Headscratchers / The Martian

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Hermes pictures

• It's a little surprising that Hermes was ordered home so soon and not tasked with first taking its own pictures of the Ares 3 site before leaving. Hermes would not be required to make its pictures immediately public, so seeing Mark's body would not have been a public relations issue, as was the excuse in the book.
  • Why wouldn't they be required to make its pictures public within 24 hours? They're still NASA. And why would the Hermes even have a ground imager capable of resolving the Ares 3 site? Its purpose is to get astronauts from Earth to Mars and back again. There's a lot of specialized equipment already in orbit to handle the imaging, so there's no need for the Hermes to have such a capability. They could interface with an existing satellite to take the picture, but the satellites are under the jurisdiction of SAT-COM, so they'd have to go through them. Otherwise they might be interrupting an experiment or observation in progress. SAT-COM would then tell them "no", because the images coming from the satellites are public.
  • NASA makes everything public partly because everything beamed at earth can be intercepted by most other countries and everyone could smell a lie or a coverup. Hermes — if it had such a camera — could take the picture and just look at it locally on the screens, then send it in a private e-mail or encrypted connection to NASA.
  • Within the time frame you're talking about, Mark was still unconscious. There would have been no proof from satellite imagery that he was alive, because he doesn't clear off the solar panels until a day or two later, once he's at least up and walking from his injury.
  • Also, the book points out that in the event of a mission scrub, Hermes leaves Mars orbit within 24 hours, because the orbital dynamics work out to make the trip shorter and safer the sooner you leave. So even if they did have cameras imaging the site, and even if they weren't required to make those pictures public, they left right away, following procedure.
  • And NASA was rightfully worried about the crew's mental state. They had eleven months of space travel ahead of them, and if they lose their composure they might die. Do you seriously think that the day after Watney's death NASA would want his best friends to photograph his dead body and then do the entire trip back to Earth with those pictures on their computer? Really? You really don't think anyone in NASA would have objections to that plan?

Radios and backups

• One thing I'm curious about. The Ares 3 mission has 4 radios: Three on the MAV, and one on the Hab. The one on the Hab broke, the 3 on the MAV left. Why doesn't the MDV have a radio? NASA is such a nanny (as shown in the book) that they'd want the MDV transmitting telemetry back to Hermes during as much of the descent as possible. Even if they can't do anything about it, at least they'd be able to see what went wrong if something did. From the time it undocks until it lands (interrupted by a short blackout due to atmospheric ionization if they don't have a martian equivalent of the TDRSS or didn't shape the MDV correctly, which I have a hard time believing), NASA would want that telemetry communicated back to them. So if it could communicate until it's landed, then when it's on the ground it would be able to communicate with satellites.
  • I went back and read over those sections of the book. The lack of radio is pretty much Railroaded because the plot demands it, and getting Pathfinder is cool. Mark says the dish has blown far away, he'll never find it, he gives up there (which is actually out of character for Mark, but early enough in the book that we don't know that). Still, he doesn't seem to do (or at least doesn't mention doing it) a 5-10 mile radius check around the hab while he's testing his rover for distance.
  • He did mention that the CO2 filters on his suits were irreplaceable and would thus limit his EVAs, so I took it to mean that he decided not to waste them on an increasingly difficult search.
  • That applies to EVAs, not the Rover. When he's testing for distance, he could have at least looked around through the windshield. It would have been killing two birds with one stone: Look for the dish (if he finds it he may not need to travel the huge distance to Pathfinder) while at the same time verifying that the Rover works.
    • Applies to the Rover too. He specifically states in the book that the Rover and the suits use the same CO2 filters, because Apollo 13 taught NASA a lot (one of the many problems was that the command module and lunar lander had different CO2 filters, and they had to kludge together a fix literally using duct tape to use one's filters in the other).
  • One point. The MDV likely had a radio..., but it would have been short range, designed to relay through the Hermes. Why put in a larger antenna with more power (i.e. more mass, more cost, less propellant margins) when you'll have the Hermes.
  • Exactly. It would have been able to contact Hermes, and thus would have been able to contact anything in Mars orbit (getting through the atmosphere is the hard part). Since it's well established that they have a communication network around Mars (the ground imaging satellites watch him 24 hours a sol with no more than a 4 minute gap in coverage at any one time), there's no reason to have a radio capable of contacting Earth directly, like Pathfinder does, just be able to contact ANY satellite and relay back to Earth without having to worry about Earth dipping below the horizon every day. He likely wouldn't have been able to send back pictures at any sort of a high speed, but all he needs is text.
  • In the book it was mentioned that the MDV's reserve chute opened during the windstorm and it was dragged away. Likely any exterior antennas were ripped off as it was dragged along the ground and possibly the radios were damaged as well.
  • Also, when the MDV landed it was directly linked to the MAV. It seems like a waste of weight to put a separate radio on the MDV when it is effectively the lower stage of a vehicle which has three radios.
  • I'm not sure I understand. Are you saying the MDV and MAV are the same ship? That's patently untrue. The MDV is how they get from Hermes to the ground. It stays behind, as it is just a lander. The MAV is sent 48 months ahead of time so it can get fuel. That's why Watney can use the next mission's without someone having landed their MDV there yet. Yes, the MAV has a descent stage that also gets left behind, but that's not the MDV that Watney gets the hydrazine from, and which should have an orbit-reaching radio.
    • Orbit-reaching isn't the same as Earth-reaching in this case. Even if the MDV had a communications array, it would be pointless to set it up with enough power to reach Earth directly. It would connect to Hermes and the facilities on Mars. Since he's alone on Mars and Hermes is out of range, that array wouldn't be useful.
• It gets worse when you consider Mark states it takes fourteen pre-supply missions to ready an Ares landing site. That's fourteen individual landers, even leaving out the MAV. Every single one of them would probably have a radio and an antenna to report a successful landing back to Houston. Even if you assume that they only have short-term batteries, it seems improbable that Watney couldn't rig at least one of them to HAB power to send messages back to Earth.
  • Mark is also in such good shape for supplies because he repeatedly states that whenever NASA could send redundancies, they did. So he has twice the amount of food the mission is supposed to need, many more CO2 filters, and so on. All this just in case some landers had issues and the supplies weren't usable or bounced out of recoverable distance. Kind of hard to believe NASA didn't send at least two copies of the main communications system, just in case one was damaged/lost in landing. Really, the whole plot revolves around NASA being a possibly-unique combination of Crazy-Prepared and holding the Idiot Ball.
    • It's likely that the backup communications solution for the Ares 3 team involved uplinking to the Hermes, which had the high gain antenna necessary to communicate with Earth. If the main array goes down, Hermes acts as their antenna. Mark's problem is that the crew took the Hermes out of range when they left for Earth, so he has nothing to which he can connect the backup communications until he goes and fetches Pathfinder. NASA sent lots of redundant equipment but it's obvious from the entire movie that they didn't anticipate that the Ares 3 crew would separate and leave someone behind on Mars.

