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The first scene of "The Martian" a storm kicks up on the surface of Mars and the crew of the Ares 3 decide they need to abandon their mission and return to the Hermes. They quickly board the MAV and launch, somehow successfully getting back to the Hermes.

It's established later that the MAV are suborbital craft, and aren't intended to reach orbit (at-least not without heavy modifications).

If the Hermes is set to orbit Mars while the crew works on the surface (assuming the Hermes is in a "sub-orbital orbit", otherwise the MAV would never reach the altitude to reach the Hermes in the first place), there seems to be a pretty good chance the Hermes is no where near where the MAV would need it to be to pick up the crew.

At-least within the context of the movie, we don't see the crew check on the position of the Hermes when they launch.

My question: What is the likely-hood the Hermes is right where it needs to be to pick up the crew. Is there any science behind the crew being able to abandon the planet at a moment's notice and be able to make it back to the Hermes safely, or would the science conclude that they could only abandon the planet in a very small window of time, when the Hermes is in prefect position to retrieve the crew?

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    areostationary orbit? en.wikipedia.org/wiki/Areostationary_orbit – kaine Oct 6 '15 at 19:17
  • @kaine Areostationary orbit would provide the science needed to allow the crew to evacuate the planet at a moment's notice (by placing the Hermes in a constant optimal position to retrieve the crew). If you where to flesh that out into a full answer and maybe provide some form of evidence that it might be the method they used in the movie, I would accept that as the answer. – onewho Oct 6 '15 at 19:48
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    I'm sorry but the reason I put this as a comment is I have don't have enough information to put is as an answer I would be satisfied posting. I thought the term I knew off hand might be useful for someone who remembers more than I do about the book but not the terminology. – kaine Oct 6 '15 at 21:14
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    Maybe it wasn't explained in the movie in enough detail but the MAV can orbit and the Hermes is normally parked in an orbit. So the MAV does not normally need to reach Hermes the first try. That's how space rendezvous are normally done. The second time around is the special case - Hermes couldn't orbit. – slebetman Oct 7 '15 at 4:27
  • @kaine Delta V from Mars surface to areostationary orbit is 5.3 km/s and that's with no gravity loss. The unmodified M.A.V. has a 4.1 km/s delta V budget. Given a 4.1 km/s M.A.V. delta V budget and making allowance for gravity loss, rendezvous would need to be in low Mars orbit. If gravity loss were zero, max altitude of a circular rendezvous orbit would be 1400 kilometers. – HopDavid Feb 29 '16 at 17:20
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The MAV was an orbital craft...it would be nearly useless if it were not. The reason why it needs to be heavily modified, and why it DOESN'T achieve orbit (even though it could) is because it has to match the insanely fast velocity of the Hermes as it does a fly-by. A little orbital mechanics, and what happened to the MAV after the rendezvous was that it escaped Mars' gravity entirely and went into a long orbit around the sun. There's a slim chance it fell into the Sun, but making that happen is harder than you think.

Much of the hard science behind the Martian and in NASA thinking in general surrounding Mars comes from a book called The Case for Mars by Robert Zubrin. While it's a policy book, it's extremely engaging.

  • I'm dying to know: why is falling in to the sun harder than one would think? – J Doe Feb 29 '16 at 22:26
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    Put simply, space is really big, and the sun, while large, is small compared to the space in between Mars and the sun. While the sun is obviously the big attractor in the solar system, the sum of possible orbits which orbit it is far greater than the sum of orbits which intersect it. – Chris B. Behrens Feb 29 '16 at 22:54
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We are talking about nominal emergency planning here - an expedition besieged by li'l green men is forced to abort-to-orbit in the MAV. The MAV ascends into a Mars orbit, and then meets up with the Hermes. Mark Watney's case (a hyperbolic rendezvous with the Hermes in Martian fly-by) is much more scary since orbital mechanics won't give him another opportunity, and won't be discussed in this answer.

This boils down to several questions:

  1. What is the inclination of the Hermes' orbital plane? Judging by the latitude of Ares 3 landing site it is not less than 31.380 degrees (equatorial). We can safely assume that it is not greater than 32 degrees (give or take several tenths due to non-spherical gravity and influence of Jupiter etc.), either, since the only purpose of the Hermes at this stage is to pick up the expedition when they come back a month later.

  2. What is the longitude of ascending node of the current orbit of the Hermes? This is the crucial item that gives us the difference between the orbital plane of the MAV and the orbital plane of the Hermes (since Mars rotates around its axis).

  3. What are the Hermes' fuel reserves to perform a plane change maneuver? This is not given in the book, but we may assume they are enough to rescue a MAV from an emergency orbit. Here's how it's done:

    • the Hermes turns on her VASIMR engines, firing prograde (in the direction of orbital motion) at a specific arc of her orbit, progressively increasing the apoapsis (highest orbital point);
    • when the Hermes is near the necessary orbital node with respect to the target orbital plane (that of the MAV), she fires the VASIMRs in normal or antinormal attitude (as required) for a while during several orbits, rotating the orbital plane closer to the target one;
    • after the plane is the same as the target's, VASIMRs are fired retrograde (against the orbital motion) in an arc near the periapsis (lowest orbital point) to circularize the orbit back.

Then, depending on how much propellant the MAV has left for the rendezvous, a rendezvous scheme is executed.

TL;DR Remote control of the Hermes must allow for an abort-to-orbit in the MAV with the Hermes doing the "heavy lifting" of orbital plane change and subsequent rendezvous maneuvers. The problem is that VASIMR engines aren't powerful enough for this to be done quickly, so MAV life support reserves must be sufficient for let's say a week of watching movies and listening to the disco (I haven't run the numerical scenario, so this is just a ballpark estimate).

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    The book mentions the training scenario involving the crew being stuck in the MAV for 6 days, which lines up with your 1 week estimate at the tail end of your answer. – user1027 Oct 6 '15 at 22:18
  • @Keen - may be just a coincidence :) I would need mass, propellant, specific impulse and thrust numbers for the MAV and the Hermes to be sure there's no mistake in ballistics. – Deer Hunter Oct 6 '15 at 22:21
  • Isn't Hermes in a parking orbit for the duration of the expedition? The Purnell maneuver is certainly designed to aim exactly at Mark's inclination. – Jacob Krall Oct 7 '15 at 0:55
  • @JacobKrall - yes, Hermes is in parking orbit while the expedition is on Mars. If the crew evacuates outside of the launch window, it falls to the Hermes to change planes. – Deer Hunter Oct 7 '15 at 4:39
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They didn't use the landing vehicle. They used the MAV (Mars Ascent Vehicle). Since this vehicle is specifically designed to launch from Mars and dock with Hermes it shouldn't have a problem doing that.

Keep in mind too that this vehicle was part of the mission abort protocol. Since using the MAV to connect with Hermes was the key to the capability of aborting a mission then NASA would have to make this work.

Also as mentioned by Keen in a comment:

In addition, the Hermes can be remote controlled. At one point in the book, Mark mentions a scenario they trained for where the MAV's second stage fails to ignite, leaving the crew in a lower orbit than the Hermes'. Mission Control would simply alter the course of the Hermes to intercept their lower orbit to pick them up.

Due to the time delay I don't think Mission Control on Earth would make this correction. A more likely candidate would be Martinez controlling Hermes from the MAV (IMO). Regardless, the ability for Hermes to adjust position to dock with the MAV is there when needed.


In a comment you stated:

Woops, that was my mistake, either way it's the same Vehicle Mark uses at the end of the movie. And is still limited to sub-orbit.

The orbital height isn't the problem when Hermes picks up Mark at the end. The problem is that Hermes has accelerated at this point to a speed where it can't orbit Mars. So the problem isn't that the MAV can't reach Hermes, the problem is that they only have one chance to recover Mark.


To back up the statements made by Chris with information from the movie, Mark mentions that the MAV will be blasted into orbit when he is sitting in the rover looking at the Ares IV MAV. That MAV will be heavily modified of course, but there is a second relevant quote. During the scene when JPL is presenting the modifications to the Ares IV MAV it is mentioned that the MAV was only designed for "low orbit." This is proof in that the MAV was able to orbit Mars without modifications.

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I haven't seen the movie yet. But I'll quote a paragraph from page 386 of the book:

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."

A low Mars orbit isn't suborbital.

The Hermes is moving 5.8 kilometers/sec when it flies by Mars, which is above Mars escape velocity.

The Hermes has ion engines. Great ISP but very low thrust. It would take too much time to put on the brakes enough to slow from a sun centered orbit to a Mars capture orbit. Thus the Hermes has to zoom by Mars a lot faster than the typical MAV rendezvous trajectory. So the MAV has to be adapted to match Hermes' 5.8 kilometer/second velocity.

Here's a quick pic (not to scale):

Hermes Fly By

Low Mars Orbit is about 3.4 km/sec. Why does Bruce say 4.1? Not sure but maybe because the MAV would incur a gravity loss penalty during ascent.

A related problem

Low thrust also means Hermes is slow to step on the gas. Neil DeGrasse Tyson's trailer for The Martian has Hermes departing from low earth orbit (1:15). At 2 millimeters/second^2 acceleration, it would take Hermes more than a month to spiral out of earth's gravity well. That wrecks Weir's 124 day trajectory from Earth to Mars. See Fact Checking Neil deGrasse Tyson.

I sure wish Tyson had Hermes depart from Earth Moon Lagrange 2 (EML2) instead of low earth orbit. Then Weir's trajectory would be plausible. And it would have been a great opportunity to give EML2 some attention. EML2 is a very interesting location.

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    Good work finding the quote in the book. – Erik Oct 7 '15 at 15:28
  • Love the diagram! – Lightness Races with Monica Oct 10 '15 at 10:22
  • It's likely that Andy Weir just counted the 124 days from the time reaching escape velocity. – Joshua Mar 15 '16 at 21:40
  • @Joshua that's what most folks do. With chemical it only takes a short time to reach escape velocity. But that's definitely not the case with ion engines. Tyson's video had Hermes leaving LEO and arriving 124 days later. Tyson has replied to my criticism in the comments. – HopDavid Mar 16 '16 at 1:58

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