Learning To Love The Bomb

One of the first spaceships I ever modelled was the Discovery from 2001 : A Space Odyssey. This was back in 2002 when I first started learning Blender, for one of those absurdly ambitious, doomed-to-failure projects that all 3D hobbyists go through. I wanted to render all of the space scenes in the novel which weren't included in the film. I failed miserably, of course, but I did learn how to model spaceships. Not very well, you understand, but it was a beginning.

More ancient historical images can be seen here.

OK, it's a crappy render, but it's still recognizable as the classic design featured in the film - a long boom connects the obvious habitation module and compact drive section, with some standard-looking rocket motors at the back. The boom turns out to be to keep the crew well-separated from the nuclear drive unit. The whole design is very practical - everything serves a purpose, but it's also elegant (well, the real movie version is, at any rate). The only major concession to form over function is the lack of waste heat radiators (we'll get to them a bit later).

But there was another, quite different design that was originally considered. This one wouldn't have used rockets. It would have used bombs. This would have been the first mass-popularisation of the Orion* drive, where nuclear bombs are used to blast a ship forwards. The whole thing is made survivable for the crew by a pusher plate connected to the main ship by huge pistons, reducing the acceleration to survivable (actually quite modest) levels of a few g.

* Absolutely nothing to do with the modern NASA vehicle, this was a conceptual study from the 1950's.

Image credit : me.

I've written about and animated Orion before quite extensively, of course (have a look at the first link if you're not familiar with it). Briefly, it was a 1950's plan to sidestep useless chemical rockets altogether and use the massively greater power of nuclear explosions (which were quite in vogue at the time) to launch truly stupendous payloads (thousands of tonnes) into space. It would also be pretty good for nipping around the Solar System - journeys could be shortened from months to weeks. So, naturally, an Orion drive was considered as an early concept for the Jupiter-bound Discovery in the 1968 masterpiece 2001 : A Space Odyssey.

Image courtesy Winchell Chung.

Looking at the design, though, it's clear why it was rejected for the film. It looks more like something constructed in Kerbal Space Program than anything that Kubrick, with his notorious use of visual clues to manipulate the audience, would ever have allowed. You can't have this sort of ship convey any kind of subtle message, or any message at all except perhaps, "YEEEEEE-HAAAAAAAH !".

No ! That's another Kubrick film entirely !

Only the spherical habitation module is recognizable from the final movie version, and it's rather crudely bolted on to the main ship by what is essentially a big stick (it was only concept art, after all). There also appear to be two, grossly asymmetrical cooling fins, which don't help matters. The rear of the ship looks far more similar to something you'd find in a James Cameron movie - a plausible, practical design that would look just fine on its own, but which is completely inconsistent with the front end.

We can deduce a few things from this drawing. Its overall ungainly appearance suggests an orbital construction instead of a ground launch. Given the size of the habitat module it appears to be about the same size as the movie version, 10-20m diameter. That's pretty small for an Orion though it would be on the long side - about the same as the movie version at 140m. The big cylindrical tanks are presumably for storing fuel (i.e. bombs), with their bulk suggesting this is a ship built for speed.

That would seriously impact the movies's depiction of a remote, isolated crew, and there'd be absolutely no need for most of them to hibernate. I also imagine that convincingly animating the movement of the pusher plate and pistons would have been extremely difficult with 1968 special effects. For all these reasons, I reluctantly conclude that it was rejected for the best. But Orion and 2001 are two of my favourite things, so I can't not model this. I would lose the same amount of self-respect as if I suddenly decided to become a naturist, join the BNP and work for a bank*.

* Though I'm not quite sure all of those are morally equivalent.

I like to think this is an improvement on my 2002 version.

I deliberately set out with the aim not to re-create the reference picture in exact detail, but to make something with all its major features in a way that Kubrick wouldn't have spat on in disgust. 2001 paid exceptional attention to detail in terms of... well, everything really, but perhaps most unusually in terms of making the spaceships realistic. Not merely believable, but almost to the point of being NASA-worthy design studies.

Nonetheless, Kubrick was not adverse to breaking realism's house and burning its legs down (or possibly the other way around) if it helped make a better movie. I've already mentioned the lack of cooling fins in the movie-version Discovery; another noticeable example is the massive size discrepancy of Space Station V in different shots (which frankly is almost as bad as in certain Godzilla movies). And rightly so : there's absolutely no point in making a realistic spaceship for a feature film if it leaves the audience cold.

So, the main change was to make the propellant magazines quite a lot smaller, no wider than the habitat module. That in turn also makes the pusher plate smaller, giving the ship a much sleeker profile and looking less like it's butt-heavy. It's still an ungainly thing, but hopefully looks more like a respectable spaceship and less like a a bunch of flying grain silos on pogo sticks.

Other changes are subtler. I don't like the crude join of the habitat and drive sections in the sketch, so I used something based on the movie version of the ship. Potentially this could also be another piston, reducing the shock that the crew experience still further.

I considered ditching the giant cooling fins and replacing them with some much smaller, recessed panels. These might actually be more accurate. Most nuclear engines produce huge amounts (gigawatts) of waste heat from their reactors, which they have to get rid of to stop the ship from melting. But not Orion - its waste heat goes straight out into space*. The ship will probably still need a nuclear reactor for powering other systems, but only a few megawatts, and even that might be a stretch. The only major power requirements would be for a powerful radar transmitter (via the AE-35 unit) or possibly a high-tech magnetic shield to deflect the solar wind. 

* Some people criticize Orion for being inefficient, since not all of the material from the bomb impacts the pusher. This is like saying solar power is inefficient since the Earth only receives a minute fraction of the Sun's energy - true, but totally irrelevant.

In the end I liked the big cooling fins too much. So they stayed, but now they're symmetrical and rather heavily braced to deal with high accelerations. They're still pretty substantial, probably overkill for a ship like this, but meh. Perhaps it uses its radar to vaporise passing asteroids instead of studying them. Maybe its RCS thrusters use nuclear power so that it can... umm... turn around... really, really quickly. Yeah.

The main thrusters are big, obvious rocket nozzles, but there are also other smaller exhausts scattered about the whole ship. So it can orient itself however is needed, albeit in a slow and stately manner. The exhaust from the main nozzles doesn't quite intersect the cooling fins, so they're safe, but it will scour the hull of the magazines. That could be unsightly, so I added some protective teardrop-shaped panels, based on Apollo design drawings.

I made a small modification to the mounts of the pistons to the pusher plate. Here I used hexagonal prisms in imitation of the rockets on the movie Discovery. I wanted to pay as much homage to the movie as possible, while still largely following the concept art.

Orion aficionados will have spotted that this version does not have a protective plasma deflection sheath around the pistons, as the original design did. The purpose of the sheath was to divert any plasma that made it through the hole in the pusher plate around the shock-absorbing pistons. A trapdoor was supposed to open and close to let each bomb through - it's never been clear to me whether this trapdoor was supposed to be on the pusher plate itself, or on the end of the ejection tube. Anyway this version doesn't have a sheath, so presumably it relies on a trapdoor on the pusher plate.

Finally, the AE-35 communications antenna. Lots of people have pointed out that the pulse unit magazines would block its field of view if it points backward. And they would, but this is a non-issue. I'm assuming that the antenna is fully steerable (as is the ship), so it can still point up or sideways. Given the movement of the planets along their orbits, there's no guarantee that Earth would always be directly behind the magazines... but more importantly, spaceships don't have to point in the direction they're travelling. And wherever the unit is placed, the ship will block part of the antenna's field of view.  

Finally finally, as you'll already have noticed, I also added the ships' name, American flag and NASA logo, none of which are visible in either the drawing or the final movie version. There's no doubt they would be if the ship were ever built though. Since another concept ship recently drew some astonishingly hostile criticism by large parts of the internet, I'd better state it in no uncertain terms : NASA isn't planning to build a ten thousand tonne spaceship propelled by enough nukes to devastate a small country. Just in case you were wondering.

On then, to the artwork. A nice thing - well, one nice thing - about 2001 is that there are different accepted versions of the story. In the movie, the ship goes to Jupiter, discovering an alien monolith in orbit.

ALL THESE WORLDS ARE YOURS EXCEPT THE ONE AT THE
BOTTOM.

In the novel, the monolith is on Japetus (or Iapetus, whatever), which turns out to be a frickin' awesome moon of Saturn. Saturn was rejected for the film because it was difficult to render in the 1960's, but that, of course, is no longer an issue.

The moon here is Enceladus, a proposed target for the original Orion project. Fewer images exist of Iapetus that were suitable for making a nice composition.

The story never describes Discovery in Earth orbit, but of course it was bound to have been there at some point.

Animation ? Duh. Winchell Chung says it best :

"Wild horses couldn't prevent Rhys from animating an Orion drive Discovery from 2001. It is just too much fun for an ordinary person, but infinitely too much fun for an Orion fanatic like Rhys."

Quite right. The main task was to get the firing sequence right. In my first video I deliberately rendered the explosions in an unrealistic way because I wanted to clearly illustrate a basic principle of the ships' operation : the shaped nuclear charges. Most of the mass of the bomb can be directed towards the ship, which is a good deal more efficient than if the debris expands in a sphere. Shaped charges can be extremely good at collimating the plasma into a respectably narrow cone of 20 degrees or less (the exact figure is still classified).

The main problem with this early rendition is that the speed of the plasma is much too low - absurdly low. So in subsequent videos I've tried to make things much more realistic. According to George Dyson's "Project Orion" book, the whole nuclear event - from detonation to impacting the pusher plate and re-expansion - takes about 1 millisecond : less than 1 frame in standard 25 fps video. Rendering only a single frame is not really a viable option (I want the audience to see the explosion, not get a subliminal impression of it), but I restricted myself to 5 frames for the whole event. That's still fast enough that it appears instantaneous to the eye (at least at 25fps, probably not in the GIF though).

Unlike my previous efforts, this version features both the spherical detonation and the plasma jet from the shaped charge.

