Monthly Archive for November, 2010

System Book 1: Katringa is now available!

I’ve been spending the past few months working for Spica Publishing on a new supplement for the Mongoose Traveller RPG called System Book 1: Katringa, and I am pleased to say that it is now available for purchase!

There’s actually some science in it too, since I used my realistic planetary system generator (which is based on the latest research) to make the system, and I made the physical details of the young A8 IV primary star, its planets, their orbits and Katringa itself as realistic as possible (I even figured out the orbital evolution of the moons of the gas giants in the system!). So if you’re looking for an interesting new planetary system for your SF games then please do check it out!

Welcome to Katringa!

Spica Publishing is pleased to announce that its latest product – System Book 1: Katringa – is now available from RPGnow and DrivethruRPG. This 30-page PDF is written by Richard Hazlewood with physical data by Constantine Thomas, and is available for $6.99.

System Book 1: Katringa is the first in a series of books from Spica Publishing that describes a complete planetary system and its society, and is designed for use with Mongoose Traveller or any science-fiction RPG. Katringa is a former corporate mining colony that is gradually being allowed more independence over time, although corruption still pervades the government. Conditions are harsh in the asteroid belt close to the system’s energetic primary star, but the riches are worth the risk.

System Book 1: Katringa includes:
– A realistic planetary system, based on current astrophysical knowledge.
– Details of the worlds in the system, including the mineral-rich Idowa Belt, the gas giants Accra and Yendi and their moons, and Olufemi and the Outer Asteroids.
– A planetary map of Katringa showing major landmarks and settlements.
– A full physical description of Katringa, including a detailed breakdown of its geographical features and timekeeping system.
– Katringa’s unique African-influenced society, history and government.
– Important NPCs that may be encountered on Katringa, from all walks of life.
– Adventure seeds to occupy Player Characters while in the Katringa system.
– Detailed asteroid mining rules.

Download it today!

“Traveller” and the Traveller logo are Trademarks owned by Far Future Enterprises, Inc. and are used with permission. The Traveller Main Rulebook is available from Mongoose Publishing.

[Book Review] Voyager, by Stephen J Pyne

Summary: If you want to learn about the Voyager mission, then this is not the book for you.

Review: Being a Voyager fan, I really had high hopes for this book. Unfortunately, it isn’t the book I thought it would be.

I was hoping that this would be a definitive book describing the Voyager mission in detail and talking about its goals, the troubles and triumphs it encountered as it travelled through the solar system, the new discoveries it made and how they revolutionised planetary science (and how it captured the imagination), as well the people involved in the Voyager project.

What I got instead was a lot of meandering, historical waffle comparing Voyager’s exploration of the solar system to past historical ages of discovery, with a few little nuggets of actually Voyager-relevant detail thrown in. I’d say that about 80% of the book is historical waffle, and 20% of it is actually about Voyager. After a while I was actively skipping pages of the historical stuff (which I really wasn’t interested in) to get to the Voyager “meat”, but in the end even that couldn’t hold my attention because there just wasn’t enough of it. The descriptions of the planetary encounters contain very little actual information and a lot of the really cool stuff is glossed over as if it was a distraction to the real purpose of the book. This is surprising given that the book is divided quite methodically into the various phases of the mission (e.g. Cruise, Asteroid Belt, Cruise, Jupiter flyby, Cruise, Saturn flyby, etc) – it’s not as if there isn’t enough science or stuff happening involving Voyager itself fill up those chapters! I got through about half of the book before giving up, because it turned into a very frustrating read – had I continued I think I would have thrown it against the wall in anger! And skipping through the rest of the book, I didn’t really see any change for the better later on either.

To be fair, its subtitle is “Seeking Newer Worlds in the Third Great Age of Discovery”. That being the case, I was expecting a little bit of historical comparison, maybe a chapter at most at the start… but the author scatters this throughout the whole book, breaking off a perfectly good narrative about Voyager itself and veering off on vaguely related tangents about Vasco Da Gama, Magellan, Cook, and Darwin’s expeditions. Even when it’s on topic, it hops randomly back and forth between Voyager itself and the politics and history of what was going on at JPL on Earth, which is somewhat jarring and more annoying if you just want it to focus on Voyager. The reviews on Amazon raise the same issues as I’m describing here, and in retrospect I really should have read them before picking this up. Oh well.

I suppose that if you were actually looking for a book that was low on actual science and high on the author’s opinion about comparisons between planetary discovery and exploration and the historical exploration of new lands on Earth then this book probably has a lot going for it in that regard. Even then though, the author wanders around all over the place with his discussion, and expounds and opines and waffles on with annoyingly flowery language that rapidly grated on me. In the end, I don’t really think it rightfully belongs in the Science section of a bookstore, but rather in the History section instead.

Maybe I’m being overly harsh, and I’ll admit that my negative review is in part because I was expecting the book to be something different to what it actually was. But even then, I think it still lacks focus and that it isn’t actually very well written either.

In short – if you want to learn more about the Voyager mission itself (rather than its historical context), I can’t recommend this book at all. I am searching for a better book on the subject, and I have it on good authority (from the Voyager2 twitter feed itself!) that “Voyager Tales” by David W. Swift or “NASA’s Voyager Missions” by Ben Evans are much more like what I hoped this book would be (the latter looks particularly promising).

What is a planet?

Eris and its moon Dysnomia (Artist's impression)
Eris and its moon Dysnomia (Artist’s impression). Image Credit: NASA, ESA, and A. Schaller (for STScI)

Recently it was revealed that Eris – that pesky worldlet that precipitated the removal of Pluto’s status as a planet – may actually be smaller than Pluto after all, albeit only by a couple of kilometres in radius. Naturally, this has got some folks thinking about the definition of a planet again.

The way that Eris’ diameter was refined was pretty clever – astronomers realised that the distant dwarf planet would be passing in front of a very faint star when viewed from certain places on Earth. They weren’t sure exactly where the occultation path would be except that it’d be visible from South America at least. As it happens, it was observed by astronomers in Chile, and at least three groups saw the light from the star blink out as Eris passed in front of it. By timing how long the occultation took, they could figure out an upper limit to Eris’ diameter, and it turns out that Eris can’t be larger than in 2340 km diameter. Pluto, by comparison, is 2344 km in diameter (give or take 20 km).

So it seems that at best Eris is actually similar in size to Pluto, and is more likely to actually be smaller. So that’s good news for Pluto, which now appears to have regained its crown as the largest object in the Kuiper Belt… but is it enough to make it a planet again?

Well… no.

When Pluto was “demoted” I have to admit I was a little put out by it, but I rapidly realised that it had to be done. What people may not recall is that Pluto has always been considered “the odd one out” since its discovery in 1930 – of the (then) nine planets, we had four terrestrial planets in the inner solar system, four gas giants in the outer solar system… and then Pluto, which was smaller even than any of the terrestrial planets. This was made worse when we started to discover other objects out there beyond Neptune – the first was 1992 QB1, a small body about 100-200 km in diameter orbiting between 40 and 45 AU (further than Pluto) that was discovered in 1992 by David Jewitt and Jane Luu. Since then, hundreds more objects have been discovered in orbits beyond Neptune – as a result, Pluto was no longer alone in the outermost reaches of the solar system, raising further questions about its planetary status, and people had been wondering what would happen if something was discovered in the outer system that was larger than Pluto anyway. Would that new object be a 10th planet? What if we found others, would they be Planets #11, 12, 13, or more? When Mike Brown and his team discovered Eris in 2005 (back then it was nicknamed “Xena”) and that seemed to be larger than Pluto, that lit the fuse on the powderkeg. Something clearly had to be done.

The IAU weighs in

In 2006, the International Astronomical Union passed a resolution defining a planet as:

The IAU…resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects [3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

It also resolved that planets and dwarf planets are actually “two distinct classes of object” so dwarf planets aren’t actually a sub-type of planets.

However, this didn’t make everyone happy – in fact it made some scientists very unhappy for a number of reasons (for those who think that scientists are all objective and unemotional about their work, they’re not – they can get very intense about things 🙂 ). Personally, I don’t like it much either. For one, it defines planets only in our own solar system (orbiting the Sun), and as we now know – and knew at the time – planets patently are found around other stars. Another problem is that it doesn’t clearly define “clearing the neighbourhood”. Another thing is that “dwarf planet” is misleading – what if we find an object that is as big as Earth, but still hasn’t “cleared its orbit” – is that a dwarf planet, even though it’s the size of Earth? If they’re called “dwarf planets” then why go on to state that they’re not actually planets at all? If that’s the case, then they should be called something else!

