Sciencing The Martian

This weekend, my family and I saw The Martian, and we loved it. Amazing movie. Matt Damon plays Mark Watney, an astronaut stranded on Mars who has to figure out how to survive, how to let NASA know he is still alive, and how – if at all – he’s going to get home.

A Hollywood movie whose most memorable line is “I’m going to have to science the shit out of this” is guaranteed to get a good deal of attention from scientists. On top of that, there has been a lot of talk about how The Martian is “based on real science”, so naturally scientists will be tempted to see whether that stacks up. So here’s my science review. There are spoilers. Just to be clear, I’ll say that again, louder:


But I’ll start with eye-candy from the trailer. NB if you scroll down past the pictures, you’ll definitely find the spoilers.

Mars. With astronaut.

Mars. With astronaut.

Mars Rover

Mars Rover

The Red Planet

The Red Planet

Solar panels for the Mars Rover.

Solar panels for the Mars Rover.

The crew leaving Mars. With one of the astronauts missing. Not a spoiler, since you know that from the trailer. 

The crew leaving Mars. With one of the astronauts missing. Not a spoiler, since you know that from the trailer. 

The Hermes, which carries the astronauts from Earth to Mars and home again.

The Hermes, which carries the astronauts from Earth to Mars and home again.

The Big Picture Overall, the science is spot on. Nothing relies on magic new technology and the movie makes reasonable-looking extrapolations from what we currently know. The broad constraints faced by any Mars mission (travel time, the equipment needed, and the overall size and scope of the project) are realistically reflected in the plot. So far so good. 

The Planet Mars looks great in the movie. Between our landers, rovers and orbiting satellites, we know what Mars looks like and the film-makers have worked hard to bring that knowledge to the screen. That said, Martian gravity is about 1/3 of what we experience on Earth, and the filmmakers have not gone out of their way to depict the low gravity environment. So, Matt Damon strides purposefully, rather than bouncing around like a kangaroo. This is how we know the movie wasn’t actually filmed on Mars.

The Weather The weather has a key role in the plot; as the movie opens, a raging storm is threatening to topple the rocket that will carry the astronauts back into space, forcing them to leave early and in a hurry. In the confusion, they lose track of Watney (Damon’s character) and leave without him, assuming he is dead. Mars does have huge dust storms. So huge that they sometimes cover half the planet and can be seen from Earth with fairly modest telescopes and we have known about them for more than 100 years. However, the Martian atmosphere is much, much thinner than the Earth’s, so while air can move at hurricane speeds on Mars, there is not enough air to make it feel like a hurricane. So, as others (e.g. Katie Mack) have pointed out, a Martian storm has no chance of toppling a rocket. If only the crew had known! (It would have been a shorter movie). 

The Spacecraft The movie describes a complex mission, with supplies “pre-positioned” on Mars ahead of the astronauts’ arrival. The astronauts travel between Earth and Mars on the Hermes, a huge spacecraft similar to the International Space Station and presumably assembled in orbit. Likewise, the “Hab” (the habitat/HQ on Mars) is generously sized. None of this is unreasonable, but: the amount of stuff being moved off Earth and round the solar system is hard to square with NASA’s subsequent struggle to send a single cargo rocket to Mars once they realise Watney is alive and needs rescuing. It’s like having a really sophisticated train and bus network, but not being able to find a taxi when you need one.

Health and Wellbeing Getting astronauts to Mars in good shape is a big challenge. The Hermes has a spinning mid-section to provide artificial gravity for the crew as they travel to the Red Planet, which would explain why Damon’s bare midriff, seen close-up in the early scenes of the movie, shows no signs of muscle-loss due to prolonged weightlessness on the voyage out. Quite the opposite, in fact, although I mostly viewed it through the cracks between my fingers, thanks to the accompanying DIY surgery – I’ll admit it, I’m squeamish. With this in mind, as well as artificial gravity, the Hermes has a well-appointed gym. Presumably being an astronaut is a bit like being in prison: you can’t go outside, so you pass the time and keep safe by bulking up.

Of far more concern to astronauts on a Mars mission is radiation: this will be a real worry for Martian explorers who will be spending years beyond the protection of the Earth’s magnetic field, with extended exposure at a level likely to make you pretty sick indeed.

Power Supplies At one point, Watney unearths the mission’s plutonium-powered generator, apparently an RTG. It wasn’t clear to me what the generator’s original purpose was, as the Habitat also gets energy from solar panels; but getting enough power to make everything work is going to be a critical challenge for any Mars mission. Nuclear power sources are compact but complex and potentially dangerous, while solar power is not especially efficient if you need a lot of electricity.

