20

Why didn't the Hermes just drop the resupply payload (after taking out what they needed to survive, of course) onto Mars somewhere near the Ares III site? This would've been safer for Watney and would've saved the Ares IV MAV.

  • 3
    There isnt enough backing evidence for this in canon material, but my immediate answer to this was simply there wasnt enough capacity on the resupply mission to both resupply the Hermes for its 18 month additional trip and supplying Watney with everything he would need until Hermes could return to earth, get resupplied again and get back to Mars to pick him up safely. This was an either-or situation in every sense. – Moo Feb 9 '17 at 15:52
25

They didn't have anything that could land on Mars. There was no way for them to drop off supplies to the surface.

The resupply probe they got from their Earth fly-by. The original plan was they were going to build it to crash land on Mars, but since the Hermes crew tipped their hand, they didn't have time to make it crash proof because now it just needed to dock with Hermes near the Earth.

The resupply probe attached to one of the airlocks.

enter image description here

When it arrives at Mars, the probe isn't still there. So they must have dumped it after getting the supplies to reduce their mass to make their journey to Mars a little lighter / faster.

Here you can clearly see the two airlocks with no probe attached to either one (it would have been the lower one in this picture).

enter image description here

Additionally, had they just resupplied Watney, it would have meant him having to spend another two years or more waiting for Hermes to return to Earth and the bring back the Aries 4 crew.

  • They had the option of dropping a supply module to the surface. – Valorum Feb 8 '17 at 14:52
  • 4
    The supply module that docked with Hermes wasn't built to land on a planet. It would have had to have been crashed into Mars, hoping it was close enough to him and that the supplies weren't destroyed. – Jack B Nimble Feb 8 '17 at 14:58
  • 2
    @Valorum Or... Crashing capability – user31178 Feb 8 '17 at 15:17
  • 1
    Except Hermes decision made them have to speed it up. No time for crash proofing – Jack B Nimble Feb 8 '17 at 15:17
  • 2
    @CreationEdge - A crash is certainly a kind of landing. – Valorum Feb 8 '17 at 15:26
18

This was addressed more fully in the film's source novel. In short, the bigwigs at NASA wanted to go with your idea, dropping the supply payload near Watney and hoping that he's able to survive until the next ship gets there on schedule.

“I see,” Teddy said. “So we have two options on the table: Send Watney enough food to last until Ares 4, or send Hermes back to get him right now. Both plans require the Taiyang Shen, so we can only do one.”
“Yes,” Venkat said. “We’ll have to pick one.”

...

“Consider degree of risk, Teddy,” Venkat said. “Mitch is right. The crash-lander is high-risk. It could miss Mars, it could reenter wrong and burn up, it could crash too hard and destroy the food…We estimate a thirty percent chance of success.”
“A near-Earth rendezvous with Hermes is more doable?” Teddy asked. “Much more doable,” Venkat confirmed. “With sub-second transmission delays, we can control the probe directly from Earth rather than rely on automated systems. When the time comes to dock, Major Martinez can pilot it remotely from Hermes with no transmission delay at all. And Hermes has a human crew, able to overcome any hiccups that may happen. And we don’t have to do a reentry; the supplies don’t have to survive a three-hundred-meters-per-second impact.”

NASA Administrator Teddy Sanders personally signed off on the "Iris 2 plan" (the supply drop option).

“It wasn’t an easy decision,” Teddy said to the assembled elite. “But I’ve decided to go with Iris 2. No Rich Purnell Maneuver.”
Mitch slammed his fist on the table.
“We’ll do all we can to make it work,” Bruce said.
“If it’s not too much to ask,” Venkat began, “what made up your mind?”
Teddy sighed. “It’s a matter of risk,” he said. “Iris 2 only risks one life. Rich Purnell risks all six of them. I know Rich Purnell is more likely to work, but I don’t think it’s six times more likely.”

But when it came down to it, the crew made the decision for them. They were willing to risk their lives to save Watney.

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.”
“Are we going to do it?” Johanssen asked.
They all looked to Lewis.

