Start with free fall
Free fall means moving under gravity without a supporting surface holding you up. A satellite in an ideal orbit is falling all the time. Its sideways motion carries it far enough that Earth's curved surface falls away beneath its path. This description is a physical explanation, not a claim that gravity disappears at high altitude.
A helpful thought experiment is throwing a ball faster and faster from a very high mountain. The analogy ignores the atmosphere and assumes a simplified Earth. Its purpose is to show how sideways velocity changes a falling object's path. Real launches must also manage air resistance, structural limits and the fact that no such mountain exists.
A spacecraft's mass does not determine its ideal orbital period around Earth. At a given position and velocity, a small satellite and a large one follow the same gravitational trajectory, ignoring other forces. Their responses to drag or propulsion can be very different.
What the orbit description tells you
Altitude
Height above a reference surface. In an elliptical orbit, altitude changes continuously; engineers distinguish the lowest point, perigee, from the highest, apogee.
Inclination
The angle between the orbital plane and Earth's equatorial plane. It helps determine which latitudes the spacecraft can pass over.
Period
The time needed for one orbit. For an ideal circular Earth orbit, a greater orbital radius means a longer period and a lower orbital speed.
These parameters are connected. You cannot independently demand any altitude, speed and period you like. They must describe a physically possible trajectory. Operational predictions also account for Earth's uneven gravitational field, atmospheric effects and the influence of other bodies.
Choose the view, then count the costs
Low Earth orbit is useful for detailed observations because the sensor is relatively close to the ground. The trade-off is motion: a spacecraft passes over a location rather than hovering above it. Ground contacts are limited, and Earth rotates beneath successive tracks. A mission must plan when it can collect data and when it can send that data home.
A geostationary satellite follows a circular, equatorial, prograde orbit with a period matching Earth's sidereal rotation. It appears nearly fixed in the sky to a ground observer. That is valuable for continuous regional communications and weather monitoring. It is not above the poles, and its large distance creates different antenna, resolution and signal-delay requirements.
A Sun-synchronous orbit uses a carefully selected inclination and altitude so that the orbital plane precesses at a useful rate. Many Earth-observation missions use this to keep a similar local solar time at overpass. It does not mean the satellite remains over one location or receives uninterrupted sunlight.
Repeat time is not useful-image time
Suppose our vegetation mission returns to the same viewing geometry on a regular schedule. That schedule alone does not guarantee a clear image. Cloud cover, instrument availability, viewing angle and ground-processing delays all influence when a usable product arrives.
Revisit can be improved with a wider sensor swath, multiple satellites or off-nadir pointing, meaning looking away from straight down. Each solution changes something else. A wider swath can complicate optics; extra satellites increase operations work; angled viewing changes image geometry and may affect measurement quality.
Worked example: raise an orbit without “driving upward”
In a simplified circular orbit, a brief forward thrust increases velocity at that point and creates an elliptical path with a higher opposite side. A second appropriately timed burn can circularize the new orbit. Real mission planning must account for propulsion limits, collision risk and navigation uncertainty. This is an explanation, not maneuver instructions.
Orbits need an end-of-life plan
The upper atmosphere is extremely thin, but its drag can still matter. Drag removes orbital energy and generally lowers an orbit over time. Solar activity changes atmospheric density, so predicting decay is not simply a matter of reading the current altitude.
Spacecraft operators also consider conjunctions: predicted close approaches between objects. Tracking estimates have uncertainty, so a conjunction alert is not a certain collision prediction. Decisions combine estimated risk, maneuver capability, mission effects and coordination with other operators.
Disposal planning belongs at the beginning of design. A mission might reserve propellant for controlled reentry, lower its orbit to shorten its remaining lifetime, or follow another disposal approach suitable for its operating region. Applicable requirements depend on the mission and licensing context; there is no single rule that makes every orbit sustainable.
The systems connection: your orbit affects sunlight, battery demand, communications windows, instrument geometry and disposal. It is a mission-wide choice, not just a line on a map.
Continue the chain
Now that the spacecraft moves through a predictable set of opportunities, ask how it exchanges information during those opportunities.
Follow the radio link