So exactly how can we simulate weightlessness without escaping the
gravitational pull of the Earth? The simplest answer is that we observe
an object in **free fall**. Free fall is when an object falls solely under the influence of gravity. Because of **air resistance**, an object can't truly be in a free fall without being in a **vacuum**.

Photo courtesy of Zero Gravity CorporationZero Gravity Corporation Parabolic flight path |

In order for passengers in a plane to experience a free fall safely,
the aircraft must climb at a steep angle, level off, and then dive,
creating a path called a **parabolic arc**, also called a **Keplerian Trajectory** or **free-fall path**. In a true parabolic arc, the only accelerative force is gravity pulling in a vertical direction -- horizontal **velocity**
remains constant. Because of air resistance, objects in Earth's
atmosphere only travel in arcs that approximate a true parabola.

Typically, ZERO-G's plane, called G-FORCE-ONE, flies between 24,000 and 32,000 feet altitude. This gives the pilot enough room to maneuver the plane safely through its flight path. The plane's descent must start at a high altitude to provide enough distance for the pilot to safely pull out of a dive. As the plane climbs to the peak of its arc, the pilot orients it at a 45-degree angle. During the climb, the plane's acceleration and the force of gravity create a pull 1.8 times the strength of gravity alone -- passengers temporarily weigh nearly twice as much as normal.

Photo courtesy of Zero Gravity Corporation A passenger aboard the G-FORCE-ONE |

**centrifugal force**exerted on the plane and everything in it cancels out the gravitational force pulling downwards. At this point, passengers experience

**microgravity**-- it feels as if you are weightless because only negligible gravitational forces are present. The sense of weightlessness lasts for about 30 seconds. Because the plane shields the passengers from the rush of air, they can experience a free fall without the interference of air resistance.

The pilot pulls the plane out of the dive so that the dip between one arc and the next is at about 24,000 feet altitude. As the plane pulls out of the dive and begins to climb again, passengers again experience the force of 1.8 times that of gravity. The typical ZERO-G flight includes 15 of these parabolic arcs, while NASA flights may include up to 100.

In the next section, we'll find out how passengers prepare for zero gravity -- and motion sickness.