Energy!

Inspired by that last Astrotropes post of mine, here is a very circular graphic showing kinetic energies of various things, fictional and non-fictional. Go and check the other post for some more details, and mind out for any spoilers.

(click to humongify)

Essentially, there are only two types of energy in the universe. Kinetic energy and potential energy. Moving objects (like asteroids and tennis balls) have kinetic energy, and if a moving object strikes something, that energy is transferred. Pretty much everything also contains potential energy, waiting to be released. That could happen by detonating it, in the case of a stick of dynamite, or by eating it in the case of a doughnut. Also, a doughnut contains more potential energy than a stick of dynamite! How wild is that!

The sizes in the graphic are scaled logarithmically. Double the size means 10 billion (1010) times as much energy. Which makes some things a little tricky to discern, but it was the only realistic way to fit a black hole collision and a cosmic ray particle on the same graphic. I mean, I tried a linear scale, but, well…

So yeah, a black hole collision releases about a hundred thousand million trillion trillion trillion⭐️ times as much energy as a cosmic ray particle, and I figure most people don’t have screens that large.

Also, yes, the most energetic cosmic ray particle ever detected was nicknamed the OMG particle. With good reason too. A single proton travelling at 99.99999999999999999999951% the speed of light, and consequently carrying the same energy as a baseball is not the kind of thing we normally detect here on Earth!

⭐️ This sounds like hyperbole, but it’s really not. That particle was travelling with 48.07 J of energy, while two colliding black holes release 5.39 × 1047 J!

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Astrotropes: Spaceship collisions

High above Earth, several United Federation starships are locked into a desperate battle against a Borg cube. On the bridge of his ship, Worf hears a grim damage report. His blood boils and he slams a console with furious fists. “Perhaps today is a good day to die,” he growls, before barking an order to his surviving crew. “Prepare for ramming speed!”

This post is loaded with massive (and high velocity) spoilers for Star Wars – Rogue One, A New Hope, Return of the Jedi, and The Last Jedi.

The above sequence with Worf fighting against the Borg, by the way, is from Star Trek First Contact. It’s not a spoiler because it happens in the first 5 minutes of the movie!

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Astrotropes: Surviving Space

Space, as a background, is a mainstay of sci fi. But space is hazarous, and if you’re to find yourself without air to breathe, that should probably be cause for alarm. Naturally, sci fi characters (much like real life astronauts) go to great lengths to avoid this. “Don’t leave your spacecraft without a protective suit” is generally some rather sound advice. Unfortunately, in many stories, that isn’t always an option.

Caution: This post has mild spoilers for Star Wars VIII: The Last Jedi, Sunshine, Gravity, and episode 7 of Cowboy Bebop.

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“We humans are capable of greatness”

Happy Birthday Carl ❤️

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Quark Fusion

When thinking about the future of energy production, aside from renewable energy like solar power, fusion is something which immediately springs to mind. Clean, efficient, and the same thing which stars have been using for around 13 billion years. But imagine for a moment, if there was another energy source available which was dramatically more potent.

A paper published recently in Nature discusses the idea of quark fusion. For anyone who’s uncertain what that means, it’ll require a brief trip down the rabbit hole.. So matter, as you probably know, is made up of atoms. And atoms are made up of electrons and nucleons. Nucleons come in two types, protons and neutrons, and are a subtypes of a whole family of subatomic particles called baryons. Baryons, in turn, are made up of quarks. Quarks, as far as we can currently prove, are the most fundamental building block of matter there is, and they come in six types. Every baryon contains three of them – so, as an example, in its most fundamental terms, a hydrogen atom is a structure made from two up quarks, one down quark, and an electron.

Still with me? Cool. So Marek Karliner and Jonathan Rosner, a pair of physicists working in Tel Aviv and Chicago respectively, published their work all about the idea of directly fusing quarks. Specifically bottom quarks.

Now, I’m not a particle physicist or a nuclear physicist, so I may be missing a few of the finer points in all of this, but the concept concerns bottom quarks – the second heaviest type. One of these quarks alone is already over 4 times as massive as an entire proton. Apparently, fusing two of them releases a hilarious amount of energy.

