What is Microgravity? Gravity is a force that governs motion throughout the universe. It holds us to the ground, keeps the Moon in orbit around the Earth, and the Earth in orbit around the Sun.Many people mistakenly think that there is no gravity above the Earth's atmosphere, i.e., in "space," and this is why there appears to be no gravity aboard orbiting spacecraft. Typical orbital altitudes for human spaceflight vary between 120 - 360 miles (192 to 576 km) above the surface of the Earth. The gravitational field is still quite strong in these regions, since this is only about 1.8% the distance to the Moon. The Earth's gravitational field at about 250 miles (400 km) above the surface maintains 88.8% of its strength at the surface. Therefore, orbiting spacecraft, like the Space Shuttle or Space Station, are kept in orbit around the Earth by gravity.The nature of gravity was first described by Sir Isaac Newton, more than 300 years ago. Gravity is the attraction between any two masses,...