Friction in space and on Earth

This post is in response to a comment I got on my previous post "More thoughts on the importance of science in science fiction" where Shannon commented/asked (I'm only quoting the question-y part of her comment):

It really is a hard concept to grasp, the no-friction-in-space thing. I don't think I really get it - I'm not sure how to visualise it, for a start - but I don't understand how a space ship - of the super-advanced, sci-fi kind - can't slow down. I mean, it's mechanical and computerised and runs on fuel; on Earth anything we build for transportation will slow down especially if there's a mechanical failure etc. I know in space you can't "stop", you'd only drift, right? I'm hoping you can explain this a bit more to me because I really do want to understand!

(The more time I have to let this concept dwell in my brain, the more I'm starting to get it. So what does happen when you, in sci-fi, go from "warp speed" or whatever they like to call it, to, well, not?)

On Earth (or really, anywhere that isn't the empty vacuum of space) moving objects slow down because they lose energy through friction — rubbing against other objects. Commonly on Earth, the source of friction would be land, water and/or air.

Some examples:

• The motor of a boat needs to stay on to keep the boat moving, because if the motor is turned off, the boat will be slowed down by the water pushing back against it.
• If you ski straight down a hill (let's say a small hill for safety reasons) you will accelerate (get faster) while you're going down hill, but once you reach the flat bit at the bottom you will eventually slow down and stop without having to stop yourself. This is because of the friction between the snow and your skis. Generally, skiing works because there's much less friction between snow and skis than, say, between shoes and dirt, but there isn't zero friction. When you were going down the hill and getting faster, there was still friction, but at that point gravity pulling you downwards was stronger.
• If you drop something from a great height (tall building, aeroplane), gravity will make it accelerate as it falls down. However, the air pushes back on it, upwards (or more generally, in the opposite direction to the movement) and eventually will prevent the object falling any faster. (With air, the friction is directly related to the size and shape of the object and how fast it's going, but I won't get into the maths.) The maximum speed the object can reach while falling is called terminal velocity.
• On the other hand, if there is no air — for example on the moon — there will of course be no friction from air and things like feathers which normally fall very slowly (because of all the little fuzzy bits catching on the air) will fall at the same speed and acceleration as a lead ball (or whatever). This will also work in a vacuum chamber where all the air has been removed. Here is a video of an astronaut on the last Apollo mission dropping a hammer and a feather at the same time:
  
  
  And a gif of the same if you can't be bothered watching and listening to the 47 second clip:
  
  
  
  
• Brakes on cars and whatnot work by intentionally increasing the friction on the axle to slow down the spinning speed of the wheels

Now let's talk about how spaceships slow down in space. I want to emphasis that my complaint with Across the Universe wasn't that the spaceship was slowing down, but that it was slowing down by itself. Things can only slow down by themselves if there is friction around (so really they're not slowing down by themselves but because of friction, but we don't usually think about or notice friction so it seems like its happening by itself).

In real life, spaceships slow down (and manoeuvre) by firing their engines in the other direction. It might be a bit easier to picture on a smaller scale. Consider an astronaut on a spacewalk. Let's pretend they're not tethered to their ship and that the ship is out in deep space away from the gravitational influence of any planets. To be able to move around, the astronaut will have a gas tank (or similar) that will allow them to press a button to move forward. The gas will shoot out backwards for a couple of seconds, and the astronaut will move forwards. At this point, if the astronaut does nothing, they will continue moving in a straight line indefinitely. Basically until they run into something. The same thing happens with a spaceship: gravity and obstacles not withstanding, after it fires its engines for a bit to accelerate, it will keep going in a straight line at the same speed until something else happens to stop it. This clip from WALL-E is a good example (thanks to Shaheen for the suggestion). Also note that once they start spinning, things will continue spinning until something else makes them change, which you can see a bit of in that clip.

That doesn't mean things can't stop or slow down in space. Our astronaut — assuming they're not unconscious — can fire their gas in the opposite direction (to manoeuvre properly they'd have to have several directional options, six for complete manoeuvrability) to slow down. The spaceship can also fire thrusters in the opposite direction to slow down (either by having two sets or by rotating the main ones). Coming to an absolute complete stop is a bit tricky because a) you would have to balance forces very exactly and b) there's not much to use as a reference for how fast you're going out in space, but matching speeds with another ship is doable. And the astronaut slowing down enough to not break a wrist colliding with his ship is also useful. My older post about turning around in space addresses some issues with why just stopping and going in the opposite direction isn't the most efficient way of doing it.

