LEO (Low Earth Orbit)
LEO refers to an orbit below approximately 2,000 km altitude, situated between Earth’s atmosphere and the inner Van Allen belts.
This region is relatively less affected by solar protons and galactic cosmic rays (GCR) but is significantly influenced by trapped radiation, especially during South Atlantic Anomaly (SAA) or polar passes.
LEO is widely used for telecommunications, weather monitoring, and scientific research missions. Most currently operating satellites are located in this orbit.
Due to its proximity to Earth, it enables high-resolution imaging and shorter communication latency. Mission durations typically range from 2 to 5 years.
For example, the International Space Station (ISS) orbits at this altitude, circling the Earth about 15.7 times per day at a speed of 27,744 km/h.
Because the radiation environment is comparatively mild, Commercial Off-The-Shelf (COTS) components can often be used, making LEO ideal for low-cost, large-scale satellite constellations.
VLEO (Very Low Earth Orbit)
VLEO lies between 180–400 km altitude.
Its proximity to Earth enables ultra-high-resolution imaging and minimal latency.
However, atmospheric drag is substantial, resulting in high fuel consumption and shorter operational lifespans.
Trapped radiation is minimal, with solar protons being the primary radiation concern.
MEO (Medium Earth Orbit)
MEO extends from 2,000 km to 35,786 km in altitude.
Satellites in this region typically have orbital periods between 2 and 24 hours. Most operate in the 10–12 hour range.
MEO offers a balance between LEO’s low latency and GEO’s broad coverage, requiring fewer satellites for global reach.
However, it intersects the Van Allen belts, exposing satellites to higher radiation levels.
Radiation-hardened components and shielding are essential.
Examples include navigation constellations such as GPS, Galileo, and GLONASS.
GEO (Geostationary Earth Orbit)
GEO is located at approximately 35,786 km altitude, where a satellite’s orbital period matches Earth’s rotation.
Satellites in GEO appear stationary when observed from the ground, making them ideal for communications, broadcasting, and weather observation.
This orbit allows fixed ground antennas and constant coverage of designated regions.
However, the high altitude results in longer signal delays and limited coverage near the poles. Launch costs are also higher.
GEO is outside the Van Allen belts but still within the magnetosphere, making it susceptible to high-energy particles during solar events.
There are two main transfer methods to reach GEO:
- Chemical propulsion: The satellite is launched into a Geostationary Transfer Orbit (GTO), then performs a high-thrust burn at apogee to circularize into GEO.
- Electric propulsion: The satellite is placed in GTO and gradually raises its orbit over several months using low-thrust engines. This method reduces launch mass and cost.
Van Allen belt (purple region) and Earth orbits
GTO (Geostationary Transfer Orbit)
GTO is an elliptical orbit with a perigee near LEO altitude and an apogee at GEO altitude.
It serves as an intermediate stage for delivering satellites to GEO.
The launch vehicle injects the satellite into GTO, and the satellite’s propulsion system performs a burn at apogee to circularize the orbit.
This is commonly achieved via a Hohmann transfer, which is fuel-efficient.
In the diagram below, the yellow ellipse represents the GTO trajectory.
Geostationary transfer orbit
Radiation Assessment by Orbit
LEO benefits from Earth’s magnetic shielding and the Van Allen belts, which reduce exposure to solar particle events (SPE) and significantly attenuate GCR.
However, trapped radiation, especially over the SAA and polar regions, remains a major source of single event effects (SEE).
In contrast, GEO lies outside the inner Van Allen belts but within the magnetosphere (~65,000 km).
During GTO transfer, satellites pass through regions of intense trapped radiation.
Although trapped particles are minimal at GEO altitude, the orbit remains vulnerable to SPE and GCR, especially during solar maximum periods.
| Item | LEO (Low Earth Orbit) | GEO (Geostationary Orbit) |
|---|---|---|
| Primary Radiation Sources | Trapped particles (protons/electrons), Van Allen belts | Solar flares (SPE), Galactic cosmic rays (GCR) |
| Trapped Particle Impact | High (especially over SAA/poles), can cause SEE | Negligible |
| SPE (Solar Proton Events) | Mitigated by geomagnetic field (except poles/SAA) | Significant threat |
| GCR Flux | Very low due to Earth’s magnetic shielding | ~6 times higher than in LEO |
| TID (Total Ionizing Dose) | LEO-Zero: ~136 rad LEO-ISS: ~430 rad LEO-Polar: ~333 rad |
GEO: ~5,930 rad GTO: ~59,630 rad (during transfer) |
| SEE Sources | Mainly trapped protons/electrons (SAA) | Solar flares, GCR |
| LET Impact | Protons (0.1–50 MeV), TID dominant | Protons (10–100 MeV+), SEE dominant |
| Shielding Requirements | Moderate (basic enclosure often sufficient) | High (≥5 mm aluminum recommended) |
| Typical Applications | ISS, CubeSats, Earth observation satellites | Communication, weather, military satellites |
References
Dalehite, K. A., & Xapsos, M. A. (2022). Mission Radiation Environment Modeling and Analysis (NASA STI/TP-20220016301). NASA Goddard Space Flight Center.