Back after hiatus; Good, Bad & Wha??
My apologies for the long hiatus; I've been overwhelmed with work and travel. Fortunately, those issues seem to have cleared up with the end of the current semester, so I'll be able to put in a bit more time here.
I thought I might also vary the tone a bit by an occasional list of "The Good, the Bad, and the Wha??" Everyone's familiar with "The Good, the Bad, and the Ugly," of course; it's been used fairly often as a rhetorical framework since the movie came out. In this case, I wanted to discuss things done right and wrong in science fiction examples - or, more fully, right, wrong, and "what were they
thinking??"
For example, let's take a basic principle too often abused in science fiction: the issue of sound in the airless void of outer space. Sound, of course, requires a medium through which to travel; if there isn't something for it to pass through - something dense, like air or water - then the sound waves just won't travel. (Note that this doesn't apply to light, which can travel without a surrounding medium.) So, some examples:
The good: Explosion of soundlessness
The television show Firefly, during its first episode, "Serenity", has a scene which handles this extremely well. In an early scene, the protagonists are working to recover salvage from a derelict spaceship. They have rigged explosives to the ship's hatch, and are maneuvering to set them off. Except for short bursts of conversation over radio channels, the maneuvers and the explosion are all in complete silence - not even background music. Although some have said that "noises in space" are necessary to add drama, this total silence is actually quite dramatic, heightening the sense of tension. (2001 also used silence to good effect in the same way.)
The bad: Spaceships go whoosh!
Lots of movies and TV shows have this particular problem, but I'll pick Independence Day as my example, because I love to pick on Independence Day. As the heroes take their little spaceship toward the Big Mother Ship, it makes a nice little whooshing sound as it goes past. Of course, this sound is supposed to emphasize to us how fast the ship is going - but surely light-effects would have done that just as well. The common argument is that this is necessary for drama, but as above, silence, properly wielded, can be very stirring.
The
What were they thinking??: Outer space, office space, what's the difference?
The booby prize goes to a 1979 movie called The Black Hole, which was a stinker of epic proportions and shall be ragged on repeatedly in this space. The characters escape their spaceship and go out into outer space without spacesuits, doing things like talking, breathing, and other things made impossible by the lack of air. News alert, folks: Not only does sound not travel in a near-vacuum, but it really makes breathing troublesome.
Moral of the story: You can sometimes do better with dramatic (and scientifically accurate) silence than with the best sound effects.
Labels: goodbadwha
More hiatus
This past week I've been preparing for a big review - thus the lack of articles from this quarter. So, naturally, next week I will be at said review, and also uncommunicative. Hopefully after that the work side will be a little less hectic, and I'll be able to contribute more.
Interplanetary Navigation For Dummies (part 1 of 186,282)
If you've ever been to Disneyworld, you've probably seen a ride called the Mad Tea Party, and if you were particularly masochistic or naive at the time, you may even have put your stomach to the test by giving it a try. For the lucky few who haven't had the "pleasure," it's basically a circular platform that spins, while the victims sit atop it in giant teacups that also spin on their own axes. Not advisable following a heavy lunch, of course, but for our purposes this isn't a bad stellar system simulator.
Suppose that you're sitting in one of these spinning teacups and you wanted to plot a straight-line trajectory that would land you in a specific seat in a specific teacup somewhere else on the big spinning platform. How fast is your teacup spinning? How fast is the destination teacup spinning? How fast is the overall platform spinning? How far from the platform's axis are each of these cups located? How much would the spinning of your own teacup "fling" you as you leave it, and in what direction? What would be the ideal moment during your teacup's spin cycle for you to launch yourself away from it? Similarly, how can you time things so that you'll arrive at the destination cup at just the right time to land in the seat you want?
Welcome to the fun world of interplanetary navigation, otherwise known in the business these days as orbit determination. This is the complicated science of getting from one planet to another within a stellar system--or more specifically, getting from an orbit around one planet to an orbit around another. (If you're traveling between stellar systems, you need interstellar navigation, which is a whole other kettle of fish we can get into another day.)
Orbit determination is without a doubt one of the hardest problems in the physics field of classical mechanics. There is no "simple" equation that describes the path from A to B, and over distances of billions of kilometers the tiniest errors get magnified to disastrous proportions. In fact, almost all of the space probes we've launched toward destinations within our solar system have relied on Earth-based course correction management--measurements of the probe's position and velocity relative to the destination as assessed from Earth, and course and thrust adjustments transmitted from Earth-based monitoring stations. I mention this to highlight just how computationally difficult this problem is--the number of variables involved is huge, and solving a set of differential equations this large is not something that can be done on the back of an envelope.
