Thursday, October 1, 2015

The Martian: A Technical Commentary


The Martian Technical Commentary
Casey Handmer, Adam Jermyn, Matthew Paragano, Peter Lommen, Jeff Nosanov

In late September 2015 (later this week!), the film adaptation of Andy Weir's debut novel "The Martian" hits the big screen. By no means the only Mars-centric slice of media to have been released this year, my friends and I saw an opportunity to show off our technical knowledge and share our passion for space by writing a detailed technical commentary of the already technical novel!

If you haven't read the novel, not much of this will make sense. It's highly recommended. An earlier draft can be found for free at http://online.wsj.com/public/resources/documents/TheMartian.pdf or in fine bookstores everywhere. (Edit: This link no longer finds the complete PDF. You'll have to be more creative.)


Executive Summary

Andy Weir has knocked it out of the park. Within a sub-genre already adored and reviled for its adherence to technical details and the laws of physics, "The Martian" is astonishingly realistic. While there are naturally numerous quibbles and minor mistakes, not a single one is inexcusable from either a technical or narratological point of view. Weir does not employ any technically impossible deus ex machinas, and his novel's mission-to-Mars architecture, while awkward in some respects, is entirely credible. We sincerely hope that the film adaptation preserves this commitment to reality.

Also, SPOILER ALERT. Duh.


Contents

We found we wanted to loosely organize our discussion around the following topics. This doesn't imply that each topic contains a serious mistake, however!
  • Mars Weather
  • Communications with Earth
  • Hydrogen, Oxygen, and Water
  • Solar Cell Efficiency
  • Mission Operations Concept - all vehicles and systems
  • Orbital Mechanics
  • NASA Processes
  • Life Support Systems
  • Botany and Nutrition
  • EVA Suits and practicality


Mars Weather

The Martian opens (Sol 6) with astronaut Mark Watney marooned on Mars, left for dead during a powerful dust storm that forced a mission abort, destroyed his communication system, and otherwise sets up the premise of the whole plot. In this respect, the dust storm is an obvious narrative device, with far more gravitas than a more probable cause of his predicament, such as "the *shiny object* broke".

In reality, while Mars has large dust storms, the Martian atmosphere is so thin that the forces experienced by people and structures on the surface are entirely trivial - no more than a light breeze on Earth. Likewise, it is correct that dust lofted high into the atmosphere by seasonal pressure changes is extremely fine and scatters light, obscuring the surface from space and reducing solar insolation, but not to the point where any of NASA's surface missions have failed due to a lack of power during a dust storm.

The Martian atmosphere enters the novel later on too! On Sol 499, Watney is busy righting rovers when he comments that he can't hear the noise of distant movement. This is not because the atmosphere is too thin to carry sound, but because its low density couples weakly into his suit. Sound can propagate quite well in the Martian atmosphere. Later that day he complains that the sun has set early because the Ramp is up against the western wall of Schiaparelli crater. But the grade of the ramp and crater wall is about 2 degrees - it should shorten the day by about 9 minutes. Watney has clearly had a long day!

On Sol 505, NASA discusses cutting out a bunch of the Mars Ascent Vehicle (MAV)'s hull to save weight. Mars' atmosphere is thin - you don't need much streamlining provided your speed is relatively low for the first few 10s of km. Most orbital rocket launches clear the atmosphere before punching the gas and then it's parallel to the surface to pick up speed while exploiting the Oberth effect. It's unclear why they cover the front with a tarp - this is a prime opportunity to ride a chair to orbit like Kerbal Space Program, or the actual Lunar Escape System https://en.wikipedia.org/wiki/Lunar_Escape_Systems.

In Chapter 26, Johanssen states that the Ares 4 MAV mooring clamps are released about 5 seconds before launch. US rocket designs tend to clamp the rocket to the launch pad until full launch thrust is achieved, whereas Russian rockets just take off as soon as thrust overcomes gravity. This clamping technique led to pogo in the early Saturn V launches, a tendency for the entire launch stack to vibrate lengthwise after stretching a few inches during main engine start. Nevertheless, to what is the MAV clamped? There is no launch gantry or pad here. The descent stage is all parachutes and empty fuel tanks at this point. Perhaps a small onboard rover runs mooring cables out and drills them into the ground, but that would confer immunity to tipping in even the wildest of dust storms!


Communications with Earth

Once Watney has patched himself up from his ordeal and taken stock on Sol 6, he comes to realise that his communication system to Earth has failed. In the novel, substantial space is given to describing how the surface coms work and why the Hab was, in the absence of the Mars Ascent Vehicle (MAV), a single point of failure. Obviously if Watney was able to talk to Earth easily, the marooning would be much less interesting. The film "Interstellar" also featured a convenient (for the plot) and highly unlikely com system failure!

Later in the novel, Watney uses the Hab transponder to communicate with the rovers, which apparently has a line-of-sight range of about 20km. Mars has a useable ionosphere, so short-wave radio can bounce beyond line-of-sight, which would substantially reduce Watney's navigational confusion.

Additionally, satellites in Low Mars Orbit are between 250km and 350km from the surface at lowest approach. Radios' data transfer rate is inversely proportional to their power - which means that even a weak radio can transmit some data. Using the inverse square law, the Hab beacon could transmit data to satellites about 100x slower than to the navigation system in the rover. Note that even 1bit/second is thousands of words a day - easily enough to help with some of Watney's most basic problems.

Even in the event that the Hab/rover beacons (or surface hand-held radios) couldn't communicate with Mars satellites (due to operating in the wrong band, for instance), they could still talk to Earth, which has all kinds of radios. During entry, descent, and landing, the NASA rovers transmit a status beacon with their low-gain antenna. For instance, Curiosity's 15W low-gain (omnidirectional) radio can transmit at up to 15b/s, while the high-gain antenna can communicate at up to 32kb/s, which is about as fast as internet was in 1994. The rover can and does more often communicate with Earth via the Mars orbiting satellites, which have a higher data transfer rate, albeit only for a few minutes a day. To put the required power in perspective, a 3G phone can transmit at up to 2 watts (a thousandth of the power of Watney's microwave), requiring a mere 10 minutes to send one SMS from Mars to Earth under ideal conditions, plus 4 minutes time of flight for the relevant microwaves. Somewhat more plausible, then, is a software or hardware problem in the relevant transmitters which requires the MAV to be present to communicate with Earth, much as cellphones occasionally require pesky SIM cards to communicate with their towers.

On Sol 33, Watney backs up his log to both rovers, presumably with USB sticks, since at this point of the story there is no other method (even coax down a pipe) to communicate with the rovers. Even later in the story, Watney needs to suit up and walk to a rover to check his email.

Also relevant is that Watney usually knows where Earth is in the sky. It is possible to build a rudimentary directional antenna from a tin can or mylar blanket or string of wire that vastly increases your odds of both receiving and transmitting messages. Back in my day kids used to make crystal radios with cans and bits of pencil lead. Software-defined radio closes this technological loop.

