Tag Archives: science fiction

On molar mass and ideal gas

I recently started reading Andy Weir’s The Martian which is supposed to be the hardest of hard science fiction, written by the son of a particle physicist and scientifically accurate in every possible respect. We’ve heard that story before, so I was not surprised to find the first error (claiming that desiccated stool would be completely free of bacteria) about 13 pages in. Then I got to page 24 and it got bad. Really, really bad. Bad enough that I wouldn’t be surprised if Weir’s physicist father disowns him.

The protagonist, astronaut Mark Watney, is stranded on Mars and believed dead. He has calculated that he has no chance of surviving until a rescue mission arrives (not least because he has no way of informing anyone that he is still alive), but decides to try anyway. He plans to grow food inside the habitat using a mixture of Martian soil, Terran soil that was brought along for experiments, and his own waste. But he needs water:

There isn’t a lot of water here on Mars. […] I’ll have to make it from scratch. […] Take hydrogen. Add oxygen. Burn.

Burning a stoichiometric mixture of hydrogen and oxygen is actually very dangerous, which is not mentioned, but Watney does reflect on the danger of extracting hydrogen from hydrazine, so I’ll let it slide. But let’s see how he plans on obtaining oxygen:

I have a fair bit of O2 reserves, but […] only enough to make 100 liters of water (50 liters of O2 makes 100 liters of molecules that only have one O each). […] That’s where the MAV fuel plant comes in. […] Once I get the fuel plant hooked up to the Hab’s power, it’ll give me half a liter of liquid CO2 per hour, indefinitely. After ten sols it’ll have made 125 liters of CO2, which will make 125 liters of O2 after I feed it to the oxygenator.

Now for hydrogen, from what’s left in the hydrazine-powered descent module’s fuel tanks:

Each molecule of hydrazine has four hydrogen atoms in it. So each liter of hydrazine has enough hydrogen for two liters of water.

The first red flag is that Watney uses units of volume instead of mass, which is inappropriate when calculating quantities for a chemical reaction. Watney is a mechanical engineer and would have been thoroughly trained in the correct use of units, even if chemistry is not really his field. I also doubt he would use the chemical formulas for carbon dioxide, water etc. in daily conversation or in a diary destined for laypeople, but I understand why Watney (or rather Weir) did it: he wants the reader to be able to count H’s and O’s and follow Watney’s calculations. Unfortunately, his calculations are unsound, because you have to add up mass, not counts.

It is not initially clear whether Watney is talking about gases, liquids or solids. Since he will be working in the habitat, close to standard conditions of temperature and pressure, it is not unreasonable to assume that the CO2, O2 and H2 are in gas form and the H2O is liquid. But it seems Watney himself is confused: when he says that the fuel plant will make “125 liters of CO2, which will make 125 liters of O2 in ten sols, he is right… if he is talking about gases, but not if he is talking about liquids (“it’ll give me half a liter of liquid CO2 per hour”).

In reality, 1 l of liquid CO2 at a density of 770 kg·m-3 contains (770 / 44) * 32 = 560 g of oxygen, barely enough for 0.5 l of liquid O2 at a density of 1141 kg·m-3. Since 1 l of water requires (1000 / 18) * 16 = 889 g of oxygen, 1 l of liquid CO2 will only provide enough oxygen for 0.63 l of water.

Meanwhile, 1 l of liquid N2H4 at 1021 kg·m-3 contains (1021 / 32) * 4 = 128 g of hydrogen, which is enough for slightly more than 1 l of water ((1000 / 18) * 2 = 111 g), not the 2 l Watney claims.

It would be different if he was operating exclusively with gases. Assuming the ideal gas law is sufficiently accurate (which depends on temperature, pressure and molecule size), and assuming conditions of temperature and pressure under which carbon dioxide, hydrazine and water are all in gas form, one liter of carbon dioxide and one liter of hydrazine vapor contain enough hydrogen and oxygen for two liters of water vapor (which is not the same as steam) plus one liter of nitrogen and a few grams of solid carbon.