So how does the movie do here? On the plus side, the film recognises the power problems, but it does not really solve them. When Watney takes a long trek in the Rover, he stops each day to charge the batteries. He apparently has maybe 20 solar panels, each about 1 metre wide and 2 metres long. Even futuristic solar technology is unlikely to be more than 50% efficient, and storing energy in batteries instead of using it on the spot leads to more losses. On earth, sunlight delivers a kilowatt per square metre, but on Mars you get only half of that, since the sun is further away. And if the panels are flat on the ground and not pointed directly at the sun you likely lose another factor of two. So Watney’s panels can deliver a maximum of about 4 kilowatts total. This might come close to letting you drive a Tesla Roadster on Mars for a couple of hours a day (although how cool would that be?), but Watney’s Rover is the size of a bus, and looks like a real battery-guzzler.

If I was on Mars, I would want a high-tech electric bike towing a small tent – a bonus relative to earthbound cycling would be reduced air resistance (the flipside of the fact that the storms wouldn’t really be able to blow over the rocket), along with easier hill-climbs in the low Martian gravity. Of course, Watney is short on food, and even a bike needs fuel. (I love electric bikes – I reckon they’d be great on Mars if your spacesuit was flexible enough for pedalling.)

Navigation I did say there would be spoilers, and here’s a big one. In the climactic sequence, the crew of the Hermes use an improvised bomb to blow open a hatch, so as to vent their air into space, and change course enough to rendezvous with Watney, who has just launched himself from Mars. So would it work? To science this properly we’d normally use the rocket equation (and yes, it is really called that). In this case we can take a shortcut, since the air lost by the Hermes doesn’t significantly change the spacecraft’s total mass, whereas the mass of a regular rocket changes dramatically as it burns through its fuel.

So here’s the sciencing: At 25 Celsius, a typical oxygen molecule is moving at 480 m/s, or about 1720 km/hr. The Hermes looks to be a cylinder about 40 metres long and maybe 5 metres in diameter (the scale is set by the capsule at the near end in the picture above), with a volume of about 600 cubic metres; we’ll bump that up to 1000 cubic metres to account for the rotating ring. By coincidence (or perhaps design), this is about the same as the volume of the International Space Station whose total mass is 420 tons. So let’s assume that the total mass of the Hermes is about the same as the ISS. At breathable temperatures and pressures, a cubic metre of air has a mass of about 1 kilogram, so the Hermes contains roughly 1000 kg of air, which is a metric ton. We now have enough information for a guesstimate: an object weighing 420 tons throwing 1 ton of material away from it at 480 m/s will pick up a speed of 480/420 m/s in the opposite direction – just a bit more than 1 m/s. (This is Newton’s Third Law: every action has an equal and opposite reaction.) We’re assuming for the purposes of convenience that all the air is moving at the same speed and in a straight line, but it is good enough for a guesstimate. (And it will be on the high side.) 

Unfortunately, the Hermes needs to change its velocity by tens of metres per second in order to pick up Watney. And science says that the “blow the hatch off” trick can give (at most) one metre per second. So that doesn’t look good. But! If you blow the hatch early enough, this manoeuvre also changes how close Hermes comes Mars, which in turn changes the total acceleration supplied by Mars’ gravitational field. How far in advance would you have to blow the hatch to profit from the gravity assist and make it just in time to catch your astronaut? I will leave this as homework – would it work? (No prizes, but finding a solution will make you a steely-eyed missile man [sic!], just like Rich Purnell. So have at it.)

It’s easy to critique movie science when it operates according to slightly different laws from real-world science. But it’s instructive to look at the corners the movie had to cut to make the plot workable. To make Watney’s challenge surmountable, the movie had to make it simpler. This works in a movie but unfortunately for us the laws of physics can’t be edited away. If we want to run an electric vehicle on Mars, we must provide it with enough power for the task. A spacecraft in orbit has very little choice about where it goes next, even if you deliberately blow a gasket. And if you’re going to run an operation on Mars with a decent (or even minimal) safety margin, you’re going to need real back-up on the ground.   

Beyond politics and shifting priorities, these challenges are part of why 50 years have elapsed since the moon landings without anyone making it to Mars and back. But in the meantime, The Martian lets us dream about what it might look like.

CODA: One last question – when it comes to whether you can grow potatoes in your own poop, I have no idea, and don’t really want to find out. 

Destiny’s Child

Until recently, I hadn’t played “computer games” with any regularity since the mid-90s, when Doom and Nethack were procrastination tools for young cosmologists. (Both games are available for the iPhone, by the way – the more things change, the more they stay the same.) However, for better or worse, my household finds itself with a Playstation 4 and the attendant opportunities for 21st century parent-child bonding. 

So I have been playing Destiny with my kids. Created by Bungie in 2014, Destiny did $500 million worth of business on the day it was released. But if you somehow avoided this cultural tsunami, Destiny is a “first person shooter” and players fight their way through the shattered remnants of human settlements on nearby planets in the solar system.