...

“If we do this,” Vogel said, “it would be over one thousand days of space. This is enough space for a life. I do not need to return.”
“Sounds like Vogel’s in,” Martinez grinned. “Me, too, obviously.”
“Let’s do it,” Beck said.
“If you think it’ll work,” Johanssen said to Lewis, “I trust you.”

  • 5
    He is asking why didn't the Hermes just dump the supply module they picked up from Earth on Mars, instead of having Watney cannibalize the Mav IV. He is not asking why the Hermes went instead of the resupply probe. – Jack B Nimble Feb 8 '17 at 15:00
  • Of course, Teddy's math is wrong - the additional risk to the Hermes crew is not the same as the risk to Watney - it is actually less than the risk the Ares IV crew will be assuming as there is no Mars landing. – Dale M Feb 9 '17 at 0:52
  • 1
    @DaleM - I think his maths is pretty sound. If they go for the capture, there's a solid chance they'll all die. If they go for the drop, there's a reasonable probability that Watney (and only Watney) will die. – Valorum Feb 9 '17 at 1:14
  • As @JackBNimble stated, I was asking why they didn't send the supply probe with Hermes on the RPM. Getting the supplies there more than a month earlier than he'd need, which would save the Ares IV MAV – Benjamin Feb 9 '17 at 17:35
  • @Jason - That isn't what you asked. – Valorum Feb 9 '17 at 17:40
12

They can't just drop the supply probe on the planet. There isn't enough delta-V. So the Hermes is swinging past Earth and already picking up speed (they're not in orbit). That's why they need such a huge booster just to get it up to speed with the Hermes. The Hermes is still accelerating and flies past Mars at an incredible speed (actually 5.8 km/s according to this answer).

That's why they had to lighten the MAV to get Watney to rendezvous with the Hermes. The MAV was designed to get you to low Mars orbit, which was "only" 4.1 km/s. You'd need to impart the full 5.8 km/s to get the supply probe to slow down enough to land (even crash landing) on Mars. If you had the probe hit Mars at 5.8 km/s there won't be anything left but molten rock mixed with melted protein bars. You still have to decelerate to a speed where the payload will be relatively intact (I'm thinking less than 0.1 km/s).

Mars' atmosphere isn't thick enough to slow down all the way, even with parachutes. Those supplies had to supply the entire Hermes mission, which means it was heavier than the probe you could have sent laden with food to Watney directly, which is why there would be enough weight left over to include a descent module to land it on Mars.

That brings us back to the choice in the movie: they can either try sending another probe directly to Watney so he can wait until Ares 4, or they can supply Hermes with enough to go get him, but they can't do both.

  • I like the point about delta-v. The probe doesn't have thrusters, and they can't use Hermes' thrusters either. +1 from me – Gallifreyan Feb 8 '17 at 19:28
  • 2
    This is probably the best answer. "Dropping" an object from orbit is a misnomer; if you let go of a wrench outside the ISS, it doesn't move away from you towards the surface. (More precisely, it does fall towards the surface, but since you're falling towards the surface too, it moves in a parallel orbit to you.) To actually get an object from orbit onto the surface, you have to slow it down. – Michael Seifert Feb 8 '17 at 19:39
  • @MichaelSeifert - only if you're in orbit. Prior to the flyby, when you were about half way between Earth and Mars you could give the object a tiny nudge and put it on a trajectory that would impact Mars (without slowing down). That's what I'm talking about in this answer. You could also try to get it to graze the atmosphere just enough to aerobrake but Mars' atmosphere likely isn't thick enough. I'm not an expert though, I just play one in Kerbal Space Program. :) – Scott Whitlock Feb 9 '17 at 17:08
6

The real question is, Why would they risk Watney's life for another two years? ...when they've already chosen option (b).

Option (a): send the probe on a fly-by trajectory towards Mars, use aerobraking and what little fuel it has to be captured, and eventually impact the surface without 1) destroying its cargo, and 2) landing more than 60 km away from Watney's position, and hope that Mark can survive another two years without 3) another airlock tearing open, 4) his water-reclaimer breaking (= dead), 5) his oxygenator breaking (= dead), 6) his hab, designed for a 31-day mission and already greatly overused, ripping open in some other location besides the airlocks (= most likely dead), and finally 7) Mark losing sanity from isolation for 2 additional years.

(Spolier:)

Remember at this point, Mark has accidentally fried Pathfinder so he once again has no way to talk to NASA. He modified the drill he was using to cut holes in the rover from using batteries to running off hab power from a cable, and leaned it up against Pathfinder with exposed wires... oops. Hab power has 6 amps, Pathfinder was only built for 0.6 amps. At this point, he had to get to the Ares IV MAV just to make contact again. In the book, Mark's great sense of humor is what keeps him sane most of the time, but two years is a serious test on anyone's sanity.

Option (b): send the Hermes on a fly-by trajectory to Earth for gravity assist to a fly-by trajectory to Mars, hoping that 1) the resupply from the Taiyang Shen is successful, 2) Mark can modify the MAV to be light enough to reach Mars' escape velocity and rendezvous with the Hermes, and 3) the Hermes can survive deep space for another 2 years past its maintenance window.

Edit: (Spolier:)

Edit: Mark still encountered plenty of difficulty trying to travel from Acidalia Planitia to Schiaparelli crater to meet his deadline of arriving at the Ares IV MAV days before the Hermes fly-by, and plenty of events threatened this: Global dust storms leaving him unable to recharge the rover for weeks at a time and missing the Hermes fly-by, rolling the rover & trailer on his way down the Schiaparelli crater, and an existential crisis during his travels of realizing he really was all alone on an entire planet by himself. This should have properly been added as risk b-2)

So essentially, you question is "why, when the Hermes was already on its way to Mars, did it not just drop off the care package and leave Mark for another 2 years?" "This would've been safer for Watney and would've saved the Ares IV MAV"

Let's call this option (c), and you will see very soon here that all we're doing is stacking the worst risks from both options (High probability of killing 1 person vs. low probability of killing 6 people) with the only real payoff being: We get to keep the Ares IV MAV.

Option (c): send the Hermes on a fly-by trajectory to Earth for gravity assist to a fly-by trajectory to Mars, hoping that b-1) the resupply from the Taiyang Shen is successful, b-3) the Hermes can survive deep space for another 2 years past its maintenance window, release the probe on approach at escape velocity towards Mars, use aerobraking and what little fuel it has to be captured, and eventually impact the surface without a-1) destroying its cargo, and a-2) landing more than 60 km away from Watney's position, and hope that Mark can survive another two years without a-3) another airlock tearing open, a-4) his water-reclaimer breaking (= dead), a-5) his oxygenator breaking (= dead), a-6) his hab, designed for a 31-day mission and already greatly overused, ripping open in some other location besides the airlocks (= most likely dead), and finally a-7) Mark losing sanity from isolation for 2 additional years.

  • 1
    There is also the added benefit of Mark not having to use the rover from the Ares III site to the Ares IV site, during which anything could've happened, including maybe a dust storm that nearly strands him in a crater. It would seem to be relatively (albeit not realistically) easy to get something from orbit into a location in the crater which Ares III landed in, which would be easy for Watney to drive to, taking only slightly more time than the trip to Pathfinder. – Benjamin Feb 9 '17 at 17:28
  • @Jason That is true, Watney encountered a global dust storm like we have observed from Mars orbit that limited his ability to recharge the rover, potentially leaving him stranded and missing the Hermes flyby. He did traverse the rim of a crater ~50km opposite directions and set solar panels to measure which way the storm was heading. Should've listed that as another risk under option (B): Watney making it from Acidalia Planitia to Schiaparelli crater alive. Also keep in mind that landing from orbit (3.3 km/s) vs. from Mars escape velocity (>5 km/s) is quite a difficulty difference. – IT Bear Feb 10 '17 at 20:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.