Space Forum / Astronomy / April 2006

Nasa scientists are investigating possibilities for follow-up missions
to the Cassini mission to the Saturnian moons Titan and Enceladus:

Encore For Enceladus! Saturn Moon Ripe For Astrobiology Exploration
http://www.space.com/scienceastronomy/060403_mystery_monday.html

Ideal would be sample return missions. A problem would be carrying
enough fuel for landing and assent. It may be we can get the required
fuel for the return from the Saturn system itself.
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.
Larry W. Esposito Larry W. Esposito
Principal Investigator Principal Investigator
UV imaging Spectrograph UV imaging Spectrograph
http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20050830_CHARM_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