One other note - even pitifully weak radio transmitters can transmit way further than they have any right to, provided they employ staggering and entirely pointless quantities of error correction. For example, https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software) discusses transmission of data with a signal to noise radio of -28dB, which is roughly equivalent to holding a normal conversation right next to an unmuffled Harley at full throttle, in the same frequency band. Just saying, Watney, you could have had internet.

The other side of the equation is NASA's interdiction of communications between Earth and Hermes, creating dramatic tension when they don't know that Watney has survived. It seems (from the trailer) that this is changed in the film, but it seems quite likely to me that a meddling radio HAM could easily transmit illicit information to Hermes, with no more difficulty than radio HAMs currently talk to the ISS. This was a key and accurate plot point in the generally much less accurate novel Seveneves by Neal Stephenson.

On Sol 97(2), Watney devises a code with Pathfinder to allow full ASCII text. 21 degrees of arc is only 50% more than the 13 degrees required for individual letters, and halves the bit rate. Watney was being too clever by half, since how often did he need to communicate a "&" or a "@". It turns out the engineers who developed telegrams had the right idea! By Sol 98, it's obvious that ASCII was a terrible mistake. Sol 98(2) betrays more problems - apparently laptops can't deal with vacuum (not sure how they got them into the Hab… perhaps they came with the crew inside their flight suits?) and their EVA suits lack cameras (until a few hundred days later…) and note-taking capability. During the flashback in Chapter 12, Beck is somehow able to do broken packet inspection in his flight suit, though. For consistency, this suggests that the suits have varying capabilities, though it isn't clear why the doctor needs to do packet inspection.

By Sol 116, they've patched the rover software and have a text connection. While they can send a 20Mb software patch, apparently even midis (10s of kb) of something other than disco is impossible. Poor Watney! I found it interesting that despite the sheer difficulty of EVAing to receive an email, Watney and NASA didn't have multi-phase communications going on, like letter writers used to in the old days.

On Sol 475, Mindy Park wakes up exhausted, with aluminium foil taped over her windows. It turns out that one Martian day, or sol, is about 24 hours and 39 minutes - uncannily close to Earth's own day. Kapton-taping aluminium foil or (more commonly) aluminized mylar over windows is standard practice for scientists and engineers at JPL running missions for the first few months after landing. A Martian sol is close enough to Earth's that plants, and it turns out, humans, are completely okay with this schedule. Most science fiction authors think that people on Mars will use Earth seconds, minutes, etc and insert a leap hour or 'time slip' each day at midnight.

On Sol 504, Watney picks up a blip from the Ares 4 MAV, despite all its communication systems being "directed". I'm surprised it wasn't designed to be more findable! Later, he complains the area has too much sand and not enough rocks to write a message. Why not write out messages or even obscene shapes in the sand? It wouldn't be the first time a NASA asset has done so on Mars!


Hydrogen, Oxygen, and Water

On Sol 16, Watney realises he doesn't have enough water to wet all his dirt. This starts a whole host of problems which are essential for advancing the plot, but it's worth pointing out that in the real world, much of this pain could be averted! In particular, Mars is actually pretty wet as little as 6" below the surface in some places. Carving out hunks of frozen permafrost and thawing them inside the Hab would provide all the water he could need and more.

On Sol 30, Watney decides that the answer to all his problems is to reduce hydrazine left over in the Mars Descent Vehicle (MDV). Luckily NASA has no idea what is going on, or else they would probably collectively die of shock. The calculations he relates in the log makes the assumption that hydrazine and water have similar density which, fortunately, they do.

Watney also decides to reduce hydrazine in the gas phase, which is really dangerous. There are numerous other chemical pathways to extract hydrogen from hydrazine, including in the liquid phase. While Vogel would have known about them, Watney could operate only by analogy with the MDV's rocket motors, which nearly, but not quite, killed him. During the sizzling process, some quantity of hydrazine would have vaporized without decomposing. Fortunately, it must have reacted with something else before Watney breathed it in.

Later in the Sol 33(2) post, Watney describes trying to burn off the hydrogen coming through a small hole. For an astronaut who did a Masters in botany with a bunch of stoners, he seems to know very little about making an excellent flame-thrower. Most Burning Man-worthy systems use a patch of steel wool as a diffuser/gust avoidance mechanism - which functions like a gas wick and prevents the flame from going out with even unsteady gas flow.

The Sol 37 post contains substantial evidence of hydrogen narcosis, since very little of it makes sense from a scientific point of view. According to his suit sensors, his Hab atmosphere was composed of 63% hydrogen and 9% oxygen. This would cause all sorts of problems, including hypoxemia (low oxygen content in the blood) and altitude sickness, noticeable voice changes from the lowered molecular weight of the air (like speaking with helium) and also hydrogen narcosis. The oxygenator would have tried to add more oxygen. Additionally the hydrogen present in the air would have already detonated from the pilot light. I'm glad Watney got the Hab atmosphere under control later, and impressed that he didn't bother to amend his confused ramblings on the log.

On Sol 41, Watney's exhaled oxygen builds up to the point where he nearly burns his face off. It turns out that a single breath only consumes 20-25% of the oxygen in the air, so that's spot on.


Solar Cell Efficiency

In Chapter 7, on Sol 63, Watney says that he has "over 100 square meters of the most expensive solar paneling ever made" with "an astounding 10.2 percent efficiency". As we will see, this is one of the more problematic aspects of the story.

Watney's solar panels are an example of photovoltaic technology, solid state devices which turn light into electricity. A photovoltaic panel is just a material which uses the energy in light to push electrons around. The solid state part is nice for space missions, because it means no moving parts, and hence very limited failure modes. As the story indicates, this makes setting them up straightforward: point them at the sun and plug them in. Sending 100 square meters to Mars, while not easy, is certainly within the realm of feasibility: photovoltaic cells are typically quite thin, extremely light, and stack (as noted in the story), which makes them basically the perfect thing to send to space. So far so good.

The problem arises with the efficiency figure. As of the time of this writing, commercial solar panels are sold with efficiencies up to 22%. Research labs can make cells with up to 46% efficiency, but because you then lose out on the benefits of mass production, these come at extreme cost. The cost of launching cells into space, however, typically outweighs any manufacturing cost, and so NASA tends to go for the most expensive/efficient panels they can get.  For example, the Mars Exploration Rovers (Spirit & Opportunity) used solar panels with beginning-of-life efficiency of 27.5%.  These rovers produced between 300 and 900 watt-hours per day from 1.3 square meters, varying primarily with season and latitude. In Mark's units, that works out to between 1.4 and 4.3 square meters per pirate-ninja.

