Wednesday, June 29, 2011

Weird Worlds: KOI 730

Before I get onto the main part of the post, I'd like to apologise for the lack of short posts over the weekend but life's been hectic. Part of the reason for this is that I'm going to be away at a conference next week. I plan to queue up a post to automatically go live next Wednesday since I'm not sure how reliable my internet access will be (also because I'm not taking my laptop and will be relying on Blogger's willingness to talk to my iPad, another thing of which I am not confident). On the other hand, said conference should give me lots of fodder for short, if not long, posts. So that's something to look forward to.

On to the topic of the week! Today I am going to be writing about another crazy exoplanetary system. However, unlike Kepler 11, this one isn't quite confirmed yet. KOI stands for Kepler Object of Interest and means that it's a system that the Kepler mission has identified as potentially containing some planets (four in this case) but they haven't been confirmed by other supporting data. Because science is all about the independent evidence. Nevertheless, this is a blog about science fiction, so we are quite at home to a little speculation. As such, please remember that all of the facts I state below about planets are as yet unconfirmed and haven't quite passed into official scientific cannon.

Kepler Object of Interest 730

First, some basic facts about the system. The star has designations KOI 730 or KIC #10227020 (one of these is easier to remember than the other, so guess which I'll be using). It is similar to the sun, but slightly larger and slightly cooler (by a couple of hundred degrees). It is more than 4200 light years away. The four planets are all very close in to the star, much closer than even Mercury's orbit in the solar system, making them conclusively uninhabitable to life as we know it.

Here is a screenshot illustrating the system from this Kepler Candidates Exoplanet app (not to be confused with the other one I've referenced before which was of confirmed planets, albeit otherwise pretty much identical).

The KOI 730 system side on (as would be seen by the Kepler telescope itself). Three of the four planets are visible in this illustration. The white line indicates the plane of the orbit (actually, it's a bit of a trail/track following each planet, but that doesn't come across too well side on). The planets are to scale relative to each other but not relative to their sun. Also, I don't think they get smaller when their on the opposite side of the star to the observer.


That's OK, because that's not the particularly interesting thing about this system. The scientifically interesting thing is that this system is locked in an orbital resonance. An orbital resonance is when two planets (or moons) orbit in such a way that they both complete an integer number of orbits in the same length of time. (An integer is a whole number such as 1, 2, 3, etc.) Some examples from the solar system are the Jovian moons Io, Europa and Ganymede, which are locked in a 1:2:4 resonance of orbital periods. This means that Ganymede's and Europa's periods (how long it takes them to complete one orbit around Jupiter) are respectively four times and twice the period of Io. (Sometimes this might be written 4:2:1 indicating that Io makes completes four orbits in the time it takes Europa to complete two and Ganymede to complete one. It depends on the convention being used.) Another example is Neptune and Pluto locked in a 2:3 period orbital resonance (so Neptune completes three orbits in the time it takes Pluto to complete two). A different sort of resonance is that experienced by Mercury, which completes two orbits in the time it takes to make three revolutions (so three Mercurian days are equal to two Mercurian years).

Orbital resonances can either make the system, or more specifically, the bodies involved in the resonance, stable or unstable. Yep, I know that sounds like they don't do anything because both possible outcomes are covered, but that's not true. What I mean is that if the resonance is unstable, you get things like gaps in the rings of Saturn (caused by some of Saturn's moons). On the other hand, the Jovian moons I mentioned before are in a stable resonance and Mercury is in its spin-orbit resonance rather than being tidally locked thanks to the gravitational tugs of other planets.

Which brings me to some of the effects of orbital resonance. Bodies locked in an orbital resonance exert a greater gravitational influence on each other than they otherwise would. For example, when Ganymede, Io and Europa are all lined up (Io and Ganymede on one side, Europa on the opposite side of Jupiter), they all experience a heightened tidal effect as the gravitational pulls of the other two planets add directly to the pull of Jupiter, causing additional friction in the planet interiors (and, for example, contributing to Io's volcanism).