But is this really what it would look like ? Perhaps not. Project scientist Freeman Dyson says (in his son's book) : 

"The debris goes out from the bomb essentially invisible. You don't see anything until the stuff is stopped... that won't produce anything very spectacular in the way of a flash until it hits the ship. Then all its energy is converted into heat and so you get about a millisecond or so of intense white flash. And very little else."

My naive understanding says that anything which is hot will radiate. And the atomic fire is very hot indeed (10,000 K in the jet, over 100,000 K when it hits the plate) so presumably something would be seen in visible light, however faintly. But I am not a plasma physcisit, so if you have more information, let me know. As for the exact shape, colour and density of the plasma, that is of course a complete guess. I chose to make it look like a gun-muzzle flash because I thought it would look cool.

The firing rate here is about 1 bomb per second, which is about the typical design spec for a ground-launched Orion. It doesn't need to be so fast for obrital manoeuvres because it's not trying to avoid crashing, but it's an aesthetically nice rate to depict. Interestingly, early on in the real-life Orion project a much higher firing rate was considered : 4 (smaller) bombs per second. Since the detonation point is something like 50-100m behind the ship, you need a powerful gas gun to shoot the 1-tonne bombs out the back. But you can't reload a gun like that four times per second. There are two solutions, both of which ought to worry even the most steely-eyed of astronauts.

Option 1 was not to shoot the bombs through the pusher plate at all. Instead, the bombs would be ejected through the side of the hull, guided along rails, and then either shot by a catapult system or propelled by rockets to their detonation point. Presumably the rockets would also adjust the orientation of the bombs to ensure they were pointed toward the pusher plate - in some designs this meant the bomb would do a full 180-degree flip. Well, what could possibly go wrong ?

Option 1 being rejected on grounds of sanity, option 2 was scarcely less dramatic. From Dyson's book again :

"... a gun a metre in diameter... 10 metres long, weighing 2.5 tonnes to project a 1.5 tonne projectile at 200g's. Obviously this can't be reloaded every quarter of a second so you need maybe 10 of them... this will probably wind up as a battery of Gatling gun-type gadgets."

That's right - they wanted a Gatling cannon that fired nukes, because science. Whether you'd need such a gobsmackingly terrifying contraption for the more leisurely rate of one bomb per second, I don't know.

Before I shut the hell up and let you watch the video, Orion always begs the question : "would it have worked ?" The answer, I think, is probably yes - with a catch. Experiments like Operation Plumbob do seem to indicate that a pusher plate (and therefore the ship) could survive the explosions, but there are plenty of unanswered questions. 

For instance, could a system be engineered to reliably eject one-tonne nukes at 200 mph, with an absolute guarantee that they wouldn't detonate too close to the ship ? What would happen if the ship veered off course and had to be destroyed ? What about if a bomb did detonate early - would it risk the others detonating too ? Even if everything worked perfectly, would the fallout from the bombs be anything to worry about ? Well, the answer to that last one is no, but - and this is the catch of course - it doesn't matter.

Orion is a scheme so monumentally audacious that barring the threat of an asteroid stike, it isn't ever going to fly, assuming it would work at all. The total explosive yield for a 10,000 tonne ship for a Mars-bound mission is something like half a megaton of TNT - enough to destroy Hiroshima thirty times over. Does anyone really believe, deep down, that it would be perfectly safe to launch that kind of devastating firepower, or that doing so wouldn't cause massive public outrage, however misguided that outrage might be ?

Of course not. Perhaps Orion could open the road to the stars, but it is, and will only ever be, a dream. Let it go, people, let it go. 

But it still makes for good science fiction. On that note, I suggest you set the volume to "deafen", watch the video and forget about crappy old reality for a couple of minutes.

... if you're one of those people who are chronically unable to forget about reality, you might be worried about the EMP. Well, don't. Unless specifically designed to cause it, a nuke at these orbital altitudes wouldn't cause enough of an EMP on the ground to do any damage unless it had a yield of about a megaton or more. These blasts are much smaller (few kilotons) and would cause problems only for nearby satellites. Or so I'm told.

Prague Peacocks Pumas Pubs And Parks

Left to my own devices I naturally revert to modelling nuclear pulse rockets, or playing Skyrim. Even when it's very sunny outside. However, occasionally even I can't stand sitting quietly in my room not talking to anyone, so I go outside and not talk to a whole bunch of people instead. Most of the time this just involves walking for miles and miles, but with a mighty effort of will I can sometimes force myself to actually go somewhere. Deliberately.

When my parents visited we discovered the fabulous white peacock of the Wallenstein gardens. On that first visit, the wretched creature chose to display its natural Elizabethan ruff the moment we saw it, for all of five seconds. That's just spiteful, really. However, on my return visit the bird had obviously overcome its fit of pique, and cooperated marvellously for the camera.

MISSION. ACCOMPLISHED.

I don't know why I find the peacock so fascinating - a regular peacock, some chloroform and a can of spray paint would do just as good a job with much less effort - but I rate it as the Greatest Peacock In The World. Even the weird-looking wall and owlery in the gardens just don't compete.

