Why Orbital Mechanics Matters for Trackers
You don't need to be a rocket scientist to track satellites — but a working understanding of orbital mechanics will make you a significantly better observer. It explains why pass windows are predictable, why TLEs expire, why some satellites always pass over the poles while others hug the equator, and why reentry is inevitable for low-orbit objects.
Kepler's First Law: Orbits Are Ellipses
Johannes Kepler's first law states that every satellite follows an elliptical orbit with Earth at one of the two foci — not at the center. For most operational satellites (including the ISS), the orbit is nearly circular, meaning the eccentricity is very close to zero. A perfectly circular orbit has eccentricity = 0; a very elongated orbit like a Molniya satellite might have eccentricity = 0.7 or higher.
Key terms:
- Perigee — the closest point of the orbit to Earth's surface
- Apogee — the farthest point of the orbit from Earth's surface
- Semi-major axis — half the longest diameter of the ellipse; directly determines orbital period
Kepler's Second Law: Equal Areas in Equal Times
A satellite sweeps out equal areas of its orbital ellipse in equal amounts of time. The practical consequence: satellites move fastest at perigee and slowest at apogee. For nearly circular LEO orbits this effect is minor, but for highly elliptical orbits it's dramatic — a Molniya satellite spends most of its 12-hour orbit near apogee (over high latitudes), providing long dwell times for communication, then races through perigee in minutes.
Kepler's Third Law: Period and Distance Are Linked
The square of a satellite's orbital period is proportional to the cube of its semi-major axis. In plain terms: the higher the orbit, the longer the orbital period.
- LEO (Low Earth Orbit, ~200–2000 km): Period of roughly 90–127 minutes. ISS orbits at ~408 km with a period of ~92 minutes.
- MEO (Medium Earth Orbit, ~2000–35,786 km): GPS satellites orbit at ~20,200 km with a 12-hour period.
- GEO (Geostationary, ~35,786 km): Period = 24 hours, so the satellite appears stationary over a fixed point on the equator.
Orbital Inclination: Why Some Satellites Cover More Ground
Inclination is the angle between the orbital plane and Earth's equatorial plane. It determines which latitudes the satellite passes over:
| Inclination | Coverage | Example |
|---|---|---|
| 0° | Equatorial only | Geostationary satellites |
| 28–52° | Mid-latitudes | ISS (~51.6°), many commercial sats |
| 90° | Polar — full Earth coverage | NOAA weather satellites (~98°) |
| 97–99° | Sun-synchronous polar | Most Earth observation satellites |
Sun-synchronous orbits are slightly retrograde (>90°) and are designed so the orbital plane precesses at the same rate Earth orbits the Sun — meaning the satellite always crosses the equator at the same local solar time, giving consistent lighting for Earth imaging.
Orbital Decay and Why Satellites Fall
In LEO, residual atmosphere creates aerodynamic drag on satellites. Each orbit, the satellite loses a tiny amount of energy, causing it to drop slightly lower. As altitude decreases, atmospheric density increases, drag increases, and decay accelerates — an unstable positive feedback loop that ends in atmospheric reentry. The ISS requires regular reboosts (from Progress supply ships or its own thrusters) to maintain its ~408 km altitude.
The rate of decay depends on:
- Altitude — lower orbits decay faster
- Ballistic coefficient — the ratio of mass to cross-sectional area (denser, compact objects last longer)
- Solar activity — high solar activity heats and expands the upper atmosphere, increasing drag at all LEO altitudes
Putting It Together
Every parameter in a TLE connects back to these Keplerian concepts. When you see a satellite's inclination, eccentricity, and mean motion in a TLE, you now know exactly what physical reality those numbers describe. This intuition is what separates a skilled tracker from someone just pointing software at the sky.