13

In the end Mark Watney played by Matt Damon escapes in a space shuttle (don't know what it is called). I have watched the movie only didn't read the book, I have some doubts

  1. From where did the space shuttle appear?
  2. Why it has only small energy so that he can't reach the crew?

Any help on these will be appreciated.

26

Watney was on the Ares III mission. Missions up to Ares V were in the works, and equipment such as the MAV (Mars Ascent Vehicle) were already in place on Mars, waiting for these missions to arrive. The amount of equipment necessary for each mission was split into separate launches to reduce the individual payloads.

Shortly after Watney discovers he's stranded he mentions the Ares IV MAV is already on-planet, though it is far away - much further away than the buggy can take him. Needing to reach the Ares IV mission site (where the Ares IV MAV is already) is what prompts him to start retrofitting the buggy for longer journeys.

The MAV is an ascent vehicle, only designed to get its occupants into Mars orbit, and not much else. The Hermes should have been in Martian orbit when the Ares IV MAV launched, and the MAV would have no trouble reaching the ship.

However to rescue Watney the Hermes' return trajectory involved a slingshot around Mars, intentionally avoiding entering orbit so they'd be able to return home. This meant the Hermes was going significantly faster than the speed the MAV was designed to intercept at. In order for the MAV to gain enough velocity, its weight had to be reduced.

This is all explained in-movie (though some of it's easy to miss), see it again and you'll catch all these details.


A previous answer had several inaccuracies that are also worth clarifying:

  • The MAV was not "designed to reach Mars", its sole purpose (hence the name) is to get the crew off Mars. It was delivered to Mars as part of a larger payload.
  • The crew does not leave in the same ship they arrive in; they leave in the MAV, which is already on-planet.
  • The Chinese rocket is not used as a booster to propel the Hermes. It contains necessary supplies to sustain Watney (and the crew of the Hermes). The Hermes is able to return to Mars via a gravity slingshot.
  • 1
    The fact that the Hermes is on a trajectory to flyby Mars instead of entering orbit does not affect the altitude. It is possible for it to perform a flyby with a very low altitude. It does affect the velocity though. Since it is not going to slow down to meet the MAV the MAV must go faster than orbital speed (which it was designed for) to meet up with it. – Austin Oct 5 '15 at 18:06
  • @Austin good point, thanks. Corrected. – dimo414 Oct 5 '15 at 18:18
  • 1
    The book explains that the MAV for subsequent missions (Ares 4) is landed by the current mission (Ares 3) years in advance so it can process Mars' thin atmosphere into fuel. – Coomie Oct 6 '15 at 2:55
  • @dimo414 : Great answer. Thanks for correcting the facts. – Ankit Oct 6 '15 at 6:10
  • 1
    @dimo414 that ship is called the Hermes. The Hermes was built for the Ares programme. Not a specific Ares mission. – Aron Oct 8 '15 at 2:20
2

Ankit is close. And maybe I'm not quite reading it correctly, but here is what I understand:

  • As Ankit stated, this rocket was 'shipped' to mars ahead of time for a future mission.
  • This rocket didn't deliver materials. Rather, it's the rocket that takes the crew off of mars into Mars' orbit when they finish their mission. This is the same type of rocket the crew escaped from Mars with in the first part of the film.
  • The reason it didn't have enough energy to reach the crew was that the crew couldn't get their ship into Mars' orbit. So they had to lighten the launcher as much as they could so they could get it as high as possible to intersect with the crew that was sling-shotting around Mars.
  • Yeah second point is quite right. I misunderstood it. I vague remembered a dialogue from Watney which led to me believe that it had supplies. – Ankit Oct 6 '15 at 6:09
1

The MAV could never have rendezvoused with Hermes at a 5 m/s relative velocity. Hermes was in a hyperbolic orbit around Mars, meaning it was at escape velocity the entire time -- why it was called a flyby. Watney's MAV could only put him into low Mars orbit, made slightly higher by stripping down the MAV. The physics problem is that at any orbital height, escape velocity is always 1.4 times orbital velocity.

If Hermes flew past Mars at 600 km altitude it would be traveling at least 1.4 times as fast as the MAV at that height, since the MAV could not achieve escape velocity. In orbit, speed translates into orbital height. Remember, he had no power after the initial thrust; he was totally in a ballistic orbit. At any altitude in in Hermes' hyperbolic orbit he could reach, he would be traveling much slower than Hermes, and any speed of Hermes he could match, he would be much lower.

You must log in to answer this question.

protected by Community Mar 27 '16 at 12:15

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged .