All matter has mass. Objects with mass exert a force on each other. Gravity is this force: it holds us on the earth, and it keeps the earth orbiting the sun.
Our sun is a star like other stars, but it is much closer than the other stars we can see in the night sky. All of our energy comes from the sun originally.
Gravitational forces operate in space and in a vacuum, although when a spacecraft is in free fall, it may seem to those inside as though there is no gravity.
The accelerational force due to gravity diminishes with distance, in accordance with the inverse square law: double the distance, quarter the effect.
Objects in free fall are always influenced by gravitational forces, mostly those of the nearest massive bodies, even if the forces are not detectable.
The escape velocity from a body depends on the mass of the body being escaped from, and the distance from it. It is usually given as a value from the surface.
One method of accelerating spacecraft for flight to other planets is the gravity assist, which adds velocity to the craft by taking some from a large body.
A rocket operates in accordance with Newton's laws of motion, using reaction as a means of propulsion. Rockets can work even better in a vacuum than in air.
A rocket is the only sort of vehicle which can operate in space, because it does not require oxygen to burn fuel, or need anything to push against.
In 1869, Edward Everett Hale foresaw space stations when he wrote The Brick Moon. This was an artificial satellite placed in orbit as a navigational aid.
In 1895, Konstantin Tziolkovsky proposed the use of liquid-fuel rockets for navigation in space, because they could be controlled, turned on and off.
In 1903, Konstantin Tziolkovsky wrote of using rockets to reach outer space, space suits, and colonization of the solar system, the use of liquid oxygen.
In 1919, Robert Goddard suggested a rocket could be used to reach the moon. He actually wanted to shoot for Mars, but felt the Moon was more plausible.
In 1952, Wernher von Braun discussed the technical details of a manned exploration of Mars in The Mars Project. Like Robert Goddard, Mars was his big dream.
In 1990, the launch of Hubble Space telescope took place, complete with faulty optics that took until 1993 to fix, when the first servicing mission flew.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsspace.htm, first created on January 25, 2006. Last recorded revision (well I get lazy and forget sometimes!) was on January 25, 2006.