It seems like a very unsatisfying, inelegant, rushed, and poorly considered definition to me and in my opinion it hasn’t really been improved over the years. And obviously, it defines Pluto as a “dwarf planet” too, which got a lot of people up in arms who didn’t like the idea of losing Pluto as a planet (there was an addendum in 2008 that defined further “plutoids” as being dwarf planets that orbit beyond Neptune’s orbit, so Pluto is now a Plutoid, but I think that’s a pretty unnecessary and rather pointless subdivision).

A better way?

There are IMO much better definitions out there – this one from 2000 by Alan Stern & Howard Levison (hereafter referred to as S&L), and this one by Steven Soter from 2006. Stern, incidentally, is one of the most vocal opponents to the IAU definition – if I recall correctly one of his main criticisms of the IAU resolution is that it combines physical definitions (the shape) with orbital dynamics (clearing the neighbourhood), which is pretty inelegant and is a valid concern. He also happens to be the Principal Investigator on the New Horizons mission currently on its way to Pluto (which was a planet when the probe left Earth, and now isn’t), so it’s probably not surprising that he has strong feelings about this! Anyway, if you’re interested in planet definitions then definitely read those papers – they’re both fairly readable and don’t contain too much horrible maths.

Both of those in my opinion come up with more logical ways to determine what is a “proper” planet and what is not. The nice thing about the S&L paper is that (unsurprisingly, given the identity of one of the authors) it does separate out the physical parts of the definition from the dynamical parts. They first define a “planetary body” as an object that cannot (and could never) generate energy through nuclear fusion (otherwise it’d be a brown dwarf or a star) and that also is large enough that its shape is determined largely by gravity rather than other force (essentially, that its mass is high enough that its gravity forces it into a spherical shape). Then they go on from there to define what “planet” is. At this point then, Pluto is a planetary body because it’s in a bound orbit around a star (or stars) – and the classical nine planets are included as such along with the largest asteroids and KBOs.

Having done that, they then continue to subdivide planetary bodies down into different types based on whether or not they’ve cleared their neighbourhoods, and define a numerical property (Λ, the upper case of the greek letter Lambda) that can be calculated that shows how well they have ‘cleared their neighbourhood’. This is where it gets really interesting – the eight largest planets have large values of Λ that are much greater than 1, and objects like Pluto and the asteroids have very small values of there of Λ that are much less than 1. So S&L set a boundary between “uberplanets” (Λ > 1) and “unterplanets” (Λ < 1) In the S&L system, Pluto is therefore actually an “unterplanet” along with Ceres… but the key point is that it’s still a type of planetary body (remember that Pluto isn’t a planet the IAU definition, it’s a “dwarf planet” instead, and they’re explicitly not a subtype of planet according to the IAU). However, it’s interesting to note that if Pluto was moved to Mercury’s distance from the sun, it would jump above the Λ = 1 line and become an “uberplanet”.

What I like about the S&L definition is that it’s methodical and consistent. It’s also measurable too, although they do get into further subdivisions that I don’t think are really that necessary.

The Soter paper comes up with another value called the planetary discriminant, with the symbol μ (the greek letter mu). Essentially, the planetary discriminant is the mass of the object divided by the total mass of the other stuff (not including itself) in its “orbital zone”. Soter defines two bodies to be in the same orbital zone if their orbits cross, their orbital periods are non-resonant, and their semi-major axes differ by less than one order of magnitude. Again, he finds that μ is very large for the big eight classical planets, and less than 1 for objects like Pluto and Ceres. He suggests that μ=100 should be the critical value that separates a planet from a non-planet – in other words, if an object is at least 100 times more massive than the other stuff that crosses its orbit, then it’s a planet.

While that seems pretty elegant, Soter then continues to define objects based on where and how they accreted and formed, which isn’t actually something that can be unambiguously gathered by observation. As a result, I think Soter’s classification system isn’t actually as useful and clear as the S&L system. It’s also not clear what happens if one has a situation where several large objects of a similar mass are in the same orbital zone – if we found a system with say five similar earth-sized objects that were somehow in the same orbital zone, then would they not be planets (μ would be ~0.25 in that case)?

