Fifth Planet

The year is 2087. The two great power blocs still face each other, with Britain having adopted the political role formerly occupied by Switzerland.

For 75 years the world has known of the approach of another solar system which will pass between our sun and its outermost planets. The sun of the other solar system is named Helios by astronomers, and it too is found to have planets. Its fifth planet, Achilles, is the one most similar to Earth; and rival rocket ship expeditions are launched by the Euro-American and Communist blocs to explore this transient neighbour...

From here on the story is weird and wonderful, the action lively; the scientific ideas are intriguing and the social satire is amusing.