I know our astronauts use big, bulky suits for outside work - but is the suit absolutely necessary for work in a vacuum - or just when they're outside the shuttle / ISS?

Could they get by with just a breathing helmet and a smaller 'skin suit' if they were just working inside an unpressurized bay?

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"Are you a mad-hatter that just types what he wishes, or have you actually any physics training?"
Occam's Ghost to Grant Hutchison.

My understanding is that they are actually working on something like this. And that the problem isn't so much protection as it is the body needs to have some external force pressing in in order to do stuff like breathe. Like the force that actually makes it possible to exhale is the pressure around the chest. I may be misunderstanding it though.

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It goes deeper than that.

You cannot get away with just a mask on Marts even. And it has atmosphere.

You run into various problems including transfer at the skin level and temperature and pressure.

A divers suit will not protect you from the harsh 'vacuum"

Related discussions...
Spray-on spacesuit
Skintight spacesuit
Is a space suit needed on Mars?
Space suit minimulist or minispace ship.
Naked eyeball survival on Mars
Walking on mars with or without spacesuit

Wal or K?

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Wal-Mars...

Why does that seems so likely in the future?

Towards the end of the "Walking on mars ..." thread, Van Rijn provided a link to an excellent paper on the development of such skin-suits.
We then had a post from a spacesuit developer, elaborating on some of the problems described in Van Rijn's link.

The problem is that you need to breathe about a fifth of an atmosphere of oxygen to stay alive and conscious: in physiological pressure units, that's ~150mmHg. If you are to be able to breathe out, your skinsuit must apply that pressure to the outside of your chest, otherwise the pressure gradient across your chest wall will hold your lungs in full inspiration, and probably pop a couple of pneumothoraces for you, too. If that pressure is being applied to your chest, it also needs to be applied to the rest of your body, too, otherwise blood will not be able to return to your heart.
The pressure needs to be applied very uniformly across the body surface, because very slight proportional variations in that pressure will be enough to cause bruising, oedema or skin necrosis: so there are major technical challenges in trying to keep the pressure constant throughout the breathing cycle, or while you bend and extend your limbs (imagine the potential change in pressure on the bone of your elbow, for instance). Our spacesuit designer made delicate reference to the male genitalia: imagine the challenge of providing uniform constant pressure in that region while the astronaut moves his legs.
We also need to consider the potential dangers of donning and doffing, which are explored in Van Rijn's paper. You're in vacuum, wearing a suit that squeezes your chest and limbs with a pressure of 150mmHg, and you're breathing oxygen at a pressure of 150mmHg above ambient vacuum. So all your physiological systems are in balance. But if you come inside the habitat and remove your pressurized breathing system, the suit now squeezes your chest and limbs with a pressure 150mmHg above ambient: you die, quickly.
So for donning and doffing there needs to be a way to incrementally ramp the compression of the suit up and down, while the wearer breathes pressurized gas at incremental pressures above ambient.

Grant Hutchison

Theoretically, you should be able to get away with 1/10 bar or so, seeing as the pressure at the top of a 14000 foot mountain is roughly 1/2 bar, of which 20% is oxygen (and I didn't die the last time I was up on one of those). Still doesn't solve the problems though.

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And considering the bad case of "airplane ears" I got recently flying out here to California which was due to relatively small pressure changes I'm sure, my ear drums (or something worse -- see the male plumbing concerns) would probably blow out trying to get my space suit on and off and messing with the pressure controls.

My ears have always been bad to stop up with pressure changes, but this was the worst case I've ever had. It hurt and the stuffed up, deaf feeling drove me nuts. I think it was a repressurization problem. On the leg out to Houston, I think the middle ear pressure managed to equalize at low pressure, but a rapid descent stopped them up well and good. They cleared right up on the flight to Oakland once we got to altitude, and I didn't have a problem on landing there.


-Richard

You have the advantage that air is 79% nitrogen, however.
When you breathe air, your lungs humidify it (47mmHg) and add carbon dioxide to it (40mmHg). At 14,000ft, the air pressure is about 450mmHg. Water and CO₂ displace 87mmHg between them, leaving 363mmHg, of which 21% is oxygen: a theoretical 76mmHg in your lung alveoli, to drive diffusion into your blood.
Now breathe 95mmHg of pure oxygen (about the equivalent of the partial pressure at 14,000ft). By the time you've humidified that and added carbon dioxide, there is only a theoretical 8mmHg left to drive diffusion into your blood.
In practice, the exchange process is a more complicated, and you'd hyperventilate to blow off carbon dioxide and get more oxygen. But the sums illustrate why you can't just make the change from air to oxygen, replacing partial pressure with pressure, without incurring extra hypoxaemia.

Grant Hutchison

http://davidszondy.com/future/space/SAS.htm
http://mvl.mit.edu/EVA/biosuit/
Here is some reading on this idea. Mind you, the second sites creators have a stake in the operation, so they are rather. . . enthused.

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Thanks Grant: That clarifies why 100 millibars is not enough, and even 200 millibars is marginal. How about a prothesis that oxygenates your blood and removes excess carbon dioxide to suppliment what the skin tight suit can do with your lungs at 6 milibars on Mars? Is there any reasonable hope that things like eye balls would eventually adapt to as low as 6 milibars? Neil

The vapour pressure of water at body temperature is 47mmHg: over 60mb. So you'll develop vapour at that pressure in any body tissues exposed to lower absolute pressures.
So you want to keep ambient pressure inside your suit higher than that threshold, and you want to keep all your tissues evenly at that pressure, to avoid unphysiological pressure gradients.

Grant Hutchison

Just for clarification the surface pressure on Mars is about 7 millibars. You would not need a pressure suit on Titan with it's thick atmosphere, just an oxygen mask and a really, really, really warm jacket.

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it varies from place to place and season to season.
The zero altitude datum for Mars is defined against the level at which the mean atmospheric pressure is 6.1mb. But with a carbon dioxide scale height of 13km and a range of altitudes greater than 30km, there's a tenfold difference between the highest and lowest atmospheric pressures on the surface of Mars.
Seasonally, it changes by around 30%.

Grant Hutchison