Back to KOI 730

So I mentioned that KOI 730 has four planets and that these are locked in an orbital resonance. According to the first articles I read about it (in New Scientist and somewhere else I can't recall) and the original paper (section 5.3 is specifically about KOI 730) the resonance scheme for KOI 730 is 6:4:4:3. Notice the two fours there? That is why there were a spate of pop science articles about this system. Those two fours indicate that two of the planets are in the same orbit since orbital periods depend only on the star's mass and the distance of the planet from the star.

Two planets in the same orbit. They're located at two of the Lagrange points you might remember me mentioning a while back. Due to their positioning, they are known as trojan planets after the trojan asteroids that follow and precede Jupiter and Saturn in these same Lagrange points. The two planets are 118º apart along their orbit and are slowly, over millions of years, edging towards each other (at least; the authors of the paper speculate that they might last billions of years).

But yes, eventually they will collide.

The configuration of these two planets, KOI 730.02 and KOI 730.03, is such that they form an equilateral triangle with the star as the third point. Unfortunately, this puts the second planet as far away from the first as the sun, making it about a quarter of the height of the full moon as seen in Earth's sky. It would also always be about two-thirds illuminated and it wouldn't move around in the sky relative to the stars. If the planets were tidally locked to their sun then the other planet would stay locked in the same position in the sky while the stars moved around it, which would be fairly cool to observer. (Just think of the mythology that could arise surrounding that set up!)

It also bears mentioning that one of the theories of Earth's creation has two proto-planets forming at Lagrange points like these trojan planets. The other proto-planet, usually labelled Theia, and proto-Earth, inched towards each other and eventually collided, merging and splashing, so to speak, to form Earth and moon as we now know them.

Later, when I googled this again in preparation for writing this blog post, I found this article from Sky & Telescope, which features one of the authors of the original paper saying that further analysis of the data suggests a 8:6:4:3 resonance might be more fitting. This turns one of the trojan planets into a different orbit, farther out, and makes the system less exciting. I mean, a system in which all the planets are in resonance with each other is still pretty notable, but it's just not quite as imagination-grabbing as TWO PLANETS IN THE SAME ORBIT. Although, there's still potential for interesting story-science there when the planets line up and whatnot.

Oh well, co-orbiting planets in KOI 730 or not, the concept was around before this paper was written and there's no firm reason to not suppose we couldn't have trojan planets somewhere else. Maybe a gas giant with, instead of trojan asteroids following/leading it around, a full-sized terrestrial planet. Or two terrestrial planets sharing a habitable orbit...

The possibilities are endless. And science is cool even when it's speculative.

Wednesday, June 22, 2011

Artificial Gravity: Space Stations

So if you've paid any attention at all to the International Space Station (ISS)—or, really, Mir and/or Skylab—you will have noticed that the astronauts and cosmonauts up there float around in microgravity. Effectively, they're in free-fall and, although Earth is still exerting a gravitational pull on them, they can't feel it because they and the space station are falling around the Earth all at the same time (also known as orbiting).

However, in books and movies we see (on screen or in our mind's eye) people on space stations walking around and generally acting gravitationally enabled. A notable hard SF example that springs to mind is Clarke and Kubrick's 2001: A Space Odessey. (I say hard SF because soft SF isn't necessarily going to stick to physics as rigorously.) How do they get away with this? The short answer is by spinning them up. The long answer follows.

Spinning Around

You may have been to some sort of theme park which has a ride called the Gravitron or something along those lines. The ride I'm talking about is a flat cylinder where you get it, stand along one of the curved walls and then it spins up and, once it's at full spin, the floor falls away or something, and you discover that you are somehow being held to the wall. It's the fictitious centripetal force, combined with friction that is holding you up. Why is it fictitious? Long story, but the short version is: there are only four real forces (two if you count electroweak as only one, but let's not go into that today) and gravity is the only one we care about for today.

