Nitpick "The Martian"

[KerikBalm]

So... "The Martian" is being made into a movie... the book is mostly hard Sci-Fi... but lets try and pick apart the physics and realism, to find where allowances were made for the purpose of the plot.

So far, I've got 3 things:

#1) The biggest one, the reason he is there... a storm on mars, throwing things around wth huge force, nearly tipping over the ship...

Sure, winds may reach 400km/h on Mars... but the air density is less than 1% of that on Earth.

The force of the wind is proportional to V^2. So 1/100th the air density means a force equivalent to 1/10th the velocity... that storm would have forces equal to about 40km/hour winds on Earth.... about 25 mph winds...

Gusty... but nothing catastrophic that is going to blow over a sitting rocket, and send stuff flying with enough force to impale a person

#2) Perchlorates in the soil would likely cause lots of problems when trying to grow stuff

#3) It shouldn't have been so hard to get water... it seems there is water ice not very far below the surface (as in... centimeters) in many places

Anyone else got something to nitpick about the science of the novel?

[Dodgey]

Firstly there is already a sizeable thread for this. Secondly the author Andy Weir had

has said that he knows about the pressure on Mars and it was a deliberate decision for the plot.

I don't know much about perchlorates however it is sounds like it might be another deliberate decision for the plot.

Once again not sure about the water.

[Bill Phil]

Things in space tend to be sensitive. Like the LM, dropping a screwdriver could go through the hull. Maybe those winds could take out a HAB.

He did add a whole bunch of stuff to the soil... I don't know what perchlorates could do, but it might not be that bad.

As for water, you need the equipment to extract it and purify it. I'm sure they didn't have any equipment there to do so.

Personally I think it's the fact that Watney survived at all. He got hit by a dish, going through him at high speed. He even comments in the book about how he should be dead. That's the biggest one to me.

[Kerbart]

#1 I agree on. Especially since later in the book the effects of a storm are said to be mellow "because of the low air pressure" (I'm paraphrasing and this is from what I remember, so I might be wrong, but I know it irked me).

#2 Yes and no -- I'm pretty sure the discovery of perchlorates is very recent and wasn't known when Andy Weir wrote the book. Of course, it kills the plot as it is so it's not like he can retroactively fix that (unless there's a way to clean the soil)

#3 Water ice is found in some places, but not everywhere. So you can defend that by stating that it's not around where he is. Besides, you'll need to melt the ice (although the RTG can take care of that)

[AngelLestat]

that storm would have forces equal to about 40km/hour winds on Earth

Now the question is how much 40km/h wind force effect on 1/3gee, that will be equivalent to bodies with much lower density here on earth.

Also the wind is 400km/h, all dust or projectile travels at that speed, which is something to have into consideration with respect to damage.

[KerikBalm]

#2 Yes and no -- I'm pretty sure the discovery of perchlorates is very recent and wasn't known when Andy Weir wrote the book. Of course, it kills the plot as it is so it's not like he can retroactively fix that (unless there's a way to clean the soil)

I checked,

https://en.wikipedia.org/wiki/Perchlorate#Perchlorate_on_Mars

Perchlorates first found on Mars in 2008, by the Phoenix lander.... more were found in 2012+ by the Curiosity rover.

The Martian was published in 2011... although maybe he started writing before it was known...

Now the question is how much 40km/h wind force effect on 1/3gee

Well mars gravity is 0.376 Gs... assuming a proportional relationship between gravity strength and when the wind can knock something over.... 1/0.376 = 2.66x as easy to knock over

sqrt 2.66 * 40km/h = 65 km/h winds.... or 40.5 mph still not very extreme.

all dust or projectile travels at that speed,

Yea, the kinetic energy of the small dust grains could get pretty high... but there's still f=MA... and a peice of equipment will need to accelerate once it comes off the surface... (if it even does come off in the equivalent force of 25 mph wind - things mechanically secured to the ground, as opposed to resting on top of the ground, won't be much affected by the lower gravity)

Things in space tend to be sensitive. Like the LM, dropping a screwdriver could go through the hull

It wasn't the hab they were worried about, it was the ascent vehicle... which would have head to deal with a lot more force than that during ascent (not to mention when it had to land)

As for water, you need the equipment to extract it and purify it

Not in the context of the book where he was just burning fuel inside the hab to increase the humidity, so that he could grow plants... melted martian ice from the soil (he was digging up lots of it) should have provided significant amounts.

