Galle Crater, as seen from Viking 1. Public domain.

In remarks at the International Astronautical Congress in Guadalajara, Mexico on September 29, 2016, SpaceX CEO Elon Musk revealed to great fanfare his company’s plans for an Interplanetary Transport System (ITS). According to Musk, the ITS would enable the colonization of Mars by the rapid delivery of a million people in groups of a hundred passengers per flight, as well as large-scale human exploration missions to other bodies, such as Jupiter’s moon Europa.

I was among the thousands of people in the room (and many more watching live online) when Musk gave his remarkable presentation, and was struck by its many good and powerful ideas. However, Musk’s plan assembled some of those good ideas in an extremely suboptimal way, making the proposed system impractical. Still, with some corrections, a system using the core concepts Musk laid out could be made attractive, not just as an imaginative concept for the colonization of Mars, but as a means of meeting the nearer-at-hand challenge of enabling human expeditions to the planet. With that in mind, I wrote a critique of the new SpaceX plan for The New Atlantis, showing how they can be corrected to benefit the near-term goal of initiating human exploration of the Red Planet, and then, with a cost-effective base-building and settlement program, the more distant goal of future Mars colonization. The following is an abridgement of that article, summarizing many of the points made therein.

Design of the SpaceX Interplanetary Transport System

As described by Musk, the SpaceX ITS would consist of a very large, two-stage,fully-reusable launch system, powered by methane/oxygen chemical bipropellant. The suborbital first stage would have four times the takeoff thrust of a Saturn V (the huge rocket that sent the Apollo missions to the Moon). The second stage, which reaches orbit, would have the thrust of a single Saturn V. Together, the two stages could deliver a maximum payload of 550 tonnes (metric tons) to low Earth orbit (LEO), about four times the capacity of the Saturn V.

At the top of the rocket, the spaceship itself — where some hundred passengers reside — is inseparable from the second stage. (Contrast this with, for example, NASA’s lunar missions, where each part of the system was discarded in turn until just the Command Module carried the Apollo astronauts back to Earth.) Since the second-stage-plus-spaceship will have used its fuel in getting to orbit, it would need to refuel in orbit, filling up with about 1,950 tonnes of propellant (which means that each launch carrying passengers would require four additional launches to deliver the necessary propellant). Once filled up, the spaceship can head to Mars.

The duration of the journey would of course depend on where Earth and Mars are in their orbits; the shortest one-way trip would be around 80 days, the longest would be around 180 days. (Musk stated that he thinks the architecture could be improved to reduce the trip to 60 or even 30 days.)

After landing on Mars and discharging its passengers, the ship would be refueled with methane/oxygen bipropellant made on the surface of Mars from Martian water and carbon dioxide, and then flown back to Earth orbit.

Improving the SpaceX ITS Plan

Some corrections for the flaws in the current ITS plan immediately suggest themselves:

2. Instead of hauling the massive second stage of the launch vehicle all the way to Mars, the spacecraft should separate from it just before Earth escape. In this case, instead of flying all the way to Mars and back over 2.5 years, the second stage would only fly out about as far as the Moon, and return to aerobrake into Earth orbit a week after departure. If the refilling process could be done expeditiously, say in a week, it might thus be possible to use the second stage five times every mission opportunity (assuming a launch window of about two months), instead of once every other mission opportunity. This would increase the net use of the second stage propulsion system by a factor of 10, allowing five payloads to be delivered to Mars every opportunity using only one such system, instead of the ten required by the ITS baseline design. Without the giant second stage, the spaceship would then perform the remaining propulsive maneuver to fly to and land on Mars.
3. Instead of sending the very large hundred-person habitat back to Earth after landing it on Mars, it would stay on Mars, where it could be repurposed as a Mars surface habitat – something that the Martian settlers will surely find extremely useful. Its modest propulsive stage could be repurposed as a surface-to-surface long-range flight system, or scrapped to provide material to meet other needs of the people living on Mars. If the propulsive system must be sent back to Earth, it should return with only a small cabin for the pilots and such colonists as want to call it quits. Such a procedure would greatly increase the payload capability of the ITS system while reducing its propellant-production burden on the Mars base.
4. As a result of not sending the very large second stage propulsion system to the Martian surface and not sending the large habitat back from the Martian surface, the total payload available to send one-way to Mars is greatly increased while the propellant production requirements on Mars would be greatly reduced.
5. The notion of sacrificing payload to achieve one-way average transit times substantially below six months should be abandoned. However, if the goal of quick trips is retained, then the corrections specified above would make it much more feasible, greatly increasing payload and decreasing trip time compared to what is possible with the original approach.

Changing the plan in the ways described above would greatly improve the performance of the ITS. This is because the ITS in its original form is not designed to achieve the mission of inexpensively sending colonists and payloads to Mars. Rather, it is designed to achieve the science-fiction vision of the giant interplanetary spaceship. This is a fundamental mistake, although the temptation is understandable. (A similar impulse disadvantaged the design of the Space Shuttle.)  The central requirement of human Mars missions is not to create or operate giant spaceships. Rather, it is to send payloads from Earth to Mars capable of supporting groups of people, and then to send back such payloads as are necessary.

