Orbital Mechanics Basics

Overview

Orbit is not just “where your satellite goes.” It determines how much sunlight you get, how often you talk to your ground station, how hot or cold you run, and what your payload can observe. Mission design starts with orbit, not hardware.

What Is an Orbit

An orbit is a path defined by energy, shape, and orientation. Three parameters matter most at the introductory level:

• Altitude — Influences atmospheric drag, orbital lifetime, and ground coverage area.
• Inclination — Determines which latitudes the satellite flies over on each pass.
• Eccentricity — Describes how circular or elongated the orbit is. Most CubeSat orbits are near-circular.

Most CubeSats orbit between 400–600 km altitude in LEO, completing one orbit every ~90–95 minutes at speeds around 7.5–7.8 km/s.

Example Orbital Parameters

Altitude	Period	Velocity	Lifetime (approx)
300 km	~90.5 min	~7.73 km/s	1–3 months
400 km	~92.5 min	~7.67 km/s	1–2 years
500 km	~94.5 min	~7.61 km/s	5–10 years
600 km	~96.5 min	~7.56 km/s	15–25 years

Kepler’s Laws (Practical Edition)

Johannes Kepler described three laws that govern orbital motion. Here’s what they mean in practice for CubeSat designers:

• Law 1 — Orbits are ellipses. The central body (Earth) sits at one focus. In LEO, most orbits are close to circular, but they are still technically ellipses with very low eccentricity.
• Law 2 — Satellites move faster near perigee. A satellite in an elliptical orbit speeds up at its closest approach to Earth and slows down at its farthest point (apogee). For near-circular orbits, speed is nearly constant.
• Law 3 — Higher orbits have longer periods. The relationship is T² ∝ a³. Raise the altitude and the orbital period increases. This directly affects pass frequency and contact time with ground stations.

Student takeaway: altitude up = longer period, usually longer lifetime. Drag down low is real and shortens missions. At 300 km, atmospheric drag can deorbit a CubeSat in weeks.

Orbital Elements

Six classical orbital elements fully describe a satellite’s orbit. These are the parameters you’ll encounter in mission planning tools, TLE data, and orbit design discussions:

• Semi-major axis (a) — Sets the size of the orbit. Directly determines altitude and orbital period.
• Eccentricity (e) — Sets the shape. 0 = circular, close to 1 = very elongated ellipse.
• Inclination (i) — How far north and south the orbit reaches. 0° = equatorial, 90° = polar.
• RAAN (Ω) — Right Ascension of the Ascending Node. Affects local time of the orbit and sun lighting patterns.
• Argument of perigee (ω) — Describes where the closest point of the orbit is located. Matters more for elliptical orbits.
• True anomaly (ν) — Where the satellite is along the orbit at a given moment.

Two-Line Element Sets (TLEs)

TLEs are a standardized format encoding orbital elements for Earth-orbiting objects. Each satellite tracked by the US Space Force has a TLE. You can find them on Space-Track.org and CelesTrak.

TLEs are used with SGP4 propagation models to predict satellite positions. They need regular updates because orbital perturbations — atmospheric drag, gravitational harmonics, solar radiation pressure — cause them to drift from reality over time.

Common Orbit Types

LEO — Low Earth Orbit

Below ~2,000 km altitude. This is where most CubeSats fly. LEO offers easier communications (shorter path loss), simpler launch access, and lower radiation exposure compared to higher orbits. The tradeoff: atmospheric drag reduces orbital lifetime at lower altitudes.

SSO — Sun-Synchronous Orbit

Typically 600–800 km altitude at ~97–98° inclination. The orbit plane precesses at a rate that keeps a consistent angle relative to the Sun. This provides consistent lighting conditions over ground targets — ideal for Earth observation and imaging missions.

Polar Orbit

Near 90° inclination, typically 300–1,000 km altitude. As Earth rotates beneath the orbital plane, a polar orbit provides full global coverage over time. Common for weather and environmental monitoring missions.

MEO / GEO

Medium Earth Orbit (2,000–35,786 km) and Geostationary Orbit (35,786 km). These are rare for CubeSats — they require significantly more delta-v to reach and expose satellites to higher radiation environments.

Orbit Type Comparison

Type	Altitude	Inclination	Best For
LEO	300–2,000 km	Any	Most CubeSats, tech demos, IoT
SSO	600–800 km	~97–98°	Earth observation, imaging
Polar	300–1,000 km	~90°	Global coverage, weather
MEO	2,000–35,786 km	Varies	Navigation (rare for CubeSats)

Ground Tracks & Pass Planning

Ground tracks show where your satellite flies over Earth’s surface. Understanding ground tracks is essential because you do not see your ground station all the time. Passes come in windows — often just 5–15 minutes each — and your ground station location strongly shapes your total downlink capacity.

Beginner Pass Analysis Workflow

1. Choose your orbit type (LEO, SSO, polar).
2. Choose your altitude.
3. Simulate passes over your ground station(s).
4. Estimate total contact time per day.
5. Convert contact time to data volume using your link budget.

Free Tools

• GMAT — NASA’s General Mission Analysis Tool. Open source, full-featured orbit propagation and mission design.
• STK — Systems Tool Kit by Ansys. Free tier available for students. Industry-standard for orbit visualization and analysis.
• Orbitron — Lightweight desktop tool for real-time satellite tracking and pass prediction.
• Online ground track visualizers — Browser-based tools for quick orbit visualization without installing software.

Knowledge Check

Test your understanding of orbital mechanics basics.