Windstorm on Mars

• More of a Plot Hole than anything, but the Ares 3 mission is forced to abort because of a windstorm that threatened the base. Later on, they are able to justify turning the Ares 4 MAV into a ragtop by pointing out the atmosphere is so thin that the air resistance from launching into space wouldn't be a problem. Admittedly, the narrative displayed that the biggest problem with the first storm was the risk of the MAV being blown over on its launch site.
  • A WMG speculated that Mars was experiencing outgassing, a phenomenon that NASA has never seen and might not have expected, and high winds were more dangerous because of extra material in the air. This seems a bit contrived, but it's a theory.
  • My assumption until the concerns about the MAV tipping were explicitly stated was that it was the dust itself that was the problem. The dust blown around by wind storms on Mars is very, very fine and could easily work its way into critical MAV systems given sufficiently high winds sustained for a sufficiently long interval. There may be a limit to how much dust can get into the MAV before it is no longer safe to launch. Without a means of measuring dust penetration directly, NASA may have been using wind speed and duration as proxy indicators, in which case they had probably been monitoring the MAV for dust accumulation for the entire time it had been on Mars and the wind storm that forced the crew to evacuate simply put it over its acceptable lifetime dust accumulation limit.
• I believe it was stated, in the film, at least, that air resistance wouldn't matter because once the MAV was going fast enough for it to matter normally, the ship would be up so high, and the atmosphere so thin, that the air resistance wouldn't matter in this instance.
• The author himself has admitted, that the windstorm cannot do the things it is depicted as doing, but he needed something to get the plot going. He said he later found out, there's lightning on Mars and he might've gone with that had he known it earlier.

MIDI files

• I know that it's most likely just a point for joke, really, but when Kapoor mentioned that they can't send new music over to Watney because of file size issue...why not just send MIDI files? Hell, it would have been even funnier if NASA DID send the MIDI files...and they're all disco music!
  • MIDI files can still be considerably larger than text messages, and in the meantime the communication line with NASA would be clogged up. NASA wouldn't want to spend that much time without communication with Watney, and it would still take considerable time to build a respectable collection of MIDI songs. And thats assuming Mark Watney has a MIDI player — who knows if they made it to the crew laptops in the HAB? MIDI players might have gotten largely forgotten in a future where data storage capabilities are getting better and transfer speeds are increasing.
• At some point they sent a new subroutine that would allow the rover to communicate with Pathfinder; it's estimated to be at least 20M. That's at least a few songs in MP3 with a decent quality.

Botanist with no equipment

• Never read the book, but just going off the movie, one big question comes to mind. Mark is a botanist. NASA picked him to be part of this expedition to Mars. Yet when shit hits the fan, he has to MacGyver together a greenhouse, and only has something to plant because someone brought potatoes to eat at Thanksgiving. Why did NASA send a botanist to Mars, and not give him anything useful for growing plants? No pots, no greenhouse materials, no hydroponic equipment, no fertilizer, no seeds, nothing! Why was he even there?
  • Mark was never intended to grow any plants on Mars. Martian soil is barren, lacking any nutrients and bacteria to grow anything. In the book, he was given Earth soil (which contains Earth bacteria) to test whether when mixed with Martian soil, the bacteria could properly reproduce and create viable soil. Supposedly, if his experiments were successful, future missions could attempt to actually grow something on Mars.
  • He was given a small selection of plant seeds for his experiments, but most of them were basic flowering plants iirc. None of them were crop food. This sample soil was also used up completely to create the soil he'd need to grow the potatoes, which is why he couldn't simply start over from scratch after the airlock failure. He had nothing left that contained living bacteria.
  • Actually, he mentions that the soil bacteria survived. However, the reason why he could not start over was because all of his potatoes were killed by the lack of oxygen and low temperatures.
    • Wdym he mentions the bacteria survived? In the book, it specifically says the bacteria couldn't have survived. "Also, the temperature is well below freezing. Not even the bacteria in the soil can survive in a catastrophe like that." (page 201 for me, I would say around page 200 is a good bet for any version of the book)
    • Later Mark mentions having done some tests on the soil, and finding that some bacteria did survive and are breeding again. His guess is that ice formed bubbles of survivable pressure, and when he got the Hab repaired, reinflated, and warmed up, the bacteria came out of hibernation and started doing their thing again.
  • As above mentions but doesn't emphasise: they did send him equipment. Plant seeds, soil and pots. He grabbed extra (ordinarily barren: had to mix it with Earth soil and fertiliser for it to work) Martian soil and didn't use the pots so that he could maximise his farming area and thus his yield.

Radiation

• Does Mark Watney suffer permanent consequences of being exposed to radiation? It's already visible that he has radiation burns.
  • In the film, the less-grim probability is instead that he has sores from malnutrition. Malnutrition and vitamin deficiencies can cause similarly ugly wounds on the body. (The point still stands, though; to what has Watney been exposed?)
  • Furthermore, what kind of organ damage has he suffered from a year and a half of what is essentially a starvation diet? How much of his lifespan did that cut short?
  • The world's oldest living man is a Holocaust survivor. Presumably he was in at least an average state of health before the Nazis got him. It's safe to say that an astronaut (who by definition is at the peak of physical health) living in the 2030's (with much better medical technology available) has pretty good odds of recovering from such an ordeal.
  • NASA estimates that a 500 day stay on Mars exposes an astronaut to about 335 milliSieverts (mSv) for 500 days on the surface (0.67 mSv/day), and about 1.8 mSv/day during interplanetary travel with no shielding. Information on the long-term effects of this exposure besides cancer risk is not easy to find, but the cancer risk increase for long-term exposure to 1000 mSv of radiation is about 5%. Presumably the Hermes is well shielded, or at least better shielded than the flimsy habs.
  • According to the book, the Hab is pretty well shielded. So well, in fact that it even deflects radio transmitions (given as a reason why Pathfinder has to stay outside). Assuming the Hab gets radiation down to almost zero (with the floors covered, no Radon in the air and no other people around), I'd estimate Watney's exposure from the time spent outside (23 days to get Pathfinder, 50 to Schiaparelli and 1 day outside for every ten days for everything else, which I think is generous) to about 85mSv. To compare, 6mSv/year is world average background radiation (although people live with as much as three times that in some regions) and radiation workers in my country are allowed an additional 20mSv/year. So yes, it's quite a bit but no risk of radiation sickness or a highly elevated cancer risk.
  • It should also be noted that, once Watney gets back to Earth, he'll probably be getting the best health care humanly possible for the rest of his life. Partly because, you know, everyone would want him to stay healthy, and partly for research purposes, given the amount of time he spent on Mars.