In our Gravitron ride, gravity is still pulling you downwards but the tendency of things to want to keep travelling in a straight line rather than around in a circle means that you are constantly accelerating and where there's acceleration, there's a force. There are a few technical details here about exactly which way the force is pointing and whether we should call it centrifugal or centripetal. The centripetal force is whatever force is keeping you going around in a circle. In the Gravitron, the wall pushing on you stops you from flying out of the ride. In space, the Earth's gravity keeps you in orbit and stops you from drifting off. The centripetal force is the force that's pushing you outward; it's the force pushing you into the wall and the force that gravity needs to balance to keep you in orbit. It's easy to see why these two are often confused or used interchangeably.

When something is travelling around in a circle, the acceleration it feels is given the square of the velocity divided by the radius of the circle:

And if you recall, we were able to characterise gravity on the surface of a planet by the acceleration due to gravity. On Earth, acceleration due to gravity, g = 9.8 m/s2. So if we equate these, we can work out how fast we have to spin something to make it feel like we're standing on Earth.

Knowing the velocity, how fast the edge of our space station is moving around, isn't that helpful for getting an idea of how much we have to spin it up. The period, how quickly it completes a full revolution, is more helpful:

T is the period.

So now, let's suppose we have two bits of space station joined by a kilometre-long cable and we want to set it up so that we have Earth-strength gravity in the two end capsules when they spin about the centre of the joining capsule. How fast would it need to spin? Plugging the numbers in, and remembering that there are 1000 metres in a kilometre we get... a period of 63 seconds. The whole thing would have to complete an entire revolution in just over a minute which, considering the distance each capsule travels in that time is about 6.3 km (the circumference of the circle is 2πr) is incredibly fast. At any given time it's linear velocity would be almost 100 metres per second which is more than 350 km/h. Whoosh!

Admittedly, once we got up to that speed it would be much easier to maintain it, but it would still be difficult to access the capsules. A better setup would be to have a tube or tunnel connecting them. That way, spaceships could dock at the centre where to match (angular) velocities with the eye they would only need to rotate once a minute (since the period is 63 seconds everywhere because it's all connected). Spinning a ship around once in a minute is much easier than a kilometre-long space station.

Of course, if you look at the last equation there, the bigger the radius of the circle, the longer the period but even then we quickly run into engineering problems. Not that our two capsules aren't engineering problem enough.

Reducing the gravity requirements doesn't help enough either. If you only require half Earth gravity, then our capsules still have to complete a revolution in a minute and a half and the speed becomes 250 km/h . Moon gravity? Two and a half minutes and 150 km/h. Maybe that last one is manageable, but are the health benefits great enough to justify the cost of setting it up as compared with a non-spun space station? (I have no idea; no one's lived on the moon for a significant amount of time yet to find out.)

Another practical problem is maintenance. On the outer edge of one of the capsules (or giant ring, as Clarke used in 2001), a maintenance worker would feel as though they were hanging from the capsule with their feet hanging down into open space. Same thing on the sides except that they'd be falling towards the outside or "bottom" side. This sets up some pretty dangerous working conditions. Add to that the fact that the space station is hurtling so fast the stars would be a spinning blur and I know what job I wouldn't want to have.

Reiterating: Why doesn't the ISS spin?

A few reasons, mostly technological:
  1. It would have to spin pretty damn fast, and we just don't have the technology to do that well yet
  2. It's powered by solar power and the panels are set up so that they're always facing the sun. This is harder to do if the whole thing is spinning; another technological limitation
  3. Even if we could get it spinning fast enough, ignoring the above, we only just finished assembling it which means we would only just now be spinning it up anyway (it would be a lot harder to assemble a space station that was constantly rotating.

Sunday, June 19, 2011

Solar eclipse: when TV gets it wrong

During dinner this evening, I caught part of Robin Hood on the ABC. (My internet-fu and ABC's handy iView tells me it was episode 1 of season 3, if you're interested.) Part of this episode involved a solar eclipse. I don't have a problem with solar eclipses. I don't object to them often having mystical significance or being used as a plot device.

What I do object to is the creators investing all their efforts into the CGI-ing of the eclipse (link to screen-cap) and not stopping to think about the immediate consequences. What am I talking about? Not five minutes after the eclipse has passed, we see Robin Hood standing on the battlements doing some heroic arrow firing with a backdrop of blue sky and a half-moon over his left shoulder.

