QUESTION:
It seems like it would be hard though to do a thorough test of an
experiment on Earth since you have gravity. There must be a lot you have
to guess on these tests, right?

ANSWER from Louis Ostrach on November 19, l997
That's a very good point. In fact, certain of the critical questions we
test on a special airplane called the KC-135. It's a large 4 engine jet
specially equipped to fly straight up and then straight down - in
parabolas just like a roller coaster only it keeps going. At the top of
each "hill" everyone and everything inside the airplane becomes weightless
for about 30 seconds. (at the bottom of the hills, everything experiences
2-g, twice normal gravity) We can test procedures, equipment, and such to
be sure we can use them effectively on orbit. By the way, if you saw the
movie "Apollo 13" you saw the effect of this simulated weightlessness
since many of the sciences of the movie were filmed on this special NASA
airplane.