Firstly there is already a sizeable thread for this

That seems to be about the movie, casting, depiction of stuff in the movie.

I mentioned the movie in the first post only as a reason to bring up this subject about the *science* of the book, which I don't see being discussed in that thread (though it is 16+ pages, maybe the subject was brought up somewhere in there)

Alternate stranding idea: meteor shower.

Someone reminscent of that comet that flew near mars, and they altered orbit of the orbiting spacecraft to minimize the risk of getting hit by debris.

Mars doesn't have much of an atmosphere, a meteor shower could pose a real threat. Order an evacuation, have him fall down a lava-tube (or perhaps have a meteor fall near him, and they think he's dead)

Plus it would make for some great flashy fx that hollywood loves.

Basically, this.... except more of them, each a bit smaller:

[SomeGuy12]

The Hermes is described as having a reactor compartment. For a nuclear-electric spacecraft with that kind of performance, you have to have the reactor out on a long pole away from the ship and use a shadow shield.

Also, the Hermes is described as using a lattice of cooling fins for the reactor. This will not work in vacuum at all - I emailed the Andy Weir and he admitted it wouldn't work, either. Basically only big flat plates where both faces can see the blackness of space make for workable radiators. A latticework, the fins will radiate heat to each other with little net loss.

Frankly, these limitations are fixable. He could have thought of another emergency that would force the crew to leave. It could have been as simple as some kind of problem with the ascent vehicle where they will not be able to leave if they didn't leave immediately. Unlike space Opera, the 5 or 10 significant science errors in the entire book could have been edited out as the core of the story is dead on correct.

[Albert VDS]

2. Perchlorates are easy to wash out of the soil.

From Andy Weir's AMA on Reddit:

1. About the wind:

Yes, that was a deliberate inaccuracy I made for dramatic purposes. I wanted "nature" to be responsible for this man-vs-nature story.

3. About water in the soil:

Well, we've learned that there is a bunch more water trapped in Martian soil. The Curiosity probe found that out after The Martian was already complete and for sale. Though I really liked the water-making scenes, so I would probably just add a bit that says "Acidalia Planitia is a desert, so it doesn't have nearly the soil water content as other parts of Mars" or something like that.

[sp1989]

Distilling liquid hydrogen is highly exothemric and he would have melted the hab, himself, and probably the rovers outside, from all the heat.

http://forum.kerbalspaceprogram.com/threads/116406-The-Martian-by-Andy-Weir

Edited June 26, 2015 by sp1989

[justidutch]

There was something in the book that I just could not wrap my head around, I found it completely unbelievable. I almost quit reading and threw the book in the trash…

he doesn't like disco?

Disco is awesome!

[andrewas]

Distilling liquid hydrogen is highly exothemric and he would have melted the hab, himself, and probably the rovers outside, from all the heat.

If I'm doing the math right, he's putting out 8.34Kw of heat on average. (Assuming he produces 47L of water per Sol, which is the rate he mentions after recovering from the explosion) Thats a lot, particularly since the peak output will be much higher, but its not unmanageable, particularly since he can halt production if the temperature gets out of hand, and sleep in the rover if the hab doesn't cool in time.

[StrandedonEarth]

When NASA said he needed to make more fuel for the MAV to reach Hermes, where does it go? Why would there be extra unused tankage on the MAV?

[peadar1987]

When NASA said he needed to make more fuel for the MAV to reach Hermes, where does it go? Why would there be extra unused tankage on the MAV?