Say Mark's panels are actually 20% efficient, to be conservative. It could be that there are conversion losses in the transformers, battery charging losses, and so on. The panels could also have degraded somewhat from radiation exposure sitting on Mars waiting for the Ares 3 crew to arrive. The main consequence of doubling the efficiency is that Mark should have twice as much power available. This means double the driving distance per driving day, as well as double the spacing between air days, cutting his road trip time by more than half. It also lends him more buffer against the sandstorm, though as mentioned above, that shouldn't have been a problem in the first place.

On Sol 211, Watney states that he can drive 100km/day with his solar panels. Actual solar panel efficiency would, ceteris paribus, translate to between 200km and 400km a day of driving distance. For plot reasons, Watney is mass and volume constrained on panels and geographically constrained on driving distance, so tweaking the efficiency is a good way to add difficulty to Watney's plight while avoiding more obvious fudging. Equivalently, the plot could have been maintained by reducing the carrying capacity of the rovers, or by requiring Watney to construct a makeshift transformer to interface the panels with the Rover's batteries, which would substantially reduce his efficiency.

On Sol 458, Watney remarks that he can (re)watch television while waiting for his panels to charge. This is actually reasonable - his life support and driving consumes hundreds of times more energy than running a laptop.


Mission Operations Concept - all vehicles and systems

An Ares mission begins with 14 uncrewed launches (probably with an Atlas or similar sized booster) dropping airbag-cushioned payloads on Mars. These would each weigh about 1000kg on launch, with up to 600kg of payload to the surface. This includes parts of the Hab and supplies.

The crewed part of the mission is mediated by the Hermes, a large vehicle for deep space with a nuclear powered ion drive designed to fly between Earth and Mars and back. The Hermes is used by every mission and was assembled in Earth orbit at (no doubt) astronomical expense.

The six astronauts of Ares 3, together with their supplies are launched from Earth to Hermes. The Mars Descent Vehicle is launched separately towards Mars at about this time.

Hermes and the MDV travel to Mars, parking in Mars orbit after 124 days in deep space. Hermes remains in orbit, uncrewed, while the MDV flies the astronauts to the surface.

On the surface, the astronauts build their Hab from airbag cushioned cargo drops and perform their mission. After 30 days on the surface, they climb into the Mars Ascent Vehicle and fly back up to orbit, where they meet the Hermes and fly back to Earth, taking 208 days to return.

Hermes parks in Earth orbit and the crew return to Earth in some re-entry vehicle like Orion or Dragon.

The MAV was launched years before, made its return fuel on Mars using electricity and ambient atmosphere, then was used for about 6 hours to get back to the Hermes.

This mission architecture is very credible, given a nuclear powered ion drive, which is technically possible but politically problematic. IMO, the architecture is inefficient given most of the hardware is used only once, Hermes is not self sufficient, and the astronauts spend only 30 days on the surface.

Mars Ascent Vehicle (MAV) is quite large. It had to be soft landed, but even empty weighs much more than all 14 presupply missions combined. Given that NASA has (in the story) developed soft-landing capability for tens of tonnes, it's not clear why stuff as mission critical as the Hab is landed relatively inaccurately in lots of parts. It could be that this simply reduces mission cost and complexity, or that there was no practical way to land something as bulky as the hab (even disassembled) in one piece.

The MAV employs In-Situ Resource Utilization (or ISRU) to make fuel and oxidizer for the return flight. Two (Earth) years of power from a 100W Radioisotope Thermoelectric Generator (RTG) is enough to make 13kg of fuel (methane) and oxidizer (oxygen) from every 1kg of hydrogen (H2) precursor brought from Earth, for a total of nearly 20T of fuel.

At various points of the novel, Weir describes the MAV as weighing 32 metric tons when fully fueled, and standing 27m tall. This implies that it is very long and skinny, which is unnecessary in the thin Martian atmosphere. Not only that, this means a lot of rocket mass relative to the amount of fuel it can carry (spherical rockets are vastly more efficient, absent significant atmosphere). Needless to say that's a bad thing. By comparison, the Falcon 9, a long and skinny rocket by usual standards, is about 70m tall and weighs about 600T on launch. The MAV could easily be a conical shape perhaps 5m wide and 10m tall.

Weir states that it has two stages, though one stage is perfectly adequate for the relatively low delta-V required to reach Low Mars Orbit (4.1km/s). Nevertheless, with a 325s Isp methane-oxygen engine, a two stage system would have a 16T first stage, a 8T second stage, and a 8T orbital module, with an implied mass fraction of 81% fuel vs 19% metal in each stage.

Towards the end of the novel, engineers at JPL describe the MAV as having an unrealistically low launch weight of 12,600kg (12.6T) - similar to a fully-loaded Dragon capsule. So we'll assume this is the dry mass. Let's assume, then, that the orbital module is 8T, the first stage is 3T, and the second stage is 1.6T, empty. The 19.397T of fuel is distributed accordingly, implying an engine Isp of 405s in order to reach 4.1km/s of Low Mars Orbit. This is low for H2/O2 engines, but extremely high for a methane-oxygen engine. Even SpaceX's planned monster Raptor engine has a notional vacuum Isp of 380s.

In order to get to 5.8km/s and intercept the Hermes, the mass of the orbital module needs to be reduced from 8000kg to 4280kg, a reduction of 3720kg. This takes into account adding 780kg of fuel, removing 500kg from the first stage (pulling off an engine), and so on. The accuracy of the numbers indicate that Andy Weir did the math, but it's not clear on what metrics he designed the MAV and its launch system.

More generally, given that the total delta-V needed to get from Mars to Earth is *only* 7.8km/s, a MAV that flies all the way back to Earth is completely possible, though it would probably need to be bigger than the MAV presented here to have adequate life support. But given that the fuel/delta-V is most easily obtained on the surface of Mars, rather than brought from Earth, a direct ascent architecture actually makes a lot of sense.

On Sol 68, Watney points out that NASA never used large RTGs on crewed missions before Ares, but during the Apollo program RTGs were deployed by astronauts to power lunar seismometers. On Sol 69, Watney states that Lewis had buried the RTG for safety reasons. A RTG stashed somewhere on the surface, however, is much less likely to overheat.

Mars Descent Vehicle (MDV). In the Sol 7 log entry, Watney mentions that the Mars Descent Vehicle (MDV) is useless to him for escaping, since its thrusters cannot even lift its own weight. This, of course, refers to its weight when fully fueled. Before landing, much of its fuel has burned off and it can achieve neutral thrust for a hovered landing. Nevertheless, it lacks (by far) the fuel capacity, thrust, and delta-V necessary to fly anything back to orbit!

In Chapter 8, Bruce and Teddy discuss potential MDV modifications. It is strongly implied, though not stated, that the design would not admit the addition of more engine clusters, and they don't have the time to invent a bigger engine. It is likely that this is a narrative device.

Rovers. Watney has two rovers at his disposal. NASA would almost certainly send two similar/identical rovers to serve as backups for each other and out-maneuver the "walk-back" requirement, that states no astronaut can drive a rover further from their ascent vehicle than they can safely walk back. During the later Apollo missions, astronauts drove up to 7.6km from the lunar lander, which would have been quite a walk!

In his Sol 7 entry, Watney describes the rovers as half-buried in sand but otherwise fine. Unlike the MAV, the rovers have a lower center of gravity, a lower profile, and are less likely to be damaged by an entirely impossible dust storm.

On Sol 192, Watney is in the process of modifying the rovers. One of them will have its pressure vessel opened, which requires drilling 760 holes and chiselling out the gaps. Given that the drill is nearly 6 feet long and overheats, and the pressure vessel is made from relatively flimsy composite, it's hard to imagine that some sort of saw, even a wire saw, could not have been devised. Or the drill adapted into an angle-grinder. Come on Watney! Drilling loses its luster well before 760 holes. On Sol 193, Watney's drill frequently overheats. Given that, I wonder how his suit manages to regulate heat from practically any level of physical activity without ever overheating.

On Sol 197, Watney devises a unit of measurement, the "pirate-ninja", which is equivalent to one kilowatt-hour-per-sol. Turns out NASA actually uses "watt-hour-per-sol", or "milli-pirate-ninja" when they're planning operations for the solar powered rovers which, as of 2015, is the Mars Exploration Rover "Opportunity". With dust constantly accumulating on its steadily degrading solar cells, balancing uses of power between motion, heating, instruments, and communicating with Earth is of vital importance.

On Sol 211, Watney describes fitting batteries in the rover, implying that its internal volume is a tube roughly 4m long and 2m in diameter.

On Sol 380, Watney is installing the RTG into the rover to provide a source of heat that will save electrical power, particularly in the oxygenator. It's not entirely clear why Watney wants to seal it in a plastic bag, perhaps to keep the electrical connections dry, but he's very keen to keep bubbles from being trapped inside. His solution is ingenious (sealing it in the evacuated airlock) but a few small bubbles would be unable to get much warmer than adjacent water on the other side of the plastic. This is borne out later, when he flips the rover and the RTG bag doesn't melt.

On Sol 502, Watney complains that the rovers can do only 5kmph while he was driving them carefully down the rest of the ramp. It would make a lot of sense to have remote control, so he could walk out the front and check the route as he went.

Hab. The Hab has amazing electric toilets of doom, described in several places. Despite the short duration of the mission, NASA has provided toilets capable of removing (and presumably recycling) the water content of waste. Water is actually really heavy and used a lot, so bringing as little as possible, in favour of amazing recycling, is a good idea.

On Sol 94, Watney describes the Hab has having fancy fabric to shield it (and him) from radiation. While a Faraday cage may explain his inability to setup a wifi network to the rovers, it would not be useful in shielding the Hab from any kind of solar or cosmic radiation - the kind that actually causes cancer. Fortunately, piles of dirt and the Mars atmosphere do, so future astronauts need not worry about excessive irradiation while on the Martian surface. 

More generally, talk of people being fried by or going crazy from radiation while on Mars are just not true. The sort of radiation exposure one experiences in space or on Mars (which is also in space) has a statistical effect on life-time risk of developing cancer. Current NASA limits would be exceeded by a trip to Mars, but current NASA limits are also exceedingly conservative. Given that about 40% of us will get cancer (and 20% die of it, if we live long enough), NASA's limit of <1% additional risk must be kept in perspective. Going to Mars involves all kinds of risks, but extra cancer would probably not make even the top ten most likely causes of unusual and untimely demise. Being an astronaut is a dangerous profession. The numbers are too low to be statistically valid, but 1 in 25 astronauts have died on missions or in training. This compares to about 1 in 60 Everest climbs or 1 in 60 BASE jumps. Bottom line: even the other Ares 3 crew that spent the most time in space (with less protection than Watney) are much more likely to die of non-space-related cancer, internecine cannibalism, yet another unplanned depressurization, an unplanned dust storm, a rover accident, burning up on re-entry, hydrazine poisoning, life support failure, acute radiation sickness from a reactor containment breach, micrometeoroid strike, untreatable renal calculi, launch failure, alcoholism, car accident, than space-related radiation-induced cancer.

In the same vein, on Sol 475 Watney talks about trolling his future grandchildren. While even Watney's extended stay on the surface is extremely unlikely to cause radiation problems, some mutation of his gametes is more likely. I anticipate that saving sperm on Earth will be standard practice for future Mars-visiting astronauts if, indeed, they are not done reproducing by then - most astronauts are already in their 40s or 50s. It's worth pointing out that his crew-mates spend the entire time he's on the surface in space, getting much more cooked. Even then, if they have a small shelter for coronal mass ejection (CME) events, their lifetime risk of cancer will have increased by about two percentage points, from 40% to 42%, assuming they're not already smokers. Probably worth it.

What's a CME? Most space radiation (cosmic, solar wind, neutrinos, UV, thermal) is either completely shielded or relatively harmless because the dose occurs over a long period of time. The exception is CMEs, or coronal mass ejections. These occur when magnetic fields on the sun go crazy and throw huge volumes of high energy protons (mainly) into space. If you were outside Earth's van Allen belts during one of these (one of the Apollo missions narrowly avoided one) you would take between 500 and 5000 rads prompt dose and very rapidly die an awful death. Fortunately, it turns out you can shield humans from CME-brand radiation with a thin (6" is heaps) jacket of water or equivalent light nuclides, like fuel. Since CMEs only a last a few hours, most mission designs have a hollow water tank that the astronauts can shelter in during a solar storm - don't forget the laptop with good movies on it! 

UV and thermal radiation is easily blocked by the pressure vessel or windows. Neutrinos are blocked by nothing but they also just fly through human without causing problems. Garden variety solar wind is strongly attenuated by a thin spaceship hull and skin and muscle - the relatively radiation sensitive bone marrow is shielded partially by the rest of the body. Cosmic radiation is much higher energy and can only be blocked by a shield of meters of water, which is too heavy for any practical spaceship design. Fortunately a full unshielded dose of cosmic background radiation is factored into the earlier calculation, and is orders of magnitude too low to cause acute radiation sickness. Funnily enough, building a shield from lead or bismuth or some other metal actually increases the radiation dose. Ordinary cosmic rays usually pass through human without causing problems - they're too high energy to interact. Having them hit lead first, though, causes a shower of low energy 'secondaries' which are the perfect energy to cook human, greatly increasing the dose. The key to radiation shielding is lots of light nuclide matter, such as water, liquid methane, or even liquid oxygen. Lithium Borate is sometimes suggested, but its worrying tendency to explode on contact with oxygen is sometimes considered undesirable.

On Sol 119, the airlock explodes from the lab. In the novel, the airlock is a vertically oriented box similar in size to a shower, and it flies and lands about 40m away. Given the relevant masses and pressures, this implies a launch angle of 25 degrees from the horizontal. Not impossible, but somewhat unlikely. Not something you want to have happen, at any rate. During the subsequent audio log, he decides to light a fire to make smoke and thus locate the crack in the leaking airlock. Later on (when resealing the Hab), he realises that he's already covered in fine powdered dust which would also do the job and much more easily. Later on, he realises that the Hab is offline, presumably because its power cables run through the airlock which just exploded?

