Hugo Best Novell Winner (1973)Nebula Best Novell Winner (1973)Locus Award Best Novel Winner (1973)Ditmar International Fiction Winner (1973)Locus All-Time Poll Best SF Novel (Before 1990) [#52 (tie)] (1998)In the year 2100, mankind on Earth, settlers in a lunar colony and aliens from the para-universe, a strange universe parallel in time to our own, are faced with a race against time to prevent total destruction of the Earth. The invention of the Inter-Universe Electron Pump has threatened the rate of hydrogen fusion in the sun, leading, inevitably, to the possibilty of a vast explosion—and the vapourisation of the Earth exactly eight minutes later . . .