The very last part of the question was:

So what does happen when you, in sci-fi, go from "warp speed" or whatever they like to call it, to, well, not?

The short answer to this is, whatever you want. Warp speed and hyperspace and other "let's cheat to go faster than the speed of light devices" aren't real. They're generally not based on real physics, or if they are, it's very extrapolated and speculative and could well turn out to be just as implausible. That said, faster than light travel is a staple of science fiction and I'm not suggesting we should eliminate it because it's implausible. If all science fiction stories used only slow or relativistic (which means close to the speed of light, when weird things happen. My post about it) then there'd be a lot of very slow stories which would get boring. Variety is nice.

As long as the rest of the science is plausible, then I don't have a problem with a bit of faster than light travel and faster than light communication. If the writer doesn't feel up to making up a semi-plausible sciencey explanation, then my personal preference is not to try explaining how the FTL works at all. Because they usually stuff up some minor point which annoys me disproportionately.

More thoughts on the importance of science in science fiction

Today I was directed to a blog post about how important science is in science fiction using the hideous crime against science example of Beth Revis's Across the Universe, which I blogged about here. (From the sound of it, the blog author may have read my post or someone else's similar reaction to the book.) The blog author asks how important is accurate science really, and is there a line? The rest of this post is based on my comment over there.

I think there is definitely a line. Stuff like faster than light travel, teleportation, artificial gravity (in some circumstances) are fair game to use in fiction with no or only hand-wavey explanations. (In fact, sometimes trying to be too specific with them can be detrimental.) Everyone either knows that stuff isn't real or can very easily google it to find out. And it has a distinct plot-based purpose: if everyone wrote relativistically accurate science fiction (no faster than light travel), it would be very boring. When getting from A to B isn't the point of the story, using an accepted trope to speed things up is totally fine. Same with power sources for spaceships. That's an area where there will definitely be heaps of progress in the future that we can't necessarily predict and so hand-waving is fine.

What isn't fine is getting basic and fundamental concepts wrong like the ship slowing down in space that Revis did. Note that she also had a hand-wavey power source in said spaceship and THAT is fine. But thinking there's friction in space? No. It's a popular book for teens and it's actively confounding a concept that's actually quite difficult to teach. Pretty much no one (and certainly no teen) has been in space and so books and movies are all most of us have to base our intuition on when it comes to how stuff in space works. For things on Earth, it's easy to think about our everyday experiences and predict (from a basic physics point of view) what will happen. On Earth, stuff DOES gradually slow down. In space it doesn't and that's a concept that some kids, when learning physics for the first time, find difficult to grasp. It's a disservice to further confuse the issue.

And for the record, usually if an author tries to do their research, it's obvious in the writing.

~

Now, when I was searching for a link to something Revis said in an interview about her research for this book (or lack there of), I came across the FAQ on her website. One of the questions and responses is:

Q: WAIT A MINUTE. I think I found a scientific error in Across the Universe.
A: Well–there’s a chance I messed up. BUT if you’re one of the ones who noticed the REALLY BIG scientific error…well, I’ll just say that there IS a sequel, and it DOES address this, and maybe it’s not that the book is wrong, but that the characters have the wrong idea…

I can only assume the "REALLY BIG" scientific error is the friction in space thing that's made me so angry. I'm not 100% convinced that it and associated sciencefails are properly addressed. I can think of one scenario that would make it "the characters are wrong but the science isn't", and from the plot of book one and the hints I've seen around the web for the events in books two and three, it doesn't seem likely.

Have any of my readers actually read the second book? Is it worth my time (and money) reading it just so I can blog about the problems in it? So far the answer to the second question has been "no" and picking up the second book in a shop and flicking through it didn't exactly fill be with the desire to jump back into that world.

Review: Blue Silence by Michelle Marquardt

This  review is posted as part of my Australian Women Writers Challenge. I have cross-posted it from my review blog. I have now completed the Australian Women Writers Challenge for 2012, and you can read my de-brief here.

Blue Silence by Michelle Marquardt was originally published in 2002 and is sadly now out of print. Although I see it's in stock at Infinitas as of this writing. It was a winner of the George Turner Prize (as my edition proclaims on the cover).

