The Space Reviewin association with SpaceNews

NSRC 2020

Humans in Mars orbit
The challenges of a human mission to Mars are not insurmountable, even when compared to missions to the Moon. (credit: NASA/JSC)

The Moon and Mars: a flawed article’s false choice

Bookmark and Share

In the recent essay “The Moon or Mars: Flawed Debate, False Choice - Part One,” Madhu Thangavelu of the University of Southern California argued that the Moon was the logical next destination for human spaceflight, given all of the challenges he identified with sending humans to Mars. That article is worthy of a response, however, because of its dubious claims and flawed reasoning.

Space is vast, beautiful, and dangerous: even given the risks of a ride on the shuttle, just about any space advocate would have loved to take a ride on one.

Thangavelu starts by discussing the frequency of missions that can be flown to the Moon versus Mars: “Current technology allows us to ply rocketships in cislunar space, i.e., between the Earth and the Moon, every day while there are only very limited windows of opportunity to depart Earth to go to Mars.” This is partially correct: while launch opportunities for Mars only come around every two years, the launch windows are large enough to send multiple craft and account for any adverse launch site conditions.

He continues: “Rocketships to the Moon are much smaller, ten to hundred times smaller, depending on what and how many crew you wish to carry on expeditions, especially propellant, food and potable water.”

This is wholly incorrect. A rocket that can send 120 tonnes to low Earth orbit can send about 59 tonnes on a trans-lunar trajectory or about 47 tonnes on a trans-Mars trajectory. That takes into account that any lunar mission must use retro-propulsion (i.e. additional rocket fuel) in both an orbital capture burn and then a landing burn to cancel velocity, while that a Mars craft can, to a large extent, use aerobraking with the far smaller mass of a heat shield to accomplish the same task. Thus, any advantages as far as payload to destination are completely negated.

Thangavelu then raises concerns about mission safety. On a lunar mission, he writes, “mission control can keep check almost instantaneously round the clock. We can even mount rescue or emergency missions in short order, should the need arise.” By contrast, “We cannot do this for Mars missions using current technology. The communications time lag during most of the Mars expedition is such that mission control on Earth can do nothing to help in an emergency. Even prayers can take 30 minutes or more to reach a transiting Mars crew in trouble.”

Thankfully, the explorers of the 15th century, along with those who explored the American continent, and those who went to the north and south poles, and the people who even now live in remote Antarctic research bases, were and are not so timid. Space is vast, beautiful, and dangerous: even given the risks of a ride on the shuttle, just about any space advocate would have loved to take a ride on one. Furthermore, there will never be a time (at least in any foreseeable future) that any relief mission could get to Mars in any period shorter than about four and a half months, so it’s not a challenge that can be ever solved by delaying action.

While clearly focused on imagining challenges to Mars exploration, Thangavelu has not been so imaginative when it comes to solving them. “And imagine this: floating around in weightlessness for five or six months, and then all of a sudden, crew are subject to gravity forces upon landing on the Martian surface. Even crew returning from much shorter trips to the ISS need a lot of time to regain their muscle and bone strength once back on Earth.”

Despite NASA’s obsession with zero-g research, there is no reason to subject astronauts on a Mars mission to zero-g. By simply tethering to the upper stage, which would already be coasting to Mars along with the primary payload, and then spinning the whole assembly, Mars- or Earth-level gravity can be created, mitigating the negative effects of weightlessness.

Despite NASA’s obsession with zero-g research, there is no reason to subject astronauts on a Mars mission to zero-g.

Some of the best features of Mars also seem to concern him, without noting that the Moon has even worse characteristics. “The saving grace about Mars is that they will experience less than half their body weight on Earth and be able to adjust to a similar diurnal rhythm of approximately 12 hours of night and day,” he notes. It’s true that Martian gravity is likely a great benefit to operations, and its day-night cycle is far more advantageous than the Moon’s two weeks of day followed by two weeks of night.

“But what use is that when you need to be fully suited and unable to breathe the almost pure CO2 atmosphere, and that too, at such a low pressure as to be of no use at all, not to mention the dust storms that can mask the sun for months at a time,” Thangavelu adds.

Yes, it is obvious an astronaut could not breathe the Martian atmosphere unaided. However, that carbon dioxide can be captured with a simple pump, and then split via electrolysis to produce breathable oxygen along with a mix of argon and nitrogen buffer gas also derived from the same Martian atmosphere. By contrast, there is no atmosphere whatsoever on the Moon.

A ready source of carbon dioxide allows Mars explorers to use the Sabatier reaction to create methane fuel if they have a source of hydrogen, either sourced locally or simply brought with them, and hydrogen in the form of water is far more common on Mars than on the Moon. The atmosphere of Mars also allows for a much more forgiving thermodynamic environment.

Thangavelu then examines power generation issues. “Solar photovoltaic arrays that power the ISS today have been the mainstay for space systems and satellites since the dawn of the space age and this technology will not suffice for Mars habitats because of dust storms in the thin atmosphere block out sunlight, and nuclear power and propulsion systems are decades away from certification by NASA,” he writes.

Solar photovoltaic arrays are a low-density power source that, on the Moon, would have to be landed with retro-propulsion along with a matching set of large radiators. Because of the lunar day-night cycle, these are only truly useable at a few limited locations on some rather perilous peaks near the polar regions. Contrary to the situation on the Moon, on Mars the day-night cycle is adequate for daily use of photovoltaics or, perhaps, a solar thermal concentrator, without requiring the heat rejection capacity of a massive radiator used for operations in a hard vacuum. Dust can be removed rather easily with a bit of old fashioned housekeeping (an EVA perhaps once or twice during long-duration Mars mission). NASA’s lack of a larger nuclear power system (on the order of 50–150 kilowatts) is not because of any lack of technical proficiency, but instead a matter of will and funding priorities. Nuclear reactors are a very mature technology and can be engineered relatively quickly if needed.

