The Lunar “Distraction”

Recent Congressional testimony and other public statements by NASA Administrator Charles Bolden indicate that he thinks missions to the Moon are a distraction from the agency’s ultimate goal—a human mission to Mars. Such an attitude is not unique to him; for years, many in the space community have said the same thing. During planning for the implementation of the 2004 Vision for Space Exploration, NASA spent more time worrying about their “exit strategy” from the Moon than they did about what they’d been charged to do on the lunar surface once they got there. Eventually, the concept was recast: Missions to the Moon are only valuable to test systems, equipment and procedures for the forthcoming mission to Mars.

As a way to avoid returning to the Moon, NASA embraced a mission concept to retrieve an asteroid with a robotic spacecraft, then put it into lunar orbit so that astronauts in the new Orion spacecraft could visit it. The ostensible value of this activity is that it “prepares us for Mars” by developing a solar electric propulsion stage—a low-thrust, high-energy system that can move heavy cargo from low Earth orbit to distant locations in cislunar space (where the energy needed to depart for Mars is much lower). It also gives the Orion spacecraft a destination that it can reach without developing new spacecraft, such as a lander or an interplanetary habitat module.

The Problem

Conducting a human mission to Mars is very difficult; you must bring with you all of the propellant, consumables (air, water and food), and equipment needed for a three-year journey. You must house and protect the crew from the hostile space environment (including radiation exposure and the extremely cold, near-vacuum of the martian surface). We have not yet solved the entry, descent and landing problem for Mars. Depending on the launch opportunity, you will need between one and two million pounds of equipment and fuel in orbit; the vast bulk of this mass (more than 80%, the amount varies) is propellant for the journey.

The total mass needed in orbit is probably the most difficult problem to address. Various options have been examined to address the issue, but all have drawbacks. Large, heavy-lift rockets can partly solve the mass delivery problem, but even with this capability to bring up pieces in larger chunks, we still face a formidable task. Staging the departure from deep space (either a high lunar orbit or one of the distant “libration points” around the Moon) helps a bit, in that much less energy is needed to put a vehicle on a Mars trajectory. Thus, the idea is to send all the big pieces (habitat, lander, equipment) as cargo-only flights by solar electric tugs, taking the “slow boat” route from low Earth orbit to cislunar space (the space between the Earth and the Moon). From here, although it requires less propellant to depart for Mars, we still need lots of it. After you’ve assembled your pieces in space and it is finally time to leave for Mars, you must fuel the Earth departure stage (the stage that inserts your spacecraft into Mars orbit), have on hand additional fuel for the lander (both descent and ascent) and finally, have enough additional fuel to return back home to Earth.

The highest energy rocket fuels are cryogenic liquids, and they must be kept very cold. Typical propellant types include liquid methane and liquid hydrogen; the latter contains the highest energy, but is also the most volatile, boiling at -253° C (only 20° above absolute zero). These cryogenic liquids will evaporate if stored in space for long periods of time. So we still have a difficulty—we save mass by transporting the heavy equipment up to its departure point via the “slow boat” cargo route using solar electric propulsion, but volatile liquid fuel is still needed (more than 50% of the mass of the stack, depending on the departure opportunity).

As most of the mass of a Mars mission is propellant—a very volatile substance—handling, transporting and using such fuel constitutes a major problem. The current plan is to launch all of the propellant needed for the journey from the surface of the Earth. One can make rocket propellant on Mars, but that’s only useful for the return journey—you have to get to the martian surface first to set up that fuel supply. Propellant for Mars departure, orbit insertion and surface landing must be present in the vehicle on departure from cislunar space. Some rocket fuels are “storable,” meaning that they are liquid at room temperatures and easier to maintain. The problem with using storables is that they contain much less energy, and their use exacerbates the problem by multiplying an already horrendous quantity of required mass. Is there any way to solve this dilemma?

The Solution

Some look at the Moon only as a testing ground for equipment and procedures, and see no compelling reason to conduct human missions there. But recently, the agency has been re-examining the value of the Moon. William Gerstenmaier, Associate Administrator for Human Exploration and Operations, has suggested that the Moon might serve as a depot for propellant for the Mars mission. We now know that water (H₂O) and other volatile substances (including methane, CH₄) exist in large quantity on the Moon at both poles. The Moon has only one-sixth the gravity that the Earth does, and it is close to the deep space staging areas where we can assemble a spacecraft before departing for Mars. Might it not make sense to get the propellant needed for the journey to Mars from the Moon (a working, refueling depot) rather than dragging it up from Earth, the bottom of the deepest gravity well in the inner Solar System?

A mission to land a small, instrumented rover near one of the poles of the Moon (Resource Prospector) is planned to examine the ice in detail and practice extracting and storing it. This mission could document the feasibility of using the volatile deposits of the lunar poles to make rocket fuel and other consumables (air and water) for long duration spaceflight. We need to know exactly how much water is at the Moon’s poles, and its physical state, before we can make detailed plans for its harvesting and use. In addition, traveling to the lunar poles from one of the Earth-Moon L-points requires the same delta-v (energy from a rocket burn) as does descent from Mars orbit to the martian surface. Thus, the Moon is not only a logistics depot, but also a nearby place to rehearse and test the actual Mars spacecraft in a deep-space environment.

After several years of the Moon being relegated to “been there, done that” status, perhaps the idea of using lunar resources to learn how to live and work productively in space is again gaining some traction. The more we learn about the properties of the Moon, the more essential our nearest neighbor becomes in our understanding of the role it can play in our ability to create new spaceflight capabilities and opportunities. I’ll go one step further and suggest that without the use of lunar-produced propellant, a human mission to Mars will not happen—the depressing arithmetic of the rocket equation suggests that it is simply too large and costly a step to undertake. The old trope that the Moon is a “distraction” on the way to Mars had it exactly reversed. The Moon contains what we need to create new capabilities in space faring and is critical to achieving Mars and all of the other interesting destinations in the Solar System.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.