A few thoughts on The Martian

Spanish theaters have recently premiered The Martian, a movie based on Andy Weir’s eponymous novel (though for some reason the movie’s name was changed to “Mars” [Marte] in Spain, adding to a long list of unfathomable translation decisions). As you might well imagine, this movie touched on subjects that bear a close relationship with some of GMV’s space projects.

The movie’s verisimilitude is patchy. Many things are portrayed very well and plausibly; others less so, often drawing on a strong vein of poetic license to rack up the tension or drive the storyline. Let’s look at a few of these dodgier features.

Brief movie credits

imagen 1Movie poster (FOX) Title: Director: Screenplay: 



The MartianRidley ScottDrew Goddard (Adapted from Andy Weir’s novel) 

Matt Damon

Jessica Chastain

Jeff Daniels

Michael Peña

Kristen Wiig

Donald Glover

Kate Mara

Aksel Hennie

Sean Bean

Chiwetel Ejiofor

Sebastian Stan

Mackenzie Davis


Main plotline

 alerta SPOILER ALERT: Some details of the movie’s plot are mentioned in this and other sections of this article, so if you want to avoid any spoilers read no further. A much better option, however, might be to see the movie or read the book first and then there’ll be no problem.


Mark Watney is in a fix. After a fierce Martian storm he is assumed by his fellow crewmembers to be dead and they set off back to Earth without him. His survival alone in this harsh environment will now depend on his science savviness, his mental strength and the ability to somehow inform the Earth that he is still alive to ensure ongoing NASA support.

The storm gods rage gentler on Mars

Let’s start with the elephant in the room. Read any online comment about the movie and they all stress the fact that the starting point of Watney’s plight could not in fact occur in real life. As we’ve already seen in the plotline, Watney gets trapped on the Red Planet by a spectacular dust storm that even lifts him into the air.

Just why is this scene so controversial? Mars does have big dust storms, so big they might stretch over the whole planet. But they would be very unlikely to be able to lift an astronaut into the air. Mars’s atmospheric density is much lower than the Earth’s so the force of the wind would be correspondingly lower too. Normal air density on Earth is about 1.2 kg/m3; on Mars this density drops right down to about 0.01013 to 0.0155 kg/m3. This is tantamount to air density at a height of about 35 km above the Earth’s surface; as such it is really very low.

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The small white arrows outline the area where dust from the storm is apparent in the atmosphere. The size is appreciable (click to enlarge) (source: Wikipedia)

To make matters worse, windspeed would never be very high on Mars. According to readings of the Viking spacecraft, which weathered a Martian storm on the planet’s surface, windspeeds were 61 km/h, gusting up to 94 km/h. This pales into insignificance compared to windspeeds of 300 km/h that might be clocked up on Earth.

It might be countered that Mars’s gravity is less too, so despite the lower windspeeds it would be easier for these feebler winds to lift Watney from the ground. This is true: Martian gravity is 37.6% of the Earth’s so the force needed to lift any object would be one third of the necessary terrestrial effort.

A quick bit of number crunching might establish who is right here. The first variable would be Watney’s weight. The actor playing him is Matt Damon, who apparently weighs in at about 84 kg. To this bodyweight must be added of course the weight of his spacesuit (which in fact would not be like the one shown in the movie but we’ll come back to this later). Let’s be generous and assume a spacesuit weight of only 20 kg. So we now have a total weight of 104 kg. Not so fast: we’re not really speaking about “weight” here but “mass”. My use of “weight” earlier was a trap. The two terms “weight” and “mass” are easily confused with each other because they are pretty much interchangeable on Earth. Weight is in general the name we give to Earths’ gravitational attraction on our mass; it is the gravitational acceleration (9.8 m/s2) times the mass. As we have already seen, gravity is much lower on Mars (only one third of Earth’s), so the force needed on Earth to lift Watney would be 1019.2 Newtons, whereas on Mars it would be only 386 N. Prima facie the storm would seem to have an easier task in hand.

