The dictionary defines gravity as the force of attraction between all masses in the universe, especially the attraction of the earth's mass for bodies near its surface. The more remote the body the less the gravity.

Every object in the universe has gravity. Most smaller objects have such a small gravitational pull that you never notice it. For example, when you walk away from your car in the parking lot, you are not pulled back toward it, unless of course you forget something.

Large objects such as the Earth, have a huge pull on objects nearby. This is why when we jump into the air, we do not fly right out into space.

The gravitational pull of the Sun keeps the Earth in orbit and the gravitational pull of the Earth keeps the moon in orbit.

When a rocket launches into space, it must have enough thrust to break free of Earth's gravity. It is slightly different when it is just going into orbit.

When astronauts travel into space, they are often seen floating around the interior of their spacecraft and people often refer to zero-gravity. However, the astronauts, such as the ones orbiting in the Space Shuttle never actually get far enough away from the Earth to escape its gravity.

Instead, it is their falling motion in orbit that creates this effect. In fact, this sensation of zero-gravity can be duplicated here on Earth. If you were to stand in an elevator that was free-falling for a significant distance, you would find that you would have a weightless sensation. Astronauts often train in a modified airplane that makes rapid dives. This falling motion also duplicates the weightless effect.

Studies have shown that prolonged living in the weightless environment of space can cause health problems. Astronauts must exercise regularly to counteract loss of bone density and other problems. This could cause problems if humans were to make the long journey to Mars or beyond.

Plants that normally grow straight and tall on Earth, grow small and wild in space, as they are unable to determine which way is up. Simple things such as eating, drinking and going to the bathroom become logistical nightmares in space. Imagine trying to drink your milk as it floated away from you. Imagine trying to sleep while floating around and getting banged up. Astronauts must be strapped in while sleeping, must drink from an enclosed container and must use an adapted bathroom waste system to avoid contaminating their living environment.

When an astronaut takes a spacewalk, he must have some way of getting back to the ship. Early spacewalks used lifelines to pull them back in. Modern spacewalks utilize jet packs which use air pressure to blow the astronauts back to safety.

These are all things that NASA had to work out before sending men into space for the first time and all things that astronauts have to overcome every time they fly into space.