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Option for Staging in Geosynchronous Transfer Orbit

One of the proposed options for the Reference Mission is staging from a geosynchronous transfer orbit (GTO) rather than from low Earth orbit (LEO). GTO has some advantages for unmanned spacecraft that can be packaged into a single launch, but it would pose some interesting problems for the manned flights as we currently have them envisioned.

Energy and Fuel

While the energy for GTO is less for the moon-bound spacecraft (hence saving lots of fuel or giving us additional payload margin), it means initially going into a higher-energy orbit. So the booster has to make up for part of the fuel we save for the translunar part of the flight.

The details of booster and upper stage performance drive the details. In general, we'll be better off assembling our moonships and storing the resupply material for subsequent flights in low Earth-hugging orbits. The Artemis Project creates a growing demand for access to LEO as the flights go on.

Orbital Inclination

Orbital inclination presents us with a bit of a problem. Orbital inclination is the angle between the plane of an orbit and the plane of Earth's equator.

Geostationary orbit, where all the commercial communications satellites fly, lies in the plane of Earth's equator. However, the International Space Station and our conceptual LEO staging base would both be located in a high-inclination orbit. (See section for discussions of the LEO staging base and the reasons for putting it into a high-inclination orbit.)

When we say "low-inclination" we mean an orbit tilted at an angle of 28.5 degrees or less. This is the latitude of the Kennedy Space Center in Florida. "High-inclination" refers to orbits tilted to cover the more extreme northern and southern latitudes; such as the 51.6-degree orbits we get when we launch a Proton rocket from the Baikonur Cosmodrome in Russia.

When you are operating close to the Earth, it takes a tremendous amount of energy (hence, fuel) to change orbital inclination. If you wanted to change between 28.5 degress and 51.6 degrees of orbital inclination, it would take less fuel to land and then take off again than to burn your rockets to push your orbital inclination up or down to get where you want to go.

Getting to the moon from any near-Earth orbit is a different matter. The moon's orbit around the Earth is tilted at 5 degrees to Earth's equator. (See section M 3.12 for more details about the orbital mechanics of the moon.) However, the moon is a quarter million miles away from Earth, so we have a much smaller difference in energy to get to the moon from a high-inclination orbit vs. a low-inclination orbit.


Radiation poses an even more significant problem. We want to launch the Artemis Project lunar spacecraft in large elements, because we don't have a booster that can put the entire stack plus the crew into orbit with one launch. That means our spacecraft will be loitering in Earth orbit before heading out to the moon. A spacecraft in an elliptical GTO will spend most of its time in earth's radiation belts.

Artemis Project lunar spacecraft assembled in Earth orbit

Since our assembly point is a manned facility -- either the International Space Station or a new space station built just for the Artemis Project moon flights -- we don't have a practical solution to the radiation environment. We would burn out our crew's radiation exposure limits in just a few orbits. In general, we want to fly a trajectory that gets through Earth's radiation belts as quickly as we can.

Low Earth orbit -- the regime of the Space Shuttle, the series of Russian space stations, and the International Space Station -- is just inside Earth's radiation belts. We can go up to orbits as high as 360 nautical miles (depending on solar activity and other vagaries of Nature) and still stay out of the radiation zone, but any higher than that puts us into an environment which is inimical to life.

One caveat about radiation belts: the South Atlantic Anomaly. This is a spot over the South Atlantic Ocean where the radiation belts tend to dip down as low as 100 nautical miles. The SAA accounts for the majority of radiation experienced by astronauts on the Space Shuttle. If we build big orbiting hotels with a permanent staff, we might want the staff to get into a heavily shielded room each time the hotel flies through the South Atlantic Anomaly. This will be a nuisance, taking 10 or 15 minutes out of every 90-minute orbit, but it will only happen occasionally as the hotel's orbit precesses around the Earth.

Low Earth Orbit still looks like the best solution

Given all these considerations, I doubt that we will use GTO for staging manned flights. This becomes even less likely as we develop larger, permanent facilities in Earth orbit.

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