A half moon. Right after a solar eclipse. What. The.

If you're feeling slightly lost at this point, allow me to explain. A solar eclipse occurs when the moon passes directly between the Earth and the sun. By a happy coincidence, the angular size of the moon in the sky is approximately the same as the size of the sun. This means that the moon, when passing in front of the sun, can cover it neatly and completely.

Anyone who has stared at the moon for any reasonable length of time will have noticed that it doesn't move around the sky all that quickly. More quickly than stars, maybe, but certainly not halfway across the sky in the space of five minutes. Five minutes after the moon has passed in front of the sun, it's still going to be near the sun in the sky. This means that, since it's day time, it will cease to be visible as the illuminated side of the moon—the part facing the sun—is most certainly not facing the Earth since it just passed between sun and Earth. For there to be a half-full moon in the sky, it needs to be about 90º away from the sun which amounts to about halfway across the sky.

Would it really have been so hard to photoshop (or whatever the movie equivalent is) the moon out of that shot? Really?

People not thinking these sorts of things through make me angry. Especially since lots of people must have been paid to think about the solar eclipse and they all apparently forgot about the mundane moon in the sky.

Wednesday, June 15, 2011

Weird Worlds: Kepler 11

This week I decided to talk about some of the weird, wacky and real exoplanets that we have discovered. I am going to just focus on a few with possibly more to come at a later date.

Kepler 11

I have mentioned it in the past and the Kepler Mission is certainly the current poster-child of the planet-spotting community. It has been staring at a patch of sky near celestial north since 2009 and in that time has found 1235 potential planets, 16 of which have been confirmed and most of which require further investigation.

There are six confirmed planets in the Kepler 11 system (the star, which was too far away and hence dim to have a better name, is designated Kepler 11). The star is similar to our sun. The planets orbiting it, however, are nothing like our solar system. Five of the planets are within what would be the orbit of Mercury if they were in our solar system. The outermost (or at least, the outermost discovered so far) is just outside the orbit of Mercury. They are also all relatively small, on the scale of previously discovered exoplanets, but much larger than our rocky worlds. They range in mass from 2.3 times the mass of Earth to slightly less than the mass of Jupiter (about 300 times the mass of Earth) and size from 1.93 Earth radii to 0.4 Jupiter radii or about 4.4 Earth radii (interestingly, the smallest isn't the lightest—Kepler 11-f is much less dense than Kepler 11-b). Their years range from 10.3 to 118.4 Earth days. For comparison, Mercury's year is about 88 Earth days.

Let's ignore the fact that all of these planets are too close to their sun to be habitable and have a think about what such a system would look like from within. From Earth, all the planets out to Uranus just look like bright stars which move across the sky. How different would the night sky look from a planet in the Kepler 11 system? For no particularly compelling reason, I'm going to work this out for Kepler 11-f which is the least massive and the second furthest out from the star.

First a picture. It's a (cropped) screenshot from this Exoplanet iPhone app, showing the Kepler 11 system. The planets aren't to scale with the star (they are very much inflated), but are to scale with each other.

Obviously the trails following the planets aren't real either, but are there just to give you an idea of the orbits. I like how the developer made it so only the side of the planet facing the star is lit up. Yay, realism.

