In article ,
Derek Lyons wrote:
A contributing factor is that for most landing sites,
a particular lunar orbit will be reachable only twice a month due to the
Moon's rotation, which is a problem if you want the option of doing an
emergency return on short notice.

How does this change if your only return criteria is to hit a reentry
corridor with particularly caring where you come down?

Not at all. The problem is that, to a first approximation, a given site
on the Moon has a launch window to a particular lunar orbit only as the
Moon's rotation carries the site through the orbit's plane. But the Moon
rotates only once a month! Except in a couple of special cases(*), you
only get two launch windows a month (assuming orbit inclination equals or
exceeds latitude, because otherwise you don't get *any*), and typically
they aren't even equally spaced.

(* An equatorial site is always in the plane of an equatorial orbit, of
course, and likewise a polar site is always in the plane of a polar
orbit.)

There is a further complication that any particular lunar orbit, unless
it's near-equatorial, has only two *Earth* launch windows a month --
roughly speaking, only when Earth passes through the orbit's plane, and
the Earth goes around the Moon :-) only once a month.

(The plane of a lunar orbit is essentially fixed in space. The Moon does
not have enough of an equatorial bulge to precess orbits at a significant
rate the way Earth does.)

Hitting a particular spot on Earth is very much a detail by comparison.
Unless you are in a big hurry and want the fastest possible trajectory,
that can usually be arranged from almost any departure, given some
aerodynamic maneuverability during reentry.

Is the difficulty orbital mechanics, or fuel?

Yes. :-) Almost any orbital-mechanics problem becomes trivial given
arbitrary amounts of fuel.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |