Appendix: The Science Behind the Story

The mission described in the story tried to convey a quirk of spaceflight that isn’t widely appreciated: some things are catastrophically hard to do, but others are surprisingly easy. The story hopes to show that if we defer what’s very hard, we can still realistically plan some amazing missions.

First let’s talk about what’s easy. Space is a unique environment, and building spacecraft entails meeting design challenges that occur in no other industry. But there are also commonalities, most notably with aircraft and submarine design. One striking similarity with commercial aircraft design is as follows: both kinds of vehicle spend much of their service life in a surprisingly benign environment. For commercial aircraft it’s high-altitude cruise, and for spacecraft it’s orbital cruise. The reason airliners can stay in the air for so much of their lifetime is cruising at high altitude induces very few loads on the airframe. The engine thrust exactly matches the drag produced by the oncoming air stream, and the net stresses on the vehicle are not significantly different from those acting on an aircraft supported by its undercarriage in a hangar on the ground. The same is true for spacecraft. Once in orbit, regardless of the speed it travels, the stresses on the craft are not that significant.

When closest to Earth, Mars is 140 times further away than the moon. That’s a long way. But it doesn’t follow that it’s 140 times technically harder to get there. It certainly would take 140 times longer to get there (assuming you didn’t upgrade your moon rocket engines). If there were humans on board you’d need 140 times as many sandwiches as the moon missions, but you don’t need 140 times as much fuel because once at speed, there’s no friction slowing you down.

The point is that the difference between a journey to Mars and the Moon is principally one of duration. It’s no co-incidence that NASA and Russia have spent the last several decades learning about long duration spaceflight, most recently aboard the International Space Station alongside European, Canadian, and Japanese colleagues. Several humans have spent the equivalent of a Mars mission journey aboard space stations, so the technical challenges here are well understood.

So, building a Mars Orbital Laboratory and sending it all the way to Mars, as described in the story is not that big a challenge. Today, a typical mission to the Space Station lasts for six months, which just happens to be how long it takes current rocket technology to get to Mars. It would make no difference to an astronaut if that time were spent endlessly circling the Earth, or on a journey that concluded in orbit around Mars.

But landing a human outpost on Mars is very probably 140 times technically harder than landing on the Moon for reasons I explore briefly in the story. If you ignore the problems with landing large masses, then a mission with a surface research outpost would require the development of Mars-capable spacesuits, tools, and vehicles, etc. Importantly, such an outpost would need to land with a fueled vehicle that can take the crew back to Mars orbit where it would dock with an Earth-bound sister vehicle, so that’s two additional spacecraft. Advocates of this plan suggest the Mars research equipment would be first tested and refined on the Moon, so that’s an additional mission (or missions).

In the story, Sasha’s dad is aware that in 2025 none of the infrastructure for a landing has been developed. The MOL plan was approved in 2011 because it was achievable without the high risk and cost of developing the Mars lander, Mars ascent stage, Lunar test missions, and the surface operations hardware. This is why he doesn’t want Sasha’s to get her hopes up about a Mars landing...

But the number one challenge for a Mars surface outpost is one that also plagues the orbital mission concept in the story, and in fact plagues every other space project: Energy.

With unlimited energy you could land large masses on Mars, and could build robust extra-safe hardware with countless backup systems. You could also get to and from Mars more quickly.

But to date, every space mission has been starved for energy because the cost of getting hardware and fuel into orbit is so high. Once in orbit, the energy available for a mission is limited by either the mass of rocket fuel brought along, or the size of solar panels, or the capacity of RTGs (radioisotope thermoelectric generators). But in every case it’s a limiting factor.

While landing an outpost on Mars requires phenomenal energy (which translates into numerous launches, and cost), the energy needed to orbit Mars is LESS than that needed to land an equivalent mass for a Moon mission. My goal for the story was to keep the realism high, so while Sci-Fi writers can assume plentiful energy, I did not. I’m confident we’ll land on Mars some time in the distant future, but in 2025 a surface outpost is not realistic, but an orbital mission is.

There are two other key technical considerations, and both are related to astronaut health.

