Why this week’s comet landing is way better than celebrity butt-fests

This week’s landing on Comet 67P/Churyumov-Gerasimenko was a landmark in space history – not because the comet apparently bore a passing resemblance to the Kardashian backside that was competing for place in the news, but because surface gravity on 67P is about one millionth Earth’s. You don’t land so much as drift in and try like hell to stay there.

Potential landing sites on the double-lobed Comet 67P/Churyumov-Gerasimenko. Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Potential landing sites on the double-lobed Comet 67P/Churyumov-Gerasimenko. Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Add to that the fact that the cometary surface is like a rugged boulder-field and you have a recipe for Ultimate Challenge. That’s what made the landing so risky – and why ESA’s Philae lander was equipped with harpoons, ice-screws, and a down-firing thruster. When they failed, Philae landed on the comet, then bounced a kilometre back into space before the comet’s lazy gravity pulled it back. It was also a funny sort of bounce because the comet isn’t a sphere – it’s more like a dumb-bell. When Philae came down a second time, it bounced again before eventually settling.

For me the three-bounce landing (at 15:34, 17:25 and 17:32 GMT on 12 November) has a wow factor well beyond landing on a comet for the first time e-v-a-h. It’s also about gravity – and that means it’s about Einstein, one of my favourite physicists. Let me explain. Gravity doesn’t just cause celebrity butt-sag, after a while. It’s also why the comet’s where it is today. Fact is that 67P/Churyumov-Gerasimenko experienced a gravitationally-driven orbit change in 1959, when an encounter with Jupiter dropped its perehelion (closest approach to the Sun) from 2.7 to 1.3 astronomical units, giving the comet its current 6.45 year period. That’s why it’s where it is now.

Gravity is also how ESA got the probe to the comet. It was boosted, during a decade-long journey, by gravity assist manoeuvres, swing-bys of Earth and Mars that exploited space-time curvatures around the planets to accelerate the probe (three times) and decelerate it (once), without burning a single gram of fuel.

Ain’t physics neat. So just what is gravity? This looks like a stupid question. Actually, it isn’t.

Rosetta's long odyssey to the comet - with slingshot gravity boosts from Earth and a de-boost from Mars. NASA, public domain.
Rosetta’s long odyssey to the comet – with slingshot gravity boosts from Earth and a de-boost from Mars. NASA, public domain.

The thing is, we think of gravity as a ‘force’. But actually, according to Einstein, it isn’t. We just perceive it as such. Here’s why. Science started looking at gravity in earnest when all-round super-geek Sir Isaac Newton worked out the math for the way gravity presented in everyday terms, which he published as part of his Philosophiæ Naturalis Principia Mathematica in 1687. His gravitational theory worked (and still works) well at everyday level – you could calculate how apples might fall, figure out planetary movements and so on (the key equation is    F = G \frac{m_1 m_2}{r^2}\ , which defines the force between two point-sources of defined mass.) Newton’s triumph came in 1838 when astronomers realised that Uranus wasn’t quite where it should have been, based on the tugs of the known planets. French mathematician Urbain Leverrier and British mathematician John Couch Adams, independently, reverse-engineered the data to pinpoint where an unknown planet should be – and sure enough, there it was. Neptune.

Albert Einstein lecturing in 1921 - after he'd published both the Special and General Theories of Relativity. Public domain, via Wikimedia Commons.
Albert Einstein lecturing in 1921 – after he’d published both the Special and General Theories of Relativity. Public domain, via Wikimedia Commons.

But as science began fielding more data, it became evident that Newton’s equations didn’t account for everything – which is where Albert Einstein comes in. His General Theory of Relativity, published in 1917, is actually a theory of gravity. General Relativity supersedes Newton’s theory and portrays gravity by a totally different paradigm. To Newton, gravity was a force associated with mass. To Einstein, gravity was not a force directly innate to mass, but a product of the distortion of space-time caused by mass/energy, which bent the otherwise straight paths of particles (‘wavicles’), including light.

The proof came in May 1919 when British astronomer Sir Arthur Eddington measured the position of Mercury during a solar eclipse. Mercury’s perehelion – the closest point to the Sun – precessed (moved) in ways Newton couldn’t account for. Einstein could – and the planet turned up at precisely the place general relativity predicted. Voila – general relativity empirically proven for the first time. I don’t expect that Einstein leaped around going ‘woohoo’, but I probably would have. And general relativity has been proven many, many times since, in many different ways – not least through the GPS system, which has to account for it in order to work, because space-time distortion also causes time dilation. (If you want to live longer, relative to people at sea level, live atop a mountain).

