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.

Quick note on terraforming Galilean moons

This post comes from an "Ask Tsana" comment.

Sam Keola asked:

Aloha from Hawai'i again Tsana! I have a hypothetical question. If in the very distant future we had the technology to terraform, would it be best to terraform Callisto and Ganymede or set up domed bases? Ganymede is suppose to have an ocean similar to Europa, but I'm not sure if that's "world wide". Your thoughts on terraforming!

The main problem with terraforming either of those moons is their gravity isn't large enough to keep any atmospheric gases for long after they're introduced. Ganymede, which is larger, has a surface gravity of close to a seventh of Earth's which is less than half of Mars's and Mars has difficulty keeping much of an atmosphere itself. Purely from that point of view, domes or something else sealed would be better.

Callisto.
Credit: Galileo Project, Voyager Project, JPL, NASA

Once you've decided to build something sealed, then it would be better for colonists to build on Ganymede, as opposed to the other Galilean moons, for a few reasons:

• It has the highest surface gravity, not by much but every little bit would prevent colonist's bodies from degrading. Actually, because the Galilean moons are less dense than Earth's moon, they have a lower surface gravity, despite being larger in volume. You're going to have low gravity-related heath problems in any case, however.
• It's not as close to Jupiter as Europa (and Io!) is. The phenomenon responsible for keeping Europa's interior liquid is tidal friction thanks to its proximity to Jupiter. It's the sort of thing that also makes the surface more unstable (prone to volcanoes -- not as much as Io, of course -- and quakes) and less hospitable to people. You can read more about it here.

On the other hand, if what you're doing is mining and the minerals etc you're interested in are found on both Ganymede and Callisto, then Callisto is the place to put your colony. It's gravity slightly lower and, more importantly, it's further from Jupiter, meaning that when you're exporting your rocks, there's less gravitational pull from Jupiter to overcome.

In terms of finding water to mine, all three moons in question (ie, not Io) have water on them, so that shouldn't be too much of a problem, especially if you're already planning to mine other things.

Of course, there are also reasons why Europa would be a desirable place for a colony, especially for scientific reasons, exploring it's subsurface ocean primary among them. There's a good chance there's microbial life there.

So there you have it, if you're going to colonise the larger Galilean moons, it's better to build a close structure on them rather than try to impart an atmosphere. It would be even harder than giving Earth's moon a permanent atmosphere.

Other Foreign Skies

This post is a response to a question I got on my Ask Tsana page.

Sam Keola asked:

Love the views of Jupiter from Ganymede and Io. How large would it appear from Europa or Callisto? And how large exactly would the sun appear? (I know tiny as hell, but another lovely picture would be amazing.)

The mathematical answer to that is explained in this old post. And my first set of Jupiter images (Io and Ganymede's skies) can be found here.

Jupiter

This time around, I used a different image of Jupiter so if you're wondering why it's rotated relative to the old pictures, that's why. For the Jovian images, I've used the same starting image because in the year since I last did this, I haven't managed to take a more suitable photo. Such is life.

The original photo with a full moon in Earth's sky.

So. Europa is the second Galilean moon out from Jupiter. It's made mostly of ice, is the smallest of the Galilean moons and might harbour life in its subsurface liquid ocean. The diameter of Jupiter as it would appear in the Europan sky is almost 24 full moons across. Remember that Europa's sky wouldn't actually look blue either since it doesn't have an atmosphere but I don't have a decent night skyline to work with. I'll do a night version eventually.

The size Jupiter would appear in Europa's sky. Or in Earth's sky if you swapped it with Europa.

You might be wondering whether Jupiter would actually be oriented the way it appears in these images. Well it depends. The direction the bands run relative to the moon's horizon would depend on where on the moon you were. Close to the equator, the bands would be vertical (although if Jupiter was high in the sky, it would be pretty difficult to tell. Perhaps better to say east-west). If you were near a pole, they'd be horizontal as in these images. And remember, the Galilean moons are all tidally locked, so Jupiter would never move, just change how much of it was illuminated by the sun.

And Callisto, the most distant of the Galilean moons. Callisto's Jupiter would appear "only" about 8.5 full moons across.

The size Jupiter would appear from Callisto. If Callisto had an Earth-like atmosphere and gum trees.

The Sun
 
The second part of Sam's question was how large would the sun appear from Jupiter. Well, on Earth, the sun and the moon appear to be approximately the same size (there's a little bit of a difference when the sun is at its closest and the moon at its furthest and vice versa). So the sun from Earth is about one full moon in diameter.

From Jupiter (or its moons) the sun would appear about 0.4 full moons across which is a little bit less than a sixth of the area of the sun as seen from Earth (remember, the moon and sun seen from Earth are on average the same size).

I cheated a little bit with these next two sun photos. They're actually two separate photos and I made the sun smaller in one of them. The reason the rest of the photo looks darker for the Jovian sun is because I was fiddling with settings on my camera. And if you're wondering why I chose sunsets, it's because those (and sunrises) are pretty much the only kinds of photos where the disc of the sun is properly visible.