The first part of the burn could be done with the descent stage, which would be nearly empty after landing and need to be refilled. If he was just using the ascent stage, he'd never be able to catch Hermes on an interplanetary trajectory, no matter how much mass he shaved off

edit: Wait, the descent stage was hydrazine-fuelled and the ascent stage methane I seem to recall. This raises the question of how he was supposed to rendezvous with Hermes at all, unless the MAV is seriously overdesigned.

edit edit: The MAV is empty on landing, to save mass, and makes its own fuel from the Martian atmosphere. It wasn't expecting to need to be full for another 3 years. When Watney turned up he needed to accelerate the process to make the rendezvous.

Edited June 26, 2015 by peadar1987

[AngelLestat]

Well mars gravity is 0.376 Gs... assuming a proportional relationship between gravity strength and when the wind can knock something over.... 1/0.376 = 2.66x as easy to knock over

sqrt 2.66 * 40km/h = 65 km/h winds.... or 40.5 mph still not very extreme.

Ok, that is close to my mental estimation, if fact I was imaging something as 80km/h, but I dint understand your math, why do you do the sqrt of 2.66, what does it means?

Yea, the kinetic energy of the small dust grains could get pretty high... but there's still f=MA... and a peice of equipment will need to accelerate once it comes off the surface... (if it even does come off in the equivalent force of 25 mph wind - things mechanically secured to the ground, as opposed to resting on top of the ground, won't be much affected by the lower gravity)

First let me notice that I dint read the book and I dont remember the trailer, just that it has some interstellar cast on it.

I was just pointing that the OP review was not complete.

About how much it can accelerate it all depends on its density. Also remember that is low G, and lightweight dust can help to transfer momentum to other bodies.

Time out.. I will see the trailer again so I can understand what are you talking about..

.............

hey.. it does not look so bad..

https://youtu.be/lQqhfq87FgY?t=39s

It show that the wind is super fast, but they walk without problem, the problem comes when matt damon is hitted by another object body (which may have lower density).

Seems accurate considering that is a movie.

[KerikBalm]

Distilling liquid hydrogen is highly exothemric and he would have melted the hab, himself, and probably the rovers outside, from all the heat.

http://forum.kerbalspaceprogram.com/threads/116406-The-Martian-by-Andy-Weir

Don't you mean burning LH2? IIRC, he had to burn a little at a time

[andrewas]

The first part of the burn could be done with the descent stage, which would be nearly empty after landing and need to be refilled. If he was just using the ascent stage, he'd never be able to catch Hermes on an interplanetary trajectory, no matter how much mass he shaved off

edit: Wait, the descent stage was hydrazine-fuelled and the ascent stage methane I seem to recall. This raises the question of how he was supposed to rendezvous with Hermes at all, unless the MAV is seriously overdesigned.

edit edit: The MAV is empty on landing, to save mass, and makes its own fuel from the Martian atmosphere. It wasn't expecting to need to be full for another 3 years. When Watney turned up he needed to accelerate the process to make the rendezvous.

The MDV was hydrazine powered, I can't find any mention of a Hydrazine engine on the MAV. It would make sense to have both stages use Metholox engines, that way you can re-use the descent stage as the first stage on the way up. There shouldn't be any excess tankage on either stage, but methane and LOX are fairly easy to store, so perhaps Watney used tanks from the Rover or MAV life support systems.

[KerikBalm]

Ok, that is close to my mental estimation, if fact I was imaging something as 80km/h, but I dint understand your math, why do you do the sqrt of 2.66, what does it means?

If the force must be 2.66x more, and the force is proportional to V^2, then V= sqrt 2.66

Also remember that is low G, and lightweight dust can help to transfer momentum to other bodies.

Low G has nothing to do with inertia.

hey.. it does not look so bad..

https://youtu.be/lQqhfq87FgY?t=39s

It show that the wind is super fast, but they walk without problem, the problem comes when matt damon is hitted by another object body (which may have lower density).

As you say... it doesn't look so bad (although why that object is flying around... when the force of the wind in only = to 25mph winds on earth... that's the problem)... but if its not so bad, do they really need to abort a 6 month mission during its first days because of it?