On Sol 201, Watney makes a bathtub and finally practices hygiene. In the log, it states that it took two hours to heat the water to 37C, which corresponds to a realistic heat-exchanger efficiency of 35%. Nice work, Andy!

Pop-tents. On Sol 122, Watney remarks that the Hab depress on Sol 119 also led to collapse of the pop-tents. Given that the hoses which connect the rovers together automatically decouple (as mentioned on Sol 431), it seems unlikely that the pop-tent hoses wouldn't also automatically shut off. Similarly, if the Hab depressurized enough to kill all the crops, why are the laptops left lying around inside now mysteriously able to function after being in a vacuum? On Sol 388 they're not explicitly mentioned as worth moving to Airlock 3 while the rest of the Hab is changed. Perhaps by now they're all dead or vacuum-proof? In Chapter 26, the Ares 3 crew moves lab mice but not laptops to a safe place during the deliberate depressurization of Hermes. Why does NASA hate laptops?

On Sol 385, Watney reconfigures the pop-tents to have somewhere to walk around while waiting for batteries to charge. They are described as having folding flooring to overcome the tendency of pressurized flexible containers to become spheres, in a vacuum, of course. It's not clear exactly how this works, though. By Sol 388, Watney describes making a shape out of a perimeter and a roof. Growing up in the US, he was inadequately exposed to soccer balls - one possible way to make a curved roof with flat, mostly tessellating segments.

Observation satellites. In Chapter 8, Mindy talks about observation satellites. Specifically, a constellation of 15 satellites is able to observe roughly every 5 minutes. I'm not sure how this works. Given an orbital period of about 125 minutes, the satellite will pass over the same point (roughly) twice a day, one of those during the night. So that would imply a gap of about 40 minutes between photographs. Of course, it is possible to shoot at an angle from adjacent orbits, but getting down to a 5 minute gap is clearly the reason Mindy got the job!

Fetching Pathfinder. On Sol 82, Watney struggles to secure Pathfinder to his rover. It's not clear to me why it can't be hitched to the other side of the rover, balancing the spare battery, but presumably this will be aptly illustrated in the film.

In Chapter 16, the Chinese offer the use of their Taiyang Shen booster to launch a probe up to 941kg to Mars. This puts it (likely the Long March 3 family) in the same class as the smaller Atlas V configurations. It's neither a particularly powerful booster, nor one for which NASA couldn't find a core on which to launch the Taiyang Shen at a later date. Perhaps this should have been part of the deal, though marching-army costs might kill the mission if the delay is too long.

Also in Chapter 16, they discuss a hard-landing probe. Incredibly, NASA has specced out missions with electronics that can survive a 10,000g impact with an asteroid, rather than spend fuel slowing down to land softly. Further, pre-impact fragmentation of the cargo would further reduce its ballistic coefficient and thus impact velocity. It turns out that Mars bars have a terminal velocity of about 10m/s on Mars - probably fast enough to breach the wrapper, but the food would still be edible (no pesky bacteria to contend with!).

Resupply probe. In Chapter 19, China successfully launches a resupply probe to the Hermes as it flies by Earth. It turns out that Russian docking mechanisms are momentum dependent for latching. Several early Soyuz-Salyut dockings went astray because the cosmonauts docked too gently. The lighter Soyuz needed to hit quite hard in order to activate the mechanism!

Hermes. On Sol 439, the in-wall heaters in Martinez' cabin break, waiting just long enough that Beck and Johanssen have begun their relationship. More generally, this sort of unfixable equipment failure is the sort of thing that makes long term mission planning hard work. In general, system longevity goes like the log of the mass of spare parts, and systems are engineered to be maintainable on the fly.


Orbital Mechanics

On the first page, Watney states that he was six days into the best two months of his life. Evidently he was confused, as later it's made clear that the surface operations of the mission were only 30 days long.

30 days on the surface is the extent of surface stay permitted under an "opposition" class mission, wherein the astronauts fly by Venus on either the outbound or inbound leg. While shortening the overall mission, opposition class missions significantly lengthen the time spent in space, and also bring the spacecraft much closer to the Sun, increasing the crew's exposure to radiation.

The alternative mission design is the "conjunction" class mission, wherein the crew takes a relatively short 4-6 month Hohmann transfer flight either way, with a ~560 day stay on the Mars surface in between launch windows. Obviously, if Watney had been stranded on a conjunction mission, he would have had no shortage of snacks!

One additional detail is that Weir's spaceship, the Hermes, employs ion thrusters throughout the mission, enabling a wider class of missions and trajectories than the traditional point-and-shoot orbital mechanics described in the previous paragraphs.

In Chapter 16, the Purnell maneuver is discussed, by which the crew can return to Mars fewer days than the 404 it would take Iris to get there. It's probably worth noting that there is a very similar delta-V requirement for Iris to get to Mars vs a resupply probe to reach the Hermes. The advantage of the Hermes approach is that Iris had to be able to do entry, descent, and landing. If this is the case, Iris could also get there faster by borrowing a basic ion thruster package from, say, the Asteroid Redirect Mission (ARM) spacecraft. It's also not clear why all the crew need to return to Mars (aside from narrative reasons) - most or all could return to Earth in the entry vehicle while Hermes takes the Purnell maneuver to Mars to pick up Watney before he starves. Although the remaining crew would then depend on a new entry vehicle being sent up to meet them on their eventual return to Earth.

In Chapter 20, Annie somewhat incongruously asks why Hermes can't wait at Mars for Watney to get there, when it seems he'll be slowed by the dust storm. Venkat points out that Hermes is on a fly by and can't slow down enough to be captured into orbit, but this is not entirely true. On Sol 505, Bruce says to Venkat that Hermes is flying by Mars at 5.8km/s. Mars escape velocity is only 5.5km/s (the Earth's is 11.2km/s by comparison) meaning that a delta-V of only 300m/s is needed to capture into orbit. Given that Hermes can accelerate at 2mm/s/s, a two day burn would be sufficient to capture into a big elliptical orbit, drastically increasing their margin of error. Perhaps, if Hermes slows enough for an orbital capture, its launch window to return to Earth will close too quickly to be useful.

Of course, the MAV was designed to reach Low Mars Orbit, with a delta-V of 4.1km/s. Getting to 5.8km/s is highly non-trivial, as discussed in the previous section describing the MAV. Of course the unmodified MAV has life support, so Watney could wait while Hermes spirals down to 4.1km/s to pick him up (~30 days, because Hermes can't exploit the Oberth effect), while in the modified MAV he gets close to 5.8km/s, making it much easier for Hermes to rendezvous. A MAV that got to, say, 5.2km/s would split the difference nicely. Either way, the most likely explanation is that maneuvering Hermes to do this would make them miss the Earth launch window.

