The poem consists of 103 cantos and relates the tragedy of a space ship which, originally bound for Mars with a cargo of colonists from the ravaged Earth, after an accident is ejected from the solar system and into an existential struggle. The style is symbolic, sweeping and innovative for its time, with creative use of neologisms to suggest the science fictional setting:
The first 29 cantos of Aniara had previously been published in Martinson's collection Cikada (1953), under the title Sången om Doris och Mima (The Song of Doris and Mima), relating the departure from Earth, the accidental near-collision with an asteroid (incidentally named Hondo, another name for the main Japanese isle where Hiroshima is situated) and ejection from the solar system, the first few years of increasing despair and distractions of the passengers, until news is received of the destruction of their home port (and perhaps of Earth). According to Martinson, he dictated the initial cycle as in a fever after a troubling dream, affected by the Cold War and the Soviet suppression of the 1956 Hungarian revolution; in another version, the first 29 cantos were said to be inspired by an astronomic observation of Andromeda Galaxy.
One of the major themes explored is the nature and necessity of art, symbolised by the semi-mystical machinery of the Mima, who relieves the ennui of crew and passengers with scenes of far-off times and places, and whose operator is also the sometimes naïve main narrator. The rooms of Mima, according to Martinson, represent different kinds of life styles or forms of consciousness. The accumulated destruction the Mima witnesses impels her to destroy herself in despair, to which she, the machine, is finally moved by the white tears of the granite melted by the phototurb which annihilates their home port, the great city of Dorisburg. Without the succour of the Mima, the erstwhile colonists seek distraction in sensual orgies, memories of their own and earlier lives, low comedy, religious cults, observations of strange astronomical phenomena, empty entertainments, science, routine tasks, brutal totalitarianism, and in all kinds of human endeavour, but ultimately cannot face the emptiness outside and inside.
Aniara has been translated in English as Aniara, A Review of Man in Time and Space by Hugh MacDiarmid and E. Harley Schubert in 1956; a new English translation has been published in 1999. The poem was referenced in Vernor Vinge's science fiction novel A Fire Upon the Deep.