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 actually knew it existed until we actually 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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The Planet Quandary II – High Definition

Take a look at these five oblate spheroidal spacey things and tell me which one is the odd one out.

Not sure? I’ll admit, it’s not too easy. They’re all unique in their own ways. One of them has a dense atmosphere. Four of them contain large amounts of subsurface water. One of them has more water than Earth does. One has lakes of liquid on its surface. Two show evidence of cryovolcanism. One has an extended atmospheric haze layer which is much larger than Earth’s.

Would it help to see them all to scale?

As it happens, the odd one out is the one to the lower left. That grey, rocky orb is Mercury, and it’s the only one which is a planet. It isn’t the largest of the group, and it’s arguably the least interesting, yet it holds a privileged place in our Solar System, being one of the Sacred 8.

In discussions about defining planets, a lot of people ask, “Why does it matter?” An important question, and one which I think is a little overlooked. Between people fighting over Pluto, and people fighting to maintain the status quo of 8 planets, the actual reason for definitions gets lost sometimes.

The thing about a good definition of something is that it lets us have some idea of what it is we’re looking at. All definitions are arbitrary and human-made. Natural phenomena don’t care what we call them. They’re supposed to be made by and for us, to help with our view of the Universe and what’s in it.

To me, this is the point of trying to properly define planets. Those five objects up there are all fascinating in their own ways. And it comes down to more than just mass and orbital characteristics. Consider Enceladus and Vesta to the right, here here. They’re both roughly the same size. One orbits a planet, and one orbits the Sun. But just by looking, you can tell that they’re very different objects.

The thing is that the Universe doesn’t play by any set of rules. When new discoveries are made, they’re frequently surprising. Things aren’t always clear cut, and there are always borderline cases. But knowing loosely where to make a distinction helps us as observers.

In a similar way, hand someone a box of crayons and they’ll probably be able to pick out a red and an orange for you. But if you ask someone to point to the exact place on a spectrum where red stops and orange begins, your mileage may vary.

When defining anything, you need to use its characteristics, and these can be conveniently subdivided into two types. Intrinsic characteristics and extrinsic characteristics. Intrinsic characteristics originate solely from the thing itself. An object’s colour, mass, and shape, for instance, are all intrinsic. Extrinsic characteristics, on the other hand, originate from outside the object in question. An object’s location, and the energy it receives from elsewhere are examples of extrinsic characterstics.

So how does this all apply to planets? Well, the thing is that the current definition for a planet is needlessly convoluted, using both intrinsic and extrinsic characteristics. In case you missed the background, you can catch up here. Long story short, the current definition of “planet”, according to most, meets the recently proposed geophysical definition:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameter.

As well as the astronomical definition:

  • Orbits its parent star.
  • Dominates its orbit in terms of mass and orbital distance.
  • Would clear out any debris in their orbit in well under 0.1 billion years.
  • Its orbit, barring any outside influences, will be stable as long as its star exists.

The problem in this, as some of us see it, is the use of extrinsic characteristics in the definition. There seem to be 3 spanners in the works here.

A stray planet is not a planet

The galaxy is a big, chaotic mess of dancing stars. As they orbit the galactic centre, they stray and occasionally interact with each other. Sometimes they come close enough to perturb each other gravitationally. Tidal interactions happen. Planetary orbits get tugged and tweaked. If two stars pass close enough to each other, a planet may be ejected into space.

Except it’s not called a planet anymore. Its planet card has been revoked. Rogue planets aren’t really planets at all. I want my money back!

In astronomy, there’s a weasel term sometimes used – planemo or PMO (an abbreviation of “planetary mass object”). Technically, a planet is actually a sub-class of planemo. But this feels like a needless subdivision, and it’s questionable whether this nomenclature would be necessary if we simply referred to these objects as planets.

A planet orbiting a planet is not a planet

This is something which has always bugged me. In astronomy, I started out looking at stars. Stars frequently orbit stars. In fact, there are a huge number of multiple star systems out there. At no point has anyone seen fit to question whether or not these all qualify as stars. Mostly because that would be ludicrous.

