What was perhaps the greatest challenge of human space flight?

Returning passengers back safely to Earth

Explanation:

i believe that it is a exoplanet

for step 7 table c on the lab "newtons law of action"   the average time it took the car to travel .25 meters   was 1.92 seconds.

the average time it took the car to travel 0.50 meters was 2.61 seconds

hope this : )