Why should humans venture to Mars? In some sense, it is to fulfill a longing in our collective psyche. Humanity has dreamed of walking on Mars since travel above the Earth’s surface became feasible… Those dreams seemed to be on the verge of being realized with the dawn of the Space Age in the mid-20th Century, but for decades Mars remained out of reach to human explorers on site. That frustrating situation has resulted in what many have come to see as Mars being perpetually “20 years away.” In the terminology used for space launches, we have been in a “hold” for the past 30 years. Today, however, we can finally say that the countdown clock is running. We are going to Mars, twenty years and counting! If we stay the course, the first humans will set foot on the Red Planet by the year 2033.
New technologies have become available that have allowed us to take a fresh look at how to approach human Mars missions and, as a result, to tackle the hardest technical problems that had previously led to a hold in the countdown. And these approaches have given us the ability to design an architecture that can be brought on line incrementally, which keeps the annual expenditures within a reasonable projected NASA human spaceflight budget.
The key to this architecture is to separate much of the supplies and equipment and send them in advance of the crew. By prepositioning equipment, supplies, and even return rocket stages (more than 80 percent of the total mass), using propulsion systems such as solar electric propulsion (SEP) that are extremely energy efficient for long-haul cargo missions, we can save more than half the costs versus transporting everything using chemical propulsion systems that are needed to reach orbit.
On the other hand, the Space Launch System (SLS) is the key to placing large blocks of payload into Earth orbit. An analogy here on Earth would be how intermodal shipping of supplies and manufactured goods is accomplished. Short haul is accomplished by trucks or rail, while long haul is accomplished by massive ships. In this same way, SLS accomplishes the short haul mission to orbit, whereas the cargo is transported over the much larger distance to Mars by solar electric transport ships. The two different propulsion systems, each of which is appropriate for its particular type of mission, are complementary. And to transport the astronauts, we add in a third element: the Orion multi-purpose crew vehicle as well as a deep-space habitat module, which together provide living space and life support systems.