It’s officially been 5 years today since Ridley Scott’s The Martian came out. Thanks to the diligent research of the author Andy Weir and everyone involved in making the film, it’s one of the most scientifically accurate sci-fi films out there and I think it will stand the test of time beyond when mankind manages to make that giant leap to the red planet. The “mistakes” are down to creative license or plot devices and that’s entirely fair – this is above all a story and if you want to watch something 100% factual there are lots of documentaries out there. They might not have Matt Damon though.
Disclaimer: almost every plant that has been entrusted to me has died so I’m not going to presume to comment on the botany. I’ll be sticking to the physics.
The Big Mistake
Let’s start with the bad news. You know how at the beginning of the film there’s this huge dust storm with uproot-a-satellite-dish strong winds and Mark Watney gets hit by said satellite dish and everybody thinks he’s dead so they unknowingly abandon him on Mars and it’s the basis for the plot of the entire film? That can’t happen. There are indeed huge dust storms on Mars but the Martian atmosphere is about 100 times thinner than Earth’s so the atmospheric pressure is much smaller. That means the force exerted by the 175kph wind of the dust storm would be equivalent to that exerted by a 15kph wind on Earth i.e. a gentle breeze just strong enough to make a leaf fly around a bit but hardly enough to break a satellite dish. Andy Weir openly admits that he knew this situation was impossible but he liked the dramatic plot device – it makes for a great opening sequence so I definitely forgive him.
Little Hiccups
- The gravity on Mars is 0.38g i.e. 0.38 of the value on Earth, so although the crew look like they’re having a tough time lugging around heavy equipment they would actually be as light on their toes as ballerinas.
- Mark Watney splits the hydrazine fuel in the Mars Descent Vehicle to isolate hydrogen and burns it with oxygen to make water. That’s cool but actually Mars isn’t altogether the dry barren wasteland it was thought to be a decade ago. Not only are there subglacial lakes at its poles, the Martian soil itself is filled with water, so all you’d need to do is heat the soil to extract the water. The book was written 2 years before all this was discovered though.
- When the Hab explodes, Mark patches it up with some canvas and duct tape. As stated above, the atmospheric pressure on Mars is low enough that this would theoretically work, but the beefed up version of the Martian atmosphere in the film would tear that poor piece of canvas apart.
What The Martian Got Right
Let’s start with the cherry on top: the beautiful visuals. The vast expanses of Martian desert and the hazy orange sky are confirmed by images from the Curiosity Rover. On Earth, a clear sky is blue due to Rayleigh scattering of light by tiny air molecules but on Mars, the large dust particles in the atmosphere absorb blue light and reflect longer wavelengths including red, orange and yellow. Also, although we established that dust storms on Mars aren’t powerful enough to knock things over, they’re powerful enough to get dust everywhere so it’s definitely plausible that Watney would have to spend time everyday ridding the solar panels of all that dust.
The Hermes ferries astronauts between Earth and Mars using ion propulsion, where a propellant like argon or xenon is ionised electrically and the resulting ions are fired out of the back of the Hermes to thrust it foward (thanks Newton’s 3rd law). This technology is already used in NASA’s Dawn spacecraft which is currently orbiting two asteroids, Ceres and Vesta. The great thing about ion propulsion is that you only need a small amount of thrust to start accelerating, then the lack of friction in space allows you to build up to massive speeds over a long time. It’s kind of like powers of two: 2 is a small number but multiply it by itself 10 times and you’re at 1024; multiply it by itself another 10 times and you’re already at over a million. Once things get going, you’re really on a roll. Another good thing is that it’s easy to fine-tune trajectories as all you need to do is eject some propellant here or there at a particular angle and the spacecraft moves in the opposite direction. This comes in handy for the “Rich Purnell manoeuvre’ in The Martian which requires flybys, gravity assists, entering low orbit and hyperbolic trajectories.
What is possibly the coolest part of The Martian is that the trajectory of the Hermes was ran through independent simulations created by Andy Weir and Laura Burke, a mission designer at NASA, so they’re completely accurate. The first part of the trajectory is an elliptical orbit known as a Hohmann transfer, which is the quickest way to go between Earth and Mars. This is the path the crew takes on the way to Mars and when they head back to Earth (minus a resourceful botanist). As the Hermes approaches Earth, it uses its ion engines to accelerate and use Earth’s gravity to slingshot itself into a hyperbolic trajectory towards Mars. The Hermes at this point is moving too fast to fall into Mars’ orbit so it performs a flyby and Watney follows a hyperbolic trajectory to meet up with the crew in the climactic rescue. Then the fully equipped Hermes makes its way back to Earth with another Hohmann transfer. A hyperbolic trajectory is one that never repeats itself so if the rendez-vous didn’t work out, the paths of the crew and Watney would never cross again – this is partially why some in the film weren’t too keen on this risky plan.
Space is unpredictable. Even if everything had gone according to plan and Watney hadn’t been stranded, the 6 month long journey would have been dangerous. But in order to save Watney, the crew spent 2 extra years in space. Not only would the extended 2.5 year long journey on the Hermes have been riddled with risks and danger for the crew but they likely would have sustained long term damage due to the immense radiation exposure they would have endured by travelling into Venus’ orbit during the manoeuvre, not to mention a lot of stress-induced grey hairs. But this is another thing that The Martian gets right: the collaboration of scientists all over the world, the love they have for what they do and that “astronauts are inherently insane. And really noble.”
DASH PHYSICS 