Here is a great write-up of his argument:
“I love The Martian. However, until they find a way to introduce water to the surface of Mars we aren’t going to be getting any space pirates.
In the 1980s, the international community signed the UN Convention on the Law of the Sea. Think of it like the world’s constitution for the oceans. UNCLOS (as it’s called for short) outlines rules applicable to different maritime zones extending from a country’s coastline. All of these zones are explicitly defined as zones ‘in the sea.’
So The Martian made two interpretive errors here when it claimed that mars is international waters: 1) it mistakenly used rules that apply only to maritime zones to a place (Mars) where there is no marine environment, and 2) it overlooked the fact that UNCLOS rules are defined with reference to maritime zones extending from a country’s coast. No countries, no maritime zones.
Our Martian should have instead looked to the Outer Space Treaty for the applicable international rules. The movie is correct that countries cannot be sovereign over any extraterrestrial body. And countries retain jurisdiction over all their objects in space (it doesn’t matter that NASA is nonmilitary).
So the question becomes whether, for the purposes of U.S. domestic law, Mark Whatley (Matt Damon’s character) is illegally commandeering a vessel when he steps into the ARES 4 MAV without first getting permission from NASA. This might be true, though the Outer Space Treaty provides that all countries should give all feasible assistance to astronauts in distress. But even if he was violating U.S. law it would only make him a space felon — not a space pirate.
And while Matt Damon’s activity might violate NASA regulations, I’m guessing they wouldn’t be bringing disciplinary action anytime soon.”