Ordinary sunset on Earth:

Sunset. A little bit more than half the sun is below the horizon.

Sunset if Earth was at the same distance as Jupiter (but yet still warm enough to have liquid water. And plants. By the way, with these two, it's probably clearer if you click on the images to enlarge and compare the sun side by side.

A more diminutive sun, less than a sixth of the area of Earth's.

And there you have it. Photoshopped images (well, actually, I used Pixelmator) depicting the sizes of Jupiter and the sun from the Galilean moons and the Jovian system, respectively.

Destroying the Earth

This post is inspired by a question I got on my Ask Tsana page. Katrina asked:

I'm trying to come up with a simple (haha) and plausible way to destroy a planet to kick things off for a story but am having trouble getting the science right.

One of the first sites I visited to figure this out was this Geocide site: http://qntm.org/geocide

Under the Geocide in fiction page (http://qntm.org/fictional), the author says, "The Sun Crusher is a relatively small ship which carries a small number of missiles, each of which is tough enough to shoot into the centre of a star and cause it to go nova, which would certainly annihilate any nearby Earthlike planet."

My question is, don't stars that get massive enough to go nova have brief lives and thus not live long enough for a habitable planet to develop? I'm just basing that on Wikipedia (http://en.wikipedia.org/wiki/Planetary_habitability#Massive_stars), though, so I was wondering if you could tell me more. Can the habitable zones of massive stars ever actually be inhabited (and then later die in a supernova)?

Excellent question!

The answer depends a bit on to what extent you want to destroy your planet. Geocide pretty much defines "destroy the Earth" as "annihilate in the particle physics sense, or dismantle/tear apart on either a macroscopic (large) or microscopic scale". He doesn't count Earth as destroyed if there's still a planet-like object there. However, for many narrative purposes, rendering the Earth entirely uninhabitable will do the trick.

So, leaving the dismantling and annihilation to Geocide (the website is amusing, although be warned that some of the details of physics are slightly off, but close enough), what are some ways of rendering Earth unfit for life?

Destroy all humans

So maybe what you want is not so much to destroy the planet as to destroy all the people on it. That's not really that hard. Or, at least, destroying most of the people isn't that hard. Some methods, which generally don't require elaboration:

• Widespread nuclear holocaust
• Some sort of plague
• Climate change. No, really, melt the icecaps and raise the temperature enough so that it is too hot and humid to survive without air-conditioning and eventually you'll run out of people. Or throw in some crazy weather disasters too. The Rhesus Factor by Sonny Whitelaw touches on this a bit (see my review here), also on the plague scenario.
• Very large volcano eruption. This is one of the things thought to have caused at least one of the prehistoric dinosaur(ish)-era extinctions. A less epically large volcano (actually, a few of them probably contributed) in 1816 caused the Northern Hemisphere (or Europe and America at least, not sure that Asia was affected as strongly, but google it if you're interested) to not thaw out in the summer. This was "the year without a summer". (And now I have that Rasputina song stuck in my head. Click the link and you will too.)
• Asteroid -- this one's a toss up between destroying all (most) humans and destroying all life. Ultimately, I suppose it's a matter of scale. Let's say this asteroid kills human life but not necessarily all the microbes. It would be somewhat similar to the volcano in the throwing dust and rubbish into the atmosphere, blocking out light and generalised doom.
• Magnetic field of the Earth turning off in the process of flipping. This is something that happens spontaneously every so often. It's bad because a whole bunch of ionising radiation (miscellaneous charged particles) from space is kept at bay thanks to our nifty magnetic field. Taking it away would give us a lot more cancer and sterility and could wipe out a large chunk of humanity. Microbes and probably a lot of (some?) sea life would be OK. Good luck artificially killing the magnetic field, though.

All of those methods probably won't wipe out all life and, frankly, it's possible/likely that some tenacious dregs of humanity will hold on. Generally, evacuation is the surest way to avoid these apocalypses. Or prevention, but that's only really applicable in two or three of those scenarios.

Destroy all life

Why aim low? Bugger humanity and everything else with one of these sterilising scenarios:

• Self-replicating nanobots (von Neumann machines) which consume all the [insert important chemical here -- carbon is popular]. This is also know as the grey goo scenario. Depending on the nanobots, this is likely to render the Earth inhospitable to life while they're still doing their thing.
• Large asteroid/comet or small moon colliding with Earth. Where a small impact would cause natural disasters (earthquakes, tsunamis) and potentially block out the sun with dust, a large impact could do many detrimental things. It could change the Earth's rotation, knock it into a slightly different orbit (or send it spiralling into the sun, but that would require a particularly large body), it could smash the Earth into chucks (which, thanks to gravity, would probably later re-collide to form Earth 2.0), render an appreciable fraction of the surface molten... Actually, I now have a brilliant mental picture of two or six asteroids hitting the Earth simultaneously from opposite sides and sort of turning it into molten goop... Not actually sure that would work with two, but six seems faintly plausible in a hand-waving way. Anyway, point is, hit Earth with something big enough and bye-bye life. Depending on conditions, it's possible life could spontaneously arise again, depending on how reliably life arises and how long it takes (before, for example, the sun goes red giant).
• Supernova/nearby gamma ray burst. The main problem with this notion is the lack of suitable supernovaing stars nearby, as Geocide mentions. However, you mentioned destroying a planet, not specifically Earth. A planet orbiting a star when it went supernova would be toast. Probably, it would be fairly inhospitable before the actual explosion, if Eta Carinae is anything to go by. A planet orbiting a non-explosive star near another star that went supernova could well end up sterilised, which is what I talked about in my post about the galactic habitable zone (and near in the astronomical sense isn't that close by). And, actually, if we're talking about Type Ia (which is to say not core-collapse supernovae; not the death throes of a large star) supernovae, which involve white dwarfs and (probably) ordinary stars going through their red giant stage, we might not even see the supernova coming. That's a slightly unsettling thought.
• Some sort of implausible doomsday device. Really, you can make up whatever rubbish you want for this one if you're so inclined. (But if you do, I don't promise not to tear your science apart if I read/see/whatever it.)

A few words on supernovae and novae

Supernovae are how stars bigger than about 8 solar masses end their lives. Novae are not small supernovae. I know, that's what I originally learnt as a child/teenager by osmosis from SF novels. I think the connection between the words nova and supernova are primarily historic; a star suddenly appeared or became much brighter and acquired the label (nova meaning new), but the different causes weren't understood until much more recently.

Stars smaller than around 8 solar masses don't explode. They expand relatively slowly (well, y'know, compared with a supernova explosion) when they run out of hydrogen to fuse in their cores, then contract then expand again when they run out of helium. At this point, a large star going through the stages much more rapidly would collapse again under its own gravity and then kaboom supernova. Smaller stars aren't massive enough to collapse again under their own gravity. Instead, after the helium is used up, leaving either carbon or oxygen (or a combination) in the star's core, what was once the stellar atmosphere will keep expanding indefinitely. Initially, it forms a planetary nebula (not actually anything to do with planets), but eventually it will all dissipate and be undetectable. What's left behind is a white dwarf; basically a small, hot star which was once the core of the red giant star.

Suppose there were two stars near each other, and one went through the red giant to white dwarf steps before the other. When the companion star undergoes its red giant phase, maybe it expands enough that some of it's outer atmosphere is close enough to the white dwarf to accrete onto it. The reason these stars didn't supernova is because they were too small. There is a very definite upper limit to how massive a white dwarf can be before it collapses in on itself and explodes. That limit is 1.4 solar masses (but remember, most of the original star's mass is lost when it's doing the expanding thing, which is why the original star can be up to 8 solar masses). If the companion star accretes too much matter onto the white dwarf, the white dwarf will go over the 1.4 solar mass limit and explode. This is a Type Ia supernova. For the record, the alternative explanation for Type Ia supernovae, which is presently gaining more traction, is two white dwarfs colliding.

Since white dwarfs are small (1.4 solar masses in a volume roughly the size of Earth), they're not very visible, especially once they start to cool down. See how we might not see that kind of supernova coming? It could potentially not be that difficult to artificially orchestrate, either, if you have enough spare matter to throw at a conveniently placed white dwarf. Well, y'know, sort of easier than some large-scale astro-engineering projects could be.

 Back to the point

 If you recall the original question, Katrina asked:

My question is, don't stars that get massive enough to go nova have brief lives and thus not live long enough for a habitable planet to develop? Can the habitable zones of massive stars ever actually be inhabited (and then later die in a supernova)?

In general, the bigger the star, the shorter its life. Our sun's total lifespan is something like 10 billion years, a blue giant could live only 10 million years, and a red dwarf's lifespan is in the trillions of years. The current theory is that it took a couple of billion years for (very basic) life to arise on Earth. It then took a long time to progress to where we are now (Earth's age is 4.6 billion years, from memory). If we see this as typical, it seems like there isn't enough time for life to arise around a much larger star. On the other hand, if there was a planet at a suitable distance from the star, it could be inhabited by sufficiently motivated humans with spaceships. They wouldn't be able to stay there indefinitely, but even a few million years is more than ages on human scales, so that's OK.

The other issue is metallicity. Planets like Earth have rocky cores, which means they have high metallicity. Remember, metallicity in an astronomical sense refers to the abundance of elements heavier than helium, not necessarily just the things chemists/sane people identify as metals. Very massive stars generally form in metal-poor environments and are metal-poor themselves. This makes the presence of heavier elements as requires for rocky planet building less likely. Not necessarily impossible, but much less likely.

So yes, you can potentially have planets around the sort of stars that go supernova and, while native life probably won't get very complex if it arises at all, said planets could be inhabited by plucky humans.