The following question is packed full of spoilers:

In the movie "The Martian" (2015), the Astrodynamics specialist Rich Purnell

comes up with a plan to save Mark by sending the Hermes back to Mars immediately, using the existing velocity to get there sooner. The Hermes isn't actually going down there to pick him up. Rather, Mark will himself use a "MAV" to launch off from Mars to intercept the Hermes during its bypass. Rich Purnell says:

"I've done the math. It checks out."

Mitch, the Hermes flight director, really loves the plan and tries hard to get it implemented. The Hermes crew say that it "would work" and that they've "run the numbers, they check out", and that "It's a brilliant course."

Alright, so several highly intelligent people who are all quite well trained with special knowledge for this type of thing have all evaluated the plan. They do point out that it may be dangerous. But nobody, not even the plan's critics, point out the most dangerous part of all. They only get to that, and start improvising to solve it, later when the decision is already made and they have almost reached Mars again.

That part is that the MAV is only designed to reach low Mars orbit. And the Hermes, which will be picking him up, can't enter low orbit, because if they do they don't have enough fuel to escape Mars' gravitational field and get back home again. Therefore, the Hermes will only be doing a bypass, and Mark has to come much higher up to meet them outside of Mars' gravitational field. This means that he has to himself modify the MAV, removing 90% of its mass, including the nose airlock, replacing it with a tarp/canvas.

When the engineer tells the NASA people about this part, they are so shocked about it that they don't even allow him to continue. When Mark hears about it he says: "Are you f--king kidding me?"

I know that the NASA director did reject the plan. But the people I'm asking about are Rich, Mitch and the Hermes crew. Why didn't they think about this?

This part of the plan did indeed lead to great difficulty during the intercept. It's only due to extreme heroism, improvisation, innovation, courage, skill and risks taken by the characters that they actually succeed. But they had to make up all these emergency plans right then and there.

So the question is: Why do they only think of this when they are already approaching Mars? I mean, nobody even brought this up while they were actually contemplating the decision. Rich, Mitch, The Hermes crew: They would all have known very well that the MAV can only reach low orbit and that the Hermes would only be able to do a flyby. So they all "do the math", say it "checks out" and go to extreme lengths to bypass the NASA director.

It's only when they're almost at Mars that people start talking about the difference in altitude between the Hermes and the MAV, and the implications about that, etc.

At the point where they decided to go with this plan, they actually had a much safer option ready: They could have used the Chinese rocket to send a bunch of supplies to Mark. That would have made him last longer, and they could have sent a rescue mission at a later time.

Shoudn't the Hermes crew, who were risking their lives by deciding to go with this plan, have at least thought about this? When they "did the numbers," why didn't they realize that the Hermes would only be doing a flyby, not going into low orbit? They, as well as Mitch and Rich, seemed to only think about this later. They then had to improvise everything, using an extremely dangerous scheme that may seem a bit far fetched, even for science fiction. Why didn't they think about these challenges before making the decision to risk their lives returning the Hermes to Mars?

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    why is the entire question behind a spoiler block? markup the things you think need to be specifically spoilered, but this is WAY too much. – phantom42 Apr 29 '16 at 12:57
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    @phantom42 There are a lot of spoilers and I felt it would be inconvenient for readers to have to move the mouse around all the time to show multiple blocks. The way it is, they only have to do it once. The question is basically 80% spoilers. – Revetahw says Reinstate Monica Apr 29 '16 at 13:25
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    You seem to have asked the question about six times. Can't you streamline this post a little? – Lightness Races in Orbit Jun 14 '16 at 23:26

Edited to reflect new info:

By that point they had exhausted every option. The flyby was very, very fast and the sheer level of calculations necessary meant a certain amount of art and flying by the seat of the pants. But by that point, the Martian was running out of food, water, air, and options.

So yes, these things didn't occur to folks until later, but the circumstances were such that they couldn't even happen until later. Modifying the MAV, for example, took the boffins at NASA quite some time to work out, and the necessities of that alone were such that they had to plan accordingly.

In short: they exhausted every other option which would have been saner and less desperate, but as a result all of those options took time and resources which didn't pan out.

Edit: Adding relevant quote here from Chapter 16:

Mitch rubbed the back of his head. “Wow…549. That’s thirty-five sols before Watney runs out of food. That would solve everything.”