Conclusion

In all of these classification systems, Pluto is still reduced from being a “proper” planet in the sense that Earth or Jupiter or Neptune is. I think that’s just inevitable, because Pluto’s nature is very clearly different to that of the major planets. We know now that it’s one of the larger members of a belt of bodies that are within its “orbital zone”, much like the asteroids closer to the sun. It’s interesting to note that when the first asteroid (Ceres) was discovered in 1801, three other large asteroids were discovered at similar distances from the sun in the next three years, eventually leading astronomers to believe that there was a belt of bodies in the region between Mars and Jupiter (though they were actually classified as planets at first!). I think that had Eris and the other TNOs been discovered around 1930 along with Pluto then it would have been clear to astronomers back then that they were looking at another asteroid belt that was further from the sun, but the 60+ year gap between the discovery of Pluto and 1992 QB1 was long enough to get Pluto entrenched in our minds as “the ninth planet”.

I think it’s also clear that it’s actually really hard to come up with a consistent, universal definition for a planet, which is surprising since it’s something that a lot of people took for granted before. I think the S&L paper does the best job out of the system considered here, but even then that still has some issues, though it’s vastly superior to the IAU definition IMO (in a later post here, I may discuss my own ideas for planet definitions to further confuse matters).

Finally, I think the most important thing to realise is that Pluto is still out there merrily orbiting the sun as it always has, and I’m quite sure that it really doesn’t care what we call it ;). When New Horizons gets there in 2015 it’s going to reveal the last frontier of the solar system to us, and I’m quite sure that Pluto and Charon (and Nix and Hydra, Pluto’s recently discovered smaller moons) will have a lot of interesting surprises for us!

Emily’s Asteroids and Comets montage

I’m working on another science post of my own, but in the meantime I thought I’d share this rather awesome montage that Emily Lakdawalla compiled over at the Planetary Society Blog, which is well worth following if you’re into solar system exploration. It shows all the asteroids and comets that have been visited by spacecraft as of November 2010, including Comet Hartley 2. The really nice thing is that it shows them all at the same scale, so you can see the differences that size makes to thinks like crater size (I think Mathilde is roughly the size of Greater London).

There’s a much larger version of the image available on the Planetary Society page that’s definitely worth checking out too, that shows the little ones in more detail – though be warned, the image is huge!

Comets and Asteroids, by Emily Lakdawalla

Credits: Montage by Emily Lakdawalla. Ida, Dactyl, Braille, Annefrank, Gaspra, Borrelly: NASA / JPL / Ted Stryk. Steins: ESA / OSIRIS team. Eros: NASA / JHUAPL. Itokawa: ISAS / JAXA / Emily Lakdawalla. Mathilde: NASA / JHUAPL / Ted Stryk. Lutetia: ESA / OSIRIS team / Emily Lakdawalla. Halley: Russian Academy of Sciences / Ted Stryk. Tempel 1, Hartley 2: NASA / JPL / UMD. Wild 2: NASA / JPL.

(if you want to comment on it, go over to the Planetary Society Blog and tell Emily! 🙂 )

Happy Sagan Day!

Today would be Carl Sagan’s birthday if he was still alive, so Kepler and SETI have declared today as “Sagan Day” and invited people to send in essays about science and our place in the universe, inspired by his “Shores of the Cosmic Ocean”.

I watched Cosmos again a few months ago and it’s still a great series and well worth watching – Prof. Sagan was a darn fine educator, and really put across the wonder of the universe and how deeply he was affected by it all. I think that’s why I like Brian Cox and his Wonders of the Solar System series, because he did pretty much the same thing (unsurprisingly, Prof. Sagan was one of his big influences too).

Cosmos didn’t get me into astronomy – I was already into it by the time I saw it (being a Brit, Sir Patrick Moore and Heather Couper were probably a bigger influence on me) – but Cosmos did leave several indellible impressions on me, and it certanly reinforced my fascination with science and astronomy:

– The biggest impression, oddly enough, was a fascination with the Periodic Table, as explained by the man himself in The Lives of Stars episode, from Cosmos – ever since then I’ve had a bizarre fascination with Praseodymium :). I love reading about the elements and their properties and their uses (webelements.com is one of my favourite sites 😉 ), and I would kill to have a “real” periodic table that had samples of all the elements like the one shown in the clip!