But now we need to factor in the force of the storm. To find this out we need to estimate the drag generated by the wind-astronaut interaction. This drag depends on the dynamic pressure (generated with the air movement); this in turn depends directly on air density and windspeed squared, and the reference area, in this case the poor astronaut (maybe we should say “aeronaut”?), plus a drag coefficient. The resulting formula would be formula, (in Spanish initials where “S” = area, but we won’t get bogged down in details here). The total area of a human being is about 2 m2, so the area exposed to the wind would be half of this. But the spacesuit would be bulky, increasing the area. We also want to make it easy for the wind, because to see Matt Damon flying through the air would be pretty neat. So let’s say an area of 1.2 m2. The human drag coefficient has been estimated at between 0.3 and 0.6, so we’ll use 0.8. We’ve already seen the Martian air density, so let’s take a slightly high value of 0.016 kg/m3. We’ve also seen the dust-storm windspeed on Mars but we’ll now imagine a storm of unprecedented force (which is not really very realistic because storms on Mars actually seem to be getting weaker but we’ll make this assumption for the fun of it) with windspeeds of 150 km/h. You can see we’re making it very easy for you, Mars.

OK, so if the drag force is higher than 386 N, Watney would fly through the air; the stronger the wind, the further he would go. If the drag force was lower, it might shake him a bit (or not even that at lower forces) and the movie wouldn’t even get off the ground.

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The astronauts struggling against a fierce Martian storm (click to enlarge) (FOX)

So how do these sums pan out? Well the resulting force is … 13.33 Newtons. Oh dear. Watney, with the same size and shape, would have to weigh 3.6 kilos to be carry-off-able by the Martian wind, and this with all the variables weighted in Mars’s favor.

When the Viking probe was weathering its storm, none of the surrounding geological features (rocks, stones, pebbles, sand) were seen to move; only the fine dust, which darkened the sky and lowered the temperatures. Indeed, so low is the Martian atmospheric density that windspeeds of from 65 to 79 km/h would be needed just to stir up this dust.

This is something that the author himself has acknowledged. He left in the implausible Martian wind effect for creative reasons. Nonetheless, there is a scene in the book that does not feature in the movie, towards the end of the story. When Watney is traveling over the Martian surface in the rover, which runs on batteries charged up by solar panels, there is a moment when he is heading for a storm. The main peril on this occasion isn’t the wind (which, as we have seen, would have very little effect), but the blocking off of sunlight in the areas of densest dust build-up, preventing the batteries from recharging properly. The rover would therefore be capable of covering less distance with the same charge, taking much more time and missing out on the rescue window. This would indeed happen in a Martian storm.

imagen 5

Darkening of the sky during several days due to a Martian dust storm (source: Wikipedia)

Nice orange sunset. Hold on . . . shouldn’t it be blue?

The Martian shows us some stunning views of the Red Planet. The views from space are in fact all uncannily realistic.

When we come down to earth (with a small “e”), however, they are very Earthly, and the lie of the land is a little exaggerated for scenic effect. This is understandable, since the idea is to transmit the difficulty and drama of travelling through this landscape. Martian buggies will not go along very quickly and will not be designed for rugged terrain but a little bit of exaggeration is understandable to make it all more watchable.

The images, after all, are very beautiful, with stunning sunsets like this one.

imagen 6

Mark, the sunset is on your left (FOX)

Fantastic, except for the fact that the sky should in truth be blue (and the sun quite a bit smaller, but we’ll overlook this). Yes, yes, I know the daytime sky in Mars is orange tinted and that sunsets are traditionally red and golden . . . but only on the Earth. Once more, I don’t want to get bogged down in details, but the sky color is produced by light scattering. Basically, different atmospheric compositions generate different colors. On the Earth the result is blue and on Mars orange-tinted . . . away from the Sun. Around the sun it would be blue; this effect, however, would be barely noticeable during the day and would come into its own only at dusk. So sunsets on Mars are bluish. Witness the following photos taken from Mars. You could check this out on any Instagram account.