Laptops

• In the novel, Mark tries to take a laptop outside but it fizzles (presumably because the LCD screen freezes / boils off from the pressure ). Why would NASA send laptops like this to Mars? One blowout and all of them exposed to Mars' atmosphere would be ruined.
  • Because the Hab blowing out is pretty much a mission scrub, get to the MAV and get out of there. Laptops breaking is a pretty minor concern in that situation. NASA never planned for leaving someone behind without communication. There's probably a lot that shouldn't leave the Hab and were probably destroyed when the Hab burst. NASA sent up iPads to the ISS, but if the ISS were to depressurize the iPad would probably break.
    • Actually, they specifically stated that the only automatic mission scrub was a problem with the MAV. A blowout of the Hab was planned for, that's what all the extra Hab material and seal strips were for, so they could repair and reinflate.
  • So why didn't the laptops all crap out when the Hab got breached due to the airlock failure? Unless at least one of them was stored some place air-tight since Mark would only need one of them.
    • I'm guessing that there's at least some storage that's air-tight, where at least a couple laptops would be kept as backups, and if NASA is smart and well-prepared, which we know they are, a lot of the storage bins and lockers will be air-tight when closed, too.
  • Who says they didn't? Mark only needed a laptop once, when he tried to make first contact with NASA. After that he kept communicating through the rover's computer.
  • It seemed to me in the film at least that the Hab breach only completely depressurized a small part of the Hab. Hopefully the guys who designed it included measures to individually seal compartments in the event of something like that actually happening (at the very least, the cupboards would be pressure sealed).
  • It's still strange why didn't they go with things like LED screen or such, though, especially that that technology's starting to be used more than just phone screens...though on this point I might just chalk it to Technology Marches On since LED screen might not be that widespread yet when the novel is written.
  • LED screens are just LCD screens that are back-lit by LEDs, so the same problem would occur with one of those. "Traditional" LCD screens are backlit by fluorescent lights instead. That said, a tube filled with gas might have its own problems in a vacuum if not designed for that.

Bomb

• The improvised bomb at the end, which they had about half an hour to make. Why did they bother giving it a cool flashing LED and a beeper?
  • Germanic Efficiency.
  • The beeper/blinker shows that the device is ready. That's helpful to know right before you're about to blow something up. And it has to have an LED because you can't hear the beeper in space after you depressurize the VAL, so you can know that depressurizing hasn't screwed something up. Where did it come from? They could have belonged to something else or have been part of a kit of prototyping components, and Vogel or Johanssen realized they would do the trick.

MAV and storms

• The nature of the MAV. Specifically - Ares III is aborted due to an intense storm that happened to hit the landing site and almost toppled the Mars Ascent Vehicle. Pre-supply for Ares IV involves sending a large number of supplies including a vertically landed MAV 4 years ahead of schedule so that it can distill fuel. Fair enough, but the MAV is mission critical hardware. If Mars can have storms sufficiently intense to topple the MAV, why take the risk that the MAV will still be standing 4 years later?
  • NASA thought they knew how strong storms could get and designed the MAV to handle that (plus a safety margin, presumably). It turned out they were wrong. Maybe they'll redesign future MAVs. Or maybe they'll figure it's a million-to-one against it happening again while a crew is on Mars.
  • Mentioned in the novel. If the crew arrives in Mars orbit on Hermes, and discovers the MAV is nonfunctional, they turn around and go home. Nonfunctional MAV is an automatic mission scrub, and they can ascertain that without landing.

Re-growing potatoes

• So after the Hab blowing out, Mark can't continue to grow potatoes, since the bacteria that made it possible have died. But shouldn't the surface of the potatoes already harvested and packed still have bacteria on it? Germs are so hard to kill off here on Earth, so couldn't have some of them survived in some more sheltered nook in the Hab, for Watney to retrieve and multiply?
  • According to the book, some of the bacteria did survive. The issue was that the potatoes themselves were killed from the vacuum desiccation and exposure to freezing temperature for about 24 hours. Still edible, but dead as Jacob Marley, nothing left that would grow.
  • But didn't he have more potatoes, like the ones he was storing to eat later? The potatoes he started with had been in storage for ages and were still viable enough for him to start the farm the first time. Why couldn't he just cut up another one as he did the first time he started?
  • The potatoes were dead. Completely. Edible but absolutely incapable of regrowth.
  • In the novel, he used all his potatoes to grow new ones, and was just about to start his first food harvest when the Hab breached. He had also planned to throw the food potatoes outside to freeze-dry in order to increase their shelf life, so they'd all be killed anyway. No potatoes were in breach-proof storage.
  • Also, even if Mark could get some potatoes to start growing again, it takes time for them to mature enough to be edible, never mind being replantable to grow more potatoes. And carting in more soil, cultivating more bacteria, spreading around his arable soil, making yet more water for the potatoes, etc. Starting over completely from scratch, he was still going to starve to death before he had more potatoes to eat.
  • This is all very easily answered if you actually read the part of the book where Watney explains how he's storing the potatoes he's harvested.
  Mark: You may be wondering how I'll store them. I can't just pile them up; most of them would go bad before I got around to eating them. So instead, I'll do something wouldn't work at on Earth: throw them outside. Most of the water will be sucked out by the near-vaccum; what's left will freeze solid. Any bacteria planning to rot my taters will die screaming.

RTG danger

• I’m at page 100-ish, and I’m scratching my head at the danger that’s been made of the RTG. Yes, the radioisotope thermoelectric generator makes heat through radiation, but the Plutonium-238 isotope being used emits alpha particles — the least dangerous kind of radiation. Alpha particles, or naked helium nuclei traveling a significant fraction of c, carry large amounts of energy but are stopped in their tracks by dead skin cells. They have practically no penetrating power, and are stopped even by a few inches of 1-atm air. The only real way the RTG could harm Mark is if he ate the plutonium or breathed in plutonium dust. Merely cracking open the case shouldn’t harm him much beyond thermal burns. Artistic License – Nuclear Physics, maybe? Or am I missing something?
  • Now, the cheaper RTGs that use the shorter-lived Strontium-90 isotope are more dangerous, because the beta particles (free electrons) spat out by its decay can produce secondary bremsstrahlung X-rays (literally, braking radiation) if improperly shielded, and beta particles are more penetrating than alphas, but they’re still stopped by mere aluminum foil.
  • It was puzzling, but reading Mark’s hilariously morbid lines like, “I’ll have so much cancer, my cancer will get cancer!” made up for it, at least.
    • The "cancer having cancer" line was in reference to Mars' lack of a magnetic field and thus the reason the Hab blocked electromagnetic waves (including radio waves, meaning he has to go outside to use Pathfinder to talk to NASA).
  • He might just not know any better. He's a botanist, not a nuclear physicist.
  • The book clearly states that intact insulation completely eliminated any radiation danger. But if broken, plutonium pellets would easily leak radioactive dust, which poses extreme inhalation danger.
  • Word of God says he wanted to give Mark a problem with the RTG, but he couldn't find any way for him not to die from it.
  • NASA has an established track record of playing it safe with RTGs. After Apollo 13's little detour, the RTG in Aquarius wound up on the floor of the Pacific Ocean, and the US has run many tests to make sure the outer casing isn't leaking radiation. Mark's just going by what he was taught in training.

