Re: Why did it take 3 - 4 days for the Apollo spacecraft to reach the moon?

Good question, Rob.

The reason is that after the Apollo spacecraft used its booster stage, the Saturn S-IVB, to accelerate to escape velocity, it then shut down and discarded the stage (after extracting the LM, or Lunar Module). So from then on, Earth's gravity was continually slowing Apollo down with respect to the Earth. By the time it reached the point where the Moon's gravity had more influence than Earth's, two days had passed and Apollo was only traveling a few thousand miles per hour. As the Moon's gravity gained greater influence, Apollo sped up again but not as fast as it had been going originally by the time they fired their engine to brake into Lunar orbit.

Similarly, when Apollo left the Moon, they had to accelerate to the slower Lunar escape velocity and were slowed down by the Moon's gravity until they reached the point where Earth's gravity was stronger. Then they were able to just coast "downhill" all the way back to Earth so that, at the end of three days, they were traveling towards Earth at just about the same speed they left.

In short, it took three days because most of the time they were traveling slower than 25,000 mph.

-Adams Douglas
Senior Developer
Infonex, Inc.