Ryan M. Patrick

When I alpha or beta read, or when I see published works involving topics that I am well-versed in from my professional life – astrodynamics, space-to-ground communications, orbits, etc – I often cringe. Most authors at least try to get the high-level stuff right, but get lost in the details, while others simply don’t understand the material. While I am by no means an expert in any of these space topics, I do have a B.S. and M.S. in astronautical engineering and 12 years of experience in the space industry.

Once a month, I’m going to go into detail on a space-related topic, with the inaugural one here dealing with the different types of orbits.

Low Orbits

Shockingly, this is the most complicated orbital regime.

“Space” is usually defined as above the Kármán line, an imaginary barrier 100 km above the surface of the Earth. But, it’s not universally accepted. The US Air Force gives astronaut wings to those who break 50 miles of altitude (roughly 80 km) while the atmosphere extends beyond the 100 km of the Kármán line. However, that’s not enough for a stable orbit – that’s closer to 150 km to be able to complete one full revolution without any kind of propulsion.

Low Earth Orbit (LEO) is generally defined as the regime between the Karman line (100 km) and an altitude of 2000 km. This is where most manmade objects in space – including the ISS – reside, and outside of the Apollo missions to the Moon, it’s where all human spaceflight has taken place.

Satellites here move fast – on the magnitude of kilometers per second – and have short orbital periods, less than 120 minutes or so. This is good for remote sensing applications, as they can pass over the same area multiple times per day, as well as megaconstellations like Starlink as the distance to Earth is shorter than in higher orbits. It’s also easier to launch into LEO – less energy is required. However, passes from the ground (where an antenna can communicate with the satellite) are short, and the satellite is out of view of a ground station for much of its orbit. There’s also atmospheric drag, continually pulling a satellite down to the Earth and requiring constant station-keeping that uses precious fuel.

Medium Orbits

Medium Earth Orbits (MEO) are from about 2000 km to 35,786 km – the altitude of a geostationary satellite (more on that later). This can be a nasty radiation environment, and isn’t used much at the current moment.

The primary user of MEO is the U.S. Space Force’s GPS constellation, currently consisting of 31 operational satellites and a few on-orbit spares. It continually broadcasts radio signals down to Earth, allowing for precise position calculation with as little as 4 (but preferably 6!) satellites. There’s other spacecraft there as well, the DSX satellite that I launched on STP-2 is no longer operational but collected valuable weather data for two years.

As LEO and GEO fill up, more uses for MEO will come to the forefront, but for now it’s pretty empty.

Stationary Orbits

At an altitude of 35,786 km above the Earth’s surface, a satellite’s orbit matches the rotation of the Earth. This shows on a ground track as a dot – the satellite is always overhead in a Geostationary Earth Orbit (GEO) at zero degrees of inclination from the Earth’s ecliptic. This is incredibly important for communications satellites such as Intelsat, as well as missile warning applications such as the U.S. Space Force’s SBIRS program. It’s a high-energy orbit – a launch or transfer vehicle has to expend a lot of fuel to get there – but once it’s in GEO, only a little bit of station-keeping is required. Some GEO birds stay in the same position, others stay in a semi-synchronous orbit that looks like a figure-8 around a GEO slot. This is the most important orbital regime for both commercial and military applications, and if a future space war is to be fought, it’ll likely be centered around the GEO belt.

Oddball Orbits – Molniya

I’ll touch on launch trajectories and geometries in a future post, but a launch site can only send a satellite into an inclination equal or greater than its latitude without doing an orbit transfer (more on that in the next section!). If you look at Russia and the former Soviet Union on a map, there’s no part of it that is near the equator – its main launch site at the Baikonur Cosmodrome is at 45.9646° N latitude, requiring an expensive and time-consuming plane change to get to a geosynchronous orbit.

How can they get the same advantages of a synchronous orbit with the geographic constraints? By what’s called a Molniya orbit, after the series of communications satellites that use it. it is heavily inclined, and has a low perigee and a high apogee as seen below. What this means in context is that the satellite spends most of its orbital period in a slow, lazy arc around the part of the Earth that the satellite is more interested in, while speeding around the other side, usually in pairs for maximum coverage.

Orbit Transfers

How can you go from one orbit to another? There’s dozens of methods, but the most energy efficient way is called a Hohmann Transfer.

In it, a spacecraft in a certain orbit – the red line above – fires its thrusters to go on a transfer orbit (purple line) that is the lowest-energy approach to the new orbit in green. There’s faster ways to get there, and other methods just as the bi-elliptic transfer that use less propellant at the cost of a much-greater travel time, but a Hohmann transfer is proven to be the most efficient orbit.