Toilet paper

• Notably never mentioned in the text, especially given Mark's obsession with poop for fertilizer, is how he managed to stretch out 31 days of toilet paper for six people out to the 18 months he was on Mars. Even assuming the Hab toilet had a bidet like a high-tech Japanese model, that wouldn't have helped him on his long rover trips, especially when his diet is reduced to just potatoes.
  • I'm in the middle of reading the memoirs of an astronaut who's flown on both the Space Shuttle and the ISS. According to him, while the ISS has Russian toilet paper available, he always went with gauze squares and wet wipes instead. We know from something Mark mentions that astronauts in the Ares program are still wearing diapers under their EVA suits, and those would be light enough that NASA could send plenty of spares. One pair of scissors, and he's set for reusable toilet wipes. (And who knows, maybe NASA sent gauze pads instead of toilet paper because they could be used for both toilet and medical purposes?)
  • Or maybe he didn't give a shit (no pun intended) after a while. In the movie at least he claims to have gone without a shower for a year and a half. Hygiene becomes less of an issue when you're trying to survive.
  • They probably sent 62 days worth for redundancy (like they did with food and probably everything else) but I refuse to do the math, so I have no idea how much of a difference this would have made.
  • Well, humans were without toilet paper far longer than they had it. However it's possible that the toilet had a bidet water sprayer.
  • The food astronauts eat is deliberately designed to be low-residue to avoid putting strain on the plumbing and put off the problem of disposing of feces in a low-gravity environment as long as possible. Combined with Watney's increasingly shorter rations, he just doesn't have to go even as often as a typical astronaut, so the toilet paper lasts longer.
  • This troper can attest from personal experience that smooth rocks make an acceptable substitute for toilet paper.

Relative velocities

• The finale is more dramatic than it needs to be. Hermes is moving 11 m/s relative to the MAV. That works out to 3.9 m/s toward the MAV and 10.3 crosswise. They need to cancel the latter, add to the former, and decelerate to about 5 m/s when they get close. Which they can do within their 31 m/s delta-v budget if they're willing to take a bit longer to get there.
  • Only really true for a free-space model. Remember that we're in orbit around Mars. Orbital mechanics is weird sometimes.
  • Well, in a hyperbolic orbit going away from Mars. Yeah, it's an issue, but their closing speed before blowing the airlock was 42 m/s, so clearly they could have used a smaller impulse.

Password

• Mark logs into a ship computer with a username and password (but can get other crew's music files from their laptops). What is the security goal here? What are they defending the ship computer against? What's the forgot-password protocol like?
  • Could be that, for simplicity, they all have the same password, and have to log in for a semblance of normality.
  • It's likely a very basic security feature for when they're actually on earth, and if it is a wireless signal, to stop someone hacking into it during the short period when they might actually be in range of something.
  • He used the password to access his video log. Since the purpose of the video logs was probably in part to allow the astronauts an outlet to express their personal thoughts, it would make sense for these logs to be kept private from one another. The laptops were for entertainment and probably research related activities, so it would be more efficient for them not to have password protection since they would be likely to share them from time to time.
  • I'm not an expert, but it also could be something to protect against any damage that occurs due to Mars's weaker atmosphere (like bit flips and such?). Though why it wouldn't be on the laptops, I don't get.

Only human on a planet

• Mark ponders being the first man to drive here, walk there, that it's the first time there's been only one human on a planet. Didn't that happen before on Earth? I.e. there was a moment when there was one categorized-as-human being on the planet? In related news, this troper was once the youngest person in the world.
  • IIRC, he said he was the first person to be alone on a planet. If there was ever just one categorized-as-human being on Earth, it wouldn't have been alone. It'd have been able to interact and reproduce with other beings. Not to mention all the other animal life around. Also, that troper shouldn't rule out the possibility than another person was born at the same time as them.
  • Evolution doesn't work that way. It's not like a human popped out of a furry primate ancestor, fully formed. Changing to humans was gradual over thousands/millions of years with lots of others to spend time and breed with.
  • Moreover, he's the only person ever to demonstrably know they're the first human to live on a planet. Whatever you choose to define as the first human on Earth, they'd have no way to know that's what they were. It's the planetary equivalent of how we'll only ever know the name of one human (Emilio Palma) who was the first to be born on a continent (Antarctica).

Poop Bags

• When Mark goes to get all of the astronauts' poop to use as fertilizer, we see that each bag is labeled with the corresponding astronaut's name. There's even a joke where Mark is particularly impressed by Johanssen's poop's smell. Why on Earth (or Mars) would they be labeling who pooped what?
  • Probably for the same reason they sealed and stored poop in the first place - to analyze it later. In real life doctors sometimes analyze patient's excrement to look for signs of specific illnesses. Space travel has multiple potential hazards and effects on humans, so there may be value in studying each astronaut's poop and how it changed over time.
  • According to Mary Roach's book Packing for Mars, they really do that. Apparently NASA has freezers full of the stuff.
  • Labeling would also be important in case of illness - if person A gets sick somehow, then being able to analyze their feces might be useful (or at the least, be useful to know which ones to avoid opening). Additionally, though unrelated to the labeling part, poop is potentially a material that could be used to protect against cosmic radiation by lining a structure (or vehicle) with packages of poop. So even outside of anything else, bagged poop is useful.
  • Weighing each individual's poop is probably also a way to monitor their health, the waste-to-intake ratio of foods each astronaut was eating, how much water they were losing to feces, etc.

Patching holes in the station

• After the station airlock has been blown off, Mark covers the hole with canvas and duct tape — and leaves it at that. Why doesn't he do something more permanent like covering the opening with sand?
  • That would be a pretty big job for one guy in a suit with a shovel. Maybe he figures he has more important things to do with the time it would take.
  • Sand would most likely just blow away and/or overload the canvas and spill into (and breach) the airlock.

Duct-tape pressure

• Would the duct-tape and plastic job hold the pressure? Assuming normal Earth atmosphere and an area of 50 square feet, that's 48 tons of force. Consistent with the airlock getting blasted away, but not with the gently billowing plastic sheet held by duct tape.
  • It's not just plastic. It's material from the Hab. If Mark doesn't trust it to keep the air in, he might as well go for a nice walk in the Martian sunlight without a helmet.
  • And it's not just duct tape. He uses cargo straps and come-alongs to secure the tarp to the airlock opening. Even one of these can handle a couple of tons of force, and it appears he uses at least six or eight. The tape is there to stiffen the tarp and to minimize the damage a tear or puncture might do long enough for him to repair it.
  • The tarp and the straps is a movie change. In the book Watney has a supply of hab canvas, and a high strength resin glue especially meant for repairing breaches. It's the same materials that the the hab was built from, in the first place.
  • In the extended version he also uses an epoxy on any spots that aren't secure enough to be airtight.

Watney's entertainment

• In the book, each astronaut is allowed to bring their own entertainment. Watney uses 70's Television shows and Disco brought by Lewis and Agatha Christie's mystery novels brought by Johanssen, but that's it. Beck only brought medical journals, Vogel's stuff was all in German and Martinez didn't bring anything. But why doesn't the Hab has any entertainment brought by Watney himself, when the original mission was still supposed to last over a year and a half?
  • There's a fan theory making the rounds that Watney forgot his entertainment memory stick on Hermes when they descended to Acidalia Planitia.
  • Confirmed in the mobile game. Watney was distracted by Vogel wanting to go over some details of the chemistry experiments at the last minute, and left his data stick in his quarters on Hermes. His data stick contained a bunch of classic rock, and also a lot of Doctor Who episodes. Later in the game, you get the choice to forward him an encouraging message from either the President, or David Tennant. The latter does a better job of boosting his morale.
  • The book says that Lewis was assigned to geology work, which required long drives in the rover to visit sample sites. She brought additional music to listen to while driving, so the majority of the music Mark had access to was disco. He started off listening to Johanssen's music, but it was all by the Beatles and he got tired of it after awhile.
  • No matter how much he likes the stuff he brought, it's bound to get old after a while. Using the other astronauts' entertainment files allows Mark to change things up a little more often.
  • This is probably much of the same entertainment he used getting to Mars in the first place. Whether or not he brought his own onto the surface, it would be refreshing at that point to have something else. Not to mention that from a storytelling perspective this tells us more about Mark's crew and reveals how he feels about their interests.