How much energy? Here’s a comparison.

238U ⟶ 92Kr + 141Ba + 3 1n ……………… ~0.9 MeV

3He + 3He ⟶ 4He + 2 1H ……………….. 12.86 MeV

Λb + Λb ⟶ Ξbb + 1n ………………………… 138 MeV

Splitting uranium into krypton and barium produces about 0.9 MeV of energy per reaction and spits out 3 neutrons (most fission reactions give out roughly this amount of energy). Fusion of helium-3 is the most energy efficient fusion reaction, producing one helium-4 atom, and 2 protons, and gives 12.86 MeV of energy.

The symbols in the third reaction may be less familiar. Λ and Ξ are types of baryon (quarks don’t really exist outside baryons or mesons), but the important part is those b symbols. Λb is a lamba baryon containing one bottom quark. Ξbb is a xi baryon containing two bottom quarks. Karliner and Rosner describe this as being a quark level analogue of a nuclear fusion reaction.

The amazing part is the energy released! The quark fusion reaction releases 138 MeV, which is 10.7 times as much as helium-3 fusion, and 153.3 times as much energy as a nuclear fission reaction!

The biggest difference is that the other reactions here are chain reactions. The additional products can go on to set off additional reactions, which is what makes them useful in energy production (and scary in weaponry). Just like Meitner, Karliner was concerned by the potential implications of such a huge energy release from a single reaction. However, quark fusion is exactly that – a single reaction. Bottom quarks only exist for a fleeting moment at most, so with no means of easily producing or storing them, and no means for a chain reaction, a quark fusion reaction like this is just a one off.

So sadly, there’s not much scope for any future source of energy here. At least not yet. Guess we’ll have to stick with solar and trying to make helium-3 fusion economically viable.

Karliner & Rosner (2017)

String theorists will argue this point, and as soon as someone has evidence that strings exist, I’ll gladly concede.

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Aurora on your Finger

I’ve been very fond of jewellery pretty much forever, so I have a pretty sizeable collection of rings by now – but this aurora borealis ring made by Secret Wood is on a whole different level.

Isn’t that gorgeous?

Secret Wood make these beautiful jewellery pieces entirely from wood and jewellery resin, with a few select pieces including other things, like flowers, trapped inside the resin.

These rings made to order and, as you might expect, each one is unique due to the natural variations in the wood and materials they’re made from. But to be certain you’re happy with your purchase, they’ll send you a photograph if your ring before shipping it. They’ll also cost you somewhere around $110-$170 US. For jewellery like this, that seems like a very good price, to me!

Their website has a whole collection of different ring styles (including limited editions) which, quite honestly, seem like the sort of thing a wizard might wear. Or if you want even more pretty things to look at, they have an instagram page too.

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Space Rocks

I stumbled across this rather lovely size comparison of small Solar System objects, and I thought I’d share. Click it for a larger view!

Taking pride of place is Ceres, empress of asteroids, with her smaller sibling Vesta, the largest asteroid in the main belt. To the left are a handful of more typical asteroids, all of which have been seen up close by robots we’ve sent out to investigate them.

Below are 3 of the Solar System’s huge collection of moons, and the ever curious asteroid Chariklo with its ring system. At the very top, near the right, are the two Martian moons, Phobos and Deimos which really does illustrate how puny they are, even compared to some asteroids.

This image was created by Daniele Bianchino, left in a comment on the Dawn Blog a couple of years ago. His website’s quite a visual treat, even if you don’t speak Italian. Nice work, Daniele!

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An Uncomfortable Truth About Spacecraft

Since Juno’s arrival at Jupiter, a lot of people noted how impressively technology has improved, with Juno being powered by solar panels. Indeed, without the progress we’ve made, Juno wouldn’t have been possible. It may not be immediately apparent to everyone, seeing as solar panels are so synonymous with satellites, but they’re one of the most distinctive things about Juno. No craft has ever been sent to the outer solar system before with solar panels as their energy source.