There being an unfortunate lack of any coastline in Prague, the closest I can get to any kind of maritime adventure is to ride a boat down the Vlatava river. Which I did, and it whiled away an hour happily enough, but it wasn't massively different from seeing the city from the riverside.

However, I soon discovered that a far more interesting approach would be to hire a barbecue boat. Yes, those are a thing. Picture a round picnic table with an umbrella and a grill in the middle. Actually don't bother, because I took a photograph.

A far more satisfying excursion was when I decided to visit the New* Town Hall for no particular reason (and no, not because it's right next to Hooters - stay classy, Prague). This turned out to be a very wise move indeed. While its claim to offer the "best view in Prague" is simply wrong, it is a very nice view. The ascent is 221 steps, but it feels a lot less because there are several floors and it's not a narrow spiral staircase like St Vitus. It's also cheaper than the Astronomical Clock tower.

* Constructed 1348.

No explanation was given as to the presence of "ye olde toilet" in the bell room.

But its main advantage is that absolutely no-one else visits it. There was not a single other person in the tower the whole time I was there - and I was at the top for a good 20 minutes. I spent most of that time pointing and laughing at the people down below, shouting* things like, "HAH ! Ya daft buggers. For 50 CZK you could have seen this fabulous view, you petty FOOLS !"

* Alright, thinking.

Putting a sexploitative diner next to medieval architecture or a bunch of hot coals on a very small boat are all well and good, but don't really compare to putting pumas in a pub. Apparently, some forward-thinking (or possibly just mad) landlord decided that a pub would also be a good place to double as a puma breeding centre. Well, why not ? I mean apart from the several dozen very good reasons, like the customers getting mauled or the pumas escaping (it's the only pub I've seen which has barbed wire above the "beer garden") or basic animal welfare.

On the first visit, the female puma had become a mother that very day, so we didn't see them (presumably successful breeding bodes well for the animal welfare issue). A month or two later and we were able to see all three. This place really needs a "beware of the cat" sign -  the pictures haven't come out too well, but trust me when I say an adult puma is a serious piece of cat.

FUN FACT : Baby pumas do not meow. In fact, they sort of quack. The closet noise I can think of is police chief Wiggum's distinctive, "waaaaahhh".

That's all for this time. Tune in next week, when I discover a café full of armadillos and see the world's only albino sheep walking around Wenceslaus Square. Or something.

The Importance Of Being Idle

I don't usually reblog science stories, but this week there's an article so interesting, and so close to my own research, that I'd feel silly if I didn't. And also I think the various websites which have re-printed the official press release have kind of missed the point of why this particular story is so interesting, so I'm going to try and redress that here. This is all about "dark galaxies", which are the very thing that led me into radio astronomy in the first place.

This is going to be a pretty long post. If you're already familiar with dark matter, skip section 1. If you already know about dark galaxies and why they might be important, skip section 2 and go straight to section 3, where I describe the latest results.

1) Dark... what now ?

Although not everyone likes the term, a "dark galaxy" is generally reckoned to be a cloud of gas sitting quietly inside a dark matter halo minding its own business. The point being that the gas isn't forming - or has ever formed - any stars, making it dark. Technically this means it's only dark to visible light - it can still emit at other wavelengths, like radio waves, so some people prefer to call them "optically dark galaxies" or "gas only galaxies".

People who worry about such things probably don't get invited to parties very much, so I'm going to stick with plain-old dark galaxies for the rest of this post.

Firstly, the dark matter. Over the years, "being mostly made of dark matter" has become the de facto definition of a galaxy. Dark matter is pretty simple really - galaxies are rotating too fast to be stable, so without something else beside the visible matter to hold them together, they should just fly apart. The links have more details if you like that sort of thing.

This is what happens if you take the dark matter away.

There are alternatives to dark matter (like different theories of gravity) and I'll admit to holding a small degree of skepticism about it even now. However, the evidence is leaning pretty far in favour of dark matter's existence (Ethan Siegel has this excellent summary), so I can't justify my skepticism rationally. For the rest of the post, let's go with the consensus and assume it does indeed exist.

Sometimes, it can be tricky to tell the difference between a cluster of stars and a genuine galaxy (I stole this idea from Robert Minchin while he wasn't looking). What gives the game away is the dark matter - if the object needs dark matter to hold it together it's a galaxy, if it doesn't, it's a star cluster. Keep a copy of this identification chart handy if you have any doubts.

Mind you, not everything without dark matter is a star cluster.

With dark galaxies we're looking for clouds of hydrogen that need dark matter to hold them together, but haven't bothered to form any stars. If "dark galaxies" is a controversial term, then perhaps we could go with "lethargic galaxies" ?

In some ways, they wouldn't be all that remarkable. In almost all cases, the hydrogen gas extends further from the center of galaxies than their stars do (usually by a factor ~2, but sometimes much further) - so in that sense, all galaxies have a dark component. But the idea of objects which don't have any stars at all is intensely controversial.


2) Are you sure you're talking about "dark matter" and not "doesn't matter" ?