Pouring oxygen

• In the book Vogel has to make the bomb in non-rotating spaceship, meaning there's no gravity. How goes he pour liquid oxygen into it?
  • In the film he did it in the gym/dining room, which is on the rotating section and thus has gravity.
  • It's not impossible to pour liquids in zero gravity, but it probably takes practice, since you don't have gravity helping you. I'm guessing Vogel, a space chemist, would figure out or have been trained on how to do it.
  • Also, you don't want to spill liquid oxygen or spray it into a microgravity environment - it'd be the cryogenic equivalent of white-hot metal fragments flying around!

What do Infocom games have to do with the signalling?

• In the film, Watney talks about Zork II and Leather Goddesses of Phobos immediately before he comes up with a way to use the cameras and signs to communicate with Earth, and a graphic similar to what he devises comes up alongside the games as he speaks about them. Did he get the idea from them, and if so, what specific parts of the games inspired him?
  • They're unrelated (as much of Watney's humorous comments are.) He figured that someone on the Ares III crew would have an ASCII dictionary around, and figured it would probably be Johannsen (since her official job, more or less, is "computer nerd.") He was just pointing out that she was an exceptional computer nerd for not only having an ASCII dictionary, but two ancient text-based RPGs on her laptop. Johannsen is not just a computer nerd, she's the queen of computer nerds (It’s hinted that she’s probably this In-Universe - the promotional poster made of her outsold those of all the other crew members put together).
  • On Mars, too!

Where was he shaving?

• At the end of the film, just before he sets off in the MAV Mark shaves his beard. He's standing in a large room with a mirror. Where is that? It's not in his rover, that's too small. And it can't be in the MAV because he's already ripped the roof off and its now not pressurized. Also for that matter how does he put on the EVA suit.
  • This is a plot hole created by poor editing. The scene should take place in the Hab. He would logically perform hygiene before embarking on a 50+ day trip in the rover.
  • Mark only depressurized the upper part of the MAV capsule. The lower part, with the ladder et all remained pressurized and was the first to go during takeoff.
  • It appears to be the locker/airlock room of the Ares IV MAV. It's not unthinkable that the room would have mirrors on the walls in case somebody was alone and needed to inspect the back or sides of their suit after putting it on.
  • Addressing the earlier point about hygiene, he did clean up a bit before leaving (that's why he didn't have clothes on in one scene), though apparently not a shower with soap. 50+ days in his rover would have rendered this moot, and the MAV probably didn't have much more than a bathroom (hence, the mirror). As for why Mark didn't shave before, he was probably saving his electric razor to clean up for the return trip so he would be presentable to his colleagues (or growing a beard out in the time that NASA couldn't tell him not to).

Hermes flight crew?

• Considering the massive expenditure required to build Hermes in the first place, why not assign a permanent crew member to maintain the ship during the surface phase? Similar to the third Apollo crew member left in the Command Module.
  • Easiest explanation is that it would add more mouths to feed to an already strenuous voyage between two planets that takes over a year.
  • With all the advances in technology, it's quite feasible the Hermes can take care of itself while in orbit, especially if most maintenance-intensive systems (like those required for keeping people alive) were shut down. Also, a crew member aboard would mean those systems would need to function for quite a bit longer, and there would need to be additional consumables. And one or two people alone on a spaceship in orbit for a month? At least Mars and the Hab are a change of scenery.
  • NASA plans to leave the transfer vehicle uncrewed and send the entire crew to the surface. Away from the potential negative effects of Earth-orbital micrometeroids and trash, there's literally nothing to need the attention of a live pilot.

why not vary the airlocks?

• The HAB has multiple airlocks. The repeated use of the one Watney used is what resulted in its failure and the death of his crops. I understood he had to sacrifice two to the pop tents (in the book) but there was a fourth. Why did Watney, a literal genius, not think to alternate them?
  • Explained in the book. Airlock One was closest to the rovers, which is where he was mostly going, so he used that one almost exclusively. He mentions much earlier about rationing his EVA time (he has plenty, but not unlimited, CO2 filters) so he was probably thinking "why waste EVA time walking halfway around the Hab?" Mark is a genius, but he sometimes outsmarts himself (witness his hydrogen kerfuffle.)
  • There's a saying in Mensa of "Mensans can do stupid things faster than 98% of the population".
  • To paraphrase Max Uriarte: Some people put most of their points in the INT stat, while others put most of their points in the WIS stat. Watney presumably has more of his points in INT (and CON, obviously).
  • It's been a while since I read the book but didn't Watney hock up the other air locks to survival tents to give his potato farm some more floor space? After the blowout this became redundant.
  • No. Watney actually laments that the pop-tents are not designed to attach to the Hab airlocks, and he can't Mac Guyver a way to make them. They can attach to the rover airlocks, but that's only because they're intended to be emergency shelters, allowing personnel in a rover to rescue astronauts in the tents. They can hook up to the Hab only because they're also designed to hook up to the rovers, and all the valves and such are standardized. Watney decides to "take the air hit" for exiting and entering the pop tents since he needs the space, but they were never attached to Hab airlocks.
  • Also remember that the HAB was designed to be lived in for only 60 sols at most. Alternating extends that out to 120 (with 2). But the problem was probably less the usage and more the inevitability. The airlocks were going to fail sooner or later because they were being overused way past their intended lifespan. Had Mark had the opportunity, he perhaps could have done inspections to check for weakness but he was already on limited time. Perhaps he does this after the explosion.
    • In the book, it's explained that that particular airlock stretched slightly during the dust storm that stranded Watney on Mars in the first place. Then Watney kept using it because it was closest to the rover charging port and he was mostly heading out to communicate with NASA via the rover's connection to Pathfinder. After the blowout, NASA has him perform a full canvas inspection and tells him to swap airlocks every time he does an EVA from that point on.
    • And he did do a full inspection after the first storm left him stranded, but the stressed portion of canvas was behind a seal-strip, so he couldn't see it. Presumably, the inspection instructions NASA sent after the airlock blowout were more thorough.

Buried pathfinder?

• Pathfinder landed in the middle of a field of rocks◊. It's very unlikely that dunes came in and buried it. Even less likely that if that were the case Pathfinder would work after being dug up.
  • This is a little bit of a long shot, but perhaps the giant sand storm that hit Watney earlier was part of a more massive weather system that buried Pathfinder. Also, Pathfinder didn't work right after being dug up; it had to be hooked up to the Hab's power supply and a heater for the electronics from Rover 2.
  • Also, it has been around 40 years since Pathfinder landed at the time of the story. Depending upon the weather, it's possible Pathfinder may have been partially or completely buried in that time.

"Hang on, cowboy. You and I, we're military."