Now some stats on the system, which I have mostly gotten from that Exoplanet iPhone app. Also, I'm going to abbreviate the names to Keb, Kec, Ked, Kee, Kef, Keg for Kepler 11-b through f. Because I can. (And if you're wondering, there's no Kepler 11-a because that designator is reserved for the star.)
  • Keb: 
    • 4.3 Earth masses, 
    • 1.93 Earth radii, 
    • orbit is 0.091 AU, 
    • closest approach to Kef is 0.159 AU
  • Kec: 
    • 0.04 Jupiter masses = 12.7 Earth masses, 
    • 3.09 Earth radii, 
    • orbit is 0.106 AU, 
    • closest approach to Kef is 0.144 AU
  • Ked: 
    • 6.10 Earth masses, 
    • 0.31 Jupiter radii = 3.41 Earth radii, 
    • orbit is 0.159 AU, 
    • closest approach to Kef is 0.091 AU
  • Kee: 
    • 8.4 Earth masses, 
    • 0.40 Jupiter radii = 4.4 Earth radii, 
    • orbit is 0.194 AU, 
    • closest approach to Kef is 0.056 AU
  • Kef: 
    • 2.3 Earth masses, 
    • 2.56 Earth radii, 
    • orbit is 0.25 AU
  • Keg: 
    • 0.95 Jupiter masses = 301 Earth masses, 
    • 0.33 Jupiter radii = 3.6 Earth radii, 
    • orbit is 0.462 AU, 
    • closest approach to Kef is 0.212 AU
Now for some calculations. Using the formula from my very first blog post, I calculated how big the other planets would appear at their closest approach to Kef. Also how big the sun, Kepler 11 (or Kea, why not) would appear. For all of these, I'm going to list them in units of the diameter of the full moon, except for Kea which I list in units of the diameter of the sun (because it's a star. Actually the angular diameter of the sun and the moon are roughly equal, which is why we get such nice looking solar eclipses). Also, this wiki table is useful for comparing how big things are in Kef's sky with how big things are in our sky. For future comparison, Venus varies in size from 0.16'-1.1' where the ' indicates arcminutes and there are 60 arcminutes in a degree. At it's closest visible point, however, Venus is a crescent that size, and full only at it's farthest point. That's about a thirtieth (0.036) of the size of the full moon. Easily resolvable with a telescope, but not quite resolvable as a disc with the naked eye.

At their furthest from Kef, all 5 other planets are similar to the size of Venus at it's furthest (and also it's fullest since that's when they are on the opposite side of Kea to Kef). At their closest approaches, however, there's a bit more variation. Remember for all of these except Keg the planets would be crescents of this size and only Keg would be full. Furthermore, I do declare that 1 FM (full moon) = 0.5º, and is the units I've used below. Because I didn't think anything would be gained by copy-pasting  "times the size of the full moon as seen from Earth" five times. (This is totally how units are invented. ;-p ) I may have also included some of the values as fractions.
  • Keb: 0.05 – 0.1 FM (max is a tenth)
  • Kec: 0.08 – 0.2 FM (max is a fifth)
  • Ked: 0.08 – 0.3 FM (max is approximately a third)
  • Kee: 0.1 – 0.7 FM (max is around two thirds)
  • Keg: 0.05 – 0.15 FM (max is almost a seventh)
I'm not really sure about Keb, Kec and Keg—I suspect they may just look like very bright stars—but the other two would definitely be visible as circles/crescents to the naked eye. The night sky of Kef would be a very different place to ours. I can't help but think that with such obvious neighbouring planets any intelligent life that evolved on Kef would work out celestial mechanics rather more quickly than we did. After all, we had to really pay attention to the sky to notice that some of the shiny points of light moved while others didn't. Not that life as we know it is likely to have evolved on Kef. Which brings me to the day sky.

Kef's sun, Kea, would be 4.7 times the size of Sol in our sky. Almost five times as big. (For those keeping track, that's the same as saying almost five times the size (width) of the full moon. That might actually be easier to visualise since it doesn't generally hurt to look at the moon.) It would also be around 22 times brighter than our sun, giving Kef 22 times as much energy as the sun gives Earth. Do you see why Kef might not be particularly habitable? Kind of like how you wouldn't want to live on Mercury, but more so.

Also, as you may have surmised from the first set of data I dumped, Kef isn't very dense. Density is given by mass divided by volume and volume is proportional to the radius cubed (actually, for a sphere, volume is 4/3 π R3). So Kef is only 0.14 times the density of Earth and, if it had a solid surface, which is highly unlikely, it would have a surface gravity of around a third of Earth's (similar to Mars's), despite being more massive. It's actually about three quarters the density of water, which isn't that surprising because if it were made of water (and I have absolutely no idea what it's composition is), we wouldn't expect water to be liquid in those conditions. What's interesting is that of the other planets, the two innermost are comparable to Mars in density (less dense, but close), the next three, Ked-f are comparable to Saturn (Saturn actually lies between Kef and Kee with Ked being a bit denser) and the outermost, Keg, is ridiculously dense. Like, denser than Osmium, the densest natural element. I think what this really highlights is how we only have an idea of planet size from the amount of dimming we see when it passes in front of the sun. The mass of the planets is worked out from the gravitational tugs they exert on each other in multi-planet systems (for transiting planets like the ones Kepler is looking for, that is). I'm willing to believe the densities of the other 5 planets, but I think I'll wait for better data before believing Keg.