Pretty much everyone, I think, accepts the idea that some dark galaxies might exist. The sticking point is : are there just a few exotic, weird objects, or are there bajillions of the little blighters ? And that brings us to the "missing satellites" problem. This is nothing to do with China blasting satellites out of the sky - it's about how many smaller "satellite" galaxies should be buzzing around large galaxies like our own Milky Way.

Theories suggest that there should be about ten times as many dwarf galaxies orbiting the Milky Way as we actually see. Huge projects like the Millennium Simulation (below) attempt to re-create the Universe inside computers. And they do very well on large scales, like filaments and voids, but fail miserably on the much smaller scales of individual galaxies.

Dark matter (used as a proxy for galaxies) distribution in an artificial Universe.

There are lots of possible explanations for this - finding galaxies can be quite hard, we may not understand the physics of galaxy formation all that well, or possibly there are just too many problems with our whole cosmological model and we should just give up and start again. That last one probably isn't as outlandish as some of us might like, but let's assume that we don't need to resort to such drastic measures. Even so, the missing satellite problem is a big problem in cosmology, and solving it would be a seriously major breakthrough*.

* It's unclear whether the newer, spectacular Illustris simulation has any answers to this - we'll have to wait and see.

Dark galaxies could offer a way out of this if most of those missing galaxies just haven't formed any stars. And, just like black holes, that would make them fiendishly difficult to spot. I'll let Red Dwarf's Holly explain why.

Quite. But it's possible that some of those dark galaxies could have hydrogen in them - just enough to detect, but not so much that they'd start forming stars (more gas => more star formation... usually). Now, if there are only a very few such objects, then perhaps they don't really matter much in the grand scheme of things. But if we can prove that even one exists, then that allows the possibility that there could be many more, potentially solving the missing satellite problem.

Proving the existence of even one dark galaxy turns out to be darn tricky. Which leads us on to the object that got me into radio astronomy in the first place : VIRGOHI21. It's not too much of a stretch to say that if this gif hadn't been shown at my PhD interview, I probably would have ended up doing something completely different*. I stole this one off Robert Minchin again. Poor Robert... allow me to compensate by saying, "go and read Robert's awesome blog !"

* This is just one particularly famous example. During my PhD I discovered 8 other objects that might be sort-of similar to this one, but we don't have such good data for these yet. Various other candidates have been proposed over the years, but none have ever quite satisfied everyone.

VIRGOHI21 is in the center, linked by a stream to the nearby galaxy NGC 4254 (the biggest, brightest blob). This is the hydrogen "data cube" for the region - for more on these, have a look here, here and here - well, throughout the whole blog, really.

Robert has a detailed post about this object, so I'll just give a short summary. Basically, it's a cloud of hydrogen in the Virgo Cluster that's rotating too fast and seems to have interacted with a the spiral galaxy NGC 4254, with a long stream linking the two. It looks as though the hydrogen cloud be a dark galaxy that's pulled some of the gas out of NGC 4254... but the interaction is the problem. It's possible that the apparent rotating "dark galaxy" was formed when some other object pulled the gas out of NGC 4254, creating the illusion of a stable, rotating cloud of gas.

And that's the crux of the matter. It's easy (ish) to prove that a gas cloud is rotating too fast and would require dark matter to be stable. It's far more difficult to prove that it actually is stable, and not just tearing itself apart.


3) Jeez Louise, get on with it and tell me about the new results already !

Which bring us on, at last, to the latest awesome press release, concerning an object called the Smith Cloud. This is an object which is interacting with our own Milky Way galaxy called a high velocity cloud - simply because it's gas that's moving more quickly than gas in the disc at a similar position on the sky.

Lots of other HVCs are known, and there are almost as many theories as to what they are - maybe just gas thrown outside the galaxy by supernovae, though for the Smith Cloud this doesn't seem very likely... it's too massive. As the authors of the study say, it would take the power of 1,000 supernovae to eject it (you know you've got something cool when you cut out the bit about one thousand exploding stars from your press release). Or perhaps they're torn off by other interacting galaxies, or maybe they're primordial gas that's condensing from the intergalactic medium. Some of course, might be dark galaxies. In all likelihood, different clouds are probably formed in different ways.

From the official press release : "If it were visible with the naked eye, 
the Smith Cloud would cover almost as much sky as the constellation Orion."

The idea that (some) HVCs could be dark galaxies is by no means new. The difficulty is proving it. In the current paper, the authors describe the result of simulations of the Smith Cloud as it orbits our galaxy. From observations, they've been able to constrain its orbit (how well, I'm not sure) and find that it passed through the disc of our galaxy about 70 million years ago. They've gone to some lengths to test lots of different models, with and without dark matter, using different gas densities, to see what should happen to a cloud this massive as it punches through the Milky Way's own gas disc.

What they find is that without dark matter, the gas density needs to be very high for anything to survive that even remotely resembles the cloud we actually see - and then it would end up having more gas than we observe. With dark matter, however, they're able to reproduce something that not only looks quite remarkably similar to the observations, but more quantitatively, also has the correct mass and density of gas. Here's their figure :

Contours indicate the dark matter.