• Commander Lewis halts Martinez when he immediately agrees to go rescue Watney by pointing out that they're both military and will be court martialed for disobeying orders. Did she forget that Dr. Beck was a Reservist Air Force Captain? Why didn't she include him when she was speaking?
  • She said "you and I" because she was talking to Martinez directly; she didn't need to include Beck because he was quietly listening rather than brazenly accepting. Note how Beck only asks questions during the scene and carefully accepts after Lewis, Martinez and Vogel throw in their support.

Surface EVA suit design

• In the film, the suits that the crew use to work on the surface are white and orange. Why orange? Everything on Mars is orange in this movie. Why not green, like the flag they use for the RTG, or some other color that stands out to human eyes? I get that the designers didn't want a buzz-lightyear lookalike like the Z1, but what gives?
  • Probably the film makers were using green screen for SFX, so they couldn't use it for a real color.

Space Pirate

• In both the book and the film, Watney makes the "space pirate" gag, saying that since he can't get NASA's permission to board the MAV until he boards it, being unable to contact NASA at the time, he would be boarding without permission, and thus be a pirate. However, in the film, Watney never lost contact with NASA, making this gag nonsensical at best
  • The isolation was likely giving him Cabin Fever. Everything about it was nonsense but he was too nutty at this point to care. Likely NASA didn't specifically say "You can take the ship, it's not stealing" and his mind just spun out from there.
  • Film!Watney was probably waiting for the chance to call himself a pirate and twisted together this rationalization to justify it.
  • Not to mention, this was the plan that NASA initially rejected and which is only being carried out because the HERMES crew committed mutiny and essentially stole the ship. Which, technically, would make them humanity's first space pirates, but don't tell Watney that.
  • Mutiny is not legally an act of piracy (though it often leads to piracy). Stealing a ship in international waters is explicitly piracy, as in it is described as "an act of piracy" in legal documents.
    • Further, what the Hermes crew did might not technically be mutiny either, which is defined as disobeying the captain's orders. In this case, the crew is following the orders of the Captain, who is instead disobeying the ship's masters, making this instead an act of barratry, yet another maritime crime.

-beard

• This is kind of subjective, but isn't Watney closer to being "Brownbeard" than "Blondbeard"?
  • Blondbeard has a comical touch more suitable to Watney's taste. It's not like NASA can correct him — they have no idea what he looks like at this point, since he's sending text messages with Pathfinder and covered in spacesuit when he's in front of the Pathfinder imager (which he's not doing because he's driving to the Ares 4 site). Kapoor just went with it because Watney will be Watney.
    • Probably also went along with it because, ya know, every little bit one could do to keep Mark's spirits up is acceptable. Getting nitpicky over this, the space pirate joke, or the promotional picture is an unnecessarily asinine thing to correct on someone stranded 130 million miles from home.

Rover Balloon

• Why did Mark put a little balloon-like thing at the top of the Rover for the final journey to the landing site?
  • Presumably the oxygenator or water reclaimer.
  • The Balloon was an oxygenator or water reclaimer?
  • No, it allows those pieces of equipment to be stored in the rover while keeping the atmosphere inside, which just knocking a hole in the roof would not.
  • In the book he has to drill a hole in the roof of the rover and cover it with hab canvas because the rover is too small to fit the oxygenator and regulator inside without the hole, and he obviously needs to keep it pressurized so that it will keep giving him air.
  • The rover balloon in the film is a plot hole. The film shows two rovers, but one is hopelessly trashed so we see Watney using only one rover throughout the film. This is likely a budget matter as we likely saw the same rover prop. But in the book, Rover 1's parts are salvaged often for Rover 2, the "Watneymobile" which is his main ride. To get to the Ares IV MAV, book-Mark transforms Rover 1 into the "trailer" for his life support. He has to cut the hole in the roof of Rover 1 to fit the large oxygenator and atmospheric regulator, handwaved in the film and not appearing inside the film's rover interior. Book-Watney had no crane or flat-top trailer like that in the film. Not only was the film's balloon unnecessary, it becomes a science gaffe. When Watney leaves, his rover door is open to the thin Martian atmosphere (the film version has no airlocks as the book's rovers) and the balloon appears fully inflated.
    • The extended version of the film mentions Watney breaking down the other Rover for parts to outfit the first one for his journey to Pathfinder.

MAV fuel?

• I've just started my second read of the book and I'm wondering: What kind of fuel does the MAV actually produce? I first thought that it ran on hydrazine like the MDV, but that doesn't work with isolating CO2 from the atmosphere. Hydrazine is N2H4, it doesn't contain C or O, nor could I find a hydrazine production reaction that needed CO2 as a catalyst or anything (can you even use something as inert as CO2 as a catalyst?). If the MAV really wanted the 2% of N2 in the Martian atmosphere, why store the CO2? The only thing I could think of is burning hydrogen and oxygen, but then the fuel plant should be able to reduce CO2 on its own and Watney wouldn't have to take the CO2 tanks into the hab to have the oxygenator do the work there. Plus, when Bruce describes the plan at the end, he says that the fuel plant can "make" 780kg of fuel from the hydrogen Watney will produce, which sounds like H2 isn't the end product. For pretty much any more complicated rocket fuel (UDMH, Kerosene etc.) you need additional ingredients not found on Mars. That's all I got. Anybody got a good idea? (Also, if anyone an tell me how to stop tvtropes from thinking formula are trope names, I'd appreciate it)
  • Methane.

Early and late communication with NASA

• Once Watney has the Pathfinder and Sojourner robots, why does he write out all his notes? Why cant he type on the laptop, then use a digital camera to take pictures of what he wrote and take the pictures to the surface with him? This is more important once he does a "d'oh" and fries the Pathfinder's electronics- why cant he take pictures of Word documents instead of moving rocks around?
  • For the second part, once his drill murders Pathfinder, the only way NASA can see his communications is with the satellites. At one point, they lament that Watney is just a different-colored blur in the satellite photo, even with NSA enhancement. There'd be no hope at all of reading a picture of computer screen. As to the first point, Mark likely doesn't have any capability to print out photos in the Hab. It's easier, cheaper, and lighter (and mass is the enemy when putting things into orbit) for all the photos the Ares teams take to come back to Earth in digital form, then NASA can print out whatever ones they think are worthwhile.

Ares 1 MAV

• So, in the book, Mark notes that he watched Martinez land the Ares 4 MAV by remote, because it's the most mission-critical piece of hardware, gets sent first so it can quietly make its fuel, and has to be remote-landed by Martinez because transmission lag time means remote landing from Earth is impossible. So, how did the Ares 1 MAV get down? Was there an "Ares 0" mission that was a just a flyby in Hermes to land the Ares 1 MAV and make sure Hermes actually worked? Was the Ares 1 MAV a different design that didn't make fuel from Martian air?
  • A couple of possible explanations, first the Hermes flyby that you suggest is plausible. Second, Ares 1 may have used a combined descent/ascent vehicle, but a fuel plant may have been sent remotely a year ahead of time that manufactured the fuel for the return journey. Since NASA successfully landed Curiosity using the aero crane, it's plausible that this method would be used to ensure a soft landing for the delicate machine, which then created enough fuel for the combined MAV/MDV to use to return to orbit.
  • There was probably an Ares 0 that landed a MAV, tested Hermes' capability to do the trip, remote-land various delicate items, and so forth. It's just not mentioned because, like the early Apollo missions that were simple moon flybys or orbit insertions/escapes, nobody cares about the first man to fly around Mars; they care about the guy landing on it first. As for why the landings were Ares? Maybe the testing missions were under a different name.