Einstein’s key field equation, as it eventually evolved, is G_{\mu\nu}\equiv R_{\mu\nu} - {\textstyle 1 \over 2}R\,g_{\mu\nu} = {8 \pi G \over c^4} T_{\mu\nu}\, – which I am not going to explain other than to point out that it could be used to calculate the space-time distortion caused by the mass of, say, a Kardashian butt. This would be a hideous waste of brain-power, but at least means I’ve managed to put both Einstein’s field equation and a reference to society’s shallow obsession de jour in the same sentence. As an aside, I also think Einstein got things right in more ways than we know. I don’t say this idly.

Philae lander departing the Rosetta probe for its historic rendezvous with the comet. Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Philae departing the Rosetta probe for its historic rendezvous with the comet. Taken by the orbiter’s OSIRIS camera. Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

One of the key things about both Newton and Einstein is that their theories treated clumps of particles – a mass such as the Earth for instance – as if the gravity originated in a mathematical point at the centre of the mass, even though the gravity (‘space-time distortion’) is produced by every particle within that mass. And that works perfectly at distance. But in detail an uneven distribution of mass –  a mountain range, for instance, or even a celebrity butt – can introduce local pertubations. Small – but calculable. It’s because of ‘mass concentrations’ that satellites we put around the Moon eventually crash, for instance.

Which brings me back to the science adventure on 67P/Churyumov-Gerasimenko, 28 light-minutes away outside the orbit of Mars. With a long-axis diameter of around 5 km and a composition of loose rocks held together by ices, 67P/Churyumov-Gerasimenko doesn’t have enough mass to bend space-time much. It has, in short, almost no gravity. Orbiting it, as Rosetta has been doing since 6 August, is more like a lazy drift around it. To land is more akin to docking than anything else. There’s not a lot to hold Philae ‘down’, and it doesn’t take much to bounce off. To that we have to add the dumb-bell shape of the comet’s nucleus, which produces complex (if gentle) space-time curvatures, meaning a ‘bounce’ on the comet isn’t going to be a simple parabola like a ‘bounce’ on Earth.

All of which underscores the tremendous technical achievement of the landing – bounces and all. The final lesson? Don’t bother with celebrity butt. Einstein and comets are FAR more interesting.

Copyright © Matthew Wright 2014

9 thoughts on “Why this week’s comet landing is way better than celebrity butt-fests

    1. Thanks. I felt I HAD to say something – it’s been virtually impossible to avoid the ‘Kardashian’ pictures, more’s the pity – rammed at us here in NZ even via the metropolitan daily, which to me is a fairly sad indictment of just how empty society seems to have become. The comet’s WAY more interesting and useful to know about. I suspect I am not alone in such thoughts!

  1. So let me get this straight. Are you comparing Kim Kardashian with something like a dumb bell? An apt description.😉 I think Kim’s butt has more gravity than 67P if the number of men who are drawn into her life is any measure.

    I’m so elated by the amazing technical achievement in landing on the comet. I read that it bounced a couple times and was happy to learn it wasn’t damaged. This truly is a major deal. Much thanks to Einstein and Newton for showing us how to figure this stuff out.

    1. Did I get ‘Kardashian’ and ‘dumb-bell’ in the same sentence? I did, didn’t I. Hee hee… The Philae landing was just stunning. Turns out it almost failed. The escape velocity of the comet is 1.6 km/h and ESA revealed today the first bounce was at 1.3 or close to. Woah! But it looks like they got their 60 hours of science as planned, plus maybe a bonus if the solar battery gets enough charge. Good stuff!

  2. MJ,

    Excellent post; really enjoyed your narrative.

    I’ve been following Rosetta and Philae on my blog starting here:


    and maybe its me, but man for a busy part-time blogger it sure has been tough getting “just the facts” on this mission! ESA has tons of just absolute nonsense; NASA JPL better; but I found more concise info on Wikipedia!

    Wish I had been following you… you get to the point and make it interesting!

    1. Thank you! And thanks for the follow. One of my very favourite fields is space science. The Philae data has yet to really emerge – but it’s going to be fantastic when it does. Already it looks like the comet is nothing less than a giant block of ice with a slight dusting of dirt. Wonderful info. And, of course, the New Horizons probe is about to wake up ready for the final run through the Pluto system. We live in astonishing times, science-wise.

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