To put it another way: The visionary goal might be to create spaceships, but the rational goal is to send payloads.

Near-Term Mars Missions Using the Improved ITS Plan

Consider what this revised version of the ITS plan would look like in practice, if it were used not for settling Mars, but in reduced scale for the nearer-at-hand task of exploring Mars. If a SpaceX Falcon Heavy launch vehicle were used to send payloads directly from Earth, it could land only about 12 tonnes on Mars. (This is roughly what SpaceX is planning on doing in an unmanned “Red Dragon” mission planned for a few years from now. While it is possible to design a minimal manned Mars expedition around such a limited payload capability, such mission plans are suboptimal. But if instead, following the ITS concept, the upper stage of the Falcon Heavy booster were refueled in low Earth orbit, it could be used to land as much as 40 tonnes on Mars, which would suffice for an excellent human exploration mission. Thus, if booster second stages can be refilled in orbit, the size of the launch vehicle required for a small Mars exploration mission could be reduced by about a factor of three.

In the ITS variants discussed above, the entire flight hardware set would be fully reusable, enabling low-cost support of a permanent and growing Mars base. However, complete reusability is not a requirement for the initial exploration missions to Mars; it could be phased in as technological abilities improved. Furthermore, while the Falcon Heavy as currently designed uses kerosene/oxygen propulsion in all stages, not methane/oxygen, in the revised ITS plan laid out above only the propulsion system in the trans-Mars ship needs to be methane/oxygen, while both stages of the booster can use any sort of propellant. This makes the problem of refilling the second stage on orbit much simpler, because kerosene is not cryogenic, and thus can be transferred in zero gravity using flexible bladders, while liquid oxygen is paramagnetic, and so can be settled on the pump’s side of the tank using magnets.

Using such a system, a manned expedition of Mars could be carried out any number of ways. For example, it could be done in a manner similar to the Mars Direct mission plan, with the first trans-Mars payload delivering an unfueled Earth Return Vehicle with an onboard propellant factory to make methane/oxygen propellant on Mars, and the second delivering a habitat module with a crew of astronauts aboard who land near the ERV, using their hab as their house on Mars. After 1.5 years of exploration they would return in the ERV, leaving their hab behind on Mars to add incrementally to the facilities of a growing Mars base as the missions proceed.

Or a different plan, closer in spirit to the SpaceX ITS, could be adopted, in which a single payload combining the hab and the ERV is sent, with the hab above and the ERV below. The ERV would use a limited amount of methane/oxygen propellant to perform supersonic retropropulsion of the combined payload upon Mars entry, bringing the assembly to subsonic speeds. Once this is done, the hab would pop a parachute, or possibly a parasail, to lift it off the ERV and then land nearby using a very small terminal landing propulsion system. The first such mission could send such an assembly out with no crew, allowing the ERV to be fueled in advance of the first piloted launch, which would then arrive two years later provided with a redundant hab and plentiful extra supplies. Once the base is well-established, the hab and ERV modules could be landed together, with the hab subsequently lifted off the ERV by a crane.

The number of such potential variations is endless. Another: In initial missions, the Falcon Heavy second stage could perform the full burn, allowing it to coast out to Mars in company with the piloted spacecraft, which could then use it as a counterweight on the opposite end of a tether to provide the crew with artificial gravity on their way to Mars (just as in the standard Mars Direct plan). This would entail expending the second stage, but it could be worth it for the first missions to have their crews in top physical strength, as they will reach a Mars with minimal support facilities. In later missions, the Falcon Heavy second stage could be left behind just short of Earth escape for ready reuse (as in the revised ITS plan I described above), and the crew be allowed to fly to Mars in zero gravity, since they would by that point have plenty of ample base facilities to provide local support for recovery from zero-gravity weakening once they reach the Red Planet.

Dawn of the Spaceplanes

Towards the end of his IAC talk, Musk briefly suggested that one way to fund the development of the ITS might be to use it as a system for rapid long-distance point to point travel on Earth. This is actually a very exciting possibility, although I would add the qualifier that such a system would not be the ITS as described, but a scaled-down related system adapted to the terrestrial long distance travel application.

The point is worthy of emphasis. For three thousand years or more, people have derived income from the sea, for example by fishing, but far more by using the sea as a favorable comparatively low-drag medium for transport. Similarly, while there is money to be made by human activities in space, there is potentially much more to be made by human travel across space, taking advantage of space’s fundamental quality as a drag-free medium for rapid travel. Indeed, it has long been known that a rocketplane taking off with a high suborbital velocity could travel halfway around the Earth (i.e. anywhere else on the planet) in less than an hour. The potential market for such a capability is enormous. Yet it has remained untouched. Why?