Impressive stuff, especially since they have the comparison simulations without dark matter that just don't work. As far as I'm aware, the paper hasn't been accepted for publication yet, but it looks extremely interesting to me. What would be really exciting about this is that it implies that at least some other HVCs could also be dark galaxies. That would really overturn a lot of current ideas and could, potentially, totally knock the missing satellite problem on the head.

But let's be cautious, and remember VIRGOHI21. When this object was announced to the world, the controversy was intense, and the debate more than a little heated - in the end, few people still think it's a likely dark galaxy candidate. To me, it looked like a very plausible candidate initially, but as more observations were completed and more simulations run, it looked less and less plausible.

I shall certainly be awaiting the final version of the Smith Cloud paper with baited breath (or in reality, with a nice cup of tea). The devil's in the details, of course... and it's always possible that this object is special, and not really representative of HVCs in general (it is, after all, a lot more massive than most of them). The authors themselves make no claims in their paper for anything as grandiose as solving the missing satellites problem - even if it is a dark galaxy, it doesn't mean there are more out there. Ultimately, though, if this object fits the bill, then it certainly makes dark galaxies look like a perfectly valid way to solve the problem.


My biggest concern is a philosophical one : the authors have found a model that works, but that doesn't mean there isn't another model they haven't considered. Such things have been known to happen in the past. Also, I'm not an expert on simulations, so I can't tell if you what they've done is sensible or bonkers. Actually they don't give very many details about the simulation setup, or much description of exactly what happens during the simulation. Some of those details could be very important.

For example, how high does the gas density get when the cloud and disc collide - high enough for star formation ? Would the simulation have allowed star formation to happen, or was it turned off to save CPU hours ? Or does the complex hydrodynamics actually make the gas density decrease in some way ? The simulation reproduces the observed cloud, yes, but that's all we're shown. It would certainly be nice to watch how the cloud evolves.

Make no mistake, I'm excited about this paper. But I also don't want to leave readers with a feeling that any minute now someone will reach an exciting or uninteresting but definitive conclusion - science isn't like that. It's a process - people have been observing the Smith Cloud for years, and it's very unlikely we'll get a decisive result anytime soon, if we ever get one at all.


So, what would it take to prove a dark galaxy, definitively ? It would be difficult, but not impossible. The gas would have to show all the features of ordered, stable rotation we see in normal galaxies. It would have show no any signs of interacting with another object - otherwise there could always be some doubt that it was formed by the interaction, somehow. It would have to be extremely isolated, to avoid any suggestion that it formed by an interaction in the past. The Smith Cloud research is very exciting, but the platonic ideal of dark galaxies has yet to be found. Until then, the hunt goes on.

D U N E : A Pug's Tale

Ah, Dune... a novel of such epic proportions and complexity that it's probably unfilmable. Not that that's stopped people from trying, however.

"They tried and failed ?"
"No, they tried and no-one liked it very much."

Unless you read the book, David Lynch's 1984 movie is, at best, a broken masterpiece. Rich, dark and with a soundtrack to match, it also just doesn't make any sense. And why should it ? About half the book consists of what characters think other characters are thinking - and what they think they're thinking isn't always what they're really thinking, I think. You can't really film that.

However, it adds at least one detail to the Dune story that puts a rather different slant on the whole shebang. I'm talking, of course, about Paul Atreides' pet pug. Knowing pugs, this alters the tone of the entire movie.

Dune is not hard science fiction, but pugs are pretty consistent with the established universe. Like Paul's tiny Bible*, pugs are small and therefore well-suited to space travel. They can withstand high accelerations due to their natural crumple zone. And, although set at least ten thousand years in the future, the survival of the breed would be assured by the Bene Gesserit sisterhood's enduring eugenics program (were they trying to breed a Kwisatz Haderpug ? we'll never know).

*This always bothered me. In the Duniverse, huge space fleets aren't a problem (though they are expensive). Compressing an 1800 page book into something the size of your thumb doesn't seem like much of a saving.

One may further infer that the survival of the pugs was guaranteed by the Buterlian Jihad - the war against the thinking machines. Because if there's one thing in the Universe that's definitely not a thinking machine, it's a pug.

I CANNOT BRAIN TODAY. I HAS THE DUMB.

Paul is clearly quite attached to little Scruffles, because he took him with him from the safe (though somewhat damp) environment of Castle Caladan to the desert furnace of Arakis. What, then, was life like in the Atreides household during Paul's youth ?

We will have to assume that Scruffles was much like any other pug. And that means that just occasionally, Duke Leto would have raged around the castle complaining that everything was covered in poop. Lady Jessica would have forbidden Paul from learning the Weirding Way "until all that *!"£@ing pug hair is cleaned up, young man !". Thufir Hawat would likely have complained that even Mentats can't concentrate "with that ugly little runt barking at leaves all day."

There is, however, something very wrong with the scene where the Atreides family leave Caladan to board the Guild heighliner, and it's that the family pug doesn't do anything. In the movie I can only assume this was done by lacing Scruffles' food with horse tranquillisers. In the narrative, I expect Paul  (or possibly Gurney Halleck) spent his last few hours on Caladan chasing his beloved pug around the castle grounds. Because if God created Arakis to train the faithful, then surely God created pugs to train the patient.