On Sol 543, Beck mentions that the modified MAV will hit 12gs during launch. While they have lightened the primary payload by about 16%, Watney also removed a spare engine, suggesting that the unmodified MAV would hit at least 10gs during launch, which is unlikely for a rocket designed to fly humans! Later, Johanssen reads out a velocity of 741m/s at an altitude of 1350m, which is staggeringly fast, implying an acceleration of 20.7gs. Perhaps she dropped a zero?

When Johanssen and Vogel talk about getting Watney to orbit, what they mean is solar orbit, since Watney will have to escape Mars entirely in order to be intercepted by Hermes.

During the intercept procedure, Ares 3 crew have to think fast to find additional sources of delta-V to move the Hermes close enough to catch Watney as he flies by. The distances and velocities mentioned during this passage in Chapter 26 are correctly calculated and almost entirely realistic.

Watney suggests making a small breach in his suit and using the stored gas as a rocket to close the velocity mismatch of 42m/s. Assuming he has 5kg of gas on board (including reserve tanks) and an exhaust velocity of 400m/s (unlike rocket exhaust, it's not hot) this confers about 17m/s of delta-V, which is just not enough. This idea is transferred to the Hermes, which will spit out its atmosphere to slow down. Assuming Hermes weighs 100T, it would have to lose about 5T of air to make up the required 29m/s of delta-V. At sea-level atmospheric conditions, this implies that Hermes has a volume of 4000 cubic meters, or a floor area of 1300 square meters, or 13,000 square feet, which is the same as a very large house. Perhaps Hermes has large pressurized volumes that aren't used much for habitation? Martinez estimates that the air will take 4 seconds to leave, which implies a relatively small hole, since the shockwave would take about 0.1s to cross a Hermes-sized volume of air. A realistic concept is that Hermes is composed of two large Bigelow inflatable modules each with a diameter of about 12m, such as the BA 2100 habitats. Also worth mentioning that the process of blowing the "Vehicular Airlock" (VAL) will send lots of airlock fragments into space, hopefully missing Watney.


NASA Process

On Sol 33, Watney talks about valves designed to release pressure on fuel tanks during quality assurance testing in construction. These actually exist and were a part of the failure of the oxygen tank that nearly ended Apollo 13 in tragedy. They remain an essential part of the design of any pressurized system.

During Chapter 15, a bunch of acronyms are listed which actually refer to specific teams in the launch process. This is a surviving cultural aspect of rockets in the 1960s - SpaceX's launch sequence is considerably slimmer. This is called the Launch Status Check, and seems to have been adapted/stolen from an older Atlas launch sequence - I'm not sure what they all mean.
CLCDR (Central Launch Commander?) checking all stations. Talker, Timer, QAM1, QAM2, QAM3, FSC (Flight Safety Control?), Prop One, Prop Two, PTO, ACC, LWO, AFLC, Guidance, PTC, Launch Vehicle Director.

EagleEye 3 Booster Delta IX was lost due to cargo liquefaction and sloshing. Several real launches have been affected or lost due to similar problems, including SpaceX's second Falcon 1 launch, and early NASA and missile test launches. The novel also describes how the payload is mounted to the booster. This is surprisingly non-trivial. The booster's skin is thinner than a soft drink can expanded to the same scale - you can't simply bolt a satellite to a lithium-aluminium wall only 6mm thick! The weight of a rocket is carried through its vertical walls, and the mounting structure is typically a hexagonal truss that ensures forces are all parallel with the booster walls, not punching through them.

"GC, Flight. Lock the doors." This is standard procedure in the event of an off-nominal occurrence. Its purpose is to get all the information locked away while everyone's memories are still fresh.

In Chapter 19, Guo Ming of the Chinese Space Agency comments that this (in the year 2036) is the first time an American spacecraft has been attached to a Chinese booster. This implies an exciting progression in US-China relations, since currently NASA is forbidden by law from working with China. That said, Guo Ming, and NASA, are probably keen to forget the one prior instance of US probe + Chinese booster, the ill-fated 1996 Intelsat 708/Long March 3B launch failure. https://en.wikipedia.org/wiki/Long_March_3B#Intelsat_708_launch_failure

On Sol 383, Watney remarks that his maps are rather low resolution. This seems highly unlikely, given how good Mars maps are, and how cheap and light data storage is, in 2015, as well as the possibility for landing off target. Of course, having the full suite of geological and surface maps would reduce the need for Watney to be ingenious and clever.

On Sol 434, Watney remarks that his life was saved by having all hose and tank connections being standardized across the mission. This was a lesson learned during Apollo 13, when the CO2 scrubbers from the Command Module had to be fitted into the Lunar Lander. The flip side of this design methodology is Murphy's Law, which states that sooner or later someone will accidentally plug some hydrazine into life support and try to breathe it - which will not go well!

On Sol 529, Watney discusses using the interchangeable systems to pump hydrogen down in the Ares 4 MAV airlock - an impressive engineering achievement given hydrogen's general misanthropy.

On Sol 462, Watney describes a nasty formula that gives longitude given a sighting of Phobos. This formula has been formalized in the computer program JPL Horizons, which which a sighting of any two celestial objects can give you your location. It's worth noting that sighting Phobos won't work north or south of 70.4 degrees latitude - where Phobos will be below the horizon.

On Sol 506, Beck and Lewis discuss the possibility of going off-tether during the Watney retrieval procedure (WRP). Lewis rejects the concept as being too risky. That said, if either suit has a functional transponder, Hermes could potentially catch up with them and pick them up a few hours later. For most of that time, though, Beck and/or Watney would be floating in the inky vacuum of space with only the gradually receding and half-full disk of Mars visible.

Map credit Dr. Fred Calef III, JPL.


Life Support Systems

Andy Weir describes a series of life support systems, each custom designed for its job. They include EVA suits, rovers, the Hab, the MDV and MAV, and Hermes. Each is an accurate representation of how such a system would be engineered. In general, such systems are trade-offs between size, demand, reliability, and the degree to which they are closed. The rovers and EVA suits, for instance, dump CO2 rather than recycling it, but are much smaller and less energy intensive to run. This is entirely realistic.

At the end of Chapter 1, Watney describes all the awful things that could happen to him if aspects of his life-preserving technology were to fail. Fortunately, the size of the Hab provides a buffer of spare time to effect a fix! By this point of his log entry, Watney is tired and hyperbolic - if the lab were to breach, he wouldn't "kind of explode", he would suffocate, dessicate, and freeze. Some surface capillaries would breach under the loss of pressure, leading to a freeze-dried statue with bloodshot eyes.

In the Sol 7 log entry, Mark describes his power and storage systems as including hydrogen fuel cells - which later mysteriously become large batteries. It turns out that a sufficiently advanced hydrogen system could be more weight-efficient than regular electrolyte-filled batteries, so it is possible that NASA has built and sent a hydrogen-mediated battery.

