Why Does An Astronaut Weigh More On Earth Than In Space?

An Astronaut weighs more on Earth than he weighs in Space because of Earth's gravity. It's this force and his weight acting towards the center of the Earth is why we experience a certain Weight which is our mass times the acceleration due to gravity. Once we're out of Earth's gravitational field we are in a zero gravity situation and we are not in the influence of a massive planet like earth then is why we do not weigh much in space.

Because on earth there is gravity but in space there is no gravity so your body weighs less

This is an old question, but both answers are wrong.  Technically, an astronaut in the Space Shuttle weighs just a tad less than he does on the surface of the planet.

The reason for weightlessness is because the astronaut is falling; he is falling *around* the earth because his and the spaceship's velocity is fast enough to not be too affected by gravity.  However, the spaceship *is* affected by both gravity and atmospheric drag (the space shuttle is technically not free of drag, since the atmosphere still rises several hundred kilometers above the shuttle!).