Why can't the second MAV reach the Hermes?

In the beginning of The Martian, the first MAV can reach orbiting Hermes just fine with 5 passengers on board. Yet towards the end of the movie, we are shown that the second MAV needs to lose weight to do the same, even with only one passenger. Why is this?

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Best Answer

As explained in this answer:

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.

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What is the MAV in the Martian?

The Mars Ascent Vehicle (MAV) is a critical mission element of the Mars Sample Return campaign that delivers the Martian sample from Mars surface to orbit for rendezvous with retrieving spacecraft and sample return to earth.

What is happening to the MAV while launching?

\u201cThe stripped down MAV will get up to 12 g's during the launch. He could be unconscious and may even have internal bleeding.\u201d The reason the Hermes could not enter orbit (as it had presumably done before) was due to fuel demands.

Liz Phair - Why Can't I? (Official Video)

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