Solar power is an abundant source of energy on Earth, but a spacecraft at Jupiter receives only about 4% as much solar energy as one in Earth orbit does. Juno’s solar panels are huge, and would harvest 12-14 kW of power in Earth orbit. Out by Jupiter though, they only produce a meagre 486 W. Which is why all previous spacecraft were actually powered by nuclear isotopes. Sadly, this fact makes the future of spacecraft visiting the outer solar system rather uncertain.

Vehicles which can’t be practically powered by solar energy, like New Horizons, or vehicles which need a lot of power, like Curiosity, are traditionally powered by radioisotope thermal generators, or RTGs for short. The type used by Cassini and New Horizons looks a bit like this:

They’re quite ingenious devices really. An RTG is a generator with no moving parts at all. Instead, it’s filled with a well chosen radioisotope. As that isotope decays, it produces heat, and an array of thermocouples surrounding the hot material convert that heat directly into electricity using the Seebeck effect. These are such a convenient way of generating a sustained amount of power that the Apollo missions used several of them, and the Soviet Union even found uses for them here on Earth. Sounds pretty useful, right? The trouble is that there are only a very few radioisotopes which work effectively in an RTG. The one traditionally used produces enough power that it can easily glow red hot using only its own internal heat. Plutonium-238.

Plutonium is an almost entirely human-made element. It’s technically the heaviest naturally occurring element, but only ever formed in trace amounts as uranium-238 naturally captures stray neutrons.  And, quite rightfully, plutonium has a bad reputation. While Dummkopf Tinyhands may like to bemoan the general badness of uranium, plutonium is much more scary. Back during the anxiety-inducing days of the Cold War, when everyone was inexplicably obsessed with nuclear weapons, one thing which nuclear reactors were producing was Plutonium-239 (Pu-239). This is the frightening weapons grade material which… well. This is a subject which keeps me up at night, which is best left for another time. The thing is, nuclear reactions are a bit like chemical reactions in that they almost never produce just one product. Most nuclear reactions tend to give byproducts.

One byproduct of the reactions to make Pu-239 is Neptunium-237 (Np-237), and if you irradiate Np-237 you can produce the very useful Plutonium-238 (Pu-238). In fact, a lot of the Pu-238 produced for power sources also contained a fair percentage of Pu-239. In other words, the power source for all the deep space exploration NASA has ever done was actually a direct consequence of the US and Soviet military producing nuclear weapons. Which is a sobering thought.

Plutonium can keep providing a spacecraft with power for a long time♣︎. Voyager 1 for instance, now classified as an interstellar mission, was launched in 1977 and is not expected to finally run out of power until 2025. It left carrying 37.7 kg of Plutonium (a schematic for Voyager’s RTG is shown to the right here) which gave it an initial power output of 470 W. As of 2015, that had dropped to about 255 W.

Mercifully, the world is producing less Pu-239 now. For decades, humans realised that having enough nuclear weaponry to extinguish all complex life on Earth’s surface and phrases like “mutually assured destruction” were probably bad. The thing is, just like the Plutonium being carried by Voyager 1, the Plutonium that stayed behind is also decaying and is gradually producing less and less useable power for RTGs. In short, as a species we’re running out of Plutonium.

NASA, of course, are well aware of this problem. In 2008, NASA Administrator Mike Griffin explained in no uncertain terms, “After [Curiosity] launches, we’re effectively out of plutonium.” Since then, dwindling reserves in the US have been supplemented by remaining reserves in Russia. Unfortunately, NASA only has about 35 kg of Plutonium left to work with. Only about 17 kg of that stockpile is still good enough to be used in RTGs, and that’s been reserved in its entirety for the next Mars rover mission, currently scheduled for 2020.

So what’s to be done?  In 2013, the US Department of Energy (DoE) reactor pool at Oakridge started making Plutonium again for the first time in decades, but it’s going to take the best part of a decade before enough Pu-238 is produced to actually be useable. The DoE stance is that they’re producing enough for currently projected NASA missions and that there’s no current shortfall, but this has been rightfully criticised as circular reasoning – of course NASA aren’t going to plan anything using energy sources they don’t currently have.