We don't see the pug again until the Harkkonen attack. In the absence of Sigourney Weaver - although honestly even if she were available it would be a tough choice - there's only one man Paul trusts to guard his childhood pet : Patrick Stewart, aka Gurney Halleck, weapons master of the Atreides. And thus do we see Patrick Stewart charging into battle, blaster in one hand and pug in the other.

"LONG LIVE DUKE LETO.... AND HIS PET PUG, SCRUFFLES !"

We don't really know what happens to Scruffles after this. However, pugs are naturally suited even to avoiding the mighty Shai-Hulud, since they're lightweight and blessed with about as much sense of rhythm as William Shatner. We can only infer that the furry little idiot survived through many adventures....

I don't know who made this image but I'd like to thank them.

... because we do know the pug survives. Right at the end of the film, when Paul has brought the empire to its knees and freed the Fremen from the Harkonnen yoke, we can just about see an attendant doing their best to stop little Scruffles from interrupting Paul during his big speech.

Is it definitely the same pug ? I don't know, but how many pugs do you think there are on Arakis ?

Ours Is Bigger, Probably

Pretty soon, Europe will be blasting the heck out of a Chilean mountain so that they can build a great big telescope there. This extremely large telescope will be so large, in fact, that they'll call it the Extremely Large Telescope. At 40m across its extreme largeness is beyond dispute. However, some articles claim it will be the world's largest telescope, so here's a friendly reminder that (despite being 40m wide) compared with some other facilities, this will still only be an Extremely Little Telescope.

Nonetheless, other names considered included, "The Really Quite Enormous Telescope",
the "Too Big to Fail Telescope", and, of course, the "Telescope of Devastation".

Because 300m > 40m. I've counted.

Now, obviously, radio telescopes and optical telescopes  work very differently, and the challenge of building a 40m diameter mirror is quite different to building a 300m diameter metal bowl. But a telescope is a telescope, and calling the ELT the world's largest is a bit like calling Snowdon the tallest mountain - sure it is, in Wales. Omitting that qualifier makes the statement.... well, wrong.

The ELT will be the world's largest optical telescope. And it will be mindbogglingly awesome. But the more general question of "which is the world's largest telescope ?" is slightly more tricky - and much more interesting - to answer.

OK, Hubble isn't a giant telescope, but let's face it, it's pretty darn neat.
Sphinx model came from here. Anyone who responds with claims about mystical pyramid energy will be given a stern glare.

The diagram above shows some notable present and planned classical telescopes - big reflective (single-dish) mirrors that focus light onto a receiver. Several other very large optical telescopes are also planned (e.g. the Giant Magellan Telescope, which will consist of seven massive mirrors stuck together, and the aptly-named Thirty Metre Telescope, the American rival to Europe's ELT). There are also several other large ground-based telescopes already operating (e.g. the Very Large Telescope) and while JWST will be the successor to Hubble, it's worth pointing out that the now inoperable Herschel is currently the largest single mirror* ever flown in space at 3.5m diameter.

* Arguably. Keep reading.

Many radio telescopes work in the same way as optical telescopes, it's just that the light they collect is a at a longer wavelength. So there's no need to make their mirrors smooth and shiny - they don't care about visible (or optical) light, and the radio waves aren't affected by small bumps in the surface. That makes it quite a lot easier to build really enormous radio receivers.

In fact, the Arecibo reflector is pretty much transparent to visible light. To save weight (and, interestingly, also to let enough light through so that the plants underneath don't die), each metal panel has lots of little holes, and you can easily see through it from underneath (just like the protective grille in a microwave oven, the radio waves are too big to pass through the holes).

See more.

And that raises an important question - does the reflecting surface have to be solid and continuous ? If you insist that the surface has to be like, watertight - then the world's largest telescope is probably the 10.4m GTC in La Palma. And by that rather contrived definition, then yes, the ELT will indeed be the world's largest telescope.

A more reasonable interpretation would be that the surface must be continuous at the wavelength it's observing. In that sense, Arecibo is a clear winner, and has been for over 50 years. But all glory is fleeting, and in a few years time the massive FAST will dwarf even Arecibo. This 500m behemoth will have the extra complication of a deformable dish - thousands of cables will pull the spherical reflector into a paraboloid, allowing it to point at different parts of the sky. Whether this crazy scheme will actually work remains to be seen.

And yet, by taking a few more liberties, even FAST won't be the largest telescope. If you don't mind your reflector having some bloomin' great gaps in it, then the remarkably obscure Russian RATAN-600 claims the prize. This crazy instrument is almost like what you'd get if you built Arecibo without bothering to find a sinkhole for the reflector - basically it's a huge ring of reflectors, 576m across, which focus the radio waves onto a central receiver.

My professional opinion is that it's freakin' weird.