And it is fun, if somewhat repetitive. From the parental perspective it is more palatable than many similar titles, thanks to cartoonish enemies and a distinct lack of actual gore. I will admit that I haven’t figured out the bewildering variety of aliens but it doesn’t matter much: you shoot them all, and my offspring are on top of the finer details if it ever matters. 

Every so often theoretical physics, gaming, and parenting overlap. Not that often, but it happens. For instance, in the final moments of The World’s Grave level of Destiny, your Ghost (an annoying sidekick that follows you around, offering advice and resurrecting you after your frequent deaths) hacks into an alien library, announcing that it holds so much information that its curators must have found a way past the Bekenstein limit. 

So what is the Bekenstein limit, asked the kids. Turns out, Dad’s got this one: its a real thing in physics – the ultimate limit on the amount of information that can be stored in a finite volume.

Jacob Bekenstein, who died last month, deduced this limit by asking what happens to the entropy of stuff (e.g. a giant dying star) that collapses into a black hole. Possibly not the most obvious of things to ask, but it unlocks a hidden door to a vast storeroom of fundamental questions.

Jacob Bekenstein, with the tools of his trade... [Wikimedia]

Jacob Bekenstein, with the tools of his trade… [Wikimedia]

Entropy is a measure of disorder in a system, and is often synonymous with degradation and decay. Even if you are not sure what entropy is, you very possibly know it increases with the passage of time or, at best, stays constant, thanks to the Second Law of Thermodynamics. (This is why your house gets messier, not tidier, if you leave it to its own devices). But wherever there’s entropy, there’s information. Information is entropy’s B-side, its secret identity: a disordered system is more complex than an orderly one, so more information is needed to describe it. 

But does stuff – and its associated entropy – disappearing into a black hole provide a loophole to the Second Law? Starting from thought experiments like this in the early 1970s, Bekenstein realised that black holes themselves have entropy, and entropy does not vanish when a black hole is formed. Not only that, it seems that no object can have more entropy than a black hole of the same mass. If it did, turning that object into a black hole would engineer a violation of the Second Law. It is this ceiling on entropy that yields the Bekenstein limit on information density which, according to the makers of Destiny, was bypassed by the Hive. 

Why the Hive would bother violating the Bekenstein limit is a different question. A sphere 1 meter in diameter holds 1070 bits of data at the Bekenstein limit, and that is a lot of data. Also unexplained is how your Ghost, an object roughly the size of your fist (at least when it is encased in an armoured gauntlet) carries this information away – presumably the Bekenstein limit doesn’t bother it, either. (If you took every atom that makes up the planet Earth and attached all the data transferred on the internet in 2015 to each and every one of those atoms – about a zettabyte apparently – you would have to pack all that information into the abovementioned 1 metre sphere for it to hit the Bekenstein limit.)

The Bekenstein limit may seem almost simple (at least for something involving black holes and thermodynamics), but its consequences are still being understood. Entropy is not some sort of sauce that can be poured over a physical system; the entropy of a system is defined by how its internal components are organised. But a black hole has no internal components, and even if it did, anything inside the black hole is supposed to be hidden from observers on the outside. So if a black hole has entropy, where does it live?

Much like a game, this question leads to a new and tougher level for physics: quantum gravity. If black holes have microstates that encode the information that corresponds to their entropy, the microstates are presumably quantum mechanical, like all the other fundamental building blocks of the universe. On the other hand, a black hole is ruled by gravity, and quantum gravity is a boss fight on the path to a “theory of everything”. One toolkit for tackling quantum gravity is string theory; and in the mid-90s, “stringy” calculations starting from a microscopic description of nature produced black hole entropy results like the ones Bekenstein and others found in the 1970s. This doesn’t prove that the universe is made out of strings, but it is one why reason why physicists are excited by string theory. 

Physicists have been playing with the connections between black holes, thermodynamics, gravity and quantum mechanics for over 40 years, and no-one knows where the adventure will end. Simply announcing that he was on the trail of a solution to the “information paradox” – the question of exactly what happens to information stored inside a black hole – during a lecture in Stockholm last month got Stephen Hawking worldwide news coverage, although the solution is at best a work in progress. (See Sabine Hossenfelder’s Backreaction blog for commentary.) 

One thing we do know is that while the phrase “Bekenstein limit” is a throwaway line in Destiny there is a huge amount of information hidden in those two words, with far more left to discover than we have already learnt. If you are trying to ask the questions that will lead to the “next big breakthrough” in fundamental physics, black holes and thermodynamics are a great place to look.“>

They’ve broken the Bekenstein limit…

Coda: The full formula for the entropy of a black hole is due to both Bekenstein and Hawking. And while a black hole represents the upper limit on the entropy and information that can be stored in a given volume it seems that the microstates of a black hole all live on its surface. This leads to a proposal known as holography, suggesting that our apparently three-dimensional universe may, at some fundamental level, need only two dimensions. But that is a story for another day.