Meanwhile, the DoE are researching new production methods, presumably because they’re no longer tied to methods for making weapons grade materials. Or perhaps as a way of actively not making such materials. One method they’re researching involves using pellets of pure neptunium oxide.

Another option may be to use a different nuclide. Unfortunately, the requirements are quite specific. The nuclide used in an RTG needs to be have a long enough half life that it provides consistent amounts of energy for a suitable amount of time, while also decaying rapidly enough that it gives out a useful amount of energy. That energy also needs to be a useful type. Alpha emission is ideal, while beta and gamma ultimately require heavy shielding. For spacecraft, it also needs a good energy density (energy output per kg) given that when you’re trying to send something as far away as Jupiter and beyond, every last gram really does matter.

Sadly, out of the 905 known nuclides with half lives of over an hour, only about 30 are known to meet these requirements and be suitable for use as spacecraft fuel. Most of them decay much more rapidly that Pu-238 too, meaning that while they provide a good source of energy, they won’t be effective for the long timescales required for long distance space travel. Additionally, none of them are safe to use with as little shielding as Pu-238.

While it’s not impossible that we may still discover another good isotope for use in RTGs – or better, a different type of power source altogether – for now, it looks like Plutonium is the best we’ve got.

Looks like we’d better keep those reactors running a little while longer.

For anyone who knows a little about how solar panels work, the thermocouples in an RTG actually work in a very similar way. Essentially, you take two metals at different points in the galvanic series and join them at two places. The electron energy levels shift differently in the two metals, with the result being an electrical current. More sophisticated designs actually use n-doped and p-doped semiconductors, making them even more similar to solar cells.

♣︎ As you might expect from such a massive element, Pu-238 is a prolific alpha emitter. The heat it generates comes from its rapid decay into U-234, and with the alpha decay being the dominant mechanism, there are relatively few complications like chain reactions and unwanted byproducts. Alpha emission is also not very penetrating, so Pu-238 powered RTGs don’t require as much shielding as some other isotopes might.

Flying long haul to Neptune is a killer.

Images:
Schematic of one of NASA’s general purpose RTG units.
One of the plutonium pellets used in Cassini’s RTG.
Older design of RTG used in the Voyager probes.
DoE image of a 250g sphere of plutonium-238 (IV) oxide.
New Horizons before launch, with its RTG on display.

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The Violent History of the Martian Moons

Mars is a funny little place, but it seems that Mars orbit may be even funnier. As we all know, Mars has two small and lumpy moons which would be no more than a pair of mundane asteroids if they weren’t orbiting Mars. Phobos and Deimos. But this seems to be only their latest incarnation.

No one’s entirely sure about the origin of the two potatoes in orbit around Mars. Sharing their planet’s orbital space with 14 satellites we’ve sent there, many people believe that they were captured main belt asteroids. They do seem to share a lot in common with C-type and D-type asteroids, but their orbits seem a little too perfect. Their orbits are quite close to being circular, and both lie perfectly in Mars’ equatorial plane. Orbital drag and tidal forces can do this, but with Mars’ low gravity and thin atmosphere, it’s difficult to make this theory work. Additionally, infrared spectra of Phobos and Mars show very similar phyllosilicate minerals, suggesting a common origin.

A slightly more interesting possibility is that Mars may once have had rings, a little like Saturn, and Phobos and Deimos coalesced out of them. This may sound outlandish, but it’s a similar hypothesis to where we think Earth’s moon came from (with the big difference being that the Earth-Moon system is huge compared to the Mars system). And even asteroids can have rings, under the right circumstances!

There’s certainly evidence for a huge impact on Mars sometime in the past. The Martian Dichotomy is the name given to the interesting fact that Mars’ northern hemisphere is dramatically different to its southern hemisphere. The northern hemisphere is 1-3 km lower in elevation, and has a crust nearly half as thick as the crust in the south. At least one theory is that the entire northern half of Mars is, essentially, a huge impact basin.

An impact big enough to produce that kind of effect would be big enough to knock a lot of material into orbit. Some believe that this caused Mars’ orbit to have once been full of Phobos/Deimos-like objects. Others think it may have caused a ring system to form.