In the books, their ascent rocket was starting to tip over due to the wind, and they had to use RCS to stop it from tipping, and in the end they had no choice but to fire the main engines and "get out of dodge"

I just don't see a 25mpg wind tipping over a rocket with a dV capability of a few thousand m/s

Also... I can't find any solid reference, but I seem to recall those great wind speeds only happen at higher lattitudes.

This raises the question of how he was supposed to rendezvous with Hermes at all, unless the MAV is seriously overdesigned.

Well, they did say that he had to make extinsive modifications to it... throwing out a lot of systems, removing bulkheads etc, to reduce its dry weight... but there did seem to be some handwaving here... they didn't bring out the rocket equation, how much more dV he needed, and how much mass he'd have to remove (I suspect as a %, it would be quite unreasonable... so the author didn't go into detail there)

Edited June 26, 2015 by KerikBalm

[peadar1987]

Well, they did say that he had to make extinsive modifications to it... throwing out a lot of systems, removing bulkheads etc, to reduce its dry weight... but there did seem to be some handwaving here... they didn't bring out the rocket equation, how much more dV he needed, and how much mass he'd have to remove (I suspect as a %, it would be quite unreasonable... so the author didn't go into detail there)

Delta-V to Low Mars Orbit is 3.8 km/s, Delta-V for a Mars-Earth transfer is 2.1 km/s, so you'd need to reduce the dry mass of the craft by 55% by my calculations. That's probably not actually impossible, although it's certainly extremely difficult

[KerikBalm]

what Isp are you using for that equation? I forget what propellants the craft was supposed to use.

55% seems pretty impossible to achieve over the whole rocket, seems pretty ridiculous even for the last stage...

Of course, maybe they designed one lander for multiple sites and had some cross range capability... lets be generous and sav the MAV should normally hav...750 m/s of spare dV?

[peadar1987]

what Isp are you using for that equation? I forget what propellants the craft was supposed to use.

55% seems pretty impossible to achieve over the whole rocket, seems pretty ridiculous even for the last stage...

Of course, maybe they designed one lander for multiple sites and had some cross range capability... lets be generous and sav the MAV should normally hav...750 m/s of spare dV?

I messed up the maths anyway. ISP doesn't matter, as it's unchanged by payload fraction, but I forgot you had to take the natural log. Essentially the delta-V has to increase by 55%, so NewFraction=OldFraction^1.55. The actual weight saving needed will depend on the original payload fraction of the MAV. If it was 20%, you'd have to reduce it to 8% to get the required increase in delta-V. If it was 10%, you'd need to reduce it to 3%. Tyranny of the rocket equation and all that!

Of course, if you just refuelled the descent stage, you effectively double your delta-V.

[AngelLestat]

If the force must be 2.66x more, and the force is proportional to V^2, then V= sqrt 2.66

Why you apply 2 times the force proportion if in this case gravity and density doesn´t has nothing to do with wind speed. We already have the force that will be applied on the body (a 40km/h wind).

Also the OP takes that the martian atmosphere is 100 times less dense than earth, but nasa said it is only 61 times less dense (dont forget that is 95% co2)

Mars atmosphere at surface, density: ~0.020 kg/m3

Earth atmosphere at surface, density: 1.217 kg/m3

1.22 / 0.02 = 61

http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html

http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html

Low G has nothing to do with inertia.

But if a body does not touch the soil very often, then it does not lose inertia so often either.

As you say... it doesn't look so bad (although why that object is flying around... when the force of the wind in only = to 25mph winds on earth... that's the problem)... but if its not so bad, do they really need to abort a 6 month mission during its first days because of it?

In the books, their ascent rocket was starting to tip over due to the wind, and they had to use RCS to stop it from tipping, and in the end they had no choice but to fire the main engines and "get out of dodge"

I just don't see a 25mpg wind tipping over a rocket with a dV capability of a few thousand m/s

Not sure what are you saying, I dont know the full plot of the book or movie.