The story opens when a mysterious ship docks with one of the space stations in orbit around Earth. The ship is, on the outside, an exact replica of one that was sent out into deep space 180 years ago, and then never heard from again. The difference? This ship has new drive technology which was only invented a couple of years ago. And instead of the seven original crew members, it's full of stasis pods and five hundred creatures, half of whom look human, half of whom look almost human.

None of the aliens know where they came from or why — they have no memories before waking up docked with the space station — and the authorities on the space station don't really know what to do with them either.

Senator Maya Russini is the leader of the group of people who first board the ship. A mission which one of the group does not return from alive. Are the aliens dangerous? What do they mean for the various political machinations happening within the space station's government and between them and other governments?

I liked Maya. She was an excellent example of a female character that doesn't need to run around kicking people in the head to gain power. She's also secretly a telepath (secret because she didn't register when she turned 21), but in a nice twist, she's the weakest kind of telepath, only able to read emotions, not thoughts. I think Marquardt has done a good job of portraying a society in which women are equal without making a big deal of it. (There are, in the end, more male characters, but that's mostly because the two main aliens are male.)

Her friend Ienne, the Minister for Foreign Affairs, also gets involved with the aliens. Unlike Maya who mostly regards them as suspicious and dangerous, Ienne is always looking for a way to use them to his advantage (there's a treaty they and another space station are wrestling over). He also goes out of his way to be rude to everyone with the occasional exception of Maya.

As I noticed when I was past half-way, Blue Silence is a very character driven story, unusually so for science fiction. The world does not need saving, nor does any war break out. Instead the action comes directly from the interactions between the characters, including two of the aliens who I don't think I can say much about without spoiling key elements. There is excitement and there's no missing the climax, but it's not like a plot driven story where all the action was building up to an inevitable climax and world-saving event. In the end, we know more about the aliens, but we don't know everything. Some answers are only hinted at or presented as speculation. In a way, this was slightly annoying because I like to know all the answers (arguably why I'm a scientist in real life), but it worked for the book. The story wasn't about the people trying to study the aliens, it was about people whose paths happened to cross theirs.

Also, the science, which I feel obliged to comment on, was well done. It wasn't a technology-oriented story, but having been published ten years ago, there was a risk the technology would feel a bit dated now. It didn't. They didn't have smart phones, but they did have pagers which were functionally mobile phones and received the equivalent of email on ubiquitous computers. There was also a discussion on the merits of different kinds of space stations (mimicking Earth versus giant building floating in space) which was interesting.

I highly recommend Blue Silence to anyone looking for something a bit different in their science fiction. It also emphasises the variety we have in the Australian science fiction field, something you might miss if you only looked at the most recent few releases.

4.5 / 5 stars

Year-long days and living in them

This blog post was inspired by an email conversation with someone regarding the possibility of a planet having year-long (or half-year long) day/night cycles. The original question was whether this is even possible and whether such a planet would be habitable.

From a purely astronomical point of view, this is definitely possible. There's no reason why you couldn't have a slowly rotating planet at around the same distance from it's sun as Earth is (well any reasons that do exist are fairly theoretical so we can ignore them). That said, if the planet is similar to Earth and its sun is similar to ours, then you kind of have to have the same length year because the length of the year (ie how long it takes to orbit the star) depends only on the mass of the star and the distance from it. This is due to Kepler's Laws, which I have previously discussed here. If you made no changes to star/planet distance, the year length would have to be the same.

Image nicked from Wiki here. The little red line
represents the same point on the surface of
Mercury. The numbers are the order in which
the positions happen: 6, 1, 2 are night for
the red line and 3, 4, 5 are day, roughly.

You could also have something similar to Mercury which has three rotations (called "sidereal days" which are measured relative to the stars, not the sun) to two years. Because it rotates so slowly, weird stuff happens with its solar days (the light/dark periods, completely ignoring the positions of stars) so that in one year it experiences half a solar day. Mercury is like this because it's so close to the sun. It could have been tidally locked (the same side always facing the sun – discussed further, including for Mercury in particular, here) but the gravitational effects of the other planets in the solar system caused this more unusual resonance.

