CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 445
politics, and the stimulation of technology than to science, the Apollo
program became a fantastic scientifi c adventure, including six expedi-
tions to the surface of the moon that changed how we all think about
Earth.
Flying to the moon is not particularly diffi cult. With power-
ful enough rockets and enough food, water, and air, it is a
straightforward trip. Landing on the moon is more diffi cult but
not impossible. Th e moon’s gravity is only one-sixth that of
Earth, and there is no atmosphere to disturb the trajectory of the
spaceship. Getting to the moon isn’t too hard, and landing is
possible; the diffi culty is doing both on one trip. Th e spaceship
must carry food, water, and air for a number of days in space plus
fuel and rockets for mid-course corrections and for a return to
Earth. All of this adds up to a ship that is too massive to make a
safe landing on the lunar surface. Th e solution was to take two
spaceships to the moon, one to make the round-trip in and one
to land in (■ Figure 21-3).
Th e command module was the long-term living space and
command center for the trip. Th ree astronauts had to live in it
for a week, and it had to carry all the life-support equipment,
navigation instruments, computers, power packs, and so on for a
week’s jaunt in space. Th e lunar landing module (LM for short)lightly scarred by impacts and must be younger than the cratered
highlands.
It is diffi cult to estimate the true age of any specifi c crater. In
some cases, you can fi nd relative ages by noting that a crater or its
rays partially covers other craters. Clearly the crater on top must be
younger than the craters on the bottom. Relative ages can be cali-
brated using radioactive ages of lunar samples, which indicate how
the cratering rate decreased when the moon was young. Combining
all this information, astronomers can study the size and number of
craters on a section of the moon’s surface and estimate that sec-
tion’s absolute age in years. Th e maria, containing few craters,
appear to be 3 to 4 billion years old, and the highlands are older.
Earth-based observations allowed astronomers to begin tell-
ing the story of the moon’s surface, but the view from Earth does
not provide enough evidence. To really understand the history of
the lunar surface, humans had to go there and bring back
samples.
The Apollo Missions
On May 25, 1961, President John Kennedy committed the United
States to landing a human being on the moon by 1970. Although the
reasons for that decision related more to economics, international
■ Figure 21-2
Details visible in photographs show that meteorite impacts long ago cov-
ered the moon with craters, but that lava fl ooded out and fi lled the largest
basins covering the craters there with smooth plains. (Hadley: NASA; Moon
disk, highlands, and mare: © UC Regents/Lick Observatory)