Post by James Nicoll Post by Johnny1A
There's also a physics issue Niven skims over in the backstory.
Supposedly, our ancestors arrived on an asteroidal slowboat. We're told
that the Pak who decided to settle the other galaxy took a roughly
cylindrical nickel iron rock, about 2 miles long by a mile across, cut
chambers inside it to carry Pak breeders in suspended animation, and a
life system for themselves, and accelerated slowly up to about .16c and
travelled out to Sol.
OK...calculate how much energy it would take to bring about 1.5 cubic
_miles_ of nickel iron to .16c. Then recall that the Pak were using
(IIRC) plutonium fission power to power the drive...my BOTEC says that
there wouldn't be anywhere close to enough energy available even if the
rock was solid plutonium.
I don't know if Niven didn't do the math on that one, or just decided to
skip it and hope it wasn't noticed.
We could do the math, though. Fission is what, up to about .1% conversion
of mass to energy? So each kg of Pu is good for 9x10E13 J? Assume because
Pak they have some way of turning that into exhaust without losses, and I
get an exhaust velocity of about 4% C. Plug that into the rocket equation
and we get a mass ratio of 55: for every kilogram of dry mass the Pak
came up with 54 kg of Pu. Call the rock 30 billion tonnes: the Pak found
a source for 1.6 trillion tonnes of Pu. Well, except they probably want
to slow down too so square the mass ratio: now it's closer to 90 trillion
tons of Pu.
Let's say the Pak homeworld has 10 ppm of fissionables in its crust (although
it wouldn't be in the form of Pu, surely?). That means they processed
~10E19 tonnes of crust material. Earth's crust masses about 6x10E19 tonnes?
So roughly one sixth of an Earth crust's worth of fissionables.
Except the text is plain that this is a cesium-based ion drive, so the
exhaust velocity is probably much, much lower than 4% C, the mass ratios
are much much much much much higher and somewhere adjacent to the Pak we
have to assume there was a very large world composed mostly of cesium.
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It would also be interesting to find out what ever happened to that asteroid-ship. I can't imagine the Protectors would leave it in Earth orbit, if the orbit ever decayed, catastrophe follows. At the same time, it would be a lot of effort to send it out-system or dump it into the Sun. So I would suspect they put it in solar orbit somewhere...which means it ought to still be findable later. I wonder if the UN/Belters looked for it...