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
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].
NASA: APOLLO 11 EVA VIDEO
Why don't the
astronauts jump any higher than they would be able to on
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].