How long a rocket takes to reach the moon?

The Moon sits about 384,400 km away, but travel time depends almost entirely on how much energy the launcher pours into the departure. Apollo used a near-escape trajectory: the Saturn V's third stage restarted in low Earth orbit, the stack coasted outward losing speed the whole way, and some three days later arrived slow enough to be captured by lunar gravity. Apollo 11 left Florida on 16 July 1969, braked into lunar orbit on 19 July and set Eagle down on 20 July. Every later crewed mission ran to much the same clock.

A probe with no crew to keep alive can be thrown much harder. Flung straight onto an escape trajectory, a spacecraft crosses the Moon's distance in well under a day — though it is then moving far too fast to stop, which only suits a flyby. The opposite extreme is propulsive economy. SMART-1 rode to a transfer orbit as a secondary payload in September 2003 and then spiralled outward on a xenon ion thruster whose push is comparable to the weight of a postcard, reaching lunar orbit in November 2004, more than 13 months later.

Every hour saved on the outbound leg is paid for twice: once to accelerate, once to brake at the far end, and propellant for both has to be lifted off Earth. Crewed flights accept that bill because consumables, radiation exposure and crew endurance are expensive in a different currency. Robotic missions, which can afford patience, increasingly buy a cheap ride to a high orbit and make up the difference with efficient engines and long spirals through the radiation belts.