# Gently, gently

I had a student yesterday (which I’ll talk about more in a later post,) which meant that I wasn’t glued to my computer watching what was going in with Philae. Philae, as you no doubt recall from an earlier post, is the lander portion of the Rosetta spacecraft, itself riding shotgun with comet 67P/Churyumov-Gerasimenko. Despite my cavalier terminology, it is actually worth making some careful distinctions; Rosetta is not in orbit around 67P/C-G, as the comet is far too small to provide enough gravity to allow anything more than dust to orbit.

[Okay, brief primer on orbital mechanics. Everything with mass has gravity, but gravity is actually a very weak force. It takes a lot of mass to provide the attractive force that we tend to think of as gravity. Orbit is essentially matching sideways velocity against the downward pull. Think of falling, but somehow getting shoved sideways while you’re falling. If you go sideways far enough, the mass pulling you downwards shifts out from under you, so the attraction starts coming from a different direction off to one side, and thus your motion shifts into a different direction. Go sideways fast enough, and you’re falling perpetually because the downward pull constantly reorients; one description I heard is, “falling towards the planet but missing.” Around Earth, spacecraft in low-Earth orbit move in the range of 29,000 kph, in relation to any given point on the surface – faster than that, and the orbit extends outward; slower, and gravity can start to draw the craft in. So when any craft reenters the Earth’s atmosphere, it does so not by ‘flying’ downward, but by slowing down enough so gravity reduces the orbital distance.

Gravity is often measured by “escape velocity” – how fast an object must be going (in any direction) to exceed gravity’s ability to draw it closer. For Earth, escape velocity is 11.2 kilometers per second, or about 40,000 kph. On comet 67P/Churyumov-Gerasimenko, with its much smaller mass, escape velocity is estimated as 1 meter per second (not kilometer, note) – that’s the rate of a fast toddler, or if it helps, 3.6 kph.]

So in order for Rosetta (the mother ship, as it were) to ‘orbit’ the comet, it would need to keep directing itself with rockets, using fuel at an alarming rate – thus it doesn’t, but simply cruises alongside, matching the comet’s own orbital velocity around the sun. The comets spins, though, so Rosetta gets the benefits of orbiting anyway. There’s the old philosophical question: if a magnetic ball rolls around a compass in a circle, with the needle always pointing to the ball, the ball obviously circles the compass, but does it circle the needle? The answer: why the hell do you want to know? Which is the answer to most philosophical questions, which spend far too much time pondering supposedly deep shit with no purpose whatsoever. Sometimes the human mind is remarkably inefficient…

Anyway, Rosetta does not orbit in the sense of a gravitationally-determined action, but can see most of the comet anyway. And now the Philae lander has been launched down to the comet, and this is a serious challenge it itself. Described as the size of a washing machine, Philae weighs, on the surface of the comet, a mere gram (I overstated the case a bit in the previous post.) For most of us, this is better described as 1/4 of a teaspoon of sugar – it is safe to say that the European Space Agency did not spend a lot of money on the shock absorbers. But it also means that not only could Philae actually bounce off if it approached too quickly, it could also be shoved off, or toppled over, by the very things that make a comet a comet, the outgassing of material. So Philae has two items to assist in the function of getting down and staying down, which are a gas rocket on top to push it down to the body of the comet, and a pair of harpoons on the underside to lance the comet and draw the lander tight.

The interesting bit is, the lander is in place now, though neither of these actually operated, and according to the news sources available as of this writing, no one is sure why yet. So while it is there for the time being, it might not necessarily stay there if anything at all happens. The point, naturally, wasn’t just the bragging rights of getting there, but to perform ongoing scientific studies over the next several months, so efforts are underway to fire those harpoons and tie the lander down (sport.)

Philae was not equipped with any kind of video capabilities, just still cameras, and power is limited and carefully rationed, much of it intended for the various experiments. Thus, we have no video of its approach and landing, and only certain status indicators of its landing performance. It has been speculated, however, that the lander touched down twice, having been lifted off again briefly from the momentum of an internal flywheel winding down. The harpoons, designed for an almost-entirely unknown surface, are likely capable of penetrating dense ‘soil’ to perhaps even rock, deep enough to anchor firmly, and so may well have enough force to throw Philae off the surface again. Not to worry – that’s why they have cables to pull the lander down. But if you’re keeping up, this might mean that Philae will ‘land’ three separate times on the same comet, without any true capability of lifting off in the first place.

(I now have this irresistible mental image of Philae being blown away from the comet by the sublimation activity as the comet approaches the sun, tethered by its harpoons, flapping around in the comet’s wake like a balloon towed by a bicycle.)

I may be back with further info, but my best recommendation is to follow Universe Today, which is likely to be the most efficient method of getting updates – I say this after having watched a 40-minute podcast from ESA just over the successful separation of Philae from Rosetta, not really generating a lot of useful information. Universe Today also has a post of the ‘song’ that the comet is producing, fluctuations in the magnetic field that, as yet, have no determined cause. I suspect it’s probably someone else’s lander flapping around on the far side…

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