Rich Schuler, an adjunct instructor and outreach coordinator in the physics and astronomy department at the University of Missouri-St. Louis, provides the following explanation.

Gravity can be thought of as a side effect of matter, because any object that has mass generates a gravitational field. If two or more objects are present, then a gravitational force arises between the bodies. This force is always attractive, so objects are always drawn together by gravity.

Isaac Newton was the first scientist to quantify the gravitational force, in 1684. Newton discovered that the gravitational force between every particle of the universe is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Therefore: whereFg is the force due to gravity, G is Newton's gravitational constant, m1 is the mass of one object, m2 is the mass of the second object, and r is the distance between the objects.

As a result, although gravitational force decreases rapidly with distance, the magnitude of the force does not reduce to zero until the separation between two bodies is infinitely large. The universe is immense; however, it is far from infinite in extent. Thus, no particle in the universe is free from gravitational forces. This includes the earth as it revolves around the sun, as well as meteors (or falling stars) and satellites.

Unfortunately, the term "zero gravity" was coined to describe the forces experienced by astronauts in orbit. But an astronaut working in the shuttle bay in an orbit of 300 kilometers (186 miles) is still subjected to the gravitational force of the earth. Because the magnitude of the force is inversely proportional to the square of the distance separating the astronaut and the planet's center, the gravitational force between the astronaut and the earth is roughly 91 percent of the value he or she would experience on the earth's surface. As a result, while in orbit astronauts experience weightlessness not because there is no gravity in space but because an orbiting body is in a constant state of free fall. The astronaut is merely falling around the earth at the same rate as the shuttle is. If the velocity of the space shuttle were to suddenly become zero, it would fall toward the earth and burn up in the atmosphere.

A meteor is a small piece of debris (or meteoroid) that burns up as it enters the earth's atmosphere and is seen by observers as a momentary streak of light. Commonly known as "shooting," or "falling," stars, these objects vary in size from mere dust specks to small bits of rock and metal with masses typically less than one gram (weight of 0.04 oz.). A meteoroid enters the atmosphere at very high speeds (between 10 and 70 kilometers a second, or 20,000 to 150,000 miles an hour) that cause the surface of the object to heat up via friction. The surface of the meteoroid vaporizes and leaves high-temperature atoms, as well as heated molecules, in its wake. The superheated meteor atoms and molecules then glow in a process that is similar to a fluorescent bulb. Thus, when you see a falling star, a piece of debris the size of a grain of sand has been converted into heat, a visible streak of light and atom-size dust particles.