On Sol 29, Watney discusses the air-equalization mechanism between rovers and pop-tents. In addition to oxygen, Watney also has a finite supply of nitrogen, and there's not that much in Mars' atmosphere. That said, if his atmospheric regulator is able to liquefy oxygen it's almost certainly capable of liquefying nitrogen (only 8K colder), so could produce more nitrogen to replenish lost/blood-let supplies at a pinch. This is especially a concern since Watney insisted on walking through the pop-tent airlock to fill it with dirt, despite the fact that opening both doors and shovelling it straight into an unpressurized tent would be much faster.


Botany and Nutrition

In the first log entry, Watney mentions that he'll self-medicate with antibiotics to prevent infection from his stab wound. While a reasonable precaution, it's also possible that those antibiotics, excreted through his skin, could affect his soil sample and kill his precious bacteria. I hope Watney took appropriate measures to prevent this from occurring!

On the topic of bacteria, steeping yourself in your own waste (as Watney describes on Sol 14) is not necessarily a good idea - E. Coli and S. Aureus (inter alia) can cause serious problems, especially if you're immunocompromised by, say, a restricted diet, stress, radiation, or bizarre gravity. On the other hand, it's not like Watney has much of a choice.

On Sol 15, Watney starts shovelling Mars regolith (as opposed to dirt, which contains lots of biomass) into the hab. Why he didn't use the rover as a bulldozer to at least load two of the airlocks with dirt, then climb in through the third, escapes me. Perhaps he just really likes his shovels?

On Sol 25, Watney describes his dietary scheme as 1500 calories, inadvertently (perhaps) recreating the https://en.wikipedia.org/wiki/Minnesota_Starvation_Experiment, which was found (unsurprisingly) to increase depression, hysteria, and hypochondria. For reference, even if Watney were 5ft tall and massed 120lbs, likely an underestimate on both counts, he would need roughly 1700 calories per day to healthily maintain his weight. Fortunately Watney is made of "the right stuff" and only got more plucky over time.

Also on Sol 25, Watney is describing infecting Mars dirt with Earth bacteria and doing hyper-intensive farming. All of this is well established botanical wisdom and will be one of the first things settlers do on Mars, even (especially!) if they bring a five year supply of ramen.

On Sol 39, Watney is still evidently confused by hydrogen narcosis from Sol 37, since he uproots 48 plants to the rover, instead of simply placing them inside a spare EVA suit and leaving them out of the way, while he de-hydrogens his Hab.

On Sol 65, Watney is pleased to discover his plants are growing well. Some other tricks he could have employed to enhance productivity include 24.62 hours of light per day and raising the ambient CO2 levels to 1200ppm, about 3x the natural level on Earth but still low enough to be harmless to humans.

In Chapter 19, Johanssen tells her father that if they miss the orbital supply rendezvous, she'll (consensually) freeze and gradually eat the other astronauts to survive. This scene is somehow much funnier than a similar theme in Neal Stephenson's recent novel Seveneves. It turns out that the caloric quantity of person can keep another person alive for around 100 days, so this is technically possible.

On Sol 444, Watney has to microwave then freeze nearly 2000 potatoes, because cooking them on the road will waste electricity. Wrapping them in foil and baking them on the RTG would require going into the other rover, which is too hard. Nevertheless, microwaving them all would take about 5000 minutes, probably voiding the microwave warranty and taking until Sol 449 (about 5 days) to complete.

On Sol 497, Watney says he's close enough to Schiaparelli crater that he can taste it. If he actually did (and he's eaten a lot of dust by now, probably) it would probably taste bitter due to the reduced (alkaline) sub-surface chemistry found on Mars by the Curiosity rover.

On Sol 502, Watney eats his third last ration pack and remarks on the amazing taste of indefinitely preserved astronaut rations. They have improved markedly over the years. In the Gemini years the rations were so terrible (pureed food in gelatin cubes) that one of the astronauts smuggled a sandwich into space - which was apparently a bit messy.


EVA Suits and practicality

The EVA suits described in The Martian are basically the same as the ones NASA has designed for future planetary missions, such as the Z series, with a bit more functionality and usability. During the novel, Watney goes on hundreds of EVAs, after which he can apparently suit up and airlock in less than 10 minutes. This seems a bit unlikely, though it does permit him to do all sorts of silly things that a more involved suiting process would probably discourage - such as taking dictation from the repurposed Pathfinder on the dust outside, instead of watching it through a window.

In his first log entry, Watney describes the suit doing something named horrifyingly "bloodletting", wherein the suit's control system vents gas directly to prevent the buildup of poisonous CO2, backfilling with nitrogen and oxygen, and eventually just oxygen. This is actually a sensible failure mode, and exists in some form on today's suits.

I'm not sure whether a nitrogen/oxygen mix is best for EVA suits on Mars, due to complications surrounding "The Bends", where nitrogen bubbles can dissolve in tissue causing pain during decompression. On the other hand, some sort of buffer gas is advisable to avoid oxygen poisoning, lung collapse from low partial pressure during strained respiration, and a host of other problems which eventually (over the course of weeks) can develop in a pure O2 atmosphere.

In The Martian, there are actually two kinds of suits. One is the full EVA suit for running around outside. The other is a more compact pressurized "flight suit", worn routinely during launch and re-entry in case the spacecraft suffers depressurization, as it did during re-entry for Soyuz 11. It is not clear to me why the crew wore their flight suits in the Hab during a possible abort sequence - I would have thought that a possible abort would involve the crew sitting tight in the MAV, to avoid having to walk across the surface, and thus exposing themselves to flying debris…

On Sol 10, Watney points out that his LiOH CO2 scrubbers are expendable and that he only has a finite supply. Of course, it is possible to bake off the CO2 by heating them to 677C outside, something within his technical capability. Fortunately, though, he has a huge supply and never needs to do the equivalent of cooking Martian meth.

On Sol 64, Watney decides that doing two 10 minute EVAs a day is easier than building a switch to switch over batteries for the rover. While I can understand the urge to have an/any excuse to not spend the whole day in the rover, the EVA process is still complicated and potentially dangerous.

On Sol 119, Watney uses his EVA suit to run life support for the airlock in which he is trapped, which is totally doable. After sealing the airlock leak, he has 13.1 hours of oxygen, but the suit leaks badly enough that it'll only have enough for 4 minutes outside the airlock. So he then spends 6-9 hours (unclear) rolling the airlock back to the Hab, but this consumed much of his remaining oxygen, giving him only a minute or two at most to find working suit components. While plausible, it seems likely that Weir double-counted here.

On Sol 121, Watney is able to use the rovers to refill his air supply. With a full supply, his leaking suit would probably last about 8 minutes, and it's implied that the air refill would take longer than that. This seems unlikely - refilling a tank with a liquid is fast, and given its role in emergencies, it should be fast!


