Sustained human expansion beyond Earth not only requires the transportation of the people, but also of the cargo (supplies) needed to sustain them. Since cargo is not as sensitive to the time spent in deep space, lower-thrust systems such as solar-electric propulsion (see Chapter 11) might be used to efficiently carry such supplies on the long voyages between planets. Taking advantage of plentiful sunlight, SEP-based vehicles might be the cargo ships of deep space. Why would one use SEP instead of chemical propulsion for this job? The short answer is: efficiency. Electric-propulsion systems are considered low thrust, which means that they cannot accelerate large masses to high speeds very quickly. However, given enough time, they can accelerate these masses to significantly higher speeds than chemical rockets and do so much more efficiently. In general, SEP systems are 10—100 times more efficient (per kilogram of fuel) than their chemical counterparts. Though they may take longer to attain high speeds, they do it efficiently—thus lowering the overall propellant requirements for the mission.
For example, consider the possibility of building a continuously occupied human base on the Moon. All elements of the base (the habitat, the vehicle that carries the crew from the Earth to the Moon and back again, the supplies, etc.) are launched into Earth orbit on board conventional chemical rockets. Before the humans depart for the Moon, the habitat and supplies are sent ahead from low Earth orbit using large solar-powered, solar-electric-propelled vehicles that take them to low lunar orbit. Once the habitat arrives at the Moon, the crew departs from Earth using a higher thrust chemical propulsion system. They descend to the surface, either in the habitat or separately, joining with it after landing. While this scenario could be accomplished without SEP, it would require more launches—just to carry the fuel that would not be required when electric propulsion is utilized.
In the far term, solar-electric propulsion might play a key role not only in transferring cargo from Earth to Mars, but also in transferring the crew. With its inherent efficiency being most advantageous on deep-space missions, very large solar-powered vehicles might one day be used to carry people. The idea of a "solar clipper" using solar arrays hundreds of meters long to power thrusters requiring hundreds of kilowatts of power is not new. Again, the benefit of using the technology comes down to the mass required to accomplish the mission. And less mass is always better. In the terrestrial real estate business, the mantra is "location, location, location." For space exploration, it is "propulsion, propulsion, propulsion." And solar-electric propulsion provides a much more efficient and economical approach than chemical rockets—it reduces the amount of fuel required, saving on launch cost and overall system mass.
It should be noted that not all solar-electric propulsion technologies are applicable to human exploration. The most widely used deep-space SEP system today is a gridded ion thruster. The physics of their design limits them to mostly lower-power, lower-mass applications—perfect for robotic exploration, but woefully inadequate to support humans. More likely, electromagnetic propulsion systems like those that use Hall and Magnetoplasmadynamic (MPD) thrusters will be used. They require much higher power, but are capable of higher thrust without losing too much of their efficiency.
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