Similarly, galaxies orbit galaxies and asteroids orbit asteroids. In all of astronomy, the only class of object which isn’t allowed to orbit another of its kind is a planet. This is not logical.

Planetary requirements vary dramatically

This seems to be the central point which I’m refuting. As I pointed out in part I, this means that the definition of a spheroidal planet sized object actually changes according to where that object is. I’ve spent awhile thinking about this, and I can’t think of a single other physical phenomenon anywhere else in science whose definition is dependent upon its location♣︎. Feel free to leave a comment if you can think of one.

Additionally, subdivide any physical phenomenon into its component parts, and they too are defined by intrinsic characteristics. Clouds on Earth are made of water droplets. An iron nail is made of metal crystals, made of atoms, made of subatomic particles. All of these things are defined intrinsically. An iron atom doesn’t cease to be an iron atom because you remove it from its crystal lattice, and a neutron doesn’t cease to be a neutron if you pluck it from an iron nucleus.

Personally, I’d be much more comfortable with a definition based on intrinsic characteristics, such as the geophysical definition I included above. The criticism of using the geophysical definition alone is, apparently, that it’s too inclusive. We’ve catalogued over 4000 comets, with estimates for the total of up to 100 billion. It’s estimated there are 1.9 million asteroids over 1 km in diameter in the main belt alone. But 110 planets is too many? I find this argument… unconvincing.

However, there’s one line in the proposed geophysical definition which I’m unsure about – “a spheroidal shape adequately described by a triaxial ellipsoid” has obviously been included so that the dwarf planet Haumea would be included. This would also include the asteroid Vesta (shown in that image above). I think it’s fair to say, Vesta’s claim to planethood is a can of worms best saved to open in a later blog post.

Of course, there is one thing the current astronomical definition can tell us about an object, and that’s its formation and history – which is what I intend to discuss in part 3 of this series. Thanks for reading!

In the meantime, for no particular reason, here’s the most hilarious photograph of Alan Stern that I could find.

Ultimately all photons are very much the same. Minuscule packets of electromagnetic radiation travelling at 2.998 × 10⁸ m/s. But still, we categorise them. Infrared. X-ray. Optical. Radio. Not just for the way we experience them, the way we perceive colours, but the way they interact with the rest of the universe. Infrared photons and ultraviolet photons may be relatively similar in energy, but their effects on matter which they interact with is vastly different.

♣︎ You could, I suppose, argue cause and effect to some extent. For instance, an Auger electron is specifically an electron ejected from the inner shell of an atom due to the Auger effect. But it’s still unquestionably an electron. And it should be called the Meitner effect anyway…

I’d look for a hilarious picture of Ethan Siegel, but to be honest, it seems difficult to find one which isn’t hilarious in one way or another.

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SCIENCE IS GOOD AND YOU SHOULD FUND IT

 

I’m pretty sure it’s something we’ve all done. You’re so intently typing something, with your desk cluttered with various printed papers, books, and notes. It’s so easy to not notice that you hit caps lock when you started typing.

Which is exactly what happened to Julie Blommaert, aka @Julie_B92 on twitter.

Who understandably found it hilarious that she’d been effectively yelling at her computer about important science things.

And so, the #ALLCAPSPROPOSAL hashtag was born! Obviously, hilarity ensued as miscellaneous scientists from various fields started yelling about science they want to do in 140 character bites.

Hopefully when we’re all done yelling, the research funding should start pouring in. In the meantime, as you might expect, the resulting tweetfest ranged from the actually quite sensible…

…to the slightly silly…

…to the delightfully absurd.

And then there’s Chissa Rivaldi.

Yeah, we’ve all been there.

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RIP Chuck Berry

(via)

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Artificial Trees, Real Energy

Take a look at this thing. It may look like an art installation of some kind, but it’s much more than that. This is a Wind Tree, made by a French startup company called New Wind, and it may hopefully be a great way to generate energy in urban environments.