Finally, Watney mentions that some of the reactions he relies on are extremely exothermic, but not that releasing liquid carbon dioxide into the habitat’s atmosphere will dramatically lower the temperature. The exterior temperature is never mentioned, so I cannot comment on the effect of bringing in soil and hydrazine, nor on the state of the hydrazine, which has a melting point of 2 °C and is therefore very likely to be frozen solid.

I’ll keep reading, for the same reason I sometimes watch CSI (but not CSI Miami): the story and characters are sufficiently engaging that I can overlook the bad science, as long as they’re not waving it in my face. The Martian is flying dangerously close to Gap territory, but at least the text flows well and the characters are likable. For now.


More-or-less aimless surfing brought me to an old blog post about math in movies, which mentions Pi and Contact. This mostly coincidental juxtaposition reminded me of the conclusion of the novel behind the latter.

When I first read the book, a zillion years ago, I thought the ending was pretty clever. Then I realized that it was incredibly dumb. It later occurred to me that it might actually be intentionally dumb, and therefore incredibly clever, because Carl Sagan really ought to have known better, but… I don’t know. I think he either genuinely goofed or assumed (probably correctly, in most cases) that his readers wouldn’t notice. Continue reading “Pi” »

Mind the Gap

I don’t get it.

I’ve started reading Stephen Donaldson’s Gap cycle, and I don’t get what’s so great about it.

I don’t mind that the characters are dicks. That’s Donaldson’s thing, and I knew it before I started reading.

I do mind his writing style, which I find flat and unengaging, but I guess it’s a matter of taste.

There are two things, however, that really bother me: plot holes and bad science. Continue reading “Mind the Gap” »

Dereliction of duty

I started on Lois McMaster Bujold‘s Vorkosigan Saga this weekend. Quite a good read; unlike many other books in the genre—say, David Weber‘s Honorverse—the societies she describes and their politics are not too far-fetched or caricatured, nor is the heroine too much of a Mary Sue.

One thing that did make me groan, though, was that the entire plot of the first four chapters of the first book hinges on no fewer than three starship commanders leaving their ships to lead what Trekkies would call the away team1. In any real military organization, this is the gravest sin a commander can commit and would be grounds for court-martial; heck, that almost happened to John Kerry when he jumped ashore for a few minutes in the heat of combat to save his ship and crew (they ended up giving him a Silver Star instead).

To add insult to injury, a few chapters later, in what is probably the linchpin of the entire novel, one of those very same commanders accuses a superior officer of dereliction of duty for doing the exact same thing.

Oh, and there are several instances of characters using a light pen to control a computer, or fiddling with it while thinking or talking. In the author’s defense, unlike light pens, touch screens weren’t all that common in 1986 :)

1 Most of TOS and a large percentage of TNG is about the Enterprise‘s entire wardroom leaving the ship to lead the away team, then returning sans about half of it.


I just finished Neal Stephenson’s Anathem—highly recommended! Like most of Neal Stephenson’s novels, it is full of little gems. There were none that stood out as much as the passage in The Confusion where Stephenson manages to place his protagonists in a situation where it is completely natural for one of them to say “I certainly did not expect the Spanish Inquisition”, but here are a couple I just had to share.

On why people stopped building particle accelerators:

Erasmas: “I always tend to assume there’s an infinite amount of money out there.”

Arsibalt: “There might as well be, but most of it is spent on pornography, sugar water, and bombs. There is only so much that can be scraped together for particle accelerators.”

On the balance of power between the protagonists’ civilization and their ostensibly extrasolar and probably hostile visitors:

Cord: “Do you need transportation? Tools? Stuff?”

Erasmas: “Our opponent is an alien starship packed with atomic bombs. We have a protractor.”

Cord: “Okay, I’ll go home and see if I can scrounge up a ruler and a piece of string.”