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Monday, June 18, 2007

"To boldly go..." - not any time soon

I came across an interesting essay on the technological (and other) limitations to human space travel and colonization of other worlds (via Slashdot).
Here's a handy metaphor: let's approximate one astronomical unit — the distance between the Earth and the sun, roughly 150 million kilometres, or 600 times the distance from the Earth to the Moon — to one centimetre. Got that? 1AU = 1cm...

Next on our tour is Proxima Centauri, our nearest star...[But that's not habitable, so to get one of those we look to] Gliese 581c, the first such to be detected (and it looks like a pretty weird one, at that). [This closest habitable planet] is roughly 20.4 light years away, or using our metaphor, about ten miles.
So to just reach the nearest habitable planet, we'd need to travel over 600,000 times further than we've ever sent a human being. To give a human being somewhere to stay whilst traveling over 20 light-years, he envisions a space capsule with all sorts of automated systems including near-magical technologies like an artificial uterus (since this is a solo mission and we'll need more colonists).
It's going to be pretty boring in there, but I think we can conceive of our minimal manned interstellar mission as being about the size and mass of a Mercury capsule. And I'm going to nail a target to the barn door and call it 2000kg in total.
So how much energy do we need to hit the nearest planet - 4.22 light years - despite its inhospitable atmosphere?

We're sending them on a one-way trip, so a 42 year flight time isn't unreasonable. ..This means they need to achieve a mean cruise speed of 10% of the speed of light...So we need to accelerate our astronaut to 30,000,000 metres per second, and decelerate them at the other end. Cheating and using Newton's laws of motion, the kinetic energy acquired by acceleration is 9 x 1017 Joules, so we can call it 2 x 1018 Joules in round numbers for the entire trip. NB: This assumes that the propulsion system in use is 100% efficient at converting energy into momentum, that there are no losses from friction with the interstellar medium, and that the propulsion source is external ...So this is a lower bound on the energy cost...

To put this figure in perspective, the total conversion of one kilogram of mass into energy yields 9 x 1016 Joules. (Which one of my sources informs me, is about equivalent to 21.6 megatons in thermonuclear explosive yield). So we require the equivalent energy output to 400 megatons of nuclear armageddon in order to move a capsule of about the gross weight of a fully loaded Volvo V70 automobile to Proxima Centauri in less than a human lifetime...

For a less explosive reference point, our entire planetary economy runs on roughly 4 terawatts of electricity (4 x 1012 watts). So it would take our total planetary electricity production for a period of half a million seconds — roughly 5 days — to supply the necessary va-va-voom.

He goes on to discuss the energy costs of an entire colony of humans, which are vastly higher. And then he points out that this idea of a colony is kind of silly anyway:
As Bruce Sterling has puts it: "I'll believe in people settling Mars at about the same time I see people settling the Gobi Desert. The Gobi Desert is about a thousand times as hospitable as Mars and five hundred times cheaper and easier to reach"...In other words, going there to explore is fine and dandy — our robots are all over it already. But as a desirable residential neighbourhood it has some shortcomings, starting with the slight lack of breathable air and the sub-Antarctic nighttime temperatures and the Mach 0.5 dust storms, and working down from there.
So we can keep dreaming (and watching Star Trek), but human space colonies are a long way off.

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