Outer Planet Mining

Self-fuelling an Enzmann starship

Jupiter was touted as the major source of He3 fusion fuel for Daedalus, some 30,000 tons of it, but Jupiter’s gravity well is HUGE and far beyond the abilities of solid-core fission rockets, stretching the capabilities of gas-core rockets in terms of thrust-to-weight ratios. So what about the other gas-giants?

Planet mass (Earths) radius (km) P-mag (sec) P-hydro (sec) Eq. velocity (km/s) Orbital vee (km/s) delta vee (km/s)
Jupiter 317.838 71492 35727.3 35618 12.573 42.098 29.525
Saturn 95.161 60268 38362.4 38196 9.871 25.088 15.217
Uranus 14.536 25559 62064 61704 2.588 15.057 12.469
Neptune 17.148 24764 57996 60120 2.683 16.614 13.931

As you can see Uranus has the most forgiving gravity field, but Neptune and Saturn aren’t out of reach either, and Saturn has proximity to the Sun and Jupiter, for gravity assists, in its favour as well as Titan, a moon with a dense atmosphere and decent gravity. The Daedalus study assumed floating factories at the 0.1 bar level in Jupiter’s atmosphere serviced by gas-core automated shuttles, but if there’s enough need for them volatiles from the big planet atmosphere can be scooped and shipped up for processing at an off-world base.

Scoop-ships could also allow starships to be self-fuelling. I have just received an issue of the October 1973 “Analog” – the one with a gorgeous Rick Sternbach cover of two Enzmann starships and the Cover article by G. Harry Stine, “A Program for Star Flight”. It’s quite a memorable article as Stine was arguing for a star flight program to begin c.1990, and the development of a massive in-space industrial base to support the effort. His initial phase would study the nearby stars with Lunar interferometers, then launch million-ton space-probes at 0.9c, and finally launch ten-ship fleets of Enzmann starships (roughly 12,000,000 tons each, mostly deuterium fuel.) Quite a major effort, but he optimistically costs it at $100 billion (in 1973$.)

A few problems arise – 0.9c from Orion-style pulse drives is a touch unlikely, even with mass-ratios over 1,000 – but over all the concept is sound. Magnetic sails might change the approach, but the basic idea of attaching starships to huge masses of propellant, rather than big tanks, is a good one. However I have read that hydrogen and deuterium ice are mechanically like Jello and thus utterly useless as envisaged. Lithium-6 is a fusion fuel and pretty strong at cryogenic temperatures, so it might be the fuel of choice. Either that or carbon nanotubes might allow very, very light weight tanks to keep deuterium Jello in. The 12,000,000 ton starships probably mass just 120,000 tons empty (the design needs BIG mass ratios for speed), but the size Stine quotes is all wrong. Deuterium’s density is 0.16 relative to water, yet the fuel sphere is described as 1000′ across meaning a density of ~ 0.8, some five times denser than deuterium Jello. A sphere 1,710′ across will do nicely.

Refining out 12,000,000 tons of deuterium from a gas giant will be quite a task. Since deuterium is about 1/2000th of the abundance of protium in Jupiter some 24 BILLION tons of hydrogen will be sifted through to collect the fuel. Bit of a tall order, but inevitable when you’re trying to fit 2,000 people on a starship that’s over 2,000′ long and push it through a delta-vee of 0.3 c.

Actually I don’t know the empty mass of the Enzmann starship is 120,000 tons which is a touch frustrating since I’m paying attention to details here. What Stine does say is that the probes will hit 0.9 c with a mass ratio of ~ 1,000, and the starships will hit ~ 0.3 c (thus a delta-vee of ~0.6 c.) A bit of non-relativistic maths, and assuming the same exhaust velocity, means the starships have a mass-ratio of 100 (=1000^(0.6/0.9).) Now fusion reactions don’t make enough particles with sufficient energy to get that sort of exhaust velocity (about 0.13 c), nor are Orion pulse-drives 100% efficient (~25%?) More realistically an Enzmann starship will hit 0.08-0.15 c which is respectable for a fusion-drive.

Author: Adam

Nothing much to say. What about you?

9 thoughts on “Outer Planet Mining”

  1. Mining deuterium from Jupiter may be inferior to mining it from Venus or Mars. The D/H ratio on both those planets is much higher than elsewhere in the solar system that we know of. The D/H ratio on Venus is 120 (!) times higher than on Earth, apparently because of escape of most of the planet’s hydrogen to space over the ages (deuterium, being more massive, has a lower loss rate than ordinary hydrogen.) The D/H ratio on Mars is five times that of Earth, but the ice caps may be a more practical location for mining than the sulfuric acid clouds of Venus.

    Having said that, once could imagine using the water clouds of a gas giant to jump start the enrichment, exploiting the fact that the equilibrium of H2 + HDO HD + H2O strongly favors the deuterium to be in the water phase. If this equilibrium is not naturally reached then a simple catalyst column could be used to enrich the water before further processing. Ammonia is another possibility. The oxygen and/or nitrogen could be recycled, so the total water requirement need not be all that large, although the total flow would be.

  2. Hi Paul

    Thanks for the comment. I suspect in both cases the D/H enhancement is due to isotopic fractionation due to protium loss to space, but we really won’t know until both planets have their volcanic exhalations tested and compared. Mars’s polar caps will be a far better prospect than the rather slim pickings from clouds on Venus, even with the enhancement.

    If it’s just deuterium we want then the cost of dragging it out of Mars or Venus is more prohibitive than shifting through seawater here on Earth. The whole point of my discussion was about mining He3 for “Daedalus” and mining gas-giants in alien star systems for starship refuelling. Saturn-to-Neptune mass gas-giants are to be preferred over Jupiter-plus type planets. Any planet heavier than Jupiter will, unless still hot, be the same radius roughly and so the gas-mining will be that much more difficult.

    This assumes we’re riding fusion rockets to the stars, which is definitely still sub judice.

  3. Hi John

    Good argument to my ears. But mining for He3 might be the preferred option for fusion power for other uses in the millennia ahead, so someone will be doing it eventually.

    Beamed propulsion, powered by solar, has so much appeal – but we don’t yet have ultra-scale solar power that is built by self-replicating machines. Thus large-scale interstellar is still a long ways off.

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