My original plan was to talk about a variety of weird worlds in this post, but I got a bit carried away with Kepler 11, it seems. Stay tuned for more weird worlds in the future.

Monday, June 13, 2011

Moon-spotting possibilities

I came across this article when I was browsing arXiv: astro-ph and it was pretty cool, so I thought I'd share it with you all.

Now, the actual paper is pretty technical so I wouldn't bother reading it unless you're really keen. In it the author, Kipping, simulates a planet-moon system transiting its primary and discusses the possibility of actually detecting the moon from such a transit.

The transit method, if you recall from last Wednesday's post, uses the fact that a planet passing in front of its sun dims the star's light a little bit. Kepler is a telescope orbiting in space which is currently searching for planets using this method.

In his paper, Kipping computes what we need to look for in the light curves (the data which shows how much light we can see from a star over time) to identify possible extra solar moons. What really caught my eye, though was that he concludes that Kepler is sensitive enough to detect some exo-moons. How cool is that? Maybe in the near future we'll be reading about the latest batch of exo-moons as well as the increasingly large database of exoplanets we're building up.

Wednesday, June 8, 2011

Planet spotting

I have in the past talked about some of the things you need to consider when you make up your own planets outside of the solar system. This week, I thought I'd talk about real exoplanets that we've discovered and how those discoveries have happened.

Methods of detection

There are several different ways in which we can determine whether a star has planet orbiting it.
  • Direct imaging
  • Spectroscopically
  • Transit
  • Microlensing
  • Pulsar timing
  • Stellar wobble

Direct Imaging

This is sort of what it sounds like. Point a telescope and fortuitously see the planet. The problem is, most planets are quite small and not very bright, so this isn't the most reliable of methods. That's not to say it hasn't had some results. For example, Formalhaut B was discovered this way when the debris cloud surrounding the star was imaged.


This method requires a little bit more background physics. The Doppler effect is what happens when something which is emitting waves (for example, sound waves) is moving with regards to the observer. For example, if you are on a train, going past a level crossing with the ding-ding-ding-ing, it sounds higher-pitched when you're moving towards it because the waves seem more "bunched up", and then, as you go past, it suddenly sounds lower-pitched because they seem more "spread out". That's the Doppler effect.

Light is also a wave (or at least, often behaves as one). When a light source is moving towards you, the light waves will appear more bunched up and hence bluer (because blue light is a higher frequency than other visible colours). And if the light source, a star, for example, is moving away from you, the light will seem more spread out and hence redder.

Planets have a non-zero mass which means that while their star gravitationally pulls on them and keeps them in orbit, the planet also pulls on the star a bit. But because the planet is going around the star, it pulls in different directions at different times, making the star wobble. When the star wobbles towards Earth, it's light will look slightly bluer, and when it wobbles away, it will look slightly redder. Fancy spectrographs on telescopes can detect these slight variations in light output and hence, we can use this method to detect plants.

This only works on sufficiently heavy planets, sufficiently close to their stars. This has been the most popular method (up until Kepler, maybe, which I'll talk about below) for discovering exoplanets. It was the reason we suddenly discovered a whole lot of "hot Jupiters"—Jupiter sized, or bigger, planets close in to their stars—and had our pre-existing theories of planetary formation turned on their heads*.

*We were basing our theories on our own solar system which has large planets far out and small rocky planets close in. Suddenly, because of the detection methods we were capable of, we were seeing a lot of large planets close to their suns which we could not easily explain. However, it's quite likely that the plentitude of these hot Jupiters is actually a selection bias (as they are the easiest to find) rather than an indication that they are actually proportionally that common in the galaxy.