imagen 7

Sunset on Mars, with the sun already below the horizon, as seen from the Viking (source: Wikipedia)

imagen 8

Sunset on Mars seen from the rover Spirit (click to enlarge) (source: Wikipedia)

Everything comes down to gravity in the end

As already mentioned, gravity on Mars is much less than on Earth. A third, in fact. It is not as low as the Moon’s, which is one sixth of the Earth’s (and hence half of Mars’s), but low enough for the difference to be noticeable. So astronauts would not bound along as they do on the Moon but neither would their gait, I’m afraid, be Earthlike. Watney’s, unfortunately, is (and the rest of the characters also have a very Earthlike gait during their short spell on Mars).

Another feature of the movie also obeys different gravitational laws from the Earth’s, namely the mothership Hermes. Hermes is driven by a nuclear-powered ion thruster. That’s spot on. Although this smacks of the most outlandish science fiction, it is in fact, believe it or not, pretty realistic (without pushing it, mind you: the reactor needed to move Hermes would be incredibly big and heavy and it’s not at all sure it could in fact be launched into space). Ion thrusters, however, have been with us for some time now. They work on a different principle from traditional chemical rockets, which can provide very high thrust for a short spell of time. Ion thrusters, on the other hand, generate very small thrust but they can keep it up for a very long time. The Hermes thruster, for example, generates 2 mm/s2. This is meagre. Accelerating from 0 to 100 km/h would take 3 hours 52 minutes. It’s obviously no roaring sportscar. But if you keep up this rate of acceleration for a long period of time (days, months…), you can build up high speed. This is the working principle of ion thrusters: keeping them turned on and accelerating for a long time.

imagen 9

Hermes overtaking the Earth from the left (FOX)

Another aspect of interest to us here is how the Hermes spacecraft simulates a sense of gravity for its crew. The effects of weightlessness on the human body are well known. One of the most important long-term effects is probably spaceflight osteopenia or bone loss, an irreversible effect at the time of writing. Prolonged weightlessness is also fairly debilitating, so it is best avoided if the crew is to get to Mars in a condition fit for work.

How is this gravity generated? Sadly, we don’t yet have artificial gravity systems as seen on Star Trek or Star Wars, so we have to stoop to cheap tricks. The go-to solution is centrifugal force (if some wiseguy physicist should tell you this force doesn’t exist, do me the favor of referring him or her to this XKCD cartoon strip). It’s a force we’ve all felt at one time or another. For example, when taking a bend at some speed in a car, we’re forced outwards. It is also the force that keeps a stone in a sling when it is swung round. On the same principle, if one part of the spacecraft is spun round, this would simulate an outwards-acting gravitational force.

imagen 10

Kate Mara doesn’t fall because in fact her “gravity” is acting upwards (click to enlarge) (FOX)

imagen 11

The spacecraft Hermes spins round in infinite space (FOX)

Is this artificial reality as represented in the movie realistic? As the starting point for our analysis we note that the book quotes gravity onboard Hermes as 0.4g, i.e., 40% of the Earth’s. Bearing in mind that Mars’s is 33%, this strikes a pretty happy compromise. The spacecraft can thus spin a little more slowly than it would do if simulating Earth’s gravity. This is important for several reasons. First of all you use less energy in reaching the necessary spinning speed, and energy-saving is always good news in space. The second reason we’ll look at later.

Achievable acceleration depends on the angular velocity (or spinning speed) and the radius of the turning circle (the formula is formula 2), and this should give us 0.4 x 9.8 m/s2, i.e.,  3.92 m/s2. The radius can be estimated from the image of Kate Mara in the window, extrapolating this to the view of the complete Hermes.

Kata Mara is said to be 1.6 m tall. Assuming she isn’t wearing high heels, and juggling with images and scales, I calculate a radius of about 22.5 meters. It should therefore spin at 0.417 rad/sec. Does it? Quite frankly, I don’t know. Wired, however, analyzed the video  (partly inspiring this analysis, though they estimate a different radius), and they reckoned that it seemed to spin at 0.109 rad/sec. Four times less. To spin at this speed it would need a radius of 329 m, and I don’t think my Kate Mara measurement is so far out. Wired concluded that they did so because spinning at a slower rate looks cool, more dramatic.

