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:

THERE WILL BE SPOILERS! 

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.