This method uses the fact that when a planet passes in front of its sun, it blocks out some (very small amount) of its light. Sensitive telescopes can pick up this dip in light output. The size of the dip gives an indication of the size of the planet and monitoring the star for long enough allows us to work out how quickly it goes around the star.

This is the method the Kepler telescope, currently in orbit, is using the discover a pile of exoplanets. And I do mean pile. So far more than 1200 planet candidates have been detected by Kepler. There are some really interesting stranger-than-fiction ones that I'll talk about in a later blog post.


Gravitational microlensing is a smaller-scale version of gravitational lensing, which I have briefly mentioned in the past. Remember (or follow the previous links to find out), if a massive object passes in front of a more distant light-source, then the massive object's gravity, which distorts spacetime a little bit, will cause the light from the background source to bend through the distortion. On a large scale, this can give us several images of the background source (example: Einstein's Cross). On a smaller scale (one might say a micro scale, heh), what happens when a moderately massive object like a star or a planet (or a star with a planet) passes in front of a background star, is the light from the background star is temporarily magnified.

If the foreground object is a star with a (sufficiently massive) planet around it, then the presence of the planet will make an extra peak in the magnification light curve. The height of this peak tells us about the mass and distance from the star of the planet.

Pulsar timing

I've put this in for completion, but there is no way for a pulsar to be human-habitable, even though some of them have planets. Pulsars are neutron stars—very, very dense stars made entirely of neutrons; sort of giant atoms. they emit radiation, mostly radio waves, from their magnetic poles which, because they spin very quickly, flashes past us in pulses, hence the name.

We can measure and time these pulses very precisely and usually vary in a very predictable way. Slight timing variations due to the gravitational tug of planets are very noticeable and some of the earliest exoplanets in the 90s were detected in this way.


The problem with most of these methods is that they require a fortuitous positioning of planets with respect to their suns and the Earth. We cannot yet look at a star and definitely say that there are no planets orbiting it. If we're lucky we can say there there definitely are planets there, but if we don't see any it could be because they're not lined up with us nicely, rather than because they're not there.

Wednesday, June 1, 2011

Lifting life off Earth

A friend suggested I blog about getting a biosphere off Earth and onto another planet (within the solar system). This is a bit of a challenge for me since, if I am anything, then I am not a biologist. As a result, I am going to focus on the transportation logistics more than what, specifically, we would need to get to said other planet.

However, if you are interested in the what and how to maintain it once there, I strongly suggest reading these two posts by Patty Jansen: So you want to be a space farmer (part 1) and Growing crops in space (part 2). She has a background in agricultural science and hence is significantly more knowledgeable than I on the matter.

Getting off the ground

Lifting anything off Earth into orbit requires a large chunk of energy. Exactly how much depends mainly on the weight (and a little bit on the size, in the sense of how big—and hence heavy—the spaceship doing the lifting needs to be). To get something off Earth (pretty much to anywhere further away than the moon, though it's not that different for the moon either), we need to give it enough energy to overcome the energy of Earth's gravitational pull.

Some basic terminology first:
  • Kinetic energy is the energy something has due to its movement. It mostly depends on how fast the object is moving, but also on its mass. A faster object will have more kinetic energy, but of two objects moving at the same speed, the heavier one will have more kinetic energy. The formula for kinetic energy is
  • Potential energy is stored energy that has the potential to turn into a more directly useful form of energy. For example, if you lift a brick off the ground, you're giving it the potential to turn gain kinetic energy when you drop it. In fact, thanks to conservation of energy, the amount of potential energy you add to it when you lift it up will be equal to the kinetic energy it gains as it falls and just before it hits the ground. What we are interested in is gravitational potential energy. You can have other types, like spring potential energy, which is the energy stored in a spring when it is stretched or compressed. The general formula for gravitational potential energy is
  • Conservation of energy is the law that says energy cannot be created or destroyed but can only change forms. Hence, potential energy can be transformed into kinetic energy and vice versa, but neither can appear out of nothing. (On a macroscopic level. Things get a little bit more complicated on a quantum scale, but that's not relevant here.)
  • Escape velocity is the velocity required to get something off the surface of a planet. It's basically the amount of kinetic energy required to overcome the potential energy stored between your object and the planet. It's found by equating the kinetic and potential energies above (ignore the minus sign, it's just a convention). Doing that, the mass for the object, m, cancels out and we find a common escape velocity depending only on the mass of the planet, M, and the radius of the planet, R. (This is assuming we're trying to get off the surface. For other distances, replace radius with distance from the centre of the planet.) Incidentally, Earth's escape velocity is about 11 kilometres per second (more than 40 000 km/h). The general formula for escape velocity is

