On Tue, Feb 23, 2016 at 7:36 PM, Tim <xxxxxx@little-possums.net> wrote:

For refuelling purposes even a few dozen metres is more
than enough for many starships. Almost all of them will have large
fractions of water ice, because it is one of the most common compounds
in the universe. Most of what isn't water ice will be methane ice,
which is also a source of hydrogen (the most common element in the
universe).

As to the earlier question of purity:

I seem to recall (from a series of threads some time back) that simply creating liquid hydrogen from water/methane would involve elimination of impurities. That is, if you manage to get any LH at all out of the process, it's going to be chemically pure.

Of course, this leads: "Well, if LH is necessarily pure fuel, then what is unrefined fuel?"

The answer (at least IMTU) is, "Water."

Richard Aiken

"Never insult anyone by accident." Robert A. Heinlein

"I studied the Koran a great deal. I came away from that study with the conviction there have been few religions in the world as deadly to men as Muhammed." Alexis de Tocqueville (1843)

"We know a little about a lot of things; just enough to make us dangerous." Dean Winchester

"It has been my experience that a gun doesn't care who pulls its trigger." Newton Knight (as portrayed by Matthew McConaughey), to a scoffing Confederate tax collector facing the weapons held by Knight's young children and wife.