Slide 1
Gravity keeps the spacecraft turning
An orbiting spacecraft continually falls under gravity while moving sideways. Its path curves around the world instead of simply dropping onto the surface.
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At the altitude of a low Earth orbit, gravity remains substantial. The sensation of weightlessness comes from the spacecraft and its occupants falling together, rather than from gravity disappearing.
Slide 2
Sideways speed matters
Reaching a great height is not sufficient to remain in orbit. The spacecraft also needs the velocity that produces a suitable path around Earth.
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A vehicle sent almost straight upward can rise far into space and then return. Its high altitude alone does not supply the sideways motion needed to keep missing the ground.
Slide 3
An orbit can stretch
In a bound two-body system, an orbit can be an ellipse, with the central body at one focus. The spacecraft moves faster when it is closer to that body.
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As its distance changes, energy shifts between gravitational potential energy and kinetic energy, the energy of motion. The engine does not need to keep firing to change the speed.
Slide 4
A brief burn changes the whole path
Changing a spacecraft’s velocity at one point changes its subsequent orbit. The resulting motion is governed by the new energy and direction of travel.
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A forward engine burn in a circular orbit can raise the opposite side of the new orbit. The spacecraft does not stay at its old altitude while travelling faster indefinitely.
Slide 5
Real orbits need a fuller model
Atmospheric drag, an uneven gravity field and other bodies perturb, or alter, ideal orbits. Mission planners track these effects to predict and manage the path.
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Low-orbit drag removes orbital energy, causing the orbit to decay unless compensated. The spacecraft can gain speed as it descends while still losing total mechanical energy.