However, if we're talking a planet as distant from the sun as Earth is, there's no danger of it becoming tidally locked in the sort of cosmological time frame we're currently living in. The time taken for the angular momentum between planet and star to be distributed into the tidally locked configuration takes longer the further apart they are (and the less massive when they're close enough). The Earth-moon system will eventually become more tidally locked: the moon already faces the same side towards us all the time, and eventually the same side of Earth will always point towards the moon.

But that's a bit of a tangent, back to planets with long days and nights. You could have a planet rotating as slowly/quickly as you like, but you should be mindful that the people living there would almost certainly have a way of distinguishing between sidereal and solar days. Ancient people on Earth already had this worked out (the difference between sidereal and solar days is why the stars move across the sky with the seasons).

Living there

Uranus: almost completely sideways.

If you did have a planet with a year-long day, the periods of day and night would be roughly equal in the same way they are on Earth, just scaled up. It could vary a bit depending on the planet's axial tilt (how much the line between the poles is tilted relative to the plane of it's orbit — Earth's is around 23º and changes slightly when earthquakes occur) so the more inclined the axis, the more extreme the seasons. If there was no or very little axial tilt, there wouldn't be seasons. The other variable in day/night lengths is the latitude. Further away from the equator sunrise and sunset would last longer and the shortness of winter days and length of summer days would be more extreme (as on Earth, but a different axial tilt could make this more so). If there was no axial tilt, the poles would be in a state of twilight permanently. The other extreme is something like Uranus which has a 90º-ish axial tilt so that during a southern summer the south pole points towards the sun and during a southern winter the south pole gets no sun at all. Spring and Autumn are the transition period. The equator is in twilight during summer and winter and has more "normal" days, like what we're used to, during spring and autumn.

Also, astronomical plausibility aside, I'm not convinced complicated life could naturally arise on a planet with a super-long day/night cycle, due to the long periods of boiling (day) and freezing (night). In terms of temperature-stability, probably only the twilight areas would be habitable. I suppose you could have migrating species (but that also has problems because in staying in permanent twilight they'd need sufficient landmasses connecting the two poles). Also, you'd probably get some sort of storms around the twilight zone, since the temperature would be in in a state of flux. I'm not an expert on atmospheres or meteorology, though, so that's a (-n educated) guess and I can't be too specific. But in short: our 24 hour days are what keeps Earth's temperature relatively temperate and suitable for life.

There's be fewer issues for microbial life to arise but I don't know that anything larger would be viable. Maybe at the poles: if the planet was slightly closer to its star than Earth is, there could be non-migratory life living near the poles and with a stable orbit and rotational period, it should survive. Since the non-polar regions wouldn't have naturally arising complex life, there could be with completely different ecosystems/forms of life at either pole with only something like microbial ancestors connecting them.

Atmospherically Speaking

Today I have another Ask Tsana post.

Brookelin asked:

Hi again, Tsana.

I was wondering - in an alternate universe, what would it take for a species to survive on Mars?

I know that it has some atmosphere, but not a whole lot. With the pressure being below the Armstrong limit, could there feasibly be large creatures (between collie and bear size) that could survive would have higher thresholds and what would they need to do so?

If the water on a human's tongue boils in space, would an alien creature in these environments be able to have eyes and mouths?

What might these species' need to overcome the intense radiation caused by Mars' weak magnetosphere?

Could bio-genetically enhanced humans ever survive these conditions outside a space suit for periods of time upwards of an hour, but less than a day?

Are these too many questions? Do you know the answers to any of them, or is this more of a medical thing?

I don't have answers to all of these questions because, as Brookelin said, some are more medical/biological and that's not my area of expertise. I will say that what we generally know a lot about is life on Earth. There are some constraints that exist for life on other planets but there is nothing to say that it has to resemble Earth life. They could have eating and seeing organs completely different to what we're used to. Even on Earth there's a pretty wide variety. I'm not sure that merely genetically enhancing a human would be enough to let them walk around on Mars. Science fictions stories have gone there, but I'm not sure genetics is up to it. I could be wrong, I'm just guessing. Hopefully my comments below on atmospheres and life on smaller planets such as Mars will answer the rest of the questions, though.

