There is currently a great interest in going someplace in the Solar System. NASA wants to go to the Moon. Elon Musk wants to go to Mars. Still others just want to go to an asteroid. These destinations require extremely complex systems and mission scenarios.

In order to get to any of these places, the easiest way is to first get into Earth orbit. This first step requires leaving the terrestrial surface on a launch vehicle, which is a complex machine designed to take payloads from a stationary point at sea level and add enough energy to achieve an altitude of a few hundred kilometers and a speed in excess of 7 km/sec.

The amount of energy expended is huge. In fact, the energy needed to achieve low Earth orbit is roughly half of the total energy to reach anywhere in the Solar System.

In the past, a single launch vehicle was able to send three astronauts to the Moon. As it turns out this is a very inefficient way to travel beyond low orbit, especially if you are going farther than the Moon.

It is much more efficient to transfer from the launch vehicle to another transporter in low orbit before leaving the vicinity of Earth. Thus, it is best to do your extraterrestrial travel in stages.

First you go from the Earth's surface to an orbiting waystation. Then you transfer to an interplanetary transporter. This vehicle takes you from low-Earth orbit on a trans-lunar or heliocentric trajectory, depending on your destination, to an orbit about the desired celestial body.

You would then transfer again to a local reusable launch/reentry vehicle that can take you to the surface of the Moon or a planet. If you are going to an asteroid, you do not need a third step, because the gravity fields around such bodies are so weak you can land directly on the surface.

When your vacation or work is completed, you can reverse the steps on your return trip to Earth. Upon arriving at the Earth-orbiting waystation, a shuttle will be waiting to take you back to a spaceport on the terrestrial surface. From there, home is a simple ride in an airplane. It is all that simple.

So, why do you need a waystation? Your reusable launch vehicle to orbit is not a good interplanetary transporter, because you would have to carry a great deal of extra weight made up of heavy structures, large rocket engines and massive reentry shielding.

It would be a big mistake to deal with that extra 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.

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