Similarly to Earth, a ring system like that is thought to have formed into a large moon. Unlike Earth, that wasn’t the end of the story.

In celestial mechanics, any object orbiting another has what’s known as a Roche limit. It’s described conveniently by this simple little equation;

where Rm is the radius of the secondary, Mm is the mass of the secondary, and MM is the mass of the primary. The equation gives d, the distance at which the secondary starts to be torn apart by gravity. In this case, the primary would be Mars, and the secondary would be its ill-fated moon.

So, one hypothesis goes that this former moon of Mars was ultimately shredded by the gravity of its parent planet. In fact, the process seems to still be occurring. After who-only-knows how many moons have been formed and crushed, Phobos and Deimos are all that remains, but Phobos is probably doomed to the same ultimate fate.

Deimos, on the other hand, is probably safe. It’s a lot further away. Any planetary orbit also has a radius called a synchronous orbit. Around Earth, we know this as a geostationary orbit – the point at which an orbiting satellite will always be staring at the same part of Earth’s surface. But there’s an interesting ramification of synchronous orbits. They’re the most stable kind. Inside this point, an orbit is destined to eventually decay, while objects outside will gradually orbit further away due to centrifugal force, and may ultimately be lost altogether.

Phobos sits, doomed, inside a synchronous orbit, while Deimos sits outside where it will eventually be free to leave. Similar situations in the past might explain why Phobos and Deimos are all that remains of the moon that once was.

Assuming, of course, that this hypothesis is true. It’s very difficult to say, until we find an errant asteroid which happens to match the composition of the current Martian moons. Additionally, while a ring of dust and small objects has been predicted to exist between Phobos and Deimos, no evidence has ever been found for such a ring.

Either way, the idea of a small, terrestrial planet like Mars having a ring system is rather pleasing!

Commanche-Outcrop-rings

Heard via Gizmodo.

Images:
Gif animation/video clip of Phobos eclipsing Deimos, as seen by NASA Curiosity.
Photomanipulations created by @InvaderXan/supernovacondensate.net using
• Comanche Outcrop observed by NASA Spirit,
• Phobos observed by NASA MRO,
Deimos observed by NASA MRO,
• Dione observed by NASA/ESA Cassini, and
• Mars observed by ISRO Mangalyaan.

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Space Dumpling

This is not a delicious 台湾水餃子 (Taiwanese boiled dumpling) drifting through space, no matter how much it may look like it. It is, in fact, one of Saturn’s moons! This is Pan. A tiny moon, only 35 km in diameter, and the second closest to Saturn itself.

As you might expect from such a tiny moon, no one even knew it existed until we sent a craft there. It was first photographed by Voyager 2 in 1981, but wasn’t actually discovered until 1990 when those images were being reexamined. Part of the reason for this is because it’s so small, and part of the reason is because of where it is.

Pan, you see, is a shepherd moon – one of a handful of tiny moons which help maintain the shape and structure of Saturn’s rings. Pan orbits in the Encke gap, a 325 km wide space in Saturn’s A ring which is free of the icy, dusty particles that make up the rest of the ring system. The reason the gap exists at all is because Pan sits there, sweeping up all of that material.

It’s also the reason why Pan is such an odd shape. That weird ridge around its equator is made up of accreted ring material. As Pan orbits, any and all stray bits of ring material which end up in its path ultimately find themselves sticking to Pan’s surface, and because Saturn’s rings are so perfectly planar, they form a surprisingly neat ridge around Pan’s middle.

That ridge is also what makes it look so much like an errant space foodstuff. Whether you see an empañada, a gyoza, a piergoi, or a ravioli, of course, probably depends on what you personally prefer to eat. Interestingly, it’s a common feature of  shepherd moons, with another of Saturn’s moons – Atlas – also having a similar equatorial ridge. If you’re curious to learn more, planetary scientist Carolyn Porco has two journal papers about it for you to read, available here and here.

Of course, that funny looking ring of material gives it more than just a passing resemblance to its parent planet Perhaps it just wants to be just like Saturn when it grows up. How adorable!

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