The first is the muscle and bone loss that occurs in weightlessness. This can only be partially alleviated when astronauts follow a vigorous exercise program while in space. Understanding and mitigating this problem drives much of the research onboard the International Space Station today. Critics of missions to Mars have suggested that after six months (or more) of weightless flight-time astronauts would arrive incapable of standing, or would at least have trouble completing the mission. The gravity on Mars is roughly one third as strong as here on Earth, so it’s not clear that such a sudden increase in gravity would be debilitating. On the other hand, since astronauts could spend up to two years on the surface, it’s also unclear if that’s sufficient gravity to prevent further deterioration. If not, explorers will find they need to spend much of their time on Mars at the gym.

While the International Space Station was designed for zero-G research, a Mars Orbital Laboratory would not have that requirement. As far back as Gemini XI in 1966 NASA experimented with inducing artificial gravity by spinning two craft around a central point, one acting as a counter-weight. Without going into details, we might reasonably expect this technique could be used to fly the MOL astronauts in a 1 G environment – just like here on Earth – for the majority of their mission. (Sadly, if the spin were maintained while at Mars, the view out of the window could make the astronauts pretty dizzy. But in 2025 it’s a good guess that the display technology, including virtual reality, will give them a spectacular view of their surroundings.)

The second consideration is potentially more grave. Here on Earth there are several mechanisms that shield us from Solar and cosmic radiation. Astronauts that head for the Moon or Mars will fly beyond that protection. There are several technologies that could mitigate this problem which are on the cusp of becoming science-fact rather than science-fiction, but for now the health of the crew is best served by simply making the mission as short as possible. Shielding the crew behind water, or polyethylene provides some protection, but because this is unknown medical territory, the first Mars explorers will once again be chosen from volunteers with the ‘right stuff.’

And we’re back to energy again. If we have energy to spare we can reduce the journey time by accelerating and then braking as we near Mars. Similarly, if we have the energy we can build a spacecraft with lots of (heavy) radiation shielding.

Most of the design studies for Mars missions assume traditional chemical rocket technology will be used. And sending the required mass to Mars needs plenty of rockets and lots of fuel. The most recent credible plan requires launching about 12 times as much payload as the Apollo lunar missions. To loft this much mass with current launchers like the Space Shuttle would take 57 separate flights. These numbers are for a surface mission, so an orbital mission will need far fewer, but the scale is still huge.

There is no doubt that at some point in the future space exploration will switch to nuclear propulsion and nuclear power; the options opened up by the availability of energy are just too enticing to ignore. The difference will be similar to the shift that occurred in commercial air transportation from propeller driven craft to jets. We could have stuck with propeller aircraft and learned to live with their limitations, but the considerable cost of developing efficient jet engines has proven to be a winning investment. So nuclear propulsion will happen, it’s just a question of when, who will do it first, and precisely what technology will be used.

One of the key findings of the Augustine Commission is NASA has been unable to drive technology development forward while busy operating the Shuttle and building the Space Station. As a result, new plans for exploration are to a large extent picking up from where we left off in the 1970s. I wholeheartedly agree with the commission; developing new technology is the right role for NASA.

The story doesn’t require it, but in one possible variant we can imagine that an aggressive nuclear space propulsion technology program begins in 2010. The technology has not been worked on much since the 1970s, but if it were funded in parallel to the other elements of the mission in the story, it might be ready by the 2020 timeframe, and the story timeline would still add up. It would be a gamble, but the payoff would be significant.

One related technology that looks promising is a plasma rocket called VASIMR. It’s far enough along that a test version will be used on the Space Station in 2012. (This is actually happening; it’s not part of my story!) It’s a small rocket, but the designers claim that if it were scaled up and driven by a 200 megawatt nuclear powerplant it could send a 60 metric tonne spacecraft to Mars in just 39 days. Progressively smaller powerplants give more modest transit times, but still beat chemical rockets by a wide margin.

The current NASA Mars Design Reference Architecture requires a 540 day stay on the surface of Mars and a 30 month mission duration. This is driven by energy limitations; once you have set up the outpost you need to wait for Mars and Earth to become aligned properly in order to get home.

With a nuclear propulsion system driving a Mars Orbital Lab, we can conceive of a 120 day shakedown mission (one month stay), returning to Earth for a refit, and then longer duration missions.

If I were Sasha’s dad, I know which one I’d choose.

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