Conclusions

We all enjoyed The Martian more than anyone should enjoy ANY book. To our credit, we didn't find very many problems when reading recreationally. Only when a few physics-textbook-toting friends and I sharpened our pencils and squared off our erasers and went to work did we truly appreciate the technical depth behind this book. We think it's just amazing how engaging a human-Mars-oriented novel that is technically accurate can be, and can't wait to see the film!

12 comments:

  1. Question to the geeks to wrote this (and I love it) - per the movie, would Nasa really have SCUBA tanks in Mars to brush off dust from solar cells? And would Nasa have black markers on board which generally contain toluene?

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    1. I think compressed air would be a great way to dust cells, it's probably much less scratchy than a broom, and easier to do in a space suit.

      Toluene - I doubt it. But definitely some way to write outside. For the film, big markers are a dramatic choice. Easier to see, more motion in their writing.

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  2. "On Sol 529, Watney discusses using the interchangeable systems to pump hydrogen down in the Ares 4 MAV airlock - an impressive engineering achievement given hydrogen's general misanthropy. "

    Hydrogen's general misanthropy. I love this succint wording.

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  3. Nice work! Some comments of my own.

    In Chapter 7, on Sol 63, Watney says that he has "over 100 square meters of the most expensive solar paneling ever made" with "an astounding 10.2 percent efficiency".
    Back in Ch.2 he said, "Two hundred square meters of solar cells, ..."
    Not strictly inconsistent -- 200 is "over 100" -- but still....

    But 100 m2 of growing plants seems problematic. Insolation on Mars is half that on Earth, even if the panels are 40% efficient, then there's the efficiency of his lighting inside the Hab, and how many light fixtures does he have anyway?

    But for traveling, he's limited by the rovers battery capacity. More efficient panels would mean he needs fewer of them. Well, and he could eliminate 'air days' entirely.

    In Chapter 26, Johanssen states that the Ares 4 MAV mooring clamps are released about 5 seconds before launch. US rocket designs tend to clamp the rocket to the launch pad until full launch thrust is achieved, ... Nevertheless, to what is the MAV clamped? There is no launch gantry or pad here. The descent stage is all parachutes and empty fuel tanks at this point.
    In Weir's design, the landing stage seems to be fairly substantial. "Though the bottom half (the landing stage) is still here. No reason to take that back up when weight is the enemy. It includes the landing gear, the fuel plant, and anything else NASA figured it wouldn’t need for the trip back up to orbit."

    I had pictured is as something like the Apollo LM, unlike the movie version. I guess the clamps are there to keep the ascent and descent sections together during the flight from Earth and landing, and then during exposure to storms for several years waiting for the expected crew. Anyway, the clamps don't hold the MAV down against the engines' full thrust; it sits there for several seconds after the clamps are released before "The MAV launched with incredible force."

    Weir states that it has two stages, though one stage is perfectly adequate for the relatively low delta-V required to reach Low Mars Orbit (4.1km/s).
    And the movie takes it further, with a detachable capsule in top of the second stage.

    On Sol 505, Bruce says to Venkat that Hermes is flying by Mars at 5.8km/s. Mars escape velocity is only 5.5km/s
    Mars's escape velocity is only 5.0 km/s. I wonder if the 5.8 figure is the delta-v needed, and like the 4.1 km/s needed to reach low orbit, allows for gravity losses, atmospheric drag, whatever.

    During the intercept procedure, Ares 3 crew have to think fast to find additional sources of delta-V to move the Hermes close enough to catch Watney as he flies by. The distances and velocities mentioned during this passage in Chapter 26 are correctly calculated and almost entirely realistic.
    I have some troubles with the intercept. Since the MAV undershot the rendezvous, shouldn't it be behind Hermes and falling further behind, rather than being overtaken?

    Also, with only an 11 m/s difference in velocity, it seems like a 31 m/s delta-v budget from the attitude thrusters would be enough without the extra boost from the 'Iron Man' maneuver; it'd just take longer.

    NASA's look at the Purnell maneuver: https://www.insidescience.org/content/inside-spaceflight-martian/3251
    And Andy Weir's original: http://galactanet.com/martian/hermes.mp4

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    1. Thanks for your input!
      The Apollo LM had much more delta-V than the MAV would need, because EDL on Mars can employ aero-braking.

      I didn't talk about the movie MAV design, which makes no sense on several levels, but ... it's just a movie.

      I think an undershot puts you into a lower orbit on Mars, which has a higher velocity, but I haven't done the math.

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  4. Thanks! This is a great article!

    In the film, the "percent oxygen" is shown at various times. Once (if I remember correctly), the percent oxygen in Watney's suit drops to 5%. Mars' atmospheric pressure is about 150x smaller than Earth's (and, presumably, Watney's suit's) pressure, so when his suit is breached, all the gases in his suit should dissipate at the same rate. This would mean that his oxygen percent would remain at 20-21%, at least until the suit gets down to Mars pressure and gas diffusion through the breached suit begins to occur. Unless I'm missing something, shouldn't his oxygen *pressure* (or perhaps partial pressure) have been the quantity falling, not the "percent?"

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    1. You're exactly right. A full blog dealing with the movie would be longer than this, but fortunately they got most of the macro-detail right.
      In this case, most of the on-screen gauges and numbers were about right (Hab pressure of 12.1 psi is fine), but the oxygen at 5% makes no sense. As soon as the pp02 falls below about 150mBar you fall asleep real fast.

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  5. On fast air refill for the suit: Surely the suit tanks would contain pressurized GOX, not LOX. Filling a compressed-gas tank fast is a good way to get on the wrong side of PV=nRT and melt/burn (or at least weaken) a hole through the tank.

    Two examples: SCUBA tanks take a long time (hours?) to fill, _and_ they're water-immersed. And Armadillo Aerospace lost a helium tank that way (despite helium's good thermal properties).

    Filling a GOX tank from a LOX tank could go much faster, but you'd have to be very careful not to put in too much LOX. And until it warmed up, you'd have very cold gas and maybe even liquid getting into systems designed for relatively warm gas.

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  6. Glad to contribute! I just thought of another fun point: some things are GOX compatible but not LOX compatible. I think we lost a couple of X planes because someone specified a treated leather gasket in a LOX line.

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    1. I think it would have to be filling in the liquid phase. That's how eg butane torches, LPG (BBQ) gas bottles, etc are refilled. Lots of steam!

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    2. Well, the vapor pressure of propane at room temperature is about 8 bar. Butane is even lower - in fact butane stoves stop working at temperatures that some people still find suitable for camping! So those gases are stored as liquids under modest pressure.

      Oxygen's critical point is at -119 C and 50 bar. I guarantee it's not stored as a liquid in space suits. So if LOX is put into the space suit tanks at all, it would have to be in carefully metered amounts to avoid overpressurizing when it boils. But I strongly suspect it's GOX all the way.

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