Have you ever wondered if sometime in the near future, if human beings will be able to colonize other planets? For centuries, people have dreamed of leaving the Earth and traveling through space to visit other planets and stars. During the past thirty years, some of these dreams have become a reality. Spacecrafts have orbited the Earth and sent back information that has helped to broaden our knowledge of the solar system. "That's one small step for a man, one giant leap for man kind" as, stated by Neil Armstrong in The New Book of Knowledge, pg 340h.
To begin with, before man could have gone to space, scientists had to figure out a way to leave the Earth. Scientists figured a spacecraft leaving Earth would have to travel fast enough to overcome Earth's strong gravitational pull. The speed needed to overcome Earth's gravity is 25,000 mph, which is about 7 miles per second. Even at this speed, that does not mean that a spacecraft has completely freed itself from the Earth's gravity, which extends far out into space. As the spacecraft continues to move away from the Earth the gravitational force weakens until it no longer has a significant effect on the spacecraft. Next, for a spacecraft to enter orbit around the Earth, it must reach a speed called orbit velocity. The orbital velocity would depend upon how far above the Earth the craft is supposed to orbit. For example, a spacecraft must obtain an orbital velocity of about 17,500mph to orbit the Earth. A slower orbital velocity would be needed to keep a spacecraft in!.
orbit farther from the Earth.
Secondly, now it is time to start back to Earth. The engine's thrust pushed the CSM out of lunar orbit and on a course toward Earth, at this phase the spacecraft is ready for re- entry. Too steep a slant would cause the spacecraft to burn as it plunges through the atmosphere. Too flat of a slant would cause the spacecraft to bounce off the atmosphere and fly into space.