The flip-around thing

After seeing the film The Martian, my sister Suzanne posed a science question about it to me, one that I can imagine many other moviegoers had as well. Here’s (an edited version of) her question and (an elaborated version of) my answer. Spoilers ahead!

Q: ‎My very first thought when NASA finally realizes Watney’s still alive was, great! It’s only been 40-something days. Surely there’s a way for the Hermes crew to go back for him. And then I waited another hour before the movie caught up.

What I don’t understand is why that idea became such a big aha/eureka moment for that dude who thought of it, why the “top minds” at NASA rejected it, and why it took so long to get behind it as a plan. Why didn’t anyone think of it sooner? Why couldn’t they expedite the rescue by boosting thrusters or whatever‎ to do the flip-around thing earlier?

A: The movie was great (and very faithful to the book), but it could have done a better job of explaining why the Hermes-return solution was such a big deal.

Star Trek makes space navigation seem like steering sea vessels. Star Wars makes it seem like driving hot rods. Other movies, video games, etc. – and even Interstellar, which took pains to depict some exotic science accurately – give the impression that it’s just a matter of pointing your ship where you want to go, and going there. For better or worse, that’s the mental frame of reference that audiences bring to movies about space.

In fact space travel (given our present technology) is much more like firing a gun. The spaceship is the bullet. You get one main chance to aim correctly, and then BANG, off you go. After that there is no changing course. Tom Hanks puts it like this in Apollo 13: “We just put Sir Isaac Newton in the driver’s seat.” (Something no real astronaut would say, understanding that there is no time Sir Isaac Newton isn’t in the driver’s seat).

In fact it’s even harder than aiming a bullet because a bullet reaches its target, or misses, in fractions of a second. A spaceship bullet is aimed at something that’s months away, at a target that’s in motion, and it travels – or more precisely, falls – through a medium governed by gravity, where the sources of gravity – the Sun, the Earth, Mars, etc. – are all in motion relative to each other, creating ever-shifting “currents” tugging the spaceship this way and that.

Once the Hermes fired her engine for the return trip to Earth, that was pretty much that. It had almost no fuel left for other maneuvering. There was no “doing the flip-around thing.” It would have taken a fair bit of inspiration even to think of looking for a return-to-Mars trajectory (let alone a return-to-Mars-and-then-Earth-again one!) plus a lot of luck that one existed, plus a lot of work to actually find it, plus a lot of daring to attempt a critical resupply during the high-speed gravity-assist Earth flyby, all of which the movie depicts a little too simply.

Hopefully, the many cool science things that the movie does depict accurately will whet audiences’ appetites to learn more and thereby discover for themselves just how audacious and unlikely the rescue plan was. Meanwhile, go play with NASA’s interplanetary trajectory browser!