That said, the science of orbit determination is coming along quite nicely, particularly as computing power becomes faster, cheaper, and available in smaller packages. These days we're able to do orbit determination with enough precision to remotely pilot a space probe through a 5x5 km window of space a billion kilometers away. Or to put it in industry slang, we're able to sink a corner shot on a billion-kilometer pool table. By doing course assessment and correction from Earth-based monitoring stations, of course, we can maneuver a space probe much more precisely as it nears its destination, so we don't have to be 100% right with that initial trajectory--99.9999995% will do.
On the surface, though, the problem of getting from one planet to another can be simplified a bit by doing orbital plots. If you want to go from Earth to Mars, for example, you could look at where both of these planets are at this moment in time, look at the plots of their respective orbits around the Sun, and express the motions of these planets as a pair of equations linked by time. You could then look at the plots and see where the two orbits come closest to one another, and once you know where the planets both need to be for that to happen, you can solve for the time that satisfies both equations. You then know that at that particular moment in time, Earth and Mars will be at their closest approach to one another.
That's not enough, of course, since it's going to take some time for your spacecraft to get from Earth to Mars, so you have to compute the travel time across that distance and arrange to launch your ship that far in advance of the closest approach. Unfortunately that's not easy either, since you have to compute the distance from where Earth was at launch time, not at closest approach time. Again, you need to solve a series of equations to find your perfect launch time. Once you do launch, your craft is aimed at an empty patch of space, with the expectation that by the time it gets to Mars' orbit, Mars will have arrived at that position. Timing, as they say, is everything.
This also makes assumptions about constant velocity--that the spacecraft is going to burn its thrusters for a certain amount of time to accelerate, then coast at a fixed velocity until it nears its destination, at which point it will turn around and burn its thrusters in the direction of travel to slow it down. Your calculations need to take into account these periods of acceleration at both ends (positive and negative).
Things get even more complicated if you decide to use gravitational assists to save fuel. By plotting a trajectory that takes your craft close enough to a planet or star to be pulled toward it, yet traveling fast enough to avoid being captured by the body's gravity, you can get a free "slingshot" out of the deal, accelerating your vessel dramatically.
On the flip side, you can use aerobraking to slow down a vessel by skimming its upper atmosphere (presuming of course that it has one, and that it's dense enough to provide a decent amount of drag). Here, though, it's a tradeoff, because you're obviously close enough at this point to get a gravitational assist whether you want it or not, and if you slow down too much you'll end up caught in the planet's gravitational pull. You have to be very certain about the composition and density of the planet's upper atmosphere, in other words, because the margin for error in either direction is very small. As such it remains largely a "cinematic" maneuver (e.g. Arthur C. Clarke's 2010).
The whole situation begins to seem hopelessly complex when you consider that every object in the stellar system exerts some gravitational force on the spacecraft, and that all of those objects are constantly in motion, their gravitational force vectors effectively changing in both magnitude and direction continuously. The Galileo probe, for example, had to maneuver its way to Jupiter over the course of six years, receiving gravitational assists from Venus once and twice from Earth, and then smaller tugs from all of Jupiter's major moons. Every time Galileo neared one of those moons, it sped up or slowed down somewhat, leaving it in a slightly different orbit of Jupiter each time. The most impressive part? Every one of those gravitational interactions was pre-calculated before launch in 1989, using several months of supercomputer time.
In short, astrodynamics and orbit determination is by no means an easy subject, but I'll be glad to visit it again in the future if there's more interest. Your astronavigator characters will thank you!
Why is there air?
So, the first question: Why is there air?
The current model for such things, or at least the favored way of explaining matters, is that the air we breathe today is Earth's "third atmosphere." This, of course, implies that there were first and second atmospheres. The whole story is quite relevant for those who're creating their own planets, too.
When the Earth was first forming, it did so by the gradual accumulation of material. The most abundant component of that material was hydrogen, followed by helium, followed by other substances. Dust clumped together into rocks, rocks into boulders, boulders into masses held together by their own gravity, called "planetesmals." Planetesmals accreted and collided into larger bodies, eventually becoming planets. In the process, they accumulated quite a lot of gas around themselves, mostly hydrogen and helium, those being, as mentioned, the most abundant elements. Thus, the forming Earth was in a haze of hydrogen and helium - its
first atmosphere.
Trouble is, hydrogen and helium are "light" gases. They were fairly tentatively held by the gravity of a small planet like Earth, particularly in the relatively warm environs at Earth's orbit. Warmer atoms and molecules move faster; the chances for hydrogen and helium to escape Earth's gravitational hold were considerable. As a result, the greater part of these gases escaped out into interplanetary space.
However, there was a replacement available. The newly-formed Earth was a hot volcanic sort of place, heated by its own gravitational compression and by the decay of radioactive elements. Gases that had been trapped deep within the Earth were released by volcanic activity, a process called "outgassing". Carbon dioxide was far in the lead of the gases thus released, with water vapor, ammonia and nitrogen coming in distant seconds. This thick carbon dioxide haze was Earth's
second atmosphere.
Venus, in fact, still has an atmosphere very much like the early one on Earth. Venus is a bit smaller than Earth, so obviously Earth's gravity was enough to maintain a carbon dioxide atmosphere. So what happened?
On a warm planet like Venus, the water remains in the form of water vapor. But on Earth, the temperature was just right for the water to condense in liquid form, creating the oceans we know and love. What've the oceans got to do with the atmosphere? Well, it turns out that carbon dioxide dissolves easily in liquid water. What with these new and abundant pools of liquid water lying around, fully half the atmospheric carbon dioxide dissolved away into the water. Yet even without half of its carbon dioxide, the resulting mix isn't at all the atmosphere we currently know and love.
The secret ingredient is - life. In that warm and hazy murk, in those new-formed oceans, life had quietly formed and was taking full advantage of that abundant carbon dioxide, which it used as an energy source. As a waste product, those handy little bacteria churned out oxygen. At first, that oxygen just combined with other elements. Eventually, however, as life spread and evolved, more and more oxygen was produced and started to accumulate in the atmosphere. As the life forms perished, they sank to the bottom of the ocean ooze, there to eventually harden into limestone and binding the carbon dioxide into the rock. Other handy bacteria subtracted out the ammonia, releasing nitrogen instead.
Thus the final product - most of the carbon dioxide and ammonia were subtracted out, while more nitrogen and new supplies of oxygen were added in. Thus our current
third atmosphere - lots of nitrogen "filler", plenty of the oxygen so useful for the animal life that later evolved to make use of it, and smaller amounts of carbon dioxide, water vapor, and assorted other.
The moral of this story: if a planet has abundant oxygen in its atmosphere, it's highly probable that at least primitive life-forms exist on that planet, to create that oxygen in the first place.
Triumphant return, or something.
Just a quick note to say that I am back, but running to catch up on things right now. I did get a couple of questions, which I shall strive to answer tomorrow, after I've slept. :)
Pause for questions?
I'm going to be away this coming week at a meeting, and I don't know if I'll have a chance to update this during that time. If not, I'll write more next weekend.
Anyhow, this seems to be a good time to solicit questions! Please, leave comments to let me know about topics you might like to discuss, questions you'd like to have answered, objections and corrections you'd like to raise, or anything like that! It doesn't have to follow along the lines I've been taking so far - anything of astronomy and science fiction would be fine. I can natter on forever, but it's probably more interesting for everyone when it's a matter of addressing specific issues.
The Doom of Stars
On to the further development of stars - and what might happen to the planets that surround them. To discuss this subject, though, we need to divide and conquer. Stars have two possible fates, once again depending on how massive the star is.
Stars below 8-10 times the mass of the Sun will go through a few giant phases, and eventually dwindle and die relatively quietly. They will shrug off their outer layers of gas, forming what's called a
planetary nebula, while the inner parts of the star shrink down to a stellar cinder called a
white dwarf.
A planet around one of these "low-mass" stars will have to deal with the initial red-giant stage of the star, of course, first. Afterwards, over the next billion-or-so-years, that star will undergo further changes. I mentioned that hydrogen fusion had stopped in the core of the star, because all its hydrogen had been turned to helium; the shrinking and heating of the inner parts of the star leads to fusion in a shell around the core, creating the red-giant stage. But as that core heats up further, it will finally arrive at a point where
helium starts to fuse. In a "helium flash", the core ignites, fusion-wise. Now you have two sources of fusion within the star - the helium-fusion core, and the hydrogen-fusion outer layer. The star heats up, becoming more "blue" overall. It will also shrink by a small amount, though remaining a "giant" star.
Alas for the star, the helium-fusion in the newly helium-rich core will use up that fuel a good deal more rapidly than the hydrogen-fusion used up the core hydrogen. When the helium is almost all converted to carbon and oxygen, the core goes "dead" again. Once more the star's inner parts start to collapse inward.
