Archive for August, 2011

Bad Times

Posted in Personal, Social Concern with tags , , on August 9, 2011 by davidnm2009

Just a small PSA regarding the current events around the UK; I’m horrified by all of it. In fact I suppose I should admit that watching the news the other day had me close to tears. This is a dreadful business, and I hope it stops soon. This violence and looting isn’t helping anybody. It’s just harming the economy, and thus people’s jobs, and it’s also destroyed the homes and property of many innocent people. It’s not making any kind of point now, and in fact may well end up backfiring badly on the people doing it. (I dread to think how this is going to combine with the immigrant-phobia meme that seems so prevalent around the country now.)

It’s time this stuff ended. Please. Before any of this gets any worse.

I hope that everyone reading this is safe, and I hope that things remain that way.

(Regarding my own situation, I’m very fortunate that where I am seems to be calm at the moment, and there’s no sign of that changing. Thank goodness.)

Moon of the Day (3)

Posted in Astronomy, Space with tags , , on August 9, 2011 by davidnm2009

Today’s pick comes from the Jovian system:

Image via: Wikipedia and the Galileo orbiter.

Ganymede is hard to miss. It’s sufficiently prominent that it was originally discovered by Galileo on the 7th of January, 1610 – in fact, it may well have been the first object ever discovered via a telescope. (We just don’t know precisely which of the Galilean Moons Galileo saw first; we do know that he saw all four of them on the same night!) Ganymede is actually fairly bright, for an outer-system object. At magnitude 4.5ish, it would actually be a naked-eye object, except for the glare from Jupiter.

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Dione

Posted in Astronomy, Space with tags , on August 5, 2011 by davidnm2009

Saturn, I have to admit, is a bit of an easy target for this sort of thing, given that it has something like 60 or so moons. With that many satellites, it’s pretty much inevitable that you’ll find some interesting oddballs. Dione is another such moon of Saturn.


Credit: Cassini/NASA, via Wikipedia

Dione was first observed in 1684 by Giovanni Cassini, who also discovered three others of Saturn’s satellites[0]. Dione wasn’t actually formally named until 1847; Cassini called his discoveries the ‘Sidera Lodoicea’ (‘Stars of Louis’) after his patron, the French king Louis XIV. (This sort of fawning was entirely normal at that point in history … luckily, it’s one practise we seem to have left behind!)

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Hyperion

Posted in Astronomy, Science with tags , on August 3, 2011 by davidnm2009

I was having a browse of some of the Cassini pictures the other day. The orbiter has returned some absolutely amazing images of the Saturnian moon system. My eye was particularly caught by Hyperion … but not because it’s pretty. In fact, it may possibly be the most ugly-looking thing in the Solar System (bar a few politicians).


(Image via Wikipedia, from Cassini’s September ’05 flyby.)

Hyperion is weird.

No, really, it’s weird.

First of all it has a massively eccentric orbit around Saturn. Secondly, despite being fairly large, the moon itself is appreciably egg-shaped (as you can clearly see in the above photo!). Its longest axis is 360 Km, its shortest is 205. Egg-shaped orbit, egg-shaped moon, apparently!

But there’s more. In terms of composition, Hyperion also appears to be a freak. Its mass is tiny. Despite being around 2/3rds the size of Mimas, it has only 15% of that other Saturnian moon’s mass. Probably this is partly why it’s so un-spherical; there’s just not enough surface gravity to flatten things out. Hyperion’s density is also rather low. In fact, some estimates claim that its insides are as much as 40% empty space.

Yup, that’s right, it appears to be partly-hollow. Presumably its interior must contain some absolutely massive caves.

Of the bits of it that actually are solid, current thinking holds that they’re mainly composed of water ice. This at least is fairly normal for Saturn’s moons. At Saturn’s distance from the Sun, things are cold enough for water to remain frozen pretty much indefinitely; it’s only once you get inside the asteroid belt that solar radiation would have been intense enough to evaporate off the early store of water. (That’s one of the reasons why there’s such a sharp compositional change between the rocky inner planets and the big, icy moons of the gas giants; basically we live inside the so-called ‘snow line’.)

However, in addition to its icy surface, Hyperion also appears to be covered in some sort of dark material. Its reflectivity is too low for pure ice; Hyperion reflects about 20-30% of incoming light. By comparison, snow reflects about 70-80%. What exactly this material is isn’t known, although there is a suggestion that its source might be another moon called Phoebe.

Another bit of compositional weirdness is the dark material that fills the bottom of Hyperion’s craters. This dark reddish substance appears to be some sort of hydrocarbon, perhaps similar to the stuff that gives Iapetus its dark hemisphere. But again, this material has never been directly-sampled, so its exact nature is uncertain.

There is one final point of high weirdness. Hyperion’s rotation is unstable. Its rotational axis flops around pretty much at random, almost as if the moon were severely drunk. This appears to be down to the conflicting tides it experiences from Saturn and Titan, and also due to the dramatic eccentricity of its orbit. (The tidal force falls off with the cube of the distance, not the square, and so is very sensitive even to small changes in orbital distance.)

Although possibly the ugliest moon in the solar system, Hyperion is an interesting object, and one which poses some interesting questions for models of planetary formation and structure. It also seems to be one of these things where we know just enough for it to be really tantalising, but not quite enough yet for any really concrete answers.