Weir's The Martian.

[VQ]

And could an astronaut make his way around the outside of a spacecraft which was accelerating with an ion drive?

I would think so (assuming there were no issues such as gamma/neutrons from the nuclear reactor from a less-shielded direction). Contemporary ion drives have a thrust of less than one Newton at an energy cost on the order of kilowatts, and achieve very high dV's at very low accelerations.

[Ranb]

And could an astronaut make his way around the outside of a spacecraft which was accelerating with an ion drive?

I suppose that there would be certain exclusion areas outside where the astronaut would not be allowed to venture while on an EVA.  Much like a nuclear powered submarine.  When I enjoyed swim call on a surfaced sub, we were restricted to the area forward of the sail so that we were kept a good distance from the unshielded portion of the hull around the reactor.

Ranb

[VQ]

I suppose that there would be certain exclusion areas outside where the astronaut would not be allowed to venture while on an EVA.  Much like a nuclear powered submarine.  When I enjoyed swim call on a surfaced sub, we were restricted to the area forward of the sail so that we were kept a good distance from the unshielded portion of the hull around the reactor.

Ranb

I had never thought about this. So SSNs rely on the water around the sub for shielding the external environment from the reactor?

[Ishkabibble]

Oh, as a self-respecting EE I should say that I object to the one line in the movie that is rapidly becoming its catchphrase:

I'm gonna have to science the shit outta this.

With all due respect, he should have said

I'm gonna have to engineer the shit outta this.

There is a difference between science and engineering, ya know. Or to paraphrase a classic Star Trek exchange:

I'm a botanist, not an engineer!
Now you're an engineer!

100% agreement.

I told my 14 year old daughter, "learn new skills, master old ones, never take anything for granted" because she balked at my teaching her how to change a tire. It is better to be resourceful than anything else. If you can think outside the box to solve a problem, then you're ten steps ahead of everyone else. Intelligence is not the end-all and be-all. Academic performance only demonstrates how well you take tests. Critical thinking and the ability to apply knowledge is the ticket. So she grumbled all the while she was changing the tire in the backyard last summer. Until this past weekend. She came in from being at a friend's house for the weekend, and her friend's mom had taken them to the movies. Yep, they had a flat. The friend's mom got on her cell to call for roadside assistance, but my daughter got out, popped the trunk, and had the tire changed before the AAA guy even got there. Came back home all puffed up and proud of herself. Not as proud as I was.

Watney didn't really "science" anything. But he engineered everything.

[Ranb]

I had never thought about this. So SSNs rely on the water around the sub for shielding the external environment from the reactor?

There is very little unclassified information on submarine shield design that I can find.  But I can say that there is a shielded passageway going through the reactor compartment (RC) for access fore and aft.  The shield is also designed to eliminate high radiation levels on the top of the hull above the reactor so that Sailors can work topside.

The thick steel hull attenuates some of the radiation while the reactor is critical.  The primary shield greatly reduces radiation levels to the ship's components in the RC, the secondary shield (bulkheads) forward and aft of the RC protects the crew.  During the brief periods that a submarine operates in port, the water around the mostly submerged hull protects anyone from the low radiation levels that make it through the steel.

Ranb

[Peter B]

Oh, as a self-respecting EE I should say that I object to the one line in the movie that is rapidly becoming its catchphrase:

I'm gonna have to science the shit outta this.

With all due respect, he should have said

I'm gonna have to engineer the shit outta this.

There is a difference between science and engineering, ya know. Or to paraphrase a classic Star Trek exchange:

I'm a botanist, not an engineer!
Now you're an engineer!

100% agreement.

I told my 14 year old daughter, "learn new skills, master old ones, never take anything for granted" because she balked at my teaching her how to change a tire. It is better to be resourceful than anything else. If you can think outside the box to solve a problem, then you're ten steps ahead of everyone else. Intelligence is not the end-all and be-all. Academic performance only demonstrates how well you take tests. Critical thinking and the ability to apply knowledge is the ticket. So she grumbled all the while she was changing the tire in the backyard last summer. Until this past weekend. She came in from being at a friend's house for the weekend, and her friend's mom had taken them to the movies. Yep, they had a flat. The friend's mom got on her cell to call for roadside assistance, but my daughter got out, popped the trunk, and had the tire changed before the AAA guy even got there. Came back home all puffed up and proud of herself. Not as proud as I was.

Watney didn't really "science" anything. But he engineered everything.

Maybe Watney didn't like Wollowitz...?

ETA: A great story about your daughter.

I'm trying the same thing in small ways with my kids. Our oldest has just turned eight, and because he's profoundly deaf and wears cochlear implants, he has a "hearing bag" with implant-related things he has to take to school (spare batteries, FM receiver, FM microphone and so on). It's his responsibility to collect all these things at the end of the school day and put them in his hearing bag to bring home each day, and he's really quite good about it. He also knows how to change the batteries if that needs to be done.

We've also been reminding him that it's his job to tell his teacher when there's a problem of some sort, and to explain the problem; in other words, he has to be his own advocate - he can't rely on us to solve his problems all the time. And as time goes by he's going to have to start taking responsibility for disassembling his implants at bed-time, putting the batteries on to charge and putting the other components in the drying box.