Thangavelu then turns his attention to constructing habitats. “The emerging robotic construction technology has huge ramifications for planetary infrastructure establishment, and that is especially true for the Moon,” he writes. “It is now possible to erect or build entire habitable structures, certify and commission them before humans arrive at the destination to occupy them.”

Robotic construction tech has limited use on the Moon. Why? Because you’d still need to land much of the material needed for such a base, along with the robotic technology (and this still exists only in a theoretical sense.) This adds an extra level of complexity, decades of development for the construction hardware, and multiple unnecessary hardware elements. This is not a good strategy to build a base on either the Moon or Mars.

Then there is the issue of radiation risks of Mars missions. “We do not have the technologies currently to keep people alive and well for the long duration missions through the deadly radiation environment that pervades interplanetary space,” he argues. “Radiation doctors and professionals know that crew will perish during transit to Mars, and that we do not yet have the technology to protect them against GCR or anomalously large solar particle radiation storms.”

The fundamental technologies required for Mars missions already exist: some will need to be refined, but there are no fundamental technologies that need to be developed.

This section is completely false. There are two types of radiation that people need to be concerned about in space. One is solar storms, which come infrequently and are mostly protons with energies of about a million electron volts. These can be blocked with less than 15 centimeters of water, or things similar enough to water to be adequate, such as food and other mission supplies.

The second type of radiation is galactic cosmic radiation (GCR), energetic particles with an energies over a billion electron volts, which cannot be blocked easily and come at a fairly constant rate from all directions in space. Earth’s magnetic field and Van Allen Belts do not protect astronauts in low Earth orbit from cosmic rays. Instead, the Earth blocks half of the sky and makes the dose rate half that what it would be in interplanetary space. Yet, even twice that rate is not a showstopper or even that large a concern: the total dose received would represent about a one-percent statistical increase of developing cancer at some later time in an astronaut’s life, from a base level of about 20 percent. For comparison, smoking increases someone’s risk of getting cancer about 20 times more than of traveling to Mars would.

Consider that there are about a dozen astronauts who have, in the course of their careers, received radiation doses equal to or greater than someone would receive during a Mars mission, yet none of them have had any negative health effects. Also consider that during a Mars mission, on which astronauts would spend only about 40 percent of their time in interplanetary space, would receive nearly the same total radiation dose as is currently experienced on board the ISS. So if someone is trying to claim that radiation in interplanetary space is an assured death sentence, they should get on the phone to NASA and let them know that their current space station program has far too much radiation for any of their crews to have survived, and they may want to check their astronauts for signs of zombification.

Thangavelu also raises concerns about radiation exposure on Mars: “But once we get to Mars surface, how to survive the solar particle radiation that is quite high even there, on the surface? Unfortunately, we do not have an answer to this lethal issue yet!” Why not simply place regolith on top of the habitat: a simple solution to a simple and rather minor issue.

Then there is the issue of planetary protection, and the risks of contamination between Mars and the Earth. “To add to all the controversy to the exploration and settlement of Mars, is the issue of contamination and quarantine,” he writes. “Some scientists believe human activity on extraterrestrial bodies will endanger potential life forms that may exist there.”

Hundreds of kilograms of material are exchanged between the two planets every year and the material is not thoroughly sterilized in this process. Thus, any contamination that could happen either way would have already happened long before our ancestors even walked upright, and it continues to this day, so these concerns can be disregarded.

Thangavelu concludes: “But all this begs the question: do we have to wait for technologies to develop, or are there worthwhile missions to do and gain invaluable experience while we get all these ‘good to have’ technologies certified and commissioned for a Mars expedition?” The fundamental technologies required for Mars missions already exist: some will need to be refined, but there are no fundamental technologies that need to be developed.

Moon and Mars exploration will also tell us a lot about the human condition, including our will to survive outside our tiny blue island in space.

There is one lunar mission that could be a worthwhile precursor: a Moon mission based on Robert Zubrin’s Moon/Mars Direct architecture that would have about 80 percent commonality between the Moon and Mars hardware and would only involve three core hardware elements (launcher, habitat, and return craft.) This mission would be followed up with an unmanned Mars flight (the Earth return vehicle), then two years (and a couple of Moon missions later) by the launch of the first Mars crew. Both the Moon and Mars could be explored using just three heavy-lift launches per year, with a long-term program average of two launches to the Moon per year and one launch to Mars per year. During the height of the Space Shuttle program, NASA launched about six flights per year, using only half that level of launch capacity we could explorer both the Moon and Mars.

Thangavelu says the Moon is right here right now: “The Moon, on the other hand, offers all the excitement, now, as opposed to the next decade or the one after.” But is it really? As noted above, they can both be done at the same time, and the development time for one (or preferably both) pursued seriously would be around a decade regardless of destination.

He also offers scientific justifications for human lunar missions: “just for those scientists aiming for the next few decades of Nobel prizes, some of the finest scientific discoveries of great and immediate import to our species anywhere on the solar system is waiting for us on the Moon.” It’s hard to believe that dating craters on the Moon is something that will excite the public, nor will the promise of a handful of Nobel Prizes awarded because of it. Many people say we should go to the Moon before Mars, yet we can do both at the same time.

Moon and Mars exploration will also tell us a lot about the human condition, including our will to survive outside our tiny blue island in space. On Mars we may find new answers and many new questions about biochemistry, the history of the most Earth-like destination within reach, and the nature of life in the universe. With an integrated approach we can open two new worlds to humanity, two new worlds that will drive human ingenuity as we rise to meet the challenges they bring.