The reason is simply this. Up till now, such vehicles have been impractical. For a rocketplane travel halfway around the world would require a DV (change in velocity) of about 7 km/s (6 km/s in physical velocity, and 1 km/s in liftoff gravity and drag losses). Assuming methane/oxygen propellant with an exhaust velocity of 3.4 km/s (it would be lower for a rocketplane than for a space vehicle, because exhaust velocity is reduced by surrounding air) such a vehicle, if done as a single stage, would need to have a mass ratio of about 8, which means that only 12% of its takeoff mass could be solid material, accounting for all structures, while the rest would be propellant. On the other hand, if the rocketplane were boosted towards space by a reusable first stage that accomplished the first 3 km/s of the required DV, the flight vehicle would only need a mass ratio of about 3, allowing 34% of it to be structure. This reduction of the propellant to structure ratio from 7:1 down to 2:1 is the difference between a feasible system and an infeasible one.

In short, what Musk has done by making reusable first stages a reality is to make rocketplanes possible. But there is need to wait for 500-ton-to-orbit transports. In fact, his current Falcon 9 reusable first stage could enable globe-spanning rocketplanes with capacities comparable to the DC-3, while the imminent Falcon Heavy (or New Glenn) launch vehicles could make possible rocketplanes with the capacity of a Boeing 737.

Such flight systems could change the world.

Colonizing Mars

In his talk introducing the ITS, Musk suggested a Mars colonization program using thousands of such systems could be used to rapidly transport a million people from Earth to Mars. This would be done to provide a large enough population to allow the colony to be fully self-sufficient. In subsequent interviews, he also said that none of these colonists would include children, since having kids around would be a burden upon the colony.

My own ideas on how the colonization of Mars could be achieved are different. Rather than a massive convoy effort to populate the planet, I see the growth of a Mars colony as an evolutionary development, beginning with exploration missions, followed by a base-building phase. As the series of missions proceeds, additional elements of the flight-hardware set would become reusable, causing transport costs to drop. Furthermore, as the base grows, its capability to produce more and more necessary items, including water, food, ceramics, glasses, plastics, fabrics, metals, wires, tools, domes, and structures, would expand — progressively reducing the amount of materials that needs to be transported across space to support each settler. This will provide the material basis for an expanding Martian population, which will grow exponentially as families are formed and children are born.

That said, Mars is unlikely to become autarchic for a very long time, and even if it could, it would not be advantageous for it to do so. Just as nations on Earth need to trade with each other to prosper, so the planetary civilizations of the future will also need to engage in trade. In short, regardless of how self-reliant they may become, the Martians will always need, and certainly always want, cash. Where will they get it?

A variety of ideas have been advanced for potential cash exports from Mars. For example, Mars might serve as a source of food and other useful goods for asteroid-mining outposts which themselves export precious metals to Earth. Or, since the water on Mars has six times the deuterium concentration as Earth’s, that potentially very valuable fusion-power fuel could be exported to the home planet once fusion power becomes a reality. Or maybe precious metals will be found on Mars, which, with a fully reusable interplanetary transportation system, it might be profitable to mine and export to Earth.

While such possibilities exist, in my view the most likely export that Mars will be able to send to Earth will be patents. The Mars colonists will be a group of technologically adept people in a frontier environment where they will be free to innovate — indeed, forced to innovate — to meet their needs, making the Mars colony a pressure cooker for invention. For example, the Martians will need to grow all their food in greenhouses, strongly accentuating the need to maximize the output of every square meter of crop-growing area. They thus will have a powerful incentive to engage in genetic engineering to produce ultra-productive crops, and will have little patience for those who would restrict such inventive activity with fear-mongering or red tape.

Similarly, there will be nothing in shorter supply in a Mars colony than human labor time, and so just as the labor shortage in nineteenth-century America led Yankee ingenuity to a series of labor-saving inventions, the labor shortage on Mars will serve as an imperative driving Martian ingenuity in such areas as robotics and artificial intelligence. Such inventions, created to meet the needs of the Martians, will prove invaluable on Earth, and the relevant patents, licensed on Earth, could produce an unending stream of income for the Red Planet. Indeed, if the settlement of Mars is to be contemplated as a private venture, the creation of such an inventor’s colony — a Martian Menlo Park — could conceivably provide the basis for a fundable business plan.

To those who ask what are the “natural resources” on Mars that might make it attractive for settlement, I answer that there are none, but that is because there are no such thing as natural resources anywhere. There are only natural raw materials. Land on Earth was not a resource until human beings invented agriculture, and the extent and value of that resource has been multiplied many times as agricultural technology has advanced. Oil was not a resource until we invented oil drilling and refining, and technologies that could use the product. Uranium and thorium were not resources until we invented nuclear fission. Deuterium is not a resource yet, but will become an enormous one once we develop fusion power, an invention which future Martians (having limited alternatives) may well be the ones to bring about. Mars has no resources today, but will have unlimited resources once there are people there to create them.

Martian civilization will become rich because its people will be smart. It will benefit the Earth not only as a fountain of invention, but as an example of what human beings can do when they rise above their animal instincts and invoke their creative powers. It will show all that infinite possibilities exist, not to be taken from others, but to be made.

No one will be able to look on it and not be prouder to be human.

A full version of this critique may be found at http://www.thenewatlantis.com/publications/colonizing-mars