How Do Orbits Work?
An orbit is an ellipse with the central body at one focus, and Kepler's three laws describe it: orbits are ellipses, an object moves fastest at its closest point, and the square of the orbital period is proportional to the cube of the orbit's size (T² ∝ a³). Three numbers carry most of the practical meaning (semi-major axis, eccentricity and inclination), and moving between orbits takes two carefully timed burns called a Hohmann transfer.
SSA-01 Module 2 gave orbital mechanics and the space environment a first pass — enough to place LEO, MEO, and GEO on a map and know why "zero gravity" is a myth. This module goes further: the actual laws governing orbits, how you move between them, and how the environment forces specific engineering decisions rather than just being hostile in general.
Kepler's Three Laws
Johannes Kepler worked these out from planetary observation data decades before Newton explained why they're true. They apply just as well to a satellite orbiting Earth as to a planet orbiting the Sun.
Orbits Are Ellipses
Not circles. Every orbit is an ellipse with the central body (Earth, the Sun) at one focus, not the center. A circular orbit is just the special case where both foci coincide.
Equal Areas in Equal Time
A line from the central body to the orbiting object sweeps out equal areas in equal time — which means an object moves fastest at its closest approach and slowest at its farthest point.
Period Relates to Size
The square of an orbital period is proportional to the cube of the orbit's size (T² ∝ a³). This is exactly why GEO sits at one specific altitude — it's the only altitude where the math works out to a 24-hour period.
Three Numbers That Define It
Real orbital mechanics uses six elements. These three carry most of the practical meaning.
Semi-Major Axis
Half the "long axis" of the orbital ellipse — effectively the orbit's overall size. Per Kepler's third law, this single number determines the orbital period.
Eccentricity
How stretched the ellipse is, from 0 (a perfect circle) toward 1 (increasingly elongated). Most operational satellites use low-eccentricity, near-circular orbits; some science and communications missions deliberately use highly eccentric ones.
Inclination
The tilt of the orbital plane relative to Earth's equator. A 0° orbit stays directly over the equator; a 90° polar orbit crosses over both poles on every revolution.
The Hohmann Transfer
Moving from one circular orbit to another isn't a straight shot — it's the most fuel-efficient two-burn maneuver between them, named for Walter Hohmann, who worked it out mathematically in 1925.
- Starting orbit — the lower circular orbit the spacecraft begins in.
- Target orbit — the higher circular orbit the spacecraft needs to reach.
- Transfer orbit — an ellipse tangent to both circles. Burn 1 (at the starting orbit) stretches the orbit into this ellipse; burn 2 (at the target orbit) circularizes it there. Two burns, no more, no less.
Illustrative — not to scale. Real burn sizes and transfer times depend on the specific altitudes involved.
Environment Drives Design
SSA-01 Module 2 named vacuum, radiation, and thermal extremes as threats. Here's specifically what engineers do about each one.
Vacuum → Material Selection
Ordinary materials outgas — slowly releasing trapped gases and vapors — in vacuum, which can fog optics and contaminate sensitive surfaces. Spacecraft use low-outgassing, vacuum-rated materials specifically to avoid this.
Radiation → Hardening and Redundancy
Radiation can flip bits in electronics or degrade solar cells over time. Spacecraft use radiation-hardened components, shielding around the most sensitive electronics, and redundant systems that can outvote a single corrupted result.
Thermal Extremes → Active and Passive Control
Multi-layer insulation (MLI) blankets — the gold or silver foil visible on most spacecraft — passively reflect radiant heat, while radiators and heaters actively manage the swing between sunlit and shadowed sides.
Micrometeoroids & Debris → Shielding
Tiny natural particles and human-made debris both travel fast enough to punch through a thin hull. Whipple shielding — a sacrificial outer layer set slightly away from the main hull — breaks up an impactor before it can reach anything critical.
Key Takeaways
- Every orbit is an ellipse with the central body at one focus; a circle is just the special case where both foci coincide.
- A line from the central body to the orbiting object sweeps equal areas in equal time, so an object moves fastest at its closest approach and slowest at its farthest point.
- The square of the orbital period is proportional to the cube of the orbit's size, which is why geostationary orbit sits at one specific altitude: the only one that gives a 24-hour period.
- Semi-major axis is the size of the orbit and sets the period, eccentricity runs from 0 (a circle) toward 1 (very elongated), and inclination is the tilt relative to the equator (0° over the equator, 90° polar).
- A Hohmann transfer is the most fuel-efficient two-burn way to move between two circular orbits: the first burn stretches the orbit into a transfer ellipse, the second circularizes it at the target.
- The space environment forces specific design choices: low-outgassing materials for vacuum, radiation-hardened and redundant electronics, multi-layer insulation for thermal swings, and Whipple shielding against micrometeoroids and debris.
Frequently Asked Questions
Are orbits circles?
No. Every orbit is an ellipse with the central body at one focus. A circular orbit is the special case where eccentricity is 0.
Why does geostationary orbit sit at one specific altitude?
Kepler's third law ties orbital period to orbit size, and only one altitude gives a 24-hour period.
What is a Hohmann transfer?
The most fuel-efficient two-burn maneuver for moving between two circular orbits. Walter Hohmann worked it out mathematically in 1925. Burn 1 puts the spacecraft on an elliptical transfer orbit tangent to both circles, and burn 2 circularizes it at the target orbit.
What do eccentricity and inclination mean?
Eccentricity describes how stretched an orbit is, from 0 for a perfect circle toward 1 for a highly elongated ellipse. Inclination is the tilt of the orbital plane relative to Earth's equator: a 0° orbit stays over the equator and a 90° polar orbit crosses both poles on every revolution.
Why does a satellite move faster at some points in its orbit?
Kepler's second law: the line to the central body sweeps equal areas in equal time, so the object is fastest at its closest approach (perigee) and slowest at its farthest point (apogee).
Key Terms
- Semi-Major Axis
- Half the long axis of an elliptical orbit — its overall size, and the single value that determines orbital period.
- Eccentricity
- How elongated an orbit is, from 0 (a circle) toward 1 (a highly stretched ellipse).
- Inclination
- The tilt of an orbital plane relative to Earth's equator — 0° stays over the equator, 90° is a polar orbit.
- Perigee / Apogee
- The closest and farthest points of an Earth orbit from the planet's center — the points where an object moves fastest and slowest, respectively.
- Hohmann Transfer
- The most fuel-efficient two-burn maneuver for moving between two circular orbits, using an elliptical transfer path tangent to both.
- Outgassing
- The slow release of trapped gases and vapors from a material in vacuum — a contamination risk that drives spacecraft material selection.
- Multi-Layer Insulation (MLI)
- Reflective foil blanketing, usually gold or silver, used to passively control a spacecraft's temperature by reflecting radiant heat.
- Whipple Shielding
- A sacrificial outer layer spaced away from a spacecraft's main hull, designed to break up a micrometeoroid or debris impactor before it reaches critical structure.
For This Module
NASA Glenn Research Center orbital mechanics and Hohmann transfer primers, NASA/ESA spacecraft materials and outgassing guidelines, and NASA orbital debris and micrometeoroid shielding technical documentation.
SSA-02 — Introduction to Space Flight
- Module 1 Rocket Propulsion Basics
- Module 2 Launch Vehicles
- Module 3 Orbital Mechanics and Space Environment
- Module 4 Applied Satellite Systems
- Module 5 Living and Working in Space
- Module 6 Non-U.S. Space Programs
Continue the Sequence
Module 4 covers applied satellite systems — navigation, meteorology, and telecommunications, end to end.