Imagine that gravity stopped. This is only a thought experiment: no technology we have can switch gravity off or block it. What people can do is feel weightless, and that feeling is the closest real thing to the imagined one, so it helps to start by separating two ideas that are easy to confuse, weight and mass. Your weight is how hard gravity pulls on you, and the pull comes from all of Earth's mass. A kilogram measures mass, which stays the same anywhere, while a pound measures the force of a planet's gravity tugging on you. Your mass stays the same even when your weight changes. What we call weight in daily life is the feeling of being supported by the ground or a seat pushing back on us.

Near Earth's surface, gravity alone makes things speed up at 9.8 meters per second every second, called 1g. Standard gravity on Earth is defined as exactly 9.80665 meters per second squared; at the equator it is a little less (9.780) and at the poles a little more (9.832). Take away the push from below, and the feeling changes completely: anything in free fall, like a dropped apple or a falling spacecraft, feels weightless because nothing is pushing back on it; on a roller coaster, free fall gives you a brief feeling of weightlessness at the top of a drop. Everything in orbit is falling around its planet: it moves sideways so fast that it keeps missing the ground.

People can also arrange free fall on purpose. To feel weightless on Earth, people ride airplanes that fly in big arcs, so everyone inside floats during the fall; gravity is never switched off. NASA Glenn's Zero Gravity Research Facility is a shaft about 500 feet deep where test packages free fall in a vacuum for just over 5 seconds, giving near weightlessness. You may have heard that gravity can be turned off in special chambers. In fact, there is no antigravity: no machine can neutralize gravity, and weightless planes and drop towers only create free fall. Free fall can also last. Astronauts on the space station look like they float because everything inside is falling together.

You may have heard that astronauts float because there is no gravity in space. In fact, gravity is still strong where the station flies: at about 250 miles up it is 88.8 percent as strong as on the ground. Gravity does not stop in space; astronauts float because they are in free fall, not because there is no gravity. Things on the station travel at the orbital speed of the station, 17,500 miles per hour (28,000 kilometers per hour). Scientists do not call it zero gravity; they call the feeling on the station microgravity, about one millionth of normal gravity. The International Space Station is in low Earth orbit and circles the planet once in about 90 minutes.

Nothing in nature or in a laboratory removes gravity completely. Gravity never quite reaches zero, although far out in deep space, away from planets and stars, its pull is very faint indeed. Gravity gets weaker the farther apart two things are, but gravity works the same everywhere in the cosmos; how strongly we feel it depends on how much mass is around. Even Earth's atmosphere shows how gravity holds things: it is only about 1/1,200,000 the mass of Earth, a thin skin held on by gravity, and some fast-moving molecules in the upper air escape the pull of gravity, over time scales of billions of years. The Moon's gravity pulls on the seas, and that makes the ocean tides go up and down.

Weight changes from place to place, even though the gravity beneath it never switches off. Everything that has mass has gravity, and things with more mass pull harder, so a planet with less mass than Earth has weaker gravity, and you would weigh less there. On the Moon you would weigh about one-sixth of what you weigh on Earth, because the Moon pulls less; the Moon has about 1.2 percent of Earth's mass, and its surface gravity is about 0.165 of Earth's. The Sun's escape velocity is 617.6 kilometers per second, 55 times Earth's, and its surface gravity is 28 times Earth's. The Sun has about 332,900 times Earth's mass.

Words to know

• weightless: no push back
• microgravity: tiny gravity
• escape: get free of
• mass: amount of matter