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Where would the spacecraft be put together? The 1993 Design Reference Mission (DRM) assumed no assembly in low Earth orbit: its plan called for heavy-lift rockets, each carrying a Mars spacecraft together with its nuclear propulsion upper stage^[1]. In the 1997 DRM, spacecraft and nuclear propulsion stages would launch separately, dock in Earth orbit, and then depart for Mars^[2].

How would the later plan handle Mars arrival? The 1997 DRM proposed an aerobrake integrated with the spacecraft hull alongside nuclear rockets^[3]. Aerobraking means passing through the upper atmosphere so air resistance slows a spacecraft, reducing the braking needed to enter orbit^[4]. Both DRMs were reference plans, not flown human Mars missions^[5][6].

On a very dark night, the book says, one or two of these star clusters may appear to the naked eye as faint spots against the black sky; others cannot be seen without a telescope^[1]. Through a powerful telescope, the little round patch takes on a startlingly different character: it is not a single light, but a gathering of minute stars crowded together^[2].

To help readers picture it, the book suggests shaking pepper onto a small circle of paper. The grains heap up at the centre, thin out towards the edge, and finally stand apart; imagine each grain shining as a star against a dark background^[3]. The book attributes to Herschel a calculation that more than 2,000 stars were visible in some such clusters. That is its historical estimate, not a present-day count^[4]. The astonishment lay in the change of scale: what the unaided eye could barely register as a smudge became, under the telescope, a densely patterned host of individual lights^[5].