Beginner · ABOUT 2 MINUTES READING
Gravity and Orbits
An orbit is continuous free fall with enough sideways motion to keep missing the central body.
Motion without a supporting surface
Gravity changes an object’s velocity, meaning its speed, direction or both. In a nearly circular orbit, the direction continually changes even when speed is nearly constant. The orbiting object is accelerating. It does not need a continuous upward engine force to stay in orbit when drag and other disturbances are negligible.
Weightlessness is not absent gravity
An astronaut and the surrounding spacecraft fall together. With little support force between them, they experience apparent weightlessness even though Earth’s gravity remains substantial. The term microgravity describes the small residual accelerations in this environment, not a region where gravity has switched off.
Changing an orbit
A thrust maneuver changes orbital energy and shape. Raising the opposite side of a simple orbit generally starts by accelerating along the direction of travel. Once settled into a higher circular orbit, the required orbital speed is lower. These two statements concern different stages of the maneuver and are not contradictory.
Key takeaways
- An orbit combines gravity and sideways motion.
- Free fall explains astronaut weightlessness.
- An orbit change must be described over the whole trajectory.
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Knowledge check
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Editorially reviewed . Mission plans can change; this lesson is an educational reference, not a live status feed.
Connect the ideas
Trace planetary orbits in 3D · Find the planets in your sky
Space Vault: Orbital Rendezvous · Mission Control · Back to your track