There are two factors that are usually overlooked when imagining science-fiction spacecraft that simulate gravity by spinning. One, as we’ve already seen, is that the “gravity” depends on the spinning radius, the gravitational effect falling towards the center. At the very center, for example, the gravity would be zero or free fall. This factor is usually taken into account. What they usually overlook, however, is that, if the radius is small, your head might well feel a different “gravitational” force than your feet. I don’t know exactly what this must feel like but I shouldn’t imagine it would be very comfortable. Let’s see what would happen in the above cases. If the spacecraft had the abovementioned radius of 22.5 meters, spinning at a rate of 0.417 rad/sec, and if your head was 1.8 m above the floor (like mine), then the artificial “gravity” at head height would be 3.606 m/s2. In other words, 8% less than the feet; this would be an appreciable difference. On the other hand, in the case of a slowly spinning, 329 m Hermes, the head-foot difference would be only 0.55%, and this would be easier to put up with.

The other factor to be taken into account is that when you set something spinning you generate not only a centrifugal force but also the much less well-known Coriolis force. I don’t want to get too technical here (some might say this wish comes too late) but this is a force that appears when you’re moving in a spinning system. Like the Hermes for example. This force runs at right angles to the speed and the spin … but it might be better to look at an example. If you are walking along the circumference, for example, against the spin, the Coriolis force would act “downwards”, hence increasing the gravitational force. With the spin, on the other hand, the gravitational force would be reduced. As we’ve already said, this force depends on the spinning speed and the speed you’re moving at. If Kate Mara, therefore, was walking along at about 3 km/h, which is slow but quite reasonable for ambling round a spaceship, her gravity could change by 18% in the small, quickly-spinning Hermes. This is appreciable. In the smaller, slower-spinning Hermes, however, it would change by 4.65%, which is no bagatelle either but would certainly be less noticeable.

It would seem, therefore, that the movie doesn’t represent this factor too well. What does seem to be clear, however, is that if we want space travel to be comfortable we should make spacecraft with the biggest possible spinning radius. Either that or develop Star Trek’s artificial gravity.

imagen 12

The Babylon 5 space station, with a radius of 500 meters, would seem a much more comfortable prospect (ignoring all the political and diplomatic complications that built up around it) (Warner Bros.)

Under pressure

The movie shows two types of spacesuits. Firstly, the bigger, more striking suits they don for spacewalks and secondly the lighter more comfortable numbers for when they’re walking on planet Mars.

Conditions in space are very harsh. Everything is out to kill you. It’s savagely cold in space and you need to protect yourself from it. On the other hand you could also heat up in sunlight, even boil up. The radiation environment is pretty intense too. Neither should we forget that in space no one would hear your cries for help . . . because there’s no air. The spacesuit has to be capable of regulating inside conditions to ensure human survival for a set period of time. That’s why they are usually so bulky and showy. No wonder. Their remit is to recreate a tiny Earthlike climate inside them and this is no easy matter. After all, it usually calls for a whole planet.

The conditions are not much better on Mars itself. Martian temperatures are pretty low. At the best of times they are in the range 2º to -70º, normally lower, though they might also soar to 35º in the equatorial summer. Furthermore, the thin Martian atmosphere is 95% CO2; oxygen accounts for only 0.14%. This makes it completely unbreathable for human beings. We’ve already seen too that the atmospheric pressure is extremely low. Unlike the suits seen in the movie, therefore, they should really be pressurized, and the life support system should be a bit showier.