Each of those “leaves” is a small turbine, trademarked as an Aeroleaf, and is capable of generating 64 watts at peak output. This gives a typical 9.2 m tall Wind Tree with 54 Aeroleaves (Aeroleafs?) a power output of over 3.5 kilowatts. I say typical, because the modular nature of this device gives some freedom for customisation, meaning the number of turbines can vary depending on your needs.

For homes in windy areas, just one Wind Tree would provide a significant amount of energy for a home. The average home in the US uses about 10800 kilowatt hours (kWh) of electricity per year. Operating at only a third of its maximum efficiency, a single Wind Tree will account for nearly 95% of that.

Thanks to the small Aeroleaf design is that the Wind Tree can make use of lower speed winds for power generation – just 2 m/s, as opposed to the 4 m/s required for more traditional wind turbines. Another benefit of these compact turbines is that an Aeroleaf is sensitive to various types of wind. Windmills operate effectively with laminar airflow – wind which comes predominantly from one direction and maintains a constant speed. This is ideal for offshore setups like those in Scotland and the Netherlands, but not so good everywhere. Because of more complex urban landscapes, city wind can be turbulent and gusty. Thankfully, turbulent airflow works just fine to power a Wind Tree.

The guy in this video clip, by the way, is Jérôme Michaud-Larivière, CEO and mastermind of New Wind.

The intention behind the Wind Tree is to provide a safe and aesthetically pleasing way to generate wind power in places where the usual kind of windmill is inappropriate. Where regular wind turbines may be inappropriate in a city, Wind Trees may be easily included on university campuses, industrial parks, outside office buildings and on rooftops to provide a convenient way to generate power.

As it happens, a typical large office building uses around 20 kilowatt hours of energy – so on a windy day, just 6 Wind Trees would yield enough energy to actually make money by selling surplus energy back to the central power grid. Of course, this doesn’t count energy storage either. Renewable energy sources like wind operate continually even when their energy is not in use, and the Wind Tree is no exception. A storage battery saves generated power, which is then converted to AC at standard voltage when needed.

Small, distributed forms of energy generation like this may ultimately be a much more effective way to generate power, particularly with the increasing energy demands of a large city. For example, based on information from a few years ago, London has approximately 680 million m² of commercial property, with around 110 million m² of that being office buildings. Taking a typical office building to occupy around 1400 m², this gives over 78500 office blocks in London. If only half of these offices installed and maintained just one Wind Tree, it would provide up to 137.4 megawatts of power. In the same region as the output of a typical UK power station. 1.1 million Wind Trees would equal the output of the ominously named Drax Power Station, the UK’s largest.

Currently, a lot of  details appear to still be concepts and plans. Future concepts include turbines shaped like flowers, apparently with solar panels in the petals, or with different Aeroleaf colours. I quite like the idea of pink ones to look like sakura petals, myself.

Previous reports have suggested that New Wind intend to start production of these lovely devices sometime this year, with the cost of a Wind Tree expected to be in the same price range as a family car – an estimated €29,500. Unfortunately, their website doesn’t currently offer details. They gave the option near the end of last year, of preordering Wind Trees for the cost of around $1000 US each, though unfortunately their store website appears to be down as I write this.

All the same, I hope they’re successful. I’d love to see these around big cities. As well as giving some visual interest to city streets, it would be deeply comforting to know that cities were generating power this way.

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moon-jupiter

I just wanted to share this gorgeous photograph – Jupiter and all four of its Galilean moons clearly visible, behind our own Moon with the sunlight illuminating the bumps and ridges on its surface. I love photographs like this. Sometimes it’s a pleasant reminder of how small our world is, to look up at night and see the other planets in the night sky. And to remember that we now know what they look like up close.

Unfortunately, I have no idea who the photographer is, and neither Google Images nor Tineye are being much help. The best I can do is to track it back to this tumblr page, which lists no other source. If you know where this photograph came from originally, please let me know so I can credit the photographer properly!

 

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