6.7 Why did the astronauts only make such low jumps?

IN A NUTSHELL: They might look low until you consider that the astronauts were wearing a suit and backpack that doubled their weight and had very limited flexibility, and that a fall on the airless Moon could have killed them.

THE DETAILS: One of the best-known lunar jumps is the one performed by John Young as he saluted the American flag during one of the Apollo 16 moonwalks. Figure 6.7-1 shows the video recording of this event, which was also photographed by Charlie Duke (Figure 6.7-2).

Figure 6.7-1. Video recording of John Young’s lunar jump during Apollo 16. The other astronaut, on the right, is Charlie Duke.

Figure 6.7-2. John Young’s lunar jump, photographed by Charlie Duke (AS16-113-18339).

Moon hoax proponents say that Young’s hop is strangely short and so are all the other lunar jumps. They argue that on the Moon, with one-sixth of Earth’s gravity, astronauts should be able to perform amazing leaps, maybe six times as high as on Earth. Perhaps the hidden wires couldn’t lift them high enough or fast enough?

Actually, there are very practical reasons for these short hops.

  • First of all, every lunar astronaut was wearing a spacesuit and a backpack that weighed, on Earth, about 80 kilograms (176 pounds): as much as the astronaut himself. Although on the Moon this gear weighs one sixth of its Earth weight, i.e., about 13 kilograms (28 pounds), it is still a substantial extra ballast that the astronaut has to lift in order to jump.
  • In space and on the Moon it is necessary to distinguish between weight and mass: weight decreases in a low-gravity or zero-gravity environment, but mass remains unchanged and therefore inertia is the same as on Earth. Therefore the leaping astronaut had to overcome the inertia of his 80-kg (176-lb) gear and of his own body just like on Earth.
  • John Young performed a standing jump, with no run-up, as shown in the video of Figure 6.7-1, and this limited the energy that he could put into the jump.
  • Young was wearing a very bulky and stiff suit, limiting his freedom of motion and therefore again the energy he could put into his leap by bending his legs and moving his arms (Figure 6.7-3).
  • More importantly, the astronaut was on the Moon, surrounded by a deadly vacuum. He was well aware that if he fell and cracked his helmet, damaged the backpack that supplied him with air and cooling or tore his pressurized inner suit, he would die by decompression or suffocation. In such conditions, it was rather wise not to try and set high-jump records.

Figure 6.7-3. John Young just before his allegedly controversial standing jump. Note the very limited bending of his legs.

Many hoax theorists also make the mistake of considering Young's jump as the highest ever made on the Moon. Actually, it was just a hop intended to take an unusual salute photograph. Other jumps were much higher and correspondingly more dangerous.

For example, Neil Armstrong reported that he jumped up to the third rung of Apollo 11's LM ladder, which was “easily five or six feet [150-180 centimeters] above the ground”*. His leaps are visible in the recordings of the TV transmission of his moonwalk (Figure 6.7-4).

* Apollo 11 Technical Crew Debriefing, 31 July 1969, in NASA Mission Reports - Apollo 11, Vol. 2, Apogee Books, p. 89 (page 10-61 in the original numbering).

Figure 6.7-4. Neil Armstrong’s leap to climb back into the LM at the end of the Apollo 11 moonwalk, in the restored footage of the live TV transmission (to the right of the flag, in the shadow of the LM, at 2:16:40).

However, Armstrong refrained from further experimentation, because he noted that “there was a tendency to tip over backward on a high jump. One time I came close to falling and decided that was enough of that.”* A backward fall would in fact have entailed the risk of seriously damaging the suit backpack, forcing to cut short the moonwalk.

* Apollo 11 Technical Crew Debriefing, 31 July 1969, in NASA Mission Reports - Apollo 11, Vol. 2, Apogee Books, p. 76 (page 10-28 in the original numbering).

Young himself and his crewmate Charlie Duke engaged in a televised high-jump contest at the end Apollo 16's third moonwalk. Duke estimated that Young had jumped “about four feet [120 centimeters] (Figure 6.7-5); Duke made an equally high jump, but fell backwards onto his backpack.

In his book Moonwalker, he reported that it was “the only time in our whole lunar stay that I had a real moment of panic and thought I had killed myself. The suit and backpack weren’t designed to support a four-foot fall. Had the backpack broken or the suit split open, I would have lost my air. A rapid decompression, or as one friend calls it, a high-altitude hiss-out, and I would have been dead instantly. Fortunately, everything held together.”*

* Moonwalker by Charlie and Dotty Duke, p. 206.

Figure 6.7-5. Young and Duke do high-jumps on the Moon and Duke falls.