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Harvesting fuel from the Saturn system.
Guest_RGClark_*
post Mar 21 2006, 07:06 AM
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This article in Science gives the estimated amount of molecular oxygen above the Saturn A ring:

Oxygen Ions Observed Near Saturn's A Ring.
J. H. Waite, Jr., T. E. Cravens, W.-H. Ip, W. T. Kasprzak, J. G. Luhmann, R. L. McNutt, H. B. Niemann, R. V. Yelle, I. Mueller-Wodarg, S. A. Ledvina, S. Scherer
Science, 25 February 2005: Vol. 307. no. 5713, pp. 1260 - 1262
http://www.sciencemag.org/cgi/content/abstract/307/5713/1260

It estimates the number of neutral O2 molecules as 10^4 to 10^5 cm^-3. However, they note it could be much higher than this range because of the limitations of the measurements. I'll take the upper number, 10^5 cm^-3. There are 10^15 cubic centimenters in a cubic kilometer so this amounts to 10^20 molecules per km^3.
The article gives the orbital velocity around Saturn at the radial distance of the A ring as in the range of 15 km/s. Actually the article explains there are magnetic effects that accelerate the various ionized molecules even faster which when exchanging momentum with the neutral molecules accelerate these faster as well. I'll use the 15 km/s number for simplicity. Then if we orbit the spacecraft in the opposite direction we would have a relative velocity with respect to these molecules in the range of 30 km/s. So if we had a scoop with a 1km x 1km opening we could collect 30 x 10^20 molecules of O2 per second. To calculate the mass of this oxygen, Avogadro's number of O2 molecules, 6.0 x 10^23, amounts to 32 grams. So 30 x 10^20 molecules is 5 x 10^-3 moles or (5 x 10^-3) x 32 grams = 160 x 10^-3 grams. This is the mass collected every second. There are 31,536,000 seconds in a year, so after a year we would have 31,536,000 x 160 x 10^-3 grams = 5,045,760 grams, or 5045.76 kilos of O2.

This article gives the density of water molecules around Enceladus:

Enceladus Eruptions Enceladus Eruptions
Larry W. Esposito Larry W. Esposito
Principal Investigator Principal Investigator
UV imaging Spectrograph UV imaging Spectrograph
http://saturn.jpl.nasa.gov/multimedia/prod...RM_Esposito.pdf

On page 21 is given the density of water molecules versus altitude. The greatest density shown is about 10^7 molecules per cubic centimeter at 200 km altitude. The methane is 1.6 percent of this amount, so 1.6 x 10^5 molecules/cc. However, you couldn't move at high velocity around Enceladus such as 15 km/sec because the orbital velocity around it is so low.
One possibility would be to put it at one of the two Lagrange points that is close to the satellite. The distance from the surface would be higher than 200km so the density of the methane would be lower. You would need pumps then to draw in the methane. You would also have to separate out the more prevalent water from the methane.

Getting back to the molecular oxygen found around the rings, the researchers also found monoatomic hydrogen and monoatomic oxygen. They give the amounts of the ionized versions, but not the larger neutral amounts.
Both of these are known to be very efficient for propulsion: pure monoatomic hydrogen has about 3 to 4 times the ISP (specific impulse) as H2 with O2 as an oxidizer. A problem though that still has not been solved is how to store the monoatomic propellants stably within a rocket.
If all you wanted to do was to accelerate the rocket from Saturn then you could just use the monoatomic hydrogen as it normally combusts by bringing the single atoms together. That is, you would not need to store it. You couldn't use it though as an onboard propellant to engage in trajectory changes or landings and launches form satellites.


Bob Clark
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gallen_53
post Mar 27 2006, 07:22 PM
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In the context of harvesting material for spacecraft utilization or an interplanetary economy, I would argue that the Saturn system is the treasure chest of the Solar System. To me, those big rings cry out: "deuterium fuel!".

Everybody should have at least one nut theory. I remain unconvinced that the equatorial ridge on Iapetus is natural. Saturn would be the logical place for an interstellar vehicle to refuel itself. The mass fractions required to acheive near relativistic velocities would require an enormous vehicle and a huge amount of fuel. If 10 million years ago, a vehicle refueld itself in the Saturn system then there should still be relics left behind by that process.

