7.9 How come Apollo didn’t reach escape velocity?

IN A NUTSHELL: Because it didn’t need to. Getting to the Moon doesn’t require escape velocity: a spacecraft only has to achieve a speed that produces a highly elongated orbit around the Earth that reaches a maximum altitude equal to the distance of the Moon, without ever escaping from the Earth’s pull.

THE DETAILS: This pro-conspiracy argument is a fine example of the misuse of science jargon and factual data to give an impression of competence and knowledge. Its premise is that the escape velocity, the speed required to escape the Earth’s gravity field, is 11.2 kilometers per second (about 7 miles per second), i.e., 40,320 kilometers per hour (about 25,000 mph). This is correct. However, NASA reported that the top speed of Apollo 11 during its climb to the Moon was about 39,000 kilometers per hour (about 24,250 mph). This, too, is factually correct.

In other words, Apollo 11’s stated maximum speed was about 1,230 kilometers per hour (765 mph) slower than escape velocity. So, the argument goes, how could the spacecraft escape Earth and reach the Moon?

The answer to this apparent contradiction is that escape velocity is required only if the spacecraft seeks to escape Earth’s attraction permanently. Anything traveling at this velocity will never fall back to Earth and will continue to climb away from it indefinitely without requiring any additional thrust (more specifically, it will escape from Earth’s gravity field yet will still be in the grip of the Sun’s gravitational attraction).

A spacecraft doesn’t actually need to reach escape velocity to get to the Moon. It just has to achieve a speed that produces an elliptical orbit around the Earth that stretches out to the distance of the Moon and is timed so that the Moon is at the opposite end of the ellipse when the spacecraft gets there. So the Apollo flights didn’t have to reach escape velocity to land on the Moon or fly around it.

Actually, staying below escape velocity is a safety bonus, because it allows to use a so-called free return trajectory (Figure 7-9): the spacecraft will fall back to Earth spontaneously, without requiring additional maneuvers or thrust from its rocket motors. This is particularly useful in case of major malfunctions, as in the case of Apollo 13.*

* More specifically, Apollo 13 began its flight on a free return trajectory and then fired its main engine to leave this trajectory and fly towards the Moon. After the onboard explosion, the thrust of the LM’s descent engine was used to inject the astronauts into another free return trajectory.

Figure 7-9. The main trajectories used by the Apollo missions. From the Apollo 11 Press Kit.