Why is an astronaut weightless?

Typically, an astronaut and their vehicle are in free fall around the Earth (orbit). This feels like going down a big hill on a roller coaster, but it doesn't stop until reentry.

If you sat on a weighing scale while riding a roller coaster, you would see your weight change with changes in momentum as the roller coaster climbed, fell, banked etc.... Say you normally weigh 150lbs, when you're climbing up the big hill, the scale would show you weighing more than 150 (your weight plus the acceleration as you gain altitude). When you crest the hill and plummet downward, the scale will read much less than 150lb, as your seat, the scale & YOU fall back towards the ground.

Likewise, an astronaut sitting on scale would weigh more than normal during lift off and climbing to orbit. Typically, astronauts will weigh 2-4 times their normal weight during this time. Once the spacecraft is in orbit, it is in continuous free fall around the Earth. The scale would read "0" (zero).