– Another big impression was the “Ship of the Imagination” (seen in some of the other videos below) that Sagan used to travel around the universe in the show. More than anything else I would have love to be able to go on such a trip, unconstrained by time or physical limits, and see the wonders of the universe first-hand. I guess that’s why I’m so into scifi :).

– There’s the Encyclopaedia Galactica, which I’m about 100% certain is what got me into Worldbuilding. Who knows what worlds are out there, waiting to be found? We’ve only just started to discover them. Meanwhile, I’ll continue to imagine what they could be like. 🙂

– And the Cosmic Calendar, where Sagan compresses all of time since the Big Bang into one calendar year, and shows us how completely insignificant all of human achievement is in time.

– And finally, a sobering reminder of our own mortality and the fragility of our planet, made vivid by the telling of a chilling dream that Sagan had. I guess he was one of the first people that I was aware of who was an environmentalist, who told us that we should be taking care of our planet and ourselves. It’s a pity that 30 years later we’re still making a mess of it all.

The other big thing was the music – the soundtrack of the series is varied and eclectic, ranging from classical pieces, to Vangelis electronica, to Bulgarian folk music. But it’s all memorable, and quite timeless.

It’s impossible to watch Cosmos without being inspired by it – Sagan’s explanations are spell-binding and enrapturing, and it’s still one of the best science shows ever made. He explains things clearly and succinctly, isn’t afraid to go into detail where necessary, puts his obvious passion across, and doesn’t patronise the audience. You couldn’t ask for a better teacher, and he’s inspirational for me in that regard as I try to get into science education myself.

So, thanks for everything, Professor Sagan (and Happy Birthday!) 🙂

Ganymede in Minecraft!

While tinkering with the textures in Minecraft (yes, I know, it’s got me too) it occurred to me that I could actually replace the moon texture with something more… interesting. So voila, Ganymede in minecraft!

Ganymede in Minecraft!

I’m half wondering if I could replace the sun with something more interesting too, though I do quite like the “pulsar sun” in the Painterly pack…

EDIT – I might as well make it available! 🙂 Right-click on this link (or left-click it to see the image), save the file as “moon.png”, find and open up minecraft.jar, and put the file in the terrain folder in there (overwriting the existing moon.png that’s in there). If you don’t know how to do that, ask on the minecraft forums. This shouldn’t screw anything up, but if it does then don’t blame me ;).

New images from Comet Hartley 2!

The Deep Impact spacecraft flew past Comet Hartley-2 earlier today, and has started to return some dramatic pictures of the comet’s icy nucleus – it looks like a roughly 2 km long peanut, with jets of material erupting from the rougher ends that look like they’re covered with boulders, and a smooth central section that may be filled in by dust. It might even be a ‘contact binary’, consisting of two separate objects that are just barely touching eachother and are held in place by gravity (so the ‘neck’ is where dust has flowed into the region between the two objects). There are lots more images coming in (apparently a total of 120,000!) and the ones we’ve seen so far aren’t even the high resolution ones, so stay tuned! 🙂

Comet Hartley 2
Comet Hartley 2
Comet Hartley 2
Comet Hartley 2, Image courtesy NASA/JPl-Caltech

More info and pictures at : http://www.jpl.nasa.gov/news/news.cfm?release=2010-371

Emily Lakdawalla at the Planetary Society blog has also made a very cool approach/departure animation of the images that have been downloaded so far, showing the nucleus tumbling through space – it’s well worth checking out!

Fun with asteroid impact simulators!

Meteor Crater, Arizona (credit: USGS)

Barringer (Meteor) Crater, Arizona (credit: USGS)

For those who like to attempt to destroy the earth with asteroids, there’s a new asteroid impact simulator online at: http://www.purdue.edu/impactearth/

I just entered a 10 km nickel/iron asteroid into the calculator, hitting crystalline rock (so… Canadian Shield?) at an angle of 90 degrees, at a (admittedly fast) velocity of 30 km/s, and got it to calculate the effects felt 1000 km away. (For comparison, 1000km is about the distance between Vancouver and Calgary in Canada, or between London and Berlin in Europe, or between Denver and Las Vegas in the US).

