A look at how rockets work to create the force needed to leave the earth.
It's been said that man has wanted to explore the stars ever since the first man stood and watched the stars twinkle in the night. Throughout history, various men have attempted to reach the heavens... though it wasn't until the 1960's that we actually succeeded. But even with all of the recent advances in space travel, many people still don't really have any idea exactly how we get from the ground to the stars.
The first real problem that is encountered when trying to leave the Earth is gravity. Mother Earth is a big planet, and everything on the Earth is attracted to the Earth's mass. In order to get into space, a rocket has to be able to reach a speed of 7 miles per second (11.2 km/sec). That's around 25,200 miles per hour! This 7 miles per second speed is known as the "escape velocity" of Earth.
Knowing that you need to travel at 7 miles per second is only the first problem that you encounter when trying to get into space. According to one of the basic principles of rocketry, in order to reach the speed of all the gasses being exhausted from the base (also known as the "exhaust velocity), then your rocket needs to have a weight before launch that is 2.72 times the weight of the rocket after all of the fuel has been used. In other words, if your capsule weighs 500 pounds and your fuel cells weigh 4500 pounds, then you'll need at least 8500 pounds of fuel... and that fuel will have to burn out of the rocket at a speed of 7 miles per second or greater. The problem with this is that there aren't any modern rockets that can create that exhaust velocity, even with that much fuel.
If a rocket needs that much thrust from it's exhaust, but we don't have any rockets that can create that much thrust, then how is it that rockets get into space (and have since the 1960's, when technology wasn't nearly as advanced?) Well, luckily enough for us, it's possible to cheat a little bit on the whole "7 miles per second" thing. Instead of having a large rocket with a lot of fuel that burns off quickly, the rockets that we use burn off a smaller quantity of fuel; once that fuel is used up, the portion where that fuel and its rockets were housed is ejected from the rocket, and a second set of rockets kicks in to push the much-lighter rocket even higher into the atmosphere. A third "booster" is sometimes used for heavier loads, pushing the rocket over the threshold and propelling it into space.
Unfortunately, once the rocket is free of the gravity of the earth, it then has to deal with the greater gravity of the Sun. The simplest way to do this is to angle the craft towards Jupiter, the largest of the planets... the gravity of Jupiter can be enough to keep the rocket from being pulled into the Sun without the use of any additional fuel. Of course, if the rocket's trajectory is figured incorrectly, then Jupiter can actually slow the rocket down, which will then trap it in the gravity of the Sun. Luckily, this has never been an issue with Earth-orbit missions, since the orbit of the craft is designed to be close enough to the Earth that the planet's gravity keeps it just close enough without pulling it in.
