Water and Wrenches, Belts and Suspenders

A Rational Approach to Building a Mars Exploration Supply Cache

by David Brin and Bud Sparhawk

Someday, Earth civilization will send emissaries to Mars. Over the long run, the task of exploration cannot be left only to robots. If we are ever to know Mars from a human perspective we must have eyes on the ground, feet in the soil. People offer unbeatable advantages of intelligence and adaptability. But getting to Mars and other deepspace destinations with crews of living men and women won’t be easy.

Among the many obstacles we must overcome are the immense cost of such an expedition and the need to ensure a much higher, almost-guaranteed probability of success. Mission designers must overcome major technical issues along with barriers both physiological and mental. A detailed plan will include procedures for orbital transfer, landing, performing science and returning to Earth. These problems will take years – likely decades – to resolve.

And yet, it turns out that some fundamentals are still the same as they were when Shackleton and Amundsen probed the Antarctic, or Hillary and Tenzig took on Everest. All of them fretted about one thing, above all. Supplies, supplies, supplies.

Ninety percent of the time, energy and expense of those exploration treks went into laying down caches of necessities. Doing so properly ensured survival when it finally came time for the blitz to the summit or pole. Failure to get it right helped doom the ill-fated Scott Antarctic Expedition. When we send human crews to Mars, we would do well to remember this and provide what they need, without scrimping, well in advance. In fact, prudence suggests that we not even launch the human components of a Mars expedition until at least twice as many consumables are already there on site, both orbiting the planet and on the surface, as they would need, even if accidents happen. (The Apollo moon landings did not fit into this pattern, because they were essentially sprints toward a much closer objective. More like Lindbergh than Amundsen.)

As it turns out, this kind of advance-provisioning needn’t delay matters at all. It will take many years, or decades, to design, critique, and build the manned components. Meanwhile, the greater bulk of material needed by any expedition will be almost completely design independent: water, food, wrenches, shovels, microscopes, sampling bags and other basic tools, and supplementary maneuvering fuels like hydrazine will be needed, whether the explorers travel by rocket or railroad. Moreover, these “general” supplies make up a great part of the mass of any expedition.

What this means is that we should undertake provisioning a Mars expedition long before the human crew is launched — possibly even before any specific mission design is decided-upon! In fact, this can be turned into a great advantage. Making such a distinction — between bulk, mission-nonspecific supplies and high value mission-specific cargoes, will have special significance if time can be traded for energy and money.

Are there alternative methods of transport that are inherently cheaper — if slower — than the standard, rocket-driven “Hohmann” interplanetary transfer orbit? If so, and given a decade’s advance notice, bulk cargoes might be conveyed to Mars by the cheapest methods possible. A slow journey and long term storage won’t damage water or wrenches or even preserved dinners. Only when such stuff is plentifully cached at Phobos, or some other convenient marshaling site, would it make sense to send human beings and their expensive, mission-specific gear, the landers and scientific instruments, by quick and costly means.

Are such methods available? On paper, it seems possible to trade time for cheap transfer with methods like ion propulsion or solar sails. Finding out if such systems can be efficiently scaled up to propel many-ton cargoes ought to be given high priority. If such experiments (delayed fifty years, in the case of photon sails went well, then it could make sense to start sending bulk supplies as soon as possible… possibly even during the next decade. If the bulk transport system were efficient enough, this approach might have considerable impact on reducing costs of a manned expedition, and/or padding in an extra safety margin.

Note also that even without slow-but-efficient solar powered transport, a standard rocket-driven transfer to Mars can still be made much cheaper, simply by hiring out the job of ferrying water and wrenches to the lowest commercial bidder, without the bureaucracy or nitpicking care required for human-rated vehicles. If a cargo of Spam and Gatorade goes astray, you shrug and send another.

Early caching of general-use supplies will also have important implications for the design-specific components — the part of the mission having to do with actually transporting people to Mars orbit, then to-from the surface. Every gram of material that was sent previously, by slow-freighter, will not have to be carried on the crewed vessel — thus also freeing up several more grams of fuel to transport it. Anything that lightens the crew transfer vehicle may make it both less expensive and easier to optimize for people-specific goals, like minimizing radiation exposure, or getting the explorers there fast. True, there are limits to our ability to speed-up a Hohmann Transfer Trajectory, with simple chemical rockets. Still, there are possibilities that are powerfully leveraged if the ship is optimized to convey people — and only people.

Consider an additional attractive feature. After the first few robot freighters arrive at the cache and are roboticaly verified to be in good condition, the mere existence of such a cache will serve as a psychological draw, a palpable on-site investment, already-made, helping to emotionally commit citizens to back the final push, sending an exploration crew. Call it cancellation insurance. Note also that the first expedition probably would not use up all the supplies that were cached for it. Hence, a substantial amount of left-over material would serve as a draw for us to send a next-followup mission… a canny, added benefit to this approach.

Some additional technologies are compatible and also needed, for example those leading to Zubrin-style in situpropellant production facilities — robotic factories that might be landed on either Mars or Phobos — or both — and put to work using local materials (e.g. carbonaceous volatiles, ices, or atmospheric CO2) to manufacture rocket fuel for the return trip. Aside from providing a new margin of safety, this will save vastly on transport costs, since fuel for the return voyage needn’t be transported from Earth. Again, the human crew would not even depart from home until mission control verified the existence of this cache, in advance. (The possibility that Phobos might contain such accessible/usable volatiles makes it potentially one of the most valuable sites in the solar system, which means that the upcoming RussianGrunt Phobos mission is a really important one, in our near future.)

Our key point here? The crucial enabling steps that humanity will need, before embarking on our long and careful plan to send humans to Mars, are not mission specific ones — like the nature of the crew or landing craft. Nor do they even require full-scale political commitment for the entire grand project. We need not wait for the study commissions to create glossy reports and billion-dollar blueprints.

Rather, what’s called for right now is to start looking into which technologies can deliver the boring stuff, e.g. the groceries and tools, to Mars in a manner that gets this vital provisioning task done efficiently and most-cheaply. If such methods are found, then at a relatively modest expenditure, NASA or some consortium of nations could begin launching advance supply ships to orbit Mars and establish a cache.

What this notion does is turn our disappointment at the glacial pace of manned exploration into an advantage. If that slow pace is acted upon and taken advantage of, then the resulting cost savings and extra safety margins might even be enough to help bring the eventual manned mission into being!

Fortunately, the first, enabling steps are simple, inexpensive, logical… and would make sense to any of our explorer forebears.


Addenda and Responses

Arlan Andrews has further proposed a “Cache Launch Race” in which various groups of contributors and/or investors sponsor launches of infrastructure and supplies for the later arrivals and colonists. It occurs to us that this might come in two phases. A modest version would be something akin to the famed X Prize, aimed only at demonstrating the alternative techniques needed to bring cargoes cheaply to Mars orbit — e.g. solar sail freighters. Perhaps even a “regatta” to attract attention from rich yachtsmen. Early cargoes might be more scientific in nature. Later, larger demonstrator missions might actually start carrying supplies, backed by bonds with government-guarante ed payment for the consumables, when a manned mission finally uses them.

Tom Ligon has done preliminary calculations suggesting that the tradeoff of time for cost and energy is real. What is further needed are calculations to see if either solar sails or solar-powered ion engines can be scaled up to mammoth size, allowing the slow but steady and cheap conveyance of multi-ton cargoes.

This popularized article briefly conveys why many believe that time can be traded for efficiency and cost savings, in the transportation of bulk cargoes around the solar system.

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