Mars. Credit: NASA, ESA, and The Hubble Heritage Team
(STScI/AURA)

It's true that Mars has a very thin atmosphere; it's about 0.6% as dense as Earth's at their respective surfaces. Part of the reason for this is Mars's lower gravity. In general, gases will expand to evenly fill the container they're in. When the container is a planet's gravitational field, we get denser air closer to the ground and less dense air higher up. This is because the air higher up is pushing down on the lower air while having less air above it to push it down. More or less.

Air is made up of particles (atoms and molecules) which move around very quickly and bounce off each other. That's why a gas is a gas and not a liquid or solid: the particles in a liquid don't move quickly enough to completely overcome the forces attracting them to each other and the particles in a solid can't move more than vibrating on the spot because the forces holding them in place are so strong. The energy that makes the particles move, for all states of matter, depends on the temperature: the hotter, the faster. The other important consideration is particle mass. At the same temperature, oxygen and hydrogen molecules (O₂ and H₂) have the same energy. However, oxygen weighs sixteen times as much as hydrogen (because the atoms are larger and heavier) so it takes more energy to move oxygen molecules at the same speed as hydrogen molecules. The result is that at the same temperature, oxygen molecules move more slowly than hydrogen molecules. And it takes less energy for hydrogen molecules to reach escape velocity (the speed required to escape the gravitational pull of Earth/whatever planet) than oxygen. And that's why there is very little hydrogen in Earth's atmosphere despite it being the most abundant element on a cosmic scale — it escapes into space. It's also the reason only the gas giants, notably Jupiter and Saturn, have any significant about of hydrogen in their atmospheres — they have the strongest gravitational fields.

So, Mars. Mars is smaller than Earth, with about a third the acceleration due to gravity at its surface. Mars is made up of similar elements to Earth, most likely because they formed so closely together, so it's likely that the same sort of lighter elements could have made up Mars's atmosphere. However, due to the lower gravity, not only hydrogen but oxygen and nitrogen would also have escaped or never been captured by the planet. I would guess the main reason there's so much frozen carbon dioxide at the poles is because it has a relatively high melting point of -78ºC rather than the much colder melting points of oxygen (-219º C) and nitrogen (-210º C). For comparison, Mars's surface temperatures vary between -143º and +35º C. So basically, even if you imported or mined enough gas to raise the air pressure to human survivable levels, it would all be lost into space and would need constant replenishing which would get tedious and be difficult to sustain. You'd also, ideally, raise the surface temperature to more consistently human survivable levels — probably using some sort of greenhouse effect to trap more of the sun's energy — but that would just hasten the atmosphere's escape.

Titan's atmosphere as seen by Cassini. Credit: NASA

But all is not lost. Heavier molecules exist, particularly those made out of carbon. Titan, one of Saturn's moons, is smaller than Mars but has an atmospheric pressure greater than Earth's by about 45%. It's colder than Mars, which allows its atmosphere to condense a bit, but it's only got a surface gravity of around a seventh that of Earth's (less than half of Mars's). According to Wiki, its atmosphere is composed mainly of nitrogen (as is Earth's) and methane with some traces of heavier carbon molecules. It's a combination of the temperature, the distance from the sun, Saturn's magnetic field and some form of replenishing methane that keeps Titan's atmosphere thick and, well, full of methane. Distance from the sun is significant by itself because Titan is far enough that the ionising solar wind is weak enough to not completely ionise and destroy the top layers of its atmosphere. The same strategy probably wouldn't work on Mars to increase the atmospheric pressure permanently unless you could find some magically resistant to solar radiation molecule to populate the atmosphere with. There are two interesting theories for what keeps replenishing the methane on Titan (which should be destroyed even by the lowered energy it receives from the sun): cryovolcanoes — volcanoes shooting icy hydrocarbons instead of lava — or biological processes using/generating methane in place of water.

The high levels of ionising radiation on Mars are as much due to its lack of atmosphere as its lack of magnetic field. (Side note: there's evidence that there was a magnetic field on Mars in the past, though I don't think we know why it went away.) Earth's atmosphere absorbs a lot of the ionising and UV radiation the sun throws at us (part of the reason the ozone layer is important). Not all of it is deflected — and things like X-rays and gamma rays can't be deflected because they don't have an electric charge — especially near the magnetic poles where the aurorae are caused by charged particles, mostly from the sun, interacting with the atmosphere. However, giving Mars a magnetic field would definitely help. Earth's is generated by molten iron in its core so it's not outside the realm of over-dramatic science fiction to drill a hole into the centre and start the core spinning. Come to think of it, Hollywood's already done that, just with Earth not Mars. (For the record, the ridiculous issues with that movie include the structural integrity of the hole and the failure to correctly represent changes in gravity.) A more feasible way to avoid radiation on Mars would be to live underground so that the ground above you did the work of absorbing harmful radiation. The reason too much radiation is bad for all forms of life is that it destroys and changes molecules. In humans this is one of the causes of cancer. In microbial life, which might only have a few cells to begin with, it's more deadly. It's why sterilising things with UV light works.

