Final answer:
Astronauts experience apparent weightlessness in both the downward-accelerating aircraft and in orbit because they are in free fall, which means no additional forces are felt by them.
The main difference is that in orbit, the free fall is continuous along a path around Earth, while in the aircraft it's a temporary condition created by a maneuver.
Step-by-step explanation:
To simulate the apparent weightlessness of space orbit, astronauts train in a cargo aircraft that is accelerating downward at g, which is the acceleration due to gravity.
They appear weightless in this scenario because they are in a state of free fall along with the aircraft, so no normal force is exerted by the scale onto their feet.
This is similar to the apparent weightlessness experienced in orbit because, in both cases, the astronaut and the surrounding environment are in free fall, accelerating at the same rate due to gravity so that the astronaut experiences no additional forces.
While the sensation of weightlessness is similar in both the aircraft and orbit, there is a difference. In orbit, astronauts are continuously falling around the Earth due to their horizontal velocity being high enough to keep them in a stable orbit, where the Earth's curvature falls away beneath them at the same rate they are falling towards it.
In contrast, the aircraft provides a temporary simulation of this condition by performing a maneuver known as parabolic flight, where it climbs sharply and then descends rapidly, creating a period of free fall for the passengers inside.