Asteroids and Comets Abundant and Dispersed Gas stations

Unlike the harbinger of battle found in the Bayeaux Tapestry, comets are much more akin to a neighborhood fueling station — waiting for vehicles to stop and fill up. The key is their water. Comets are nature's filling stations; created at the beginning of our solar system, they circle the Sun endlessly, and are available for any who have the ability to catch them and use their precious frozen water for drinking, radiation shielding, or fuel.

As we will use water found at the Moon or on Mars, we will similarly make use of the water frozen in comets. Because that is where we typically see them, we tend to think ofcomets as a resource for only the inner solar system. If so, we think incorrectly! Circling the solar system, and orbiting the sun well beyond the orbit of Pluto, is a vast halo of comets, collectively known as the Oort Cloud. Named after its discoverer, Jan H. Oort, the cloud extends outward from the Sun to nearly 30 trillion kilometers—beyond half the distance to the nearest extrasolar system. Once we get our rocket ship beyond the planets of our solar system, this potential source of water might be available for journeys beyond.

Clearly, we are in the earliest stages of our use of chemical propulsion for deep-space exploration. The propulsion systems in use today are barely adequate for most of the missions we task them to perform. These inspace stages are ignited and burn most of their fuel within the first few hours, if not minutes, of launch. Following this is the very long coast through space until the spacecraft either ignites a set of engines to slow down and be captured into orbit around its destination or not. The "not" results in mission failure. We still lug all the fuel for the mission with us from the moment the rocket leaves the launch pad, paying the penalties imposed by the rocket equation on whatever missions we are flying.

But it does not have to be that way. The concepts, techniques and processes described in this chapter are not new, nor are they conceptually difficult. They will, however, require that we change our way of looking at space exploration missions. They will drive us to take the long view, planning for multiple missions and the establishment of an in-space transportation infrastructure. Until these "filling stations'' are established on the Moon, Mars, a comet or an asteroid, the status quo will remain. And the status quo is clearly unacceptable.

Propulsion Meteor


Space- and Time! now I see it is true what I guess'd at, What I guess'd when I loafd on the grass, What I guess'd while I lay alone in my bed,

And again as I walk'd the beach under the paling stars of the morning.

Walt Whitman, from Song of Myself

IN 2004, United States President George Bush announced that American astronauts will be returning to the Moon before the year 2020, and then setting their sights upon the planet Mars. When this was announced, the advanced space technology community was energized. After all, it has been many years since Neil Armstrong first placed his footprint in the lunar dust. Surely a return to the Moon will use the latest technologies to provide an infrastructure for sustained lunar exploration and operations. Having gone to the Moon in the 1960s with 1940s rocket propulsion technology, one could imagine a 2015 human lunar return using solar- and nuclear-thermal rockets, solar-electric-

propelled cargo tugs efficiently carrying cargo from low Earth orbit, and piloted vehicles aerocapturing back into Earth orbit for rendezvous with the International Space Station.

Unfortunately, the budget and schedule realities soon became apparent and the notion that new technologies would play a central role in the near-term return to the Moon faded. With international commitment to completion and utilization of the International Space Station and the need to do this with the existing space shuttle orbiter fleet requiring a significant fraction of the NASA budget, not much is left over for a lunar return. Add to this the inherent time required to develop, test, and implement new technologies (see Chapter 9, "Technological Readiness") and it becomes painfully clear that in order to meet the deadline of returning to the moon before 2020, there simply is not enough time to use much, if any, new technology. NASA had to begin designing flight hardware now—using mostly existing technologies—and innovation would have to wait.

America's vision for space exploration is larger than simply the human lunar return. Robotic scientific exploration of the solar system will continue and it is here that we will begin to learn how to use new technologies to live off the land in space, one day transferring that capability to the larger and more propulsive intense needs of human exploration. Many of the technologies described in this book will first be used on smaller, less-expensive, and certainly riskier robotic missions of exploration in deep space. Once they are proven and widely used by our robotic explorers, they will be considered mature enough to be considered with that most valuable of cargo—human beings.

Which technologies might be scalable from robotic missions to human? First some numbers. The European Small Missions for Advanced Research and Technology-1 (SMART-1), launched in 2003, was propelled to lunar orbit by a solar-electric propulsion system. The SMART-1 spacecraft weighed 367 kilograms and was launched on an Ariane-5 rocket. It was a one-way trip to the Moon. By contrast, the Apollo 10 mission launched in 1969 sent three astronauts into lunar orbit for two days and then returned them safely to the Earth. The Command and Service Modules together weighed over 30,000 kilograms and were launched on the Saturn V rocket. These are two missions that reached and orbited the same destination. However, the robotic mission did not have to consider a safe return to the Earth nor did it carry three astronauts! It also used a state-of-the-art solar-electric propulsion (SEP) system, using the free energy provided by the Sun to produce electrical power and drive the Hall thruster-based primary propulsion system. Clearly, the propulsion system required for a 30,000-kilogram human-piloted vehicle is different from that required for its smaller robotic kin. Fortunately, many technologies have a growth path that will some day enable them to be used for human exploration.

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