Teddy leaned forward. “Run us through it, Venkat. What would it entail?”

“Well,” Venkat began, “if they did this ‘Rich Purnell Maneuver,’ they’d start accelerating right away, to preserve their velocity and gain even more. They wouldn’t intercept Earth at all, but would come close enough to use a gravity assist to adjust course. Around that time, they’d pick up a resupply probe with provisions for the extended trip.

“After that, they’d be on an accelerating orbit toward Mars, arriving on Sol 549. Like I said, it’s a Mary flyby. This isn’t anything like a normal Ares mission. They’ll be going too fast to fall into orbit. The rest of the maneuver takes them back to Earth. They’d be home two hundred and eleven days after the flyby.”

“What good is a flyby?” Bruce asked. “They don’t have any way to get Watney off the surface.”

“Yeah…,” Venkat said. “Now for the unpleasant part: Watney would have to get to the Ares 4 MAV.”

A little bit later in the spaceship:

“Would this really work?” Martinez asked.

“Ja.” Vogel nodded. “I ran the numbers. They all check out. It is brilliant course. Amazing.”

“How would he get off Mars?” Martinez asked.

Lewis leaned forward. “There was more in the message,” she began. “We’d have to pick up a supply near Earth, and he’d have to get to Ares 4’s MAV.”

“Why all the cloak and dagger?” Beck asked.

“According to the message,” Lewis explained, “NASA rejected the idea. They’d rather take a big risk on Watney than a small risk on all of us. Whoever snuck it into Vogel’s e-mail obviously disagreed.”

“So,” Martinez said, “we’re talking about going directly against NASA’s decision?”

“Yes,” Lewis confirmed, “that’s exactly what we’re talking about. If we go through with the maneuver, they’ll have to send the supply ship or we’ll die. We have the opportunity to force their hand.”

Later, just After Mark arrives at the MAV:

“So is it ready?” Venkat asked.

“Yes, it’s ready. But you’re not going to like it.”

“Go on.”

Bruce steeled himself and stood, picking up his briefcase. He pulled a booklet from it. “Bear in mind, this is the end result of thousands of hours of work, testing, and lateral thinking by all the best guys at JPL.”

“I’m sure it was hard to trim down a ship that’s already designed to be as light as possible,” Venkat said.

Bruce slid the booklet across the desk to Venkat. “The problem is the intercept velocity. The MAV is designed to get to low Mars orbit, which only requires 4.1 kps. But the Hermes flyby will be at 5.8 kps.”

Venkat flipped through the pages. “Care to summarize?”

“First, we’re going to add fuel. The MAV makes its own fuel from the Martian atmosphere, but it’s limited by how much hydrogen it has. It brought enough to make 19,397 kilograms of fuel, as it was designed to do. If we can give it more hydrogen, it can make more.” ... “This is getting really risky, Bruce.”

Bruce sighed. “I know. There’s just no other way. And I’m not even to the nasty stuff yet.”

Venkat rubbed his forehead. “By all means, tell me the nasty stuff.”

“We’ll remove the nose airlock, the windows, and Hull Panel Nineteen.”

Venkat blinked. “You’re taking the front of the ship off?”

“Sure,” Bruce said. “The nose airlock alone is four hundred kilograms. The windows are pretty damn heavy, too. And they’re connected by Hull Panel Nineteen, so may as well take that, too.”

“So he’s going to launch with a big hole in the front of the ship?”

“We’ll have him cover it with Hab canvas.”

“Hab canvas? For a launch to orbit!?”

Bruce shrugged. “The hull’s mostly there to keep the air in. Mars’s atmosphere is so thin you don’t need a lot of streamlining. By the time the ship’s going fast enough for air resistance to matter, it’ll be high enough that there’s practically no air. We’ve run all the simulations. Should be good.”

“You’re sending him to space under a tarp.”

“Pretty much, yeah.”

“Like a hastily loaded pickup truck.”

“Yeah. Can I go on?”

“Sure, can’t wait.” ... Mark reacts as you might imagine

>[08:41] MAV: You fucking kidding me?

>[09:55] HOUSTON: Admittedly, they are very invasive modifications, but they have to be done. The procedure doc we sent has instructions for carrying out each of these steps with tools you have on hand. Also, you’ll need to start electrolyzing water to get the hydrogen for the fuel plant. We’ll send you procedures for that shortly.