Concerning the scooping up of fuel by a fly-by spacecraft: Remember that momentum and energy must be conserved. If you scoop up fuel while flying through at 30 km/sec then you needed to accelerate that fuel to your frame. The energy lost in scooping up the fuel was probably far greater than any chemical energy contained in the fuel. A better approach would be to slow down to Saturn's frame and then transfer tons of fuel to storage tanks. Again the best fuel would be deuterium to be burned in a fusion reactor. One might use vaporized iron as a working fluid, e.g. vaporize iron and then accelerate it through a plasma thruster to relativistic speeds using a deuterium fueled fusion reactor as the enrgy source. The iron would serve as vehicle structure until it was dissassembled, vaporized and blown out through the plasma thruster.
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qraal
post Jun 6 2006, 01:03 AM
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Hi Gallen

Instead of iron use lithium 6 as your structural metal and then fuse it in the reactor.

A better fuel extractable from Saturn is Helium 3 which can be fused all by itself without the neutron flux from fusing it with deuterium (He3+D itself is aneutronic, but D+D side reactions are quite plentiful.)

Still I reckon z-pinch fusion pulse-bombs are better than a fusion reactor for starflight - mass penalty is a lot lower and no need for breeding tritium in lithium heat exchangers to then have to extract to keep fuelled up.

Simpler yet... leave the power-plant at home and push the ship with mini-laser sails.

As for mining Saturn it has a lot of advantages. The boost to low orbit is only 15 km/s, unlike Jupiter's 29.5 km/s - Jupiter's rotation gives a boost of 12.5 km/s, and Saturn's gives 10 km/s, but the gravity well is a lot deeper. Also Saturn's atmosphere is nowhere nearly as turbulent as Jupiter's.

Uranus and Neptune can also be mined and several nuclear ramjet designs are floating around the web for long-duration probes and mining missions. Aerostats and simple balloons are hopeless for mining facilities as they're sitting ducks for any kind of weather, so nuclear ramjets are the best bet. I dislike both the ice giants for mining because they're too damned far away and have no drawcards like Titan and Enceladus - Triton is a chill second to Titan IMHO.

Of course throwaway ramjet probes are possible - if not mandatory - for all four.

Adam

Oh and I forgot to mention my own nut theory - that the 'ridge' on Iapetus is really the 'earthworks' for a maglev launch ramp. Some of the big 'craters' look suspiciously like ice-quarries.

Of course a convincing natural explanation is yet to be presented wink.gif
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ljk4-1
post Jun 6 2006, 02:45 AM
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In this article on using the antiprotons trapped by planetary magnetic
fields to make antimatter:

http://www.centauri-dreams.org/?p=674

There is a mention in the NIAC report linked to the above (scroll down
to the near end of the article) that Saturn would be an ideal planet to
refuel an antimatter spacecraft for the following reasons:

Rings are the largest source of locally generated antiprotons in the
solar system. Nearly a half milligram of antiprotons are trapped.

Primarily formed by the decay of ring produced antineutrons in the
magnetosphere. Antineutrons generated in the A&B rings (20100
gm/cm2, Nicholson and Dones, 1991) do not have to be
backscattered for trapping which drastically increases the efficiency.

Rings are also a significant source of antiprotons which are directly
produced but which are reabsorbed by the rings after one or more
bounce periods.

Maybe we don't need a starship to find evidence of ETI, assuming
we've had visitors in the past who needed to refuel.


--------------------
"After having some business dealings with men, I am occasionally chagrined,
and feel as if I had done some wrong, and it is hard to forget the ugly circumstance.
I see that such intercourse long continued would make one thoroughly prosaic, hard,
and coarse. But the longest intercourse with Nature, though in her rudest moods, does
not thus harden and make coarse. A hard, sensible man whom we liken to a rock is
indeed much harder than a rock. From hard, coarse, insensible men with whom I have
no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

- Henry David Thoreau, November 15, 1853

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