Saturday, July 31, 2010


The image above is a composite compiled by the Planetary Society's Emily Lakdawalla, handily showing the relative sizes of all the asteroids and comets that have been visited by spacecraft to date. Check out the full-size version here. The largest by far is Lutetia, which the Dawn* spacecraft has just zipped past, on its way to even larger targets - Vesta in 2011-12 and Ceres, the largest body in the asteroid belt, in 2015. Vesta is about four times the size of Lutetia; Ceres, at an average 950 kilometres in diameter, is seven times larger. If it orbited Saturn it would be the gas giant's sixth largest moon (it's just a little bit smaller than Tethys), and is classified as a dwarf planet rather than an asteroid. Both Vesta and Ceres are massive enough to have been pulled into a spherical shape by their own gravity, and both are differentiated bodies with metallic cores and stony crusts. Vesta's surface is mostly rock, modified by flows of lava; Ceres is surfaced with water ice and clays and carbonates, and it's possible that there's an ocean of liquid water beneath its crust.

Asteroids are fantastically varied objects. Those imaged close-up all seem mostly similar, spattered with craters large and small, and with dusty intercrater plains. But while Eros, Ida and Itokawra are stony asteroids composed of iron and magnesium silicates, Mathilde is coated in primitive carbonaceous compounds, and tiny Braille is composed of olivine and pyroxene (Gaspara's composition appears to be midway between Braille and the stony asteroids). Lutetia is presently classified as a M-type asteroid; its dusty surface may cover a chunk of nickel-iron and other metals derived from the core of a larger body shattered during the early history of the Solar System. Itokawa, visited by the Hayabusa probe, which recently returned to Earth after a difficult five-year journey, is a rock pile, with a surface of dust and boulders mostly unmarked by impact craters.

Hard to think of making any kind of comfortable living on or in something as frankly ugly as that, but it's easy to think of tenting over a small crater of one of the other asteroids, or burrowing into their surfaces, or hanging sealed tenement buildings at the edges of cliffs. Smaller asteroids could be completely built over - mini-Trantors. (Many small asteroids are rock piles like Itokawa, putting paid to the cherished sf idea of manufacturing a habitat by hollowing out a small rock with a few nuclear bombs.) Although the total mass in the asteroid belt is just 4% of the mass of the Moon, the surface area is far greater. At an average diameter of around 130 kilometres, Lutetia has a surface area of some 31000 square kilometres, one and a half times the size of Wales. Plenty of room for settlements and cities. Tiny disputatious kingdoms and empires brought together and flung apart by orbital mechanics, trading with each other and with Earth and Mars, chasing after comets for water and organics . . . It's been done before, of course. Many times. But I can't help thinking that it may be time to do it again.

*EDIT My bad - it was the Rosetta spacecraft that zipped past Lutetia, on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. Dawn is, however, on course for Vesta and Ceres. Hat-tip to Phil, for pointing out my confusion.


Anonymous James Davis Nicoll said...

Tiny disputatious kingdoms and empires brought together and flung apart by orbital mechanics, trading with each other, chasing after comets for water and organics, with Earth and Mars . . . It's been done before, of course. Many times. But I can't help thinking that it may be time to do it again.

Balkanized asteroid belts are somewhat thin on the ground for some reason. Gilliand's belt wasn't unified, Anderson had one in A Bicycle Built for Brew, it's implied in Pournelle's "Tinker", and there were at least two polities in the belt in Schizmatrix but otherwise I am drawing a bit of a blank. There's a lot of untapped potential in a belt made up of thousands of pocket nations.

July 31, 2010 3:27 PM  
Anonymous Anonymous said...

Actually Paul - it was ESAs Rosetta that just bullseyed Lutetia on its way to a comet in 2014. Dawn is due to go into Orbit around Vesta in 11 months, then onward to Ceres, and despite denials, I'm hoping it might have enought fuel after that for one very out of plane flyby of Pallas (probably not though).

BTW new visitor - just read 'The Quiet War' and LOVED IT, It got to me in the same way as KSRs 'Mars' trilogy did. So looking forward to 'Children' next...


August 02, 2010 8:23 AM  
Anonymous Anonymous said...

Ooops, 'Gardens...' not Children. First post and I'm already making a fool of myself!


August 02, 2010 8:36 AM  
Blogger Paul McAuley said...

Hi Phil, Thanks for pointing out my confusion. It's very cool that there are two spacecraft cruising the asteroid belt right now, but silly of me to get them mixed up.

August 02, 2010 9:44 AM  

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