He's still the dreamy-headed eight-year-old in other respects, but hopefully pushing this problem-solving side of things and praising him when he does it will spread to other aspects of his life.

[Peter B]

And could an astronaut make his way around the outside of a spacecraft which was accelerating with an ion drive?

I would think so (assuming there were no issues such as gamma/neutrons from the nuclear reactor from a less-shielded direction). Contemporary ion drives have a thrust of less than one Newton at an energy cost on the order of kilowatts, and achieve very high dV's at very low accelerations.

Yeah, I was thinking solely from the point of view of acceleration, not radiation.

It's just that, given how risk-averse NASA is these days (and presumably likely to get even more so in the future) I wondered whether their SOP would be for no EVAs while the engine was lit.

And now another question comes to mind: what would the trajectory of the Hermes look like as it breaks out of orbit (either of Earth or Mars)? Given the acceleration is small but constant, I assume it would look like some sort of spiral? Would the presence of the Moon be an issue to deal with? And from the time the engine is lit while in Earth orbit, how long would it take to reach escape velocity? Hours? Days?

[Ranb]

Well if Hermes would need a delta V of 3.6 kps, at 2 mm/second squared it would take nearly 50 hours to get to escape velocity.  If it takes a month or two to get to Mars, two days in Earth orbit isn't a big deal.

Ranb

[ka9q]

So SSNs rely on the water around the sub for shielding the external environment from the reactor?

Water is an excellent radiation shield.

The gamma dose from a bundle of spent reactor fuel (especially if it's freshly out of the reactor) would quickly give a lethal dose to anyone nearby, but they are regularly handled underwater in spent fuel pools with workers standing next the pool. A few meters of water makes all the difference.

[VQ]

Water is an excellent radiation shield.

The gamma dose from a bundle of spent reactor fuel (especially if it's freshly out of the reactor) would quickly give a lethal dose to anyone nearby, but they are regularly handled underwater in spent fuel pools with workers standing next the pool. A few meters of water makes all the difference.

Yes, I wasn't necessarily critiquing the design decision. BTW, I suppose any reactor-powered stealth platform will be obsolete when sufficiently accurate neutrino detectors are developed?

[ka9q]

Unless somebody figures out a much better way to shield neutrinos.

Of course, the same property of the neutrino that makes it hard to shield makes it hard to detect. Works both ways.

As I understand it, it's not fission itself that generates neutrinos, it's the beta decay of (most of) its fission products. More accurately, it generates anti-neutrinos.

So you'd want to get rid of your fission products as quickly as possible, before they decay inside the reactor. One way to do that is with a nuclear fission-fragment rocket. You make the fuel thin enough that most of the fragments can directly escape the fuel, then use strong electric and/or magnetic fields to deflect these positively-charged ions out the back as your reaction mass. With each fission releasing about 200 MeV, I_sp can be quite high.

[Dalhousie]

I suppose that there would be certain exclusion areas outside where the astronaut would not be allowed to venture while on an EVA.  Much like a nuclear powered submarine.  When I enjoyed swim call on a surfaced sub, we were restricted to the area forward of the sail so that we were kept a good distance from the unshielded portion of the hull around the reactor.

Ranb

I had never thought about this. So SSNs rely on the water around the sub for shielding the external environment from the reactor?

I am surprised at this, as ship reactors are heavily shielded in all directions.  They have to be, as they come along side in harbours, and also have to be protected against collision.

Mass is not an issue in ships, and SNNs displace thousands of tonnes.

[Ranb]

While the neuron shield surrounds the reactor on the sides and the top, the coolant loops are not shielded 100%.  All water cooled reactors emit lots of N-16 which has a half-life of only 7 seconds but produces high energy gammas that are the primary source of radiation penetrating the secondary shields.

Mass is still an issue with the shields weighing hundreds of tons.  Image how submarine performance would be affected if only the forward shield was in place to protect the crew (with the propulsion plant operated remotely) and nothing for the aft and outboards areas.  The weight savings could be directed towards a more powerful reactor/turbines or for a thicker hull for greater operating depth.

Ranb

[Northern Lurker]

I went to see the movie with my daughter. We both enjoyed it a lot.

I was bit bothered about flapping tarp holding pressure and surviving the abrasion from martian sand storms. Another obvious mistake was sunlit parts of the Hermes and stars in the same shot but I guess a space movie can't avoid that 

I was left wondering about rotating artificial gravity compartment. Wouldn't it act like gyroscope and resist all attempts at changing attitude? I didn't see any contra-rotating weights. Would such gyroscopic forces damage the ship if attitude was forcefully changed?

Lurky

[ka9q]

I was left wondering about rotating artificial gravity compartment. Wouldn't it act like gyroscope and resist all attempts at changing attitude?

That occurred to me too. A simple fix is to have two counter-rotating artificial gravity wheels.

It's not so much the difficulty of changing the attitude of the wheel once it is turning, it's the difficulty of getting it to turn in the first place. You'd either spend a lot of fuel on thrusters, or put a thruster on the end of a very long boom, or spin up and fling off some counterweights. The latter method is routinely used to despin spent upper stages of the Delta launch vehicle.