It seems like everyone wants to go someplace in the Solar System. President Trump wants to go to the Moon. Elon Musk wants to go to Mars. Others want to go to an asteroid.

So, what is the easiest way to go anywhere in the Solar System? Well, most people don't know this, but the answer is to do it in stages. One approach that NASA is considering is the Lunar Gateway, a large spacecraft in orbit around the Moon. This would be modular and be able to support human missions to the lunar surface with reusable lander elements. For the first time NASA and its international partners would have access to more of the lunar surface than ever before.

The Space Launch System (SLS) and Orion spacecraft would form the backbone of the Gateway and would transport astronauts and supplies to and from Earth. The Gateway could become the waystation for solar system exploration. Planetary passengers and cargo could be transferred from the Gateway to an interplanetary transporter.

Another approach is to establish a LEO Gateway in low-Earth orbit. Such a gateway would be only a few hundred kilometers above Earth, permitting easy servicing and, if necessary, easy rescue. This gateway has several advantages for planetary passengers. A two-stage dedicated launch system could have a very large payload capacity, be at least partially reusable and more affordable.

Remember that the energy to achieve low Earth orbit is about half the total energy to go any place on the solar system, while the energy to go the lunar orbit is almost enough to escape Earth's gravity well. Planetary passengers who stop at the Lunar Gateway have to travel first to the Moon and then escape the lunar gravity in order to reach a planet. This sequence leads to a great deal more required mass and energy.

The LEO Gateway offers a different sequence. Once in low Earth orbit, passengers can easily travel to the Moon or to a planet using a single vehicle transfer. For planetary trips a transfer vehicle would take you from low-Earth orbit on a heliocentric path to an orbit about the destination celestial body. You might then transfer your bags to a local reusable launch/reentry vehicle that can take you to the surface of your desired planet. If you are going to an asteroid, you do not need a third step, because gravity fields around such bodies are so weak you can land directly on its surface.

When your vacation or work is completed, you can reverse the steps to get home to Earth. When you get back to the LEO Gateway a shuttle would be waiting to take you back to a spaceport on the terrestrial surface. From there, home is a simple ride in an airplane. So, why do you need a gateway in the first place? As noted, the energy needed to get to low-Earth orbit is about half of the total energy to get anywhere in the Solar System.

Second, your launch vehicle to low orbit is not a good interplanetary transporter, because you would have to carry all that extra weight required for the heavy launch structures, large rocket engines and massive reentry shielding. It would be a big mistake to deal with all of that weight on your transit through the Solar System. Instead, it makes all kinds of sense to use a planetary transfer vehicle that needs no reentry shielding nor large rockets.

Of course, there are variations on the plan depending on where you want to go. For example, placing a LEO Gateway over the equator offers added advantages. Assuming an equatorial launch site, the energy to orbit is minimized. There would be no "launch window" constraints due to orbit plane orientation. All ancillary vehicles such as servicing satellites and fuel depots would all be in one plane, minimizing propellant and maneuvering requirements.

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NASA Aids Testing of Boeing Deep Space Habitat Ground Prototype in Alabama
Huntsville AL (SPX) May 02, 2019
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