So basically, the easiest way to get people living and wandering around on Mars is to have them live in airtight structures and give them suits for walking around outside it. The suits wouldn't have to be as extreme as space suits though, so that's something. I'm not saying it's completely impossible to walk around on the surface with less protection, just very difficult. And because someone will mention it in the comments if I don't, I've heard that Kim Stanley Robinson's Mars books, starting with Red Mars, do a good job of talking about the terraforming process, although I haven't read them. Ben Bova's Grand Tour of the solar system books (eg Mars or Saturn and Titan) explore alternative forms of life all over the solar system. If you can stomach a bit of sexism, some of them are worth a read.

Turning around in space

Another ask Tsana question today. (And a relatively shortish response, sort of. Gasp!) Keep 'em coming, guys :-)

Anon asked:

How hard would it be to turn around in space... Say for some reason, Curiosity needed to turn around midflight and return to earth. Would BURNING fuel on some sort of reverse thruster work or would it have to make the trip to Mars, orbit the planet and break orbit to return

This is for a picture book that I feel impelled to be at least somewhat based in reality... which may be dumb.

Hi Anon,

It's absolutely NOT dumb to try to make picture books or any sort of books for kids plausible or semi-plausible. Especially when it comes to these sorts of areas where they can't possibly have any hands-on experience. Hollywood bombards them (and all of us) with so much inaccuracy that any little bit of truth helps. If they remember your book when they come to learn about these things later on, it will help the science stick. If all they have to go on are poorly researched movies which have given them wrong "intuition" about these things, it makes it a lot harder for them since they have to unlearn the rubbish first.

On to the actual question part!

It's pretty tricky to turn around in space. Because there's no friction, you have to use the same amount of energy it took to speed up to slow down by the same amount (so to come to a stop, say). This is a huge waste of fuel. Changing course more subtly isn't as difficult, however.

Apollo 13 Movie poster. (Nabbed from Wiki)

For something specifically like Curiosity: an unmanned probe sent to another planet, I can't think of a reason they'd try to get it back to Earth (unless a sample return was specifically part of the mission plan, but I don't think that's what you're asking). If something went wrong, they'd be more likely to cut their losses and abandon it. Also, almost all of that kind of probe's fuel is used up during take off, leaving only enough for minor course corrections and landing. In that case, plausibility would dictate that attempting a gravitational slingshot around Mars would be the only way to maybe get it back. You'd also have the issue of how to collect it from Earth's orbit since a) Earth would have moved a lot while it was travelling and b) if you were lucky enough to get it to pass close to Earth, it would be travelling quite fast and probably wouldn't have enough fuel to go into orbit around Earth for collection. It would definitely be tricky.

A very good example of a scenario relating to your question is the movie Apollo 13. If you haven't seen it, I recommend that you do. As far as I can remember (and I freely admit it's been many years since I watched it, so don't hold me to this), the physics in it was pretty accurate. In that, things go wrong with the (real life) 70s moon mission and, among other fixes, the astronauts have to slingshot around the moon to get safely back to Earth.

In the end, I'd say it depends on the nature of your mission as to what would be done. If it was a manned mission to Mars, for example, they might try harder to bring them back early, but physics would not be on their side.

Hope that answers your question!

Gravity and atmospheric pressure

I have another response to an "Ask Tsana" question today.

Brookelin asked:

I was wondering... with planets like Europa and possibly Ganymede, who possible have oceans, if humans made future settlements under said oceans, would the pressure from the water above counteract the effects of reduced gravity on the human body?

Interesting question. A preliminary point: it's Jupiter's moons Europa and Callisto that probably have sub-surface oceans (especially Europa), not Ganymede which is a solid rocky moon.