But we can't stop there. If we're allowing gaps, we may as well allow arrays of telescopes and not just single dishes. Then we can get very large telescopes indeed. The 27 antennas of the so-called Very Large Array form a telescope up to 22 miles (35 km) across using a technique called interferometry, and it's pretty common to connect multiple telescopes spread across the entire planet*. And yet those wonderful, crazy Russian scientists refuse to let a little thing like the size of the planet stand in their way...

* There's a price to be paid, of course - you can use multiple telescopes to get very high resolution, but you lose sensitivity by having so much empty space (especially to low density material).

I suppose it is fairly large.

Radio Astron takes interferometry to extremes. This is a 10m diameter Russian radio telescope... iinnnn sppppaaaaacce ! 10m is too large to launch a conventional reflector, so, like a giant umbrella, it was folded up during launch and unfolded once in space. Working in conjunction with ground-based telescopes, it can effectively form part of a telescope 350,000 km across. Given the existence of a telescope larger than the Earth, the question of the "world's largest telescope" seems a tad petty.

Because the world is not enough.

Then again, up until now we've been biased by assuming that telescopes have to collect light emitted by distant objects. This isn't always the case. Neutrinos are particles (so not even part of the EM spectrum at all) of almost zero mass that almost never interact with anything - most of them pass straight through the Earth unhindered. Building a camera to detect them isn't really an option. So neutrino telescopes work in a completely different way : they detect the light emitted on the (very) rare occasions when a neutrino does happen to interact with, say, a water molecule.

Yes, that's a boat. Yes, there was a missed opportunity for a Bond movie here.
The spheres house the sensors that will detect the light emitted by neutrinos colliding with water molecules (when the entire tank is filled). 

The Super-Kamiokande neutrino telescope seen above holds 50,000 cubic metres of water in a cylindrical tank about 40m tall and 40m across. For more "normal" telescopes, the area of the reflector determines how many photons of light you can detect. For neutrino telescopes, it's volume that matters, not area. Which makes them fundamentally different, so we can't really compare them to classical telescopes. That would be completely unfair, so let's do it anyway.

If we emptied all the water of Super-Kamiokande into Arecibo, it would only fill the reflector to a depth of about 8m. Assuming it somehow didn't just drain away, which it would.

Critics agreed that budget cuts to the remake of Goldenye were a mistake.

So, can radio astronomers point at laugh at all the other astronomer's puny instruments ? Possibly, but it would be inadvisable. For one thing, there's another, much bigger neutrino telescope : IceCube. Instead of building an enormous tank filled with liquid water, this is an even more audacious project using the ice of Antarctica. Buried over a mile below the surface are 86 boreholes each filled with 60 detectors, enclosing an entire cubic kilometre (1 billion cubic metres) of ice. That's a mass of 900 million tonnes - easily enough to crush Arecibo a like a puny little bug.

LIKE A BUG !!!

Still, although its mass is much, much greater, a mere cubic kilometre is paltry compared to the 350,000 km baseline of Radio Astron. For now, then, radio astronomers can laugh at other scientists as long as they're very very careful about it. Neutrino telescopes may have the most physical mass, but radio telescopes have the biggest reflectors and the largest diameters. They're likely to retain the prize for biggest mirrors for the foreseeable future, but another exotic type of astronomy has the crown for largest diameter firmly in its sights.

Gravitational waves are nothing less than ripples in space itself, produced by any moving object. These distortions should be detectable using a system of lasers and mirrors - roughly speaking, if a wave passes through, the length between the mirrors changes, altering the time it takes the laser to travel along its path. The path of the laser beam needs to be as long as possible since the changes are damnably small. Current detectors have laser paths a few kilometres in length.

LIGO detector in Louisiana.

Much to the disgust of other astronomers, none of these facilities have ever detected anything. In fact, it's the radio astronomers - yet again - who can point and laugh, since the only convincing evidence for gravitational waves comes indirectly from observations of binary pulsars (first measured, by the way, at Arecibo), whose orbital decay is in perfect agreement with the predictions of general relativity assuming they're emitting gravitational waves.

However, unless something is staggeringly, astonishingly wrong with the theory, given enough time it will be possible to directly detect gravitational waves. Fed up of other astronomers calling them "gravy waves", the GW community has hatched a foolproof plan to massively increase sensitivity and make absolutely sure if the pesky things are real or not. And that involves another giant space telescope - one that makes even Radio Astron look just a little bit pathetic. If it works.

The LISA gravitational wave observatory will use lasers to create a telescope ~5 million km across. This is a no-lose situation. If it works, we'll have a whole other way of studying the Universe. If it doesn't - I mean if the telescope itself functions correctly but fails to detect anything - it will be the greatest non-detection since the aether, and a scientific revolution is sure to follow. And, best of all, until it's built the rest of the astronomy community can continue to poke fun at the gravy wavers with all their oh-so-interesting noise measurements.


So, there you have it. The world's largest telescope is a tricky question to answer, and maybe even irrelevant. Already we have telescopes as big as or even larger than the world, in some sense, with more on the way. And that's just artificial telescopes. Arguably, even more exotic techniques like pulsar timing arrays and gravitational lensing use entire galaxies to form natural telescopes. But that's another story.