It makes a final crater about 230km in diameter, and 1.5 km deep. 8.3 seconds after impact , I get ignited by the blast of heat from the impact, which even at this distance is not unlike a nuclear blast – everything catches fire. If I’m still around after that, then around the 3 minute mark I feel the residual shaking from the 10.4 earthquake that the impact triggered (not enough to collapse buildings at my distance, but enough to be felt). Ejecta arrives at my location about 8 minutes after impact (a light dusting of millimetre-sized stuff with the occasional larger fragment). Then around the 50 minute mark (the heat is dying down at this point) I get deafened by the sound and any buildings still standing around me get blown away by the 1000 mph winds of the blast wave. If I crank down the impact velocity to a more reasonable 11 km/s then the heatblast is avoided and the crater is smaller, but the quake is still felt and the blastwave still does significant damage.

Either way, it’s not pretty :). And this is just from a comparatively tiny (relative to Earth) 10 km wide bit of nickel/iron. Thankfully there aren’t too many of those flying around on an earth-crossing orbit! It’s interesting to note that if this happened on a world without an atmosphere then the blast wave wouldn’t be an issue, and that’s what causes a lot the physical damage.

Though while I think it’s fine for smaller impacts, I’m not so sure about larger ones – I tried a 6000 km diameter object with a density of 5000 kg/m³ (i.e. about the same size and density as Earth) at 11 km/s and it made a 30,000 km diameter crater (so, about 3/4s of the Earth’s circumference) and yet it didn’t have any significant global effects other than to possible slow our rotation down? It really should result in the total eradication of everything on the surface, atmosphere blown away, probably the entire surface turned into a magma ocean, and the tilt maybe changing significantly (assuming the Earth doesn’t actually breaking apart due to the impact). So I’m not sure I’d trust it for really big impacts.

Though check out the pulldown list of possible impactors – there’s a surprise in there ;).

NASA-themed Lego sets on the way!

Oooo. NASA and Lego are going to be teaming up to produce some NASA-themed lego sets! They’ll be making four apparently – I guess one is going to be a lego space shuttle (since they’re apparently sending up a lego space shuttle on the next real shuttle!), I really hope that one of them will be a Mars Rover (that’d be really cool if done Technic-style). Wonder what the other two might be?

Full info here: http://www.nasa.gov/home/hqnews/2010/nov/HQ_10-285_NASA_LEGO_Team.html

Blue Saturn and Rhea Transit

As Emily points out on this post on the Planetary Society blog, it can be fun to have a look through the Recent Cassini Images page and see what’s arrived from Cassini recently. So here’s a couple of interesting images I found from today’s batch.

First, here’s a nice picture of Saturn with its satellite Rhea passing in front of it. This is actually “true colour” (actually taken with red, green and blue camera filters), though since the component images haven’t been calibrated and corrected for camera distortions yet they’re not quite what we’d see with our own eyes – but it’s close. Saturn’s a little yellowish but there’s not much colour visible. The coloured ‘afterimages’ of Rhea are caused by the fact that it moved across the field of view while each component picture was being taken (it’s moving from right to left). Below the ringplane you can see a foreshortened dark dot on Saturn – that’s a shadow of another moon passing between Saturn and the sun, so if you were in the cloud deck at that location then you’d see a solar eclipse! The component images are: 65795 (blue), 65794 (green), and 65973 (red)
This next one’s kinda fun – Saturn’s gone blue! It’s actually a false-colour image made by using a Methane2 (727nm) filter instead of a Red filter (649nm) in the red channel of the image. This filter is sensitive to infrared wavelengths, which allows us to see the details in the cloud deck more clearly (compare with the true colour image above, and you’ll notice that you can’t see the banding in the atmosphere there). Because the Methane image has more varied contrast than the Green and Blue images, it tints the planet a rather nice shade of blue as well – though it’s amazing how much of a difference that 78 nanometres makes! This is why it pays to have a variety of different filters on a spacecraft. The component images are 65788 (blue), 65789 (green), 65790 (methane2)

The neat thing is that the Cassini Images site gets updated pretty much on a daily basis, so there’s always something new to look at there!