>[09:09] MAV: You’re sending me into space in a convertible.

[09:24] HOUSTON: There will be Hab canvas covering the holes. It will provide enough aerodynamics in Mars’s atmosphere.

[09:38] MAV: So it’s a ragtop. Much better.

The crew, meanwhile, has been practicing their maneuvers using the simulators, which haven't been going well:

Beck held a coil of metal wire in one hand and a pair of work gloves in the other. “Heya, Commander. What’s up?”

“I’d like to know your plan for recovering Mark.”

“Easy enough if the intercept is good,” Beck said. “I just finished attaching all the tethers we have into one long line. It’s two hundred and fourteen meters long. I’ll have the MMU pack on, so moving around will be easy. I can get going up to around ten meters per second safely. Any more, and I risk breaking the tether if I can’t stop in time.”

“Once you get to Mark, how fast a relative velocity can you handle?”

“I can grab the MAV easily at five meters per second. Ten meters per second is kind of like jumping onto a moving train. Anything more than that and I might miss.”

“So, including the MMU safe speed, we need to get the ship within twenty meters per second of his velocity.”

“And the intercept has to be within two hundred and fourteen meters,” Beck said. “Pretty narrow margin of error.”

“We’ve got a lot of leeway,” Lewis said. “The launch will be fifty-two minutes before the intercept, and it takes twelve minutes. As soon as Mark’s S2 engine cuts out, we’ll know our intercept point and velocity. If we don’t like it, we’ll have forty minutes to correct. Our engine’s two millimeters per second may not seem like much, but in forty minutes it can move us up to 5.7 kilometers.”

As it approaches, they discuss it more and more but ultimately there's not much more they can do. Then:

Directly in his field of view, the Hab canvas patch flapped violently as the ship exponentially gained speed. Concentration became difficult, but something in the back of his mind told him that flapping was bad.

The remotely controlled MAV begins to have difficulty being controlled because of the loose canvas.

“VELOCITY SEVEN hundred and forty-one meters per second,” Johanssen called out. “Altitude thirteen hundred and fifty meters.”

“Copy,” Martinez said.

“That’s low,” Lewis said. “Too low.”

“I know,” Martinez said. “It’s sluggish; fighting me. What the fuck is going on?”

“Velocity eight hundred and fifty, altitude eighteen hundred and forty-three,” Johanssen said.

“I’m not getting the power I need!” Martinez said.

“Engine power at a hundred percent,” Johanssen said.

“I’m telling you it’s sluggish,” Martinez insisted.

The canvas finally rips away and the controls stabilize.

“I’M GETTING more response now,” Martinez said.

“Back on track with full acceleration,” Johanssen said. “Must have been drag. MAV’s out of the atmosphere now.”

“It was like flying a cow,” Martinez grumbled, his hands racing over his controls.

“Can you get him up?” Lewis asked.

“He’ll get to orbit,” Johanssen said, “but the intercept course may be compromised.”

“Get him up first,” Lewis said. “Then we’ll worry about intercept.”

“Copy. Main engine cutoff in fifteen seconds.”

“Totally smooth now,” Martinez said. “It’s not fighting me at all anymore.”

“Well below target altitude,” Johanssen said. “Velocity is good.”

“How far below?” Lewis said.

“Can’t say for sure,” Johanssen said. “All I have is accelerometer data. We’ll need radar pings at intervals to work out his true final orbit.” ...

“We’re working on getting you,” Lewis said. “There was a complication in the launch.”

“Yeah,” Watney said, looking out the hole in the ship. “The canvas didn’t hold. I think it ripped early in the ascent.”

So in the book at least, this was discussed back at NASA at the beginning. They knew it was a Hail Mary Pass as I understand the term. They don't super work out the details so much as "This is how it's going to have to happen, lets get them moving in the direction they need to go and in the meantime we can figure out how we're going to get this to work."

In the end with all the MAV modifications, the tarp tore loose screwing up the aerodynamics significantly such that it wasn't able to attain the velocity necessary to get to the higher orbit that was necessary.

