Bad Astronomy: Misconceptions and Misuses Revealed, from ...

164 ARTIFICIAL INTELLIGENCE
astronauts’ boots leave imprints in dust? All that dust should be
gone!
Both of these claims are wrong. First, the engine was capable
of 10,000 pounds of thrust at maximum, but it wasn’t simply a
roman candle that burns at full thrust when lit. The engine had a
throttle, basically a gas pedal, which could change the amount of
thrust generated by the engine. When high over the surface of the
Moon, the astronaut flying the lander would throttle the engine
for maximum thrust, slowing the descent quickly. However, as the
lander slowed, less thrust was needed to support it, so the astronaut
would throttle back. By the time the lander touched down,
the astronauts had cut the thrust to about 30 percent of maximum,
just enough to compensate for the lander’s own weight on the
Moon.
Three thousand pounds of thrust still might sound like a lot,
but the engine nozzle of the lander was pretty big. The bell was
about 54 inches across, giving it an area of about 2,300 square
inches. That 3,000 pounds of thrust was spread out over that area,
generating a pressure of only about 1.5 pounds per square inch,
which is really pretty gentle, less than the pressure of the astronauts’
boots in the dust. That’s why there is no blast crater under
the lander; the pressure was too low to carve out a hole.
The second claim about dust near the lander is interesting.
Why was there dust so close to the center of the landing site that
both the lander legs and the astronauts’ movements left tracks?
This defies common sense, which says the dust should have all
been blown away. However, our common sense is based on our
experience here on Earth, and it pays to remember that the Moon
is not the Earth.
Once again, we have to understand that the Moon has no air.
Imagine taking a bag of flour and emptying it on your kitchen
floor (kids: ask your parents first). Now stand over the flour, stick
your face an inch or two above it, and blow as hard as you can.
When you stop coughing and sneezing from having flour
blown into your nose, take a look around. You should see flour
spread out for a long way on your floor, blown outward by your
breath.