It is the initial minimum speed that an object needs to escape the gravitation of an astronomical body and continue moving without having to make another propelling effort. The escape velocity is generally given in terms of launch speed without regard to aerodynamic friction.
Objects that move at a speed below 0.71 times the escape velocity cannot achieve a stable orbit. At a velocity equal to 0.71 times the escape velocity, the orbit is circular, and at a higher velocity, the orbit becomes an ellipse until it reaches the escape velocity and then, the orbit becomes a parabola. Therefore, the escape velocity is also called parabolic velocity.
The escape velocity of an object from a spherical astronomical body is proportional to the square root of the body's mass, divided by the distance between the object and the center of the body. The approximate escape velocity of the Earth is 11.2 kilometers per second.
The Earth continues conserving its atmosphere, after more than 4,500 million years of its formation, because the average speed of the gas molecules that compose the atmosphere is much lower than they would have to have to overcome the gravitational attraction of the Earth.
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