But about that object, like I said, maybe is big, not aerodynamic and light.

Like this:

Rockets here on earth doest not take off with higher winds either, even a small change on angle of attack can cause severe dynamic pressure.

Also... I can't find any solid reference, but I seem to recall those great wind speeds only happen at higher lattitudes.

Yeah, but well, we dont know so much mars as earth, in earth we can have hurricanes of 350km/h, they are very rare, but happens.

And you know how is this.. if in reality something is rare.. in movies happens all the time.

Edited June 26, 2015 by AngelLestat

[KerikBalm]

I messed up the maths anyway. ISP doesn't matter,

It sure does. The change in dry mass needed will be less the higher Isp is.

Consider a dry mass of 1.0, and a wet mass of 1.1.

ln 1.1/1 = 0.0953

Now reduce dry mass by 10%

Ln 1.0/0.9 = 0.10536

You get .10536/.0953 = 10.5% more dV

If you needed 3,000 m/s, from a wet;dry mass of 1.1, your Isp would have to be pretty high

3,000= 9.8* Isp * 0.0953 = 3211

A 10% reduction in dry mass would get you ~337 m/s more

Now if you had a much lower Isp, your wet mass may have been 10 tons, and your dry mass 2 tons.

ln 10/2 = 1.61

3,000 = 9.8* Isp * 2.30

Isp = 190 (ok, you can easily get better than this.. but anyway.

Now, if you remove 10% of the dry mass, its now 9.8/1.8

ln 9.8/1.8 = 1.695

dV difference = ln (10/1.8) / ln (10/2) = 1.053

You get 10.5% more dV if you reduce dry mass by 10% at an Isp of ~3200 (assuming original design specs were for 3km/s), but only 5.3% dV if you reduce dry mass by 10% at an Isp of ~190.

Also, it will be harder to reduce dry mass at lower Isps... because then more of the dry mass is tanks, and not things like seats and extra life support that you don't need because you're only sending 1 person, not a full crew.

Of course, if you just refuelled the descent stage, you effectively double your delta-V.

Note really, because the descent stage would have had substantial aerobraking. All it would need is a deorbit burn, and then <1,000 m/s for a retroburn because the parachutes being used are meant to be deployed at supersonic velocities.

Also, if they are using ISRU, it would likely already be in the design that the descent stage is refueled.

in this case gravity and density doesn´t has nothing to do with wind speed. We already have the force that will be applied on the body (a 40km/h wind).

Gravity has nothing to do with the force of the wind, correct. Gravity does have to do with the force required to overcome friction, or to make something tip over.

If wind could push something along or tip it over at martian gravity, then it would take 2.66x the force to do the same thing in Earth gravity.

So the comparable "Earth wind" must be stronger to tip something over or make it slide along the ground.

Thats why I'm comparing a martian wind to a stronger wind on earth

Also the OP takes that the martian atmosphere is 100 times less dense than earth, but nasa said it is only 61 times less dense (dont forget that is 95% co2)

Mars atmosphere at surface, density: ~0.020 kg/m3

Earth atmosphere at surface, density: 1.217 kg/m3

That's rather simplistic... as mars topology is more varied that Earth's. If you compare the bottom of Hellas planitis to the average, its about 2x the pressure... compare it to the top of olympus mons... whoah...

Also, atmospheric density at given pressure varies with temperature, and mars has a much larger surface temperature range than earth.

Cold air is denser than warm air. It gets *really* cold during martian nights. We should consider the density during martian days, near the equator.

Here is the crater where this takes place in the story... far from the lowest spot on mars:

Even if it was 1/61 the density... instead of 1/100 -> sqrt 100/61 = 1.28x the wind speed -> 51km/h -> <32 mph wind force.

But if a body does not touch the soil very often, then it does not lose inertia so often either.

No... but there is a very small area where there are objects to be blown around. Aside from there landing site... there is nothing else on the entire planet bigger than dust grains to get blown into them. If something 20 meters away comes loose, it won't have a lot of time to accelerate.

