Science, Space

The science of Ad Astra

It’ll come as no surprise that I love sci-fi movies, especially if they’re set in a near future where humanity has managed to colonise the rest of our solar system. Ad Astra, released in the UK just last month, fits the bill perfectly. Brad Pitt stars as astronaut Roy McBride, who is brought in by US Space Command to deal with some destructive bursts of cosmic rays coming from the edge of the solar system. It is believed that the rays are linked to the long-lost ‘Project Lima’, a mission on which Roy’s father disappeared 16 years earlier, and which has been rediscovered out near Neptune. Space Command believes that Roy’s father might still be alive, and Roy must travel to the Moon, Mars and beyond to make contact with him.

There’s quite a lot about Ad Astra that borders on the pretentious (the voice over!), but for me it never fully crossed that line, and I was quite happy to go with it, especially as it included some really cool world-building and beautiful space cinematography. But as with all quasi-realistic sci-fi films, it was the science that most stood out to me… and the ways in which the film got it wrong.

Of course, Ad Astra isn’t trying to be super-scientifically accurate – it’s a story first and foremost – but it is interesting to see what the film’s mistakes can teach us. So with that in mind, let’s dive into some of the scientific missteps I noticed in Ad Astra. Beware, there are spoilers ahead!

ad astra moon
Photo credit: Fox Movies/Kino Check

Magically growing cosmic rays

The bursts of cosmic rays that are coming from Neptune are dangerous and must be stopped. The reason they’re so destructive, as we learn during McBride’s briefing with Space Command, is that they get stronger as they travel away from Neptune, so by the time they reach Earth they are powerful enough to cause untold havoc.

This seems pretty unlikely. In physics, there is something known as the Inverse Square Law, which states that the intensity of a physical force or energy radiating out from a source will decrease as the distance from the source increases. This is because the force or energy, which starts from a relatively small point, is effectively diluted as it radiates out into much bigger, 3D space.

Imagine standing in the middle of a field at night, holding a candle – the candle will look quite bright because it’s right next to you. But if you leave the candle in the middle of the field and walk away from it, it will appear to get dimmer. The candle itself isn’t changing – the intensity of light it’s producing is still the same – but that light is now spread out over a much wider area, and so the candle looks dimmer from the edge of the field.

Cosmic rays getting stronger the further they travel from their source, then, seems pretty unlikely and kind of breaks the laws of physics.


The mission-ending slowdown

On the way to Mars, Roy McBride’s ship picks up a distress call. The captain decides to stop and help (and this does not end well for him). However, it’s worth noting that stopping and starting is a pretty tricky thing to do in space, and taking a break on the way to Mars is probably a terrible idea.

Newton’s First Law of Motion states that, ‘unless acted upon by a force, an object will remain at rest or continue to move at a constant velocity’. This means that, if an object is still, it will stay still – but, crucially, if the object is already moving, it will continue to move (at a constant speed, in a constant direction) unless another force is exerted upon it. This law is very important in space travel. When it comes to getting a rocket into space, most of the energy (i.e. fuel) is spent on getting it through Earth’s atmosphere and away from the influence of Earth’s gravity. A spaceship doesn’t have to contend with any of the slowing forces that, say, a car does (the friction of the surface of the road, the drag of the air it is passing through), and so, once started, it will glide through space forever without using a drop of fuel. This is vital, because fuel is heavy and therefore costly to get into space, so you want to take as little of it as you can get away with.

So, setting aside the problem of how much fuel it would require to slow down and stop in space (‘putting on the brakes’ would involve firing some engines in the opposite direction to your direction of travel), you’re going to have a bit of a problem when it comes to setting off again. You’re going to need a lot of fuel to get a whole spaceship moving – fuel which you already spent getting yourself off the surface of a planet and hurling yourself towards your destination, and which you’ve now wasted by stopping to deal with some ravenous space monkeys.


Climbing a ladder over lit rocket engines

There’s not much to say about this one. Roy McBride climbs a ladder to sneak onto a spaceship, and he is on that ladder – right above the engines – when they ignite. Rocket engines burn at over 3,000°C. Yeh, he dead.

Image result for melting face gif


Escaping the heliosphere

The aim of Project Lima, Roy’s father’s mission, was to travel to the edge of the solar system to try and detect signals from deep space, in order to find out whether humanity is the only intelligent life in the universe. But why travel so far? Space Command explains that the heliosphere – basically a vast bubble of plasma and solar wind emitted by the Sun – could be interfering with our detection of alien signals, which means that we must get beyond it in order to talk to ET. And so they go to… Neptune.

The problem is, the heliosphere doesn’t end at Neptune. In fact, it extends beyond the orbit of Pluto, out to about 121 AU from the Sun.* In contrast, Neptune’s average distance from the Sun is a mere (!) 30 AU. So going to Neptune and claiming that you’ve escaped the influence of the Sun is pretty laughable – in fact, Project Lima only got a quarter of the way there.


Deciding that we are not alone

The big emotional payoff of Ad Astra relies on the idea that Project Lima didn’t detect any alien signals, and this means that humanity is alone in the Universe. Setting aside the Neptune/heliosphere debacle for a moment, the idea that one mission could gather enough data within the span of a single human lifetime to determine that no aliens exist anywhere in the Universe is an extraordinarily bold claim.

Jill Tarter, an astronomer who worked for SETI (the Search for Extra-Terrestrial Intelligence), came up with a pretty good analogy for this, which it’s worth quoting in full:

If you build a mathematical model, the amount of searching that we’ve done in 50 years is equivalent to scooping one 8-ounce glass out of the Earth’s oceans, looking and seeing if you caught a fish. No, no fish in that glass? Well, I don’t think you’re going to conclude that there are no fish in the ocean. You just haven’t searched very well yet. That’s where we are.

In Ad Astra, Project Lima is just one mission and it has spent far fewer than 50 years trying to detect alien signals from space. And yet somehow the people on board conclude – confidently enough to drive everyone mad – that we are entirely alone. If the aliens were watching, they’d probably suggest that we jumped the gun just a little with that conclusion.

* 1 AU, or astronomical unit, is the distance from the Sun to Earth.

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