Commander Lewis' service Branch

• In the book, we are told Commander Lewis served in the Navy, this troper assumed that she would have been a naval aviator who then transferred to the astronaut corps. However, in the Blu Ray extras, in an interview with a psychologist after a 10 day isolation, Lewis says that she served on board a submarine. Would a Navy pilot ever serve aboard a submarine, or would a submariner possess skills that would make them a viable astronaut candidate? I'm guessing that experience working with a nuclear reactor, and living in a confined pressurized vessel for several months would be useful experience to have on a prospective astronauts resume.
  • I think you answered your own question.
  • More to the point, a months-long voyage to Mars probably suggested a different set of experience than a suborbital or orbital voyage of a few days or weeks on the outside. Also note that she wasn't the mission pilot, so piloting experience probably wasn't that important unless she was to serve as an alternate pilot. On a historical note, she wouldn't be the first submariner to find themselves in a different career: Admiral Chester Nimitz was a submariner before he became famous as an admiral commanding aircraft carrier operations in WWII.

Hiding Watney's survival

• NASA, being a public organization, is forced to tell the populace that Watney is still alive on Mars but chooses to keep these secret from the crew. But despite not telling the crew until after four months, wasn't there the risk that the crew might still find out from family emails? When Vogel receives an email from his wife (really from Mitch), he doesn't consider it unusual to receive one from her, only that the subject header was misspelled. So how come in four months, none of their families mentioned anything about Watney's survival?
  • NASA is controlling the communication lines between Earth and Hermes. I don't think it would be that hard for NASA to tell them not to mention Watney's survival, or simply to expunge any mentions to it from the e-mails.
  • When the crew is told that Mark is alive, in both the book and the movie Mitch Henderson also tells them their emails were being censored. They didn't find out sooner, because nobody want to ruin their lives to smuggle the information to them. Meanwhile for the Rich Purnell maneuvers, there wasn't any public knowledge the crew wasn't supposed to know, so nobody was going through personal emails to censor them. Which meant Mitch could slip them the information under the guise of being a personal email, because nobody would be checking the contents before it was sent.

The Farmer in the Sky

• Why was the Ares mission dependent on packaged rations? From a weight point of view, being able to do some farming en route would have been helpful, essentially recycling waste back into food. NASA has already experimented with hydroponic growing on the Shuttle and ISS.
  • Perhaps the technology to grow food in space wasn't dependable enough for an Ares mission. Plus, packaged food wouldn't get ruined by decompression or solar radiation.
  • In the books it is mentioned that Watney was supposed to do some experiments with exceedingly simple plants (ferns and the likes), I take it that growing food on Mars was planned as a way to produce food for future missions eventually and Watney was supposed to do some groundwork on that. He did, just not as intended.
• Agreed, given how common and cheap the Aero Garden type home hydroponic setups are, it seems an obvious thing to include on the Hermes ship itself. A crew would certainly welcome fresh produce! Whether they would then try to transplant one or two of those plants to Mars is another matter.

Growing more food

• After one of the Hab's airlocks blows out, the resulting pressure and temperature changes destroy the plants Watney has managed to grow. He cleans out the room they were in and manages to cover up the breach from the missing airlock, and the loss of this food source is seen as a setback in NASA's rescue attempts - why doesn't he just grow more food? Did I...miss something?
  • All the potatoes died when they were flash frozen. Their eyes couldn't be used to grow more. More importantly all the soil bacteria that supported them were also killed, which means the martian soil was now sterile again. Watney seeded the soil with Earth soil transported to Mars for his botany experiments, he had none left to start over with, and using his poop for fertilizer again wouldn't have given the new soil the right bacterial requirements.
  • Ah, I see. I missed the part about some of the soil he used coming from Earth, is all. That makes sense.
    • Actually, in the books he mentions that some bacteria survived (even though the hull breech lasted way longer in the book) but his potatoes were dead as a dodo. There is some mention of other seeds, but sadly that's never brought up again.
    • The other seeds were simple flowering plants, not edible food crops.
  • In the book, Watney goes through his food rations to note what he can grow. He finds beans and peas but chooses the potatoes because they had more calories. After the Hab breech, around Sol 116, with the potatoes flash-frozen to death, it's probable that Mark has already eaten the beans and peas, realized they couldn't germinate, or would not be worth the effort to grow enough of them for food in the time available, revived soil bacteria or not. Mark never cleans out the Hab's soil in the book. As for the soil's presence after the farm's loss: Mark's back is always bugging him in the book, and getting soil into the Hab with small sample shovels was what starts that running joke. He notes that he wouldn't bother to remove the soil for those reasons.
    • It's also noteworthy that beans and peas are a lot more fragile compared with potatoes. One of the reasons why potatoes are a staple food source in rocky terrain and mountainous environments is because they are much, much hardier than most other food sources relative to their calorie count. They can survive on less nutrient-rich soil and with less water. They're probably the only plant that could survive in Mark's improvised farm with the water he has available through his furnace.

Discovering Watney

• Forgive me if this was addressed somewhere and I missed it...In the film, a NASA employee gets a message or something telling her to check out a set of coordinates amongst the satellite images of the Martian surface. She looks at the image, notices things have changed from older versions, and phones in the others to tell them that Watney must be alive. My question is, who sent her those coordinates and why? If they knew that something in the photos had changed, why not report it themselves, or at least mention it in the message?
  • Recall that in the book and movie that Kapoor was pushing Teddy for a sixth Ares mission, on the logic that most of the mission assets for one were at the Ares 3 site, unused because they left early. However, they needed reconnaissance photos to evaluate the status of the area. The orders to take the photos came from Kapoor, or at least his office. They weren't looking for evidence that Watney was alive, they just needed to know if the Hab and other major pieces of equipment had survived the storm.
  • In fact, the main reason why they didn't immediately take pictures of the Ares 3 site after the crew evacuated was because they didn't want to have pictures of what they assumed would be a dead astronaut all over the news. Hence why it wasn't until Sol 54 that they discovered Mark was still alive.
  • IIRC there was one NASA worker who basically looked at those images (during the night shift or something) and then had a "wait, something here isn't quite right" moment, then looked further into it and ultimately gathered several separate pieces of evidence for Watney still being alive (e.g. the rover having been moved) and then she called her superiors with a "you have to take a look at this" - note that she did not come with flimsy evidence and neither did she say "Watney is still alive" because that would likely have been dismissed. Instead she basically said "Come, it's important" and then laid out her case.
  • Got it. I just wasn't sure what the circumstances were for her to be checking those coordinates and who sent them to her. I understand it now.
  • Also under the impression that the next mission might at least make a visit to the site, they might have wanted to at least determine if Mark's body was potentially available (and could be taken home) or lost for good. The reason being that weight is always a constant concern in space so having to account for a body or not would be important.
  • You can see the email had Vincent's name on it if you look closely.