Neither do I really get the idea of them being orange on Mars. It must be so that the stains don’t show up.

imagen 13

Jessica Chastain, raring to go boldly into space and live to tell the tale (FOX)

imagen 14

Jessica Chastain, ready to tread Mars and probably have a rough time of it (FOX)

Life-saving Thanksgiving potatoes

One of the Watney’s main challenges in his Martian ordeal is to avoid dying of hunger. This is where he has two strokes of luck. Firstly, he is the mission’s botanist, so he’s no mug at growing things. Secondly the Martian sojourn coincided with Thanksgiving so the crew took some potatoes onboard to celebrate it. To make sense of this the author had to perform some tricky date juggling, as he explains in this article .

Using these Thanksgiving spuds, Martian earth, earth brought from Earth precisely to gauge the growing possibilities on Mars, his own feces and those of the rest of the crew (no time for squeamishness) and also huge inputs of water, Watney pulls it off. All this is pretty realistic in general.

There is one thing the author didn’t take into account because it wasn’t known at the time. Martian earth contains perchlorates, a salt that is harmful to humans. Luckily, it could be removed by rinsing the earth with abundant water. Obtaining water from hydrazine, as he does in the movie, is plausible, but bearing in mind that hydrazine is extremely toxic, it would have been much better to have done so outside the habitat.

imagen 15

Mark Watney hungrily checking out his crop (FOX)

To infinity and beyond

I fear this article has already got pretty lengthy and there are still a few things to mention. Briefly, then, the buggies appearing in the movie are in general fairly realistic, though in the real world they would probably be less bulky (the bigger the volume, the more expensive the launch; saving space is saving money). There’s a big problem both in the book and in the movie with radiation. Everything is simply said to be protected but bearing in mind that the walls of the inflatable Hab are made mainly of plastic tarp, this would seem to be unlikely. And on Mars there is a lot of radiation.

One thing comes across clearly, both in the book and the movie: duct tape is good for any emergency. It can be used for running repairs on a helmet, a spacesuit or the habitat walls. Given that duct tape works in a vacuum, this is not implausible (though other adhesives are also used in the book).

The idea of using the Pathfinder for establishing communications with the Earth, strange as it may seem, could work. The old lander does not have the brilliant LEDs of the movie but it does apparently have a port that would allow communication of text messages (always bearing in mind that a message would take from 3 to 20 minutes to get from Mars to Earth and the same time to get back again).

Towards the end of the movie the idea of removing the rocket’s metal covering and replacing it with tarp is not out as outlandish as it might seem. As we have already seen, atmospheric density is very low on Mars so launch impact would be small. Another thing we need to look at briefly is the idea of using escaping air as a mini thruster, both by the astronaut and on Hermes. Blowing out part of the spacecraft so that the escaping air generates thrust would be an extremely dangerous maneuver, but it could work. After all a rocket works on just that principle, thrusting gas in one direction to move in the opposite direction. Mind you, I’m not so sure they would achieve the 29 m/s thrust mentioned in the book, and the whole maneuver would be hard to control. But escaping air from the glove of the spacesuit would not work very well as a thruster. As soon as the suit is pierced it would probably stick to the hand, plugging the gap and cancelling out the thrust. In the book, for example, this idea is ruled out completely. Not only that, no one would perform any spacewalk maneuvers without being tied by a cable to part of the spacecraft. If they did we would in all likelihood see a completely different movie: not so much The Martian but more Gravity 2: The Red Revenge.


The movie is really interesting. Like any Hollywood blockbuster it mixes up realistic things with more fanciful ideas. I would ardently recommend reading the book. It complements the movies splendidly and is a great read. It gives a better idea of Watney’s wry sense of humor and also explains very clearly some of the technical elements of the story (in my opinion of the description of the cause of the rocket failure is masterly).

The most important conclusion to be drawn from this movie is clear. If you’re going to any sort of harsh environment take along duct tape. Meters and meters of duct tape.

And lots of potatoes.

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Author: Javier Atapuerca
Head of Mission Analysis and Studies Section (GMV)

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Las opiniones vertidas por el autor son enteramente suyas y no siempre representan la opinión de GMV
The author’s views are entirely his own and may not reflect the views of GMV

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