This time, the gas "shell" where hydrogen fusion had been occurring is now rich with helium. As this shell shrinks and heats, helium fusion begins in this layer. On top of that - more or less literally - the layer of gas above the helium shell (which is still almost all hydrogen) will now start hydrogen fusion. So you end up with an inert carbon-oxygen core; a helium-fusing shell around the core; and a hydrogen-fusion shell around both. This "double-shell burning" star puffs out even further, becoming still larger. It cools down at the surface again, as a result, becoming a double-shell burning red giant.
These puffed-out layers are not fated to remain with the star forever. Vast and varying outflows of radiation and particles - "stellar winds" - will carry off this gas little by little (or in some stages, rather more than a little). The gas is dispersed in rings and shells around the dying star, carrying off a sizeable percentage of the star's mass over time. Eventually, the shrouds of gas will be seen from afar as a
planetary nebula. (These nebulae have little to do with actual planets; astronomers named them that because, to earlier observers, they looked like planet-shaped disks in the sky.)
Meanwhile, the carbon-oxygen core of the star continues to dwindle until something rather strange happens. By now, the material has been so squashed that instead of the gas we started with, the star's core almost resembles a solid - atomic nuclei and their surrounding electrons arranged in a matrix-like array. Essentially, the atoms run into a law of quantum mechanics called the Pauli Exclusion Principle. This law states that no two electrons can occupy the same quantum-mechanical state in the same time. WIthout going greatly into detail, this means that there's a firm limit in how far the mass of the core can squash the matter in the core before the electrons "refuse" to let the atoms come any closer together. This "refusal" is called "degeneracy pressure", and it halts the collapse of the star's remains.
What's left is something that might once have had the mass of the Sun, now crushed into a sphere the size of the Earth. White-hot, its radiation will heat up the shrouds of gas thrown off by the dying star, making the planetary nebula glow with gorgeous colors. The core is now a
white dwarf, and will remain that way for a long, long time. Although it generates no heat of its own, now - both fusion and collapse having stopped - it will be very slow to cool. A long time hence it will no longer give off appreciable radiation, becoming a black dwarf, a truly dead star that will shine no more.
So imagine all this from the point of view of a planet in distant orbit around some low-mass star. At first, the sky is dark, though one particular star in the planet's sky shines more brightly than any of the others, orange or yellow or white. Then after billions of cold years, that brightest star expands, becoming thousands of times brighter, warming our icy world considerably. It becomes a glowing red source of light for our distant world.
Over the next few hundred-million to billion years, though, more changes are in store. The star will abruptly become bluer than its previous red, though still pouring radiation onto our hypothetical planet. Then after a period of more-or-less stable output, once more it will begin to expand and redden, giving off still more radiation. But this bounty of radiation is not without cost, and the star will diminish over time. Our planet will see its surroundings embedded in a very light haze of gas. As this trend accelerates, the bright star dwindles down into something far fainter; the bountiful radiation is lost, and the planet is left to freeze even deeper than its original state - lost, as it were, in the outer darkness.
The high-mass stars are another matter entirely - they end not with a whimper but with a bang. But this is already too long, so more on that next time.
Dying stars and living planets
So when we left our heroes, I was mentioning that sure, a star's expansion to a red giant, as it leaves the main sequence, could certainly warm a planet into a new, possibly life-enabling, climate. But there are drawbacks...
First, the "giant" stages of a star (and I do mean stages plural - more on that later) only last perhaps 10% of the star's main-sequence lifetime. Our Sun, for instance, is thought to have a main-sequence lifetime of perhaps 10 billion years; we've used up about 4.6 billion of that time so far, during which life formed on Earth and evolved into a number of interesting critters, like humans. Once it becomes a red giant, it will only last a billion years in the various giant stages.
From an evolutionary standpont, this is a bit of a drawback! If it takes several billion years for evolution to produce an intelligent species, then life on a planet thawed by a red giant might just have figured out how to go multi-celled when the whole works putters out again. Bummer.
Still, you might be able to jigger things just right by using a smaller star - lower mass than our Sun. Such a star would evolve more slowly, as those low-mass stars do. A star with a 30 billion year main-sequence lifetime (one of those li'l M stars) might just last long enough in the giant stages to produce an intelligent species.
It's not going to be a story with a happy ending, though. We here on Earth can look forward to several billion years more of our friendly yellow Sun (though it is very very slowly increasing in brightness, it shouldn't be enough to harm Earth for a long time to come). But our newly intelligent species will have come into being during the twilight of their planet, where the source of their life is approaching its own death. Doom for their species is just around the corner... it's rather Norse, now that I come to think of it. Or if that's too gloomy, imagine the space race multiplied by millions, when a species realizes they've got to get very good at space travel very quickly, before their time is up!
What happens when time's up? I'll save that for next time.