Rockets here on earth doest not take off with higher winds either, even a small change on angle of attack can cause severe dynamic pressure.

Even more reason for them not to fly off during a dust storm, and leave the guy behind.

if in reality something is rare.. in movies happens all the time.

So we can have another reason for the sudden abort... like a meteor shower... or him falling into a hole,

https://en.wikipedia.org/wiki/Martian_lava_tube

maybe he can fall into a tube, in the middle of a dust storm, and after the storm is over, they discover something is leaking on the MAV (may or may not even be related to the storm - a leaking seal, a line got hit by an object, or just moved around enough for a seal to become loose, or they wer doing something when the storm hit, and due to dust the thing didn't seal right... whatever).... and if they wait around too much (Oxidizer?) will have leaked, and they won't make orbit. So then they have to go then and there while the MAV can still make it... and they think the guy died anyway and because of the storm, they couldn't really tell where he disappeared.

Edited June 27, 2015 by KerikBalm

[Beowolf]

Of course, if you just refuelled the descent stage, you effectively double your delta-V.

I figured the descent stage had to be hydrazine-powered because LOX would boil off during the long trip.

[peadar1987]

It sure does. The change in dry mass needed will be less the higher Isp is.

Consider a dry mass of 1.0, and a wet mass of 1.1.

ln 1.1/1 = 0.0953

Now reduce dry mass by 10%

Ln 1.0/0.9 = 0.10536

You get .10536/.0953 = 10.5% more dV

If you needed 3,000 m/s, from a wet;dry mass of 1.1, your Isp would have to be pretty high

3,000= 9.8* Isp * 0.0953 = 3211

A 10% reduction in dry mass would get you ~337 m/s more

Now if you had a much lower Isp, your wet mass may have been 10 tons, and your dry mass 2 tons.

ln 10/2 = 1.61

3,000 = 9.8* Isp * 2.30

Isp = 190 (ok, you can easily get better than this.. but anyway.

Now, if you remove 10% of the dry mass, its now 9.8/1.8

ln 9.8/1.8 = 1.695

dV difference = ln (10/1.8) / ln (10/2) = 1.053

You get 10.5% more dV if you reduce dry mass by 10% at an Isp of ~3200 (assuming original design specs were for 3km/s), but only 5.3% dV if you reduce dry mass by 10% at an Isp of ~190.

Also, it will be harder to reduce dry mass at lower Isps... because then more of the dry mass is tanks, and not things like seats and extra life support that you don't need because you're only sending 1 person, not a full crew.

Ah, I get you, you're thinking about it practically, I was thinking purely mathematically. Of course a greater percentage change in dry mass is going to have greater effect on payload fraction at higher ISPs.

Note really, because the descent stage would have had substantial aerobraking. All it would need is a deorbit burn, and then <1,000 m/s for a retroburn because the parachutes being used are meant to be deployed at supersonic velocities.

Also, if they are using ISRU, it would likely already be in the design that the descent stage is refueled.

Right on that one as well, turns out the Martian atmosphere is better for aerobraking than I assumed. Curiosity got down from orbital speed to 200 m/s with aerobraking and the drogue.

I figured the descent stage had to be hydrazine-powered because LOX would boil off during the long trip.

It's hydrazine, as he uses descent stage fuel to make water for his plants

[andrewas]

It's hydrazine, as he uses descent stage fuel to make water for his plants

He uses the fuel plant from the MAV to produce C02, the Hydrazine and catalyst come from the MDV.

Theres no suggestion of any part of the MAV using hydrazine. Unless - the MAV brings hydrogen with it to produce fuel, but hydrogen is worse for boiloff than LOX is. Its possible that there is a descent stage, and excess hydrazine is decomposed to produce the hydrogen the fuel plant needs. However, Mars Direct, which much of the mission in The Martian was based on, claims that hydrogen boiloff can be reduced to 1% per month using insulation and gelling the hydrogen with methane.