Credit: Courtesy of NASA
License: CC BY-NC 3.0
We know that gravity is the force that keeps us from falling away from the surface of the earth. The force of gravity reaches a significant distance, though as the distance increases, the force decreases by the square of the distance. However, Earth's mass is so large, and its gravity so strong, that the force reaches beyond the surface and atmosphere into space. The Gemini 4 flight of June 1965 was NASA's second manned space flight, and circled the Earth 66 times in four days. Astronaut Edward H. White II was the first American to take a space walk, held in place orbiting the planet because of the force of gravity.
Kepler described the motion of the planets back in the 17th century, but it wasn't until Isaac Newton created his Law of Universal Gravitation that we had a solid explanation of the why of planetary motion. This chapter explained Kepler's Laws of Planetary Motion, and explained that all objects, regardless of size or distance, exert a gravitational pull on each other. Incorporating the lessons of circular motion from earlier, we explored satellite orbits and why astronauts experience weightlessness in space.