Orbital Mechanics 101: How Satellites Stay in Orbit

Every satellite in orbit is, in a very real sense, falling. It is falling toward Earth at every moment — but moving forward so quickly that the curve of its fall matches the curve of the planet beneath it. That elegant balance between gravity and velocity is the foundation of orbital mechanics, and understanding it is essential for anyone working in the space industry. This guide walks through the core principles in plain language, with enough technical depth to be genuinely useful.

Newton's Cannonball: The Core Idea

Isaac Newton imagined a cannon on top of a very tall mountain. Fire a cannonball horizontally, and it arcs toward the ground. Fire it faster, and it travels farther before hitting the surface. Fire it fast enough, and the cannonball falls at exactly the rate the Earth curves away beneath it — it never lands. It just keeps falling around the planet. That is an orbit.

This thought experiment captures the essential insight: an orbit is not about defying gravity. It is about falling with enough sideways speed that you perpetually miss the ground. Gravity provides the centripetal force that bends the satellite's path into a curve; the satellite's velocity determines the shape and size of that curve.

Kepler's Three Laws of Planetary Motion

Johannes Kepler described three laws in the early 1600s that still govern every orbit today:

First Law: Orbits Are Ellipses

Every orbit is an ellipse with the central body (Earth, the Sun, etc.) at one of the two foci. A circle is a special case of an ellipse where both foci coincide. Most practical satellite orbits are nearly circular, but transfer orbits, highly elliptical orbits (HEO), and Molniya orbits are deliberately elongated for specific coverage or efficiency reasons.

Second Law: Equal Areas in Equal Times

A line drawn from the satellite to the center of Earth sweeps out equal areas in equal time intervals. The practical consequence: satellites move faster when they are closer to Earth (at perigee) and slower when they are farther away (at apogee). This is why Molniya orbit satellites spend most of their time over high-latitude regions — they slow down near apogee, which is positioned over the coverage area.

Third Law: Period Squared Proportional to Semi-Major Axis Cubed

The orbital period (time for one complete orbit) is related to the size of the orbit by T² = (4π²/GM) × a³, where a is the semi-major axis and GM is Earth's gravitational parameter. This law is what makes geostationary orbit special: at an altitude of approximately 35,786 km, the orbital period is exactly 24 hours, matching Earth's rotation. The satellite appears to hover over a single point on the equator.

Orbital Velocity: The Numbers

For a circular orbit, the required velocity is v = √(GM/r), where r is the distance from the center of Earth. At key altitudes:

• LEO (400 km): ~7.67 km/s (27,600 km/h) — ISS orbital speed. Period: ~92 minutes
• MEO (20,200 km): ~3.87 km/s — GPS satellite speed. Period: ~12 hours
• GEO (35,786 km): ~3.07 km/s — geostationary speed. Period: 24 hours
• Escape velocity (surface): ~11.2 km/s — speed needed to leave Earth's gravitational influence entirely

Notice the counterintuitive relationship: higher orbits require lower velocity. This is because gravitational pull weakens with distance. However, reaching a higher orbit requires more energy overall because you must first climb out of the deeper part of Earth's gravity well.

Common Orbit Types and Their Uses

Low Earth Orbit (LEO): 200-2,000 km

Used by: Starlink, ISS, Earth observation satellites, most CubeSats. LEO offers low latency (ideal for broadband internet), high-resolution Earth imaging, and lower launch costs. The trade-off is limited coverage per satellite (requiring large constellations) and orbital decay from atmospheric drag, especially below 500 km.

Medium Earth Orbit (MEO): 2,000-35,786 km

Used by: GPS, Galileo, GLONASS, O3b/SES. MEO provides a balance between coverage area and latency. Navigation constellations operate here because 24-30 satellites can provide global coverage with reasonable signal travel times.

Geostationary Orbit (GEO): 35,786 km

Used by: Communications satellites (SES, Intelsat, Viasat), weather satellites (GOES), missile warning systems. A single GEO satellite can see roughly one-third of Earth's surface, and just three can cover nearly the entire planet. The trade-off is high latency (~600ms round trip) and expensive launch costs.

Sun-Synchronous Orbit (SSO): 600-800 km, ~98° inclination

Used by: Earth observation satellites (Planet, Maxar). SSO satellites pass over any given point at the same local solar time on every orbit, providing consistent lighting conditions for imaging. This orbit precesses at exactly the rate Earth orbits the Sun, maintaining its orientation relative to the Sun throughout the year.

Orbital Maneuvers: How Satellites Change Orbits

Changing an orbit requires changing velocity — a quantity called delta-v (Δv). The most common maneuver is the Hohmann transfer, which moves a satellite between two circular orbits using two engine burns:

1. First burn: At the starting orbit, fire the engine prograde (in the direction of travel) to raise the opposite side of the orbit to the target altitude. This creates an elliptical transfer orbit.
2. Second burn: At the highest point of the transfer orbit, fire prograde again to circularize at the new altitude.

The Hohmann transfer is the most fuel-efficient two-burn maneuver for coplanar orbit changes. For a transfer from LEO (400 km) to GEO, the total delta-v is approximately 3.9 km/s — a significant fraction of the satellite's total velocity budget.

Other maneuvers include plane changes (rotating the orbital plane, extremely expensive in delta-v), phasing maneuvers (adjusting position within an orbit), and station-keeping burns (small corrections to maintain precise orbital parameters against perturbations).

Why Orbits Aren't Perfect: Perturbations

In reality, orbits are not perfect Keplerian ellipses. Several forces perturb satellite orbits over time:

• Atmospheric drag: In LEO, residual atmosphere creates drag that slowly lowers the orbit. The ISS loses about 2 km of altitude per month and requires regular reboosts. Drag increases dramatically during solar maximum when the atmosphere expands.
• Earth's oblateness (J2): Earth is not a perfect sphere — it bulges at the equator. This causes orbital planes to precess (rotate) over time. Sun-synchronous orbits deliberately exploit this effect.
• Third-body effects: Gravitational pull from the Moon and Sun creates long-period perturbations, especially significant for GEO satellites.
• Solar radiation pressure: Photons from the Sun exert a small but continuous force on satellites, particularly those with large solar arrays or high area-to-mass ratios.

Why This Matters for Space Professionals

Understanding orbital mechanics is not just academic. It directly affects business decisions in the space industry:

• Constellation design: Choosing the right altitude, inclination, and number of orbital planes determines coverage, latency, and total system cost
• Launch planning: Orbital mechanics dictates launch windows, trajectory design, and the amount of propellant needed — all of which drive launch costs
• Satellite operations: Station-keeping budgets, collision avoidance maneuvers, and end-of-life disposal all depend on delta-v calculations
• Spectrum management: Orbital altitude and constellation geometry affect interference patterns, link budgets, and regulatory coordination

SpaceNexus provides tools to explore these concepts hands-on. Use our Orbital Calculator to compute orbital parameters, velocities, and transfer maneuvers for any orbit.

Try the SpaceNexus Orbital Calculator