mercury was probably that of Athanasius Kircher in his Itinerarium Exstaticum (1656), and it was generally included in other round tours of the planets, including Emanuel Swedenborg's The Earths in Our Solar System (1758) and George Griffith's A Honeymoon in Space (January-July 1900 Pearson's as "Stories of Other Worlds"; exp 1901). John Munro's A Trip to Venus (1897) includes a detour to Mercury. The earliest novel in which Mercury came into principal focus was Relation du Monde de Mercure (1750; trans Brian Stableford as The World of Mercury 2015) by Le Chevalier de Béthune; the first novel in English to be set there was William Wallace Cook's Satire Adrift in the Unknown (December 1904-March 1905 Argosy; 1925). E R Eddison's fantasy novel The Worm Ouroboros (1922) is likewise set on Mercury, but the name is used purely for convenience and the supposed planetary location soon forgotten.
Genre SF rarely employed Mercury as a milieu for exotic adventure, preferring Mars and Venus, but it does feature in Homer Eon Flint's "The Lord of Death" (10 May 1919 All-Story Weekly; in The Lord of Death and the Queen of Life, coll of linked stories 1965), Ray Cummings's Tama of the Light Country (13-27 December 1930 Argosy; 1965) and its sequel Tama, Princess of Mercury (27 June-18 July 1931 Argosy; 1966), and Clark Ashton Smith's The Immortals of Mercury (1932 chap). An invasion from Mercury is thwarted in J M Walsh's Vandals of the Void (1931), and Leigh Brackett set one of her exotic romances there, "Shannach – the Last" (November 1952 Planet Stories). Attempts to use Mercury in more thoughtful stories with some fidelity to astronomical knowledge were likewise infrequent in the pre-World War Two pulps, the first significant examples being Clifford D Simak's "Masquerade" (March 1941 Astounding; vt "Operation Mercury" in Tales of Outer Space, anth 1954, ed Donald A Wollheim) and Isaac Asimov's "Runaround" (March 1942 Astounding).
After World War Two, however, things picked up a little. Three juvenile novels featuring Mercury are Lester del Rey's Battle on Mercury (1956 as by Erik van Lhin), Isaac Asimov's Lucky Starr and the Big Sun of Mercury (1956 as by Paul French; vt The Big Sun of Mercury), and Mission to Mercury (1965) by Hugh Walters. Alan E Nourse's memorable "Brightside Crossing" (January 1956 Galaxy) represents a journey across the dayside of the planet as an adventurous feat akin to the then-recent conquest of Everest. The nightside of Mercury features ironically in Larry Niven's "The Coldest Place" (December 1964 If), unfortunately based on the then scientific belief that Mercury always keeps the same face towards the Sun. It emerged in 1965 that the planet has a 3:2 spin-orbit resonance, rotating three times for each two orbits around the Sun: there is no permanent nightside. John Varley's "Retrograde Summer" (February 1975 F&SF) uses this updated science and presents Mercury as a place of beauty and danger where, with appropriate Force Field garb, one can swim in pools of boiling mercury and admire the ionized mercury-vapour glow above. Oddly enough, there is a closely similar Mercury episode – bathing playfully in liquid metal while protected by an impalpable suit – in Franz Werfel's Stern der Ungeborenen (1946 Austria; trans Gustave O Arlt as Star of the Unborn 1946). "Retrograde Summer" may be in part a conscious homage to this novel. Ben Bova pays a less magical visit in a late volume of his loose Tales of the Grand Tour sequence: Mercury (2005). On the Robot-colonized of Charles Stross's Saturn's Children (2008), tracked Cities follow the dawn around the little world to avoid extremes of heat and cold.
More usually, recent sf employs Mercury as merely a convenient place to site bases for studying the Sun, like the one in David Brin's Sundiver (1980). Perhaps the most enduring sf image of Mercury, though, is from Kurt Vonnegut Jr's The Sirens of Titan (1959), which offers an account of the Harmonia, cave-dwelling lifeforms thriving on vibration and introduced to music by a stranded astronaut. [BS/DRL]
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