Europa, one of Jupiter's moons, has a vast ocean beneath
its surface. Credit: Galileo Project, JPL, NASA;
reprocessed by Ted Stryk

So, how do pressure and gravity work? In this context, gravity is the force that holds a planet/moon/star together and which attracts other objects to it. So we're all being pressed into the surface of Earth due to Earth's gravity. Pressure is the force a surrounding fluid (air, water, etc) exerts on something. So the atmospheric pressure we feel on Earth is pushing at us from all sides (well, OK, not out from the ground) and is due to all the air in Earth's atmosphere.

When you go swimming, the further you dive down, the higher the water pressure around you gets. This is because the deeper you are, the more water is above you to press down on you and the more water is above the bits of water on either side of you, also pressing into you. If you've ever been snorkelling (or scuba diving, I suppose but I can't vouch for that due to lack of experience) you might have noticed that it gets harder to breath the deeper you go (assuming a long enough snorkel). This is due to the water pressing down on your chest. Air does the same thing, but we're used to it, so we don't notice. The other thing that happens under water is that the water underneath you pushes up on you: this is called the buoyancy force and it's why things (people, tennis balls, icebergs, etc) float.

The higher up you go from sea level on Earth, the thinner the atmosphere gets (basically, the less atmosphere left above you). To halve the atmospheric pressure you experience, you need to go 5 km above sea level. (On the other hand, to double the pressure, you only need to be about 10 metres under water.) At that height, gravity is still pretty much the same as at sea level (the difference is about an eighth of a percent) and your main problems are getting enough oxygen (not a huge problem if your lung capacity is OK) and possibly altitude sickness (potentially a problem).

We need some amount of air pressure around us to survive which is part of the reason astronauts wear space suits. However, there is a range at which we can still function and that range increases if we have extra oxygen (and don't get altitude sickness). People have climbed Mt Everest (8.8 km above sea level) which has an atmospheric pressure of about a third that at sea level at it's peak without oxygen, but even doing it with oxygen requires training and acclimatisation and isn't something anyone can just decide to do one morning (well, unless they also decide to put in all the training).

On the surface of Europa or Callisto, there is no atmosphere and hence no atmospheric pressure. The ground is frozen water (probably not pure water, if only due to meteorite bombardment, but that's beside the point), but let's suppose we somehow got under the surface and set up a habitat. Since we're human and breathe air (a particular mix of mostly nitrogen, with some oxygen, carbon dioxide and misc) we'd have to have some sort of bubble habitat under the sea. But it's not just the air part that we need, we also need it to be around one (Earth) atmosphere of pressure. So we build a habitat with solid walls and fill it with the right amount of air... and then we're inside an air bubble and the water outside the bubble is having no effect on our bodies directly. The only way it would is if we went out into the water without pressure suits. Which probably wouldn't be the best idea in the world for a variety of health and safety reasons that don't necessarily have to do with the water pressure.

Now let's talk about gravity. The main way we detect small changes in pressure is though our ears, for example when they pop on taking off and landing in aeroplanes. The main way we detect changes in apparent gravity (which is the same as changes in acceleration) is when we feel lighter or heavier. If you're standing, this might manifest as extra strain on your legs, if the apparent gravity has increased, or a feeling like your stomach is moving upwards (possibly accompanied by nausea), if the apparent gravity has decreased. You don't experience the same feeling underwater or up a tall mountain because the gravity doesn't change in those places although the pressure does.

So what I'm ultimately trying to say is that the effects of gravity and atmospheric pressure are different. You can't compensate for a decrease in gravity by increasing pressure. Pressure is a force applied from all directions simultaneously, while gravity acts in just one direction. We know about the effects of Earth gravity, high gravity (from fighter pilots for example) and zero/microgravity (like on the space station) on people but much less about the effects of gravitational fields less than Earth's and more than zero. Europa's and Callisto's accelerations due gravity at the surface are about 13% Earth's and for comparison, the moon's is about 17% Earth's) so while we have had some experience with the moon landings during the Apollo missions, we don't really know how serious the health problems associated with spending prolonged periods at such low accelerations would be. There almost certainly would be some, but they probably wouldn't be as severe as zero gees. So while we can't use water pressure to compensate for gravity, it's not impossible for people to live on one of the moon's of Jupiter. We just don't know enough about what long term problems might arise.