OK, so that's the basics. To get something off the ground and into space, we need to make it go pretty fast to overcome Earth's gravitational pull. However, we don't do it all in one go; it's just not logistically a brilliant idea. Among other things, the faster you go within Earth's atmosphere, the greater air resistance (the friction air exerts on the spaceship) is. This is why rockets usually have stages. The space shuttles, for instance, had two initial boosters to get it off the ground, another large rocket to get them out of the atmosphere, then some small rockets which stay attached to the shuttle (the others are discarded when the fuel is used up) for orbital manoeuvring and coming back down to Earth. Here is an infographic from Wiki.

The tricky thing, when we're talking about getting the elements of an ecosystem off the ground, is how much they weigh. I don't know of any reason why crops wouldn't be transported as seeds which, compared with plants weigh a lot less and take up a lot less room. Depending on where you're taking them, giving them enough water and the right kind of soil is likely to be much more of a problem. In most cases, I think it would be best to mine the necessary water from whatever nearby source you can (the rings of Saturn, mayhaps?) and possibly ditto with the minerals needed for soil, but see Patty's posts linked to up top because I'm far from an expert.

Animals, however, would be a lot harder. With our current technology, we can't really transport a bunch of animals in foetus form and then grow them when we get to wherever like we can with plant seeds. Animals weigh a lot and eat a lot and produce a lot of waste products. That sort of thing (while making good fertiliser for our off-world plants) would be very difficult to transport.

To put this in a bit of perspective, let's look at the weight and lifting capacity of the space shuttle (which have almost all been decommissioned now with only Atlantis having one mission left). According to Wiki, an empty space shuttle weighs close to 70 000 kg, has a maximum payload weight of 25 000 kg and the payload bay is 4.6 times 18.3 metres (doesn't say how tall, but let's assume tall enough for cows). Twenty-five thousand kilograms may seem like a lot, but remember that the shuttles have been used to lift several bits of International Space Station into orbit. So a cow weighs around 500 kg, depending on the type, but let's run with this number because it's round and convenient. That means theoretically, we could squish 500 cows into a space shuttle and lift them into orbit. Well, that's not very helpful because we're ignoring all the food and water they'd need. I also think they wouldn't quite actually fit into the payload bay. Let's say cows need a metre by two metres of space to stand around in. That leaves us with only around 70 cows in our cargo bay. Well, OK, that means we could use the rest of the space for that pesky food and water I keep mentioning...

Let's say a cow eats 50 kg of hay a day... WolframAlpha tells me that hay weighs around 380 kg for a cubic metre (when pressed, because anything else would be silly in this context...) Our 70 cows would eat more than nine cubic metres of pressed hay a day and since getting to anywhere other than the moon takes at least months... we quickly run into problems being able to carry enough, even without worrying about the water.

It's fair, at this point, to mention that the space shuttles were obviously not designed to carry cows anywhere. They were designed to carry bits of ISS and other space-based equipment into orbit and not further. But in terms of lifting power they and the Soyuz rockets are all we've currently got. Also, cows probably aren't the best thing to start off carrying to other planets, I was just trying to make a point.

It takes a lot of energy to launch anything into space, let alone an ecosystem. Unlike the ISS which was built by launching bits up in manageable chunks, it's not really practical to do that with live animals. Flora poses less of a challenge and the difficult part becomes setting it up sensibly on the other end. Also the part where we haven't actually sent people on very long space flights yet.

More on this topic at a later date.


Related Posts Plugin for WordPress, Blogger...