Really nothing else to burn?

• In the film, Watney needs something to burn to generate oxygen. He ends up shaving wood off a crucifix one of the other astronauts left behind. Wasn't there paper (wouldn't there have been paper for note-taking, or pages from books people brought), cloth (from clothing the others left behind) or any other material that would also burn, or did it have to be wood?
  • In the book there is a rather lengthy discussion how - due to very good real life reasons - there is very close to nothing even remotely flammable on space missions. If memory serves even the crucifix was more "smuggled in" than actually allowed aboard. So yes, it was the only possible thing he could burn. Or at least the only thing he could think of.
  • For tropers who haven't read the book (you should) or don't want to leaf through it, the above answer is fundamentally correct, but here are the details: In the wake of the Apollo 1 fire, everything flammable was removed from spacecraft, and that's been the rule since. All cloth is fire-retardant synthetic that won't even smolder. Paper is a fireproof variant that has no wood pulp at all (you can actually use NASA flight paper to smother a fire. Insulation, tape, adhesives - nothing NASA uses in crew spaces is flammable anymore with the exception of the astronauts themselves. So the "nothing to burn" thing is really by deliberate design. That said, there's still one thing Mark could've set fire to: his food. Most foods with low water content are varying degrees of flammable - anyone who's ever burned popcorn knows that for sure - but when you're facing a critical food shortage in the first place, there are other reasons to discard that plan. So Martinez's crucifix really was the only option.
  • He wouldn't want to burn paper in case he needed it for toilet paper! (see above)

Only one rocket in the world?

• One of the plot points is that Mars and Earth are way out of position for an economical rocket launch to Mars, which requires the Eagle Eye 3 and later the Tiayang Shen boosters to send a resupply probe to Mars. Admittedly Space "X" had only just tested the Falcon 9 in 2010, and The Martian was published in 2011, but the construction of the Hermes implies several heavy lift launches to get all the pieces in orbit. It seems unlikely that only one or two heavy lift rockets would be available to launch the Iris and Iris 2.
  • It's probably not numbers so much as availability. Between launches, everything goes through recovery, maintained, and so forth before being launch ready again (and even then, most reusable only have a limited number of uses). So while technically a bunch might have been in existence, the question is probably more if they were ready to launch again. And skipping safety checks for a rocket is even less ideal than skipping it for a probe.
  • In the real-world, Space Exploration Technologies had just started ISS resupply flights at the time Weir began writing. As to the booster availability: Mars and Earth were not in good positions to make a fuel-conserving Hohlmann transfer, which is shorter. As any "Kerbal Space Program" player will tell you, you can "brute-force" an encounter with another planet, but that requires much, much more fuel, which was why only ULA's largest booster were needed. (A fictitious one: The "Delta IX" doesn't exist.) Weir also likely knew that ULA (or as their partner entities Boeing and Lockheed Martin) fly the only boosters which have sent practically every NASA probe to-date to Mars. No other launch provider Weir knew of at the time has boosters with sufficient brute-force power. China would be a logical alternative for a heavy booster as real-world shows they are continuing increasing their launch vehicle power, and that's because they barred from interaction with the US space program for political reasons since the 1990s.
  • I still think this is one of the most legitimate head-scratchers in the story. It feels like everyone in space is in this near-future state with lots of equipment up there while Earth sits in what seems to be in "present day (2010's) launch technology" despite having heaved thousands of pounds into orbit to Mars and back many times previously to maintain the Ares program. Tropes Are Tools though and you need this to maintain drama for story.

What exactly does Sols mean?

• Years? Months? Days? I can't really tell.
  • "Sol" is the accepted term for one Martian Day, equaling 24 hours and 39 1/2 minutes. So when Mark was rescued on Sol 549, he'd actually been on Mars for 564 Earth days, or 18 1/2 months.
  • You seriously couldn't tell? It's obviously days, because the other two options presented mean he'd be stuck on Mars for either 47 years or five centuries.

Taiyang Shen

• In the book, after deciding to give their booster to NASA, the Chinese space program mourns the probe it was supposed to be used for (a solar study I think?), and agrees that having one of their taikonauts on a future Ares mission is a piss poor trade. They're mainly upset at the loss to science, as they barely got the funding for the original booster and will never be able to send their probe out now. If only they knew a space organization that owes them a rocket booster and would be pretty interested in the results of new space research. If NASA can repay them with a crewman on an Ares mission, what's stopping CNSA from saying "Thanks, but it'd be better if you could spot us a new booster for our solar probe, and we'll let you see the results too since you helped". With how much the world loved CNSA and NASA collaborating to save Watney, it's not like either government would want the backlash from saying no.
  • It was probably political. The real scientists know that a replacement rocket would be more valuable, but the higher-ups who make the decisions want the prestige of having someone from China on Mars. The fact that the scientists cut a deal on their own doesn't stop them from wanting to avoid getting in serious trouble for doing it.

Radioactive Bathwater

• So when Watney set up the RTG to give himself a hot bath, did he actually irradiate the water to the point that he's probably given himself a fatal dose of cancer? I know the RTG is supposed to provide heat and energy, but how does the radiation translate to heat without actually being radioactive?
  • The RTG doesn't release any radioactivity unless something gets damaged. In fact, both the outer case and one or more of the individual pellets inside would have to be breached for Watley to be in any danger. It's another example of how NASA is hyper-safety-oriented, while Watley becomes more inured to low-level dangers with experience.
  • The radiation in the RTG fuel pellets heats them up, allowing power to be produced from it. It IS radioactive, but it's not shedding radioactive particles. Plus, water is a great radiation shield. Using it to heat your plumbing is fine. Cuddling up to it is not fine, but the thermal burns would kill you before the cancer. The Other Wiki explains in detail how they work.

Bio-monitor

• So, here's something that's brought up multiple times in the book (I haven't watched the movie so just base any explanations on the book), but not really explored further: when Watney gets hit, the bio-monitor sends a packet of vitals. But what it sends isn't readable without Johannsen doing something (what it is isn't explained), and when it's decoded the message isn't complete. So... it got damaged. Meaning it got hit by debris. The fact that Beth had to do some computer stuff to get to something that was ordinarily readable should have tipped them off a little bit. They wouldn't have time to act on it, sure, but surely, as the sysop, Johannsen should have mentioned the equipment failure in a report, and it happened during a storm, so debris isn't implausible. Did NASA really not get clued in earlier? Or was that not enough to get them to authorize the imaging even with the risk of seeing Watney's body?
  • The crew knew Watney was hit by debris — this is brought up in the flashback to Sol 6 midway through the book. The thing is, they also heard his decompression alarm (which is presumably separate from the bio-monitor) go off. So they know that Watney has a hole in his suit, and given he was flung out of suit telemetry range, they can assume he's probably injured as well and not in any condition to fix the breach himself. Humans can survive less than a minute in a vacuum, so as far as they know he's as good as dead just from that. Even Watney admits that he only survived due to being absurdly lucky that he landed just right to slow down the air leaking out of his suit. So the crew and NASA knew Watney's suit lost pressure and that would have killed him very fast regardless of what else was happening. At that point, the partial packet showing Watney has flatlined just seems to confirm the obvious.