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Why do the astronauts alternate between bounding and walking?

It depends on how fast they wanted to move. For short distances the astronauts found they could walk fairly normally. As soon as they sped up, they were unable to sustain a walking motion. "It's not like an earth run here, because you are taking advantage of the low gravity," said Apollo 11 commander Neil Armstrong [Reports11b, 77]. Apollo 11 lunar module pilot Buzz Aldrin added, "You can't move your feet any more rapidly than the next time you come in contact with the surface. In general you have to wait for that to occur" [Ibid.].

Consider the mechanism of bipedal locomotion. You propel your center of gravity forward and then "catch" yourself with your oustretched foot. The rear foot swings forward to catch you the next time. When you run, the same principle is at work except that your rear foot leaves the ground before your front foot contacts it. Obviously this process would be greatly affected by the amount of gravity on the moon. If you didn't fall back toward the surface as fast, your front foot would be hanging there six times longer than on earth, and this would be awkward.

Normally your leg motions while walking and running fall into a certain rhythm. You swing your leg forward with a customary amount of effort, and it's right there at the apex of its swing when it hits the ground. Imagine what it would be like to have to hold it there until the weaker gravity had time to act.

NASA originally proposed a "kangaroo hop" whereby the astronauts would hop with both feet and then land with both feet. Aldrin found this to be very awkward and unnatural. The "lope" (as Armstrong named it) turned out to be a good compromise. This is the characteristic Apollo stride whereby the astronaut puts one foot in front of the other, pushing off with one foot and landing on the other foot, but not separating the feet as in a normal walking stride.

Many people confuse weight with mass. The former is affected by gravity and the latter is not. But things like momentum and inertia are a function of mass. Consider a 175-pound (80-kg)astronaut wearing 120 pounds (55 kg)of equipment for a total of some 300 pounds (136 kg) on earth. On the moon he would weigh only one-sixth that amount, 50 pounds (23 kg). But he would still have 300 pounds (136 kg) of mass.

Why is that important? Because in order to get moving you have to overcome inertia, and you have the same intertia on the moon as you do on earth. And in order to stop you have to overcome momentum, which also is the same on earth as on the moon. We've probably all had our bad moments with momentum as we try to stop a full grocery cart before it bangs into the heels of the nice lady in front of us. On the moon your ability to stop and start is impaired by your relatively light weight. Your grip on the lunar surface would be a matter of friction, and friction depends on how hard you're being pulled down against the surface. Low gravity means a weak force pulling you against the surface, and that means less friction, and that means less grip. So if you tried to run you'd just slip and slide. And if you tried to stop you'd skid.

"In general," said Aldrin, "it would take a couple of steps to make a good sideways change in motion" [Ibid., 76].


Why don't the astronauts jump any higher than they would be able to on earth?

Who says they couldn't? Just because they generally didn't doesn't mean they weren't able to.

Neil Armstrong reported that he was able to jump to the third step of the lunar module ladder, which he estimated to be five or six feet from the lunar surface [Reports11b, 89]. "I did some fairly high jumps," said Armstrong, "and found 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" [Ibid., 76]. Falling over backward would risk damaging the PLSS.

At left are three frames taken about a second apart from the video of Apollo 11's moonwalk. At the top frame you see Armstrong on the footpad. In the middle frame he is in midair (midvacuum?) with his feet behind him. In the bottom frame he has landed on the LM ladder at a height consistent with that estimate.

The EVA videos show the astronauts falling and landing on their knees. Wouldn't this risk puncturing the space suit?

There's always a risk, of course. The Apollo 11 astronauts were not allowed to kneel down except in an emergency because of this [Reports11b, 79]. Armstrong and Aldrin reported that they had difficulty reaching items they had dropped without the ability to kneel. It was also not known if an astronaut wearing a lunar space suit (EMU) would be able to get back up again if he knelt or fell down. Apollo 11 astronauts helped verify that this was possible.

When the space suits were brought back and examined, it was decided that they were durable enough to allow the astronauts to casually kneel. Apollo 11 was really the final experiment and so they had to act conservatively. As the equipment was examined and modified, more roughness became allowable [Ibid., 79].

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