Going to stop adding more quotes for fear of copyright issues at this point :/

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    In the film, the fact that the tarp blew away seems to have resulted in a lower acceleration factor, resulting in a slightly lower orbit. – Valorum Apr 29 '16 at 11:07
  • @Richard Exactly, so the plan was extraordinarily difficult in the first place. It took a miracle to even give them a chance. That's the point of my question. Didn't they think about this extremely difficult altitude thing when they evaluated the plan, and favored it over sending Mark supplies with the Chinese rocket? – Revetahw says Reinstate Monica Apr 29 '16 at 11:26
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    @Richard as memory serves, in the book it's the same- the tarp screwed up the acceleration so he didn't end up as high in orbit as he should have, so things needed to be changed on the fly. – Broklynite Apr 29 '16 at 11:42
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    @Broklynite "the tarp screwed up the acceleration so he didn't end up as high in orbit as he should have, so things needed to be changed on the fly." I think that should either be made prominent in our answer, or become the core of your answer. It says to me, "without the problem of the tarp, he'd have been much closer (in both distance and vector) to where they were planning on meeting him." I'll have to watch it again. Or maybe read the book. :P – Andrew Thompson Apr 29 '16 at 12:06
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    @Fiksdal we didnt see everything going on in these peoples lives, only snapshots of moments - its useful to remember that. There is no "aha!" moment for this issue either, its just introduced as another issue to overcome later on in the story. Go and read the Apollo 13 accounts - its full of people saying "we got over this immediate issue and worried about something three steps away only when we actually got to that step". Solve an issue. Is the crew still alive? Yes? Ok, solve the next issue. – Moo Apr 29 '16 at 12:07

The book and movie make it clear that the "safer" option - the one that Teddy("I'm the director of NASA") is for - is so chancy that nobody actually believes it will work. (Crash landing the food container on Mars from orbit? Leaving nothing but protein-flecked steel splinters all over the landscape...) But when they're working on it, it is the only thing they've got, until the steely-eyed missile man Purnell comes up with his trajectory.

At that point they know it will be tough to get Mark Watney to orbit. But this is where the reputation of JPL carries things along: Yeah, right now we don't know how to get Mark to the fly-by altitude at the right speed, but by the time the Hermes gets to Mars those genius rocket scientists at JPL will figure it out!

And look, in the book/movie alone, not even considering their past (historical) deeds - the JPL guys have pretty much come through every time. Getting the comm link to the Pathfinder - last heard from 38 years ago - working! Getting the comm link through the Pathfinder to the Rover working! Getting the first supply mission built in record time (*)! Getting the second supply mission built in even more record time! They've got a can-do attitude and lots of coffee and they're on a roll! Of course they'll figure out how to get Mark to orbit! What's your problem, anyway?

(*) Yeah, they screwed up a bit by not figuring out that the protein bars in the first supply mission would liquefy under vibration + change of Gs, but that's just a minor thing. Except for the total mission failure, of course. But I dare you to do better.

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  • Yes, I agree that the "safer" option also seems sketchy. Since these are highly trained professionals, I agree that it may be reasonable to guess that they should have thought about the altitude problem, yet still concluded that it was better. Btw, Mitch is the Hermes flight director. The NASA director is called Teddy. Can you provide any quotes from the movie or book that show that anyone thought about the altitude diffrrence before the crew made the decision? Also, the MAV didn't just need to go into orbit (it was already designed for that.) it needed to leave Mars' gravitational field. – Revetahw says Reinstate Monica Apr 30 '16 at 4:27
  • @Fiksdal - fixed the Teddy problem, how could I have messed that up? And I understand what you're saying ... but I don't consider it as critical as you. Mainly because of two things: one, I believe they knew that was an issue but believed it solvable and they'd solve it in due course, but they needed to act now to get things going, and two, dramatic purposes in the book/movie means that not everything is spelled out in timeline order in order to build suspense, etc. (I know that second answer is unsatisfying to those of us here who really get a kick out of in-universe answers.) – davidbak Apr 30 '16 at 4:41
  • Alright. So we can speculate that they had thought about this, but we don't have anything concrete in the book/movie to back that up. It's alright. If they considered the current, Chinese supply rocket plan, extremely unlikely to work. Remember that the Hermes crew sacrificed the current plan when they went for the Rich plan. – Revetahw says Reinstate Monica Apr 30 '16 at 6:41

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