From NEO to GEO --- an SPS scenario

"In particular, the main problem with a NEO is the lack of launch
windows, they can be several years apart.

On the other hand, we can shuttle back and forth between LEO, GEO and
Lunar L-4/5 any time we want to.
"

There may be NEOs located at the Sun-Earth L4 and L5 libration points
(so-called "Earth Trojans"). Currently, there are 5 known Mars Trojans
and 1007 known Jupiter Trojans (c.f.,
http://cfa-www.harvard.edu/iau/lists/Trojans.html ). If Earth Trojans
were to exist, then there would exist continuous launch windows to them
from Earth with constant delta-V.

I've heard that it is very tough to detect Earth Trojans from the surface
of the Earth because of the viewing angles. (If I'm envisioning my
geometry correctly, the L4/L5 points of the Sun-Earth system are not
visible from Earth at night. Recall that L4/L5 are each 60 degrees away
from the Earth in the same orbit. ) That is, the lack of any known Earth
Trojans at this time may not so much be an indication that no such object
exists as it is an indication that such an object, even if it were it to
exist, would be much more difficult to detect from Earth than NEOs in other
orbits.

Even sans Earth Trojans, we need to examine the term "launch window"
carefully, I think, for the case of NEOs whose orbits are very similar to
Earth's. Launch windows are periods of time during which launch should
take place in order to achieve a certain low delta-V value, given an
assumed mission profile. Launches at other times are not impossible, but
their delta-V values will be higher. It is conceivable to design a
spacecraft that would allow a range of possible delta-V values, with more
cargo or less cargo delivered depending on whether the required delta-V
were smaller or larger. All of the delta-V required for HEO->NEO->HEO
transport could be provided with high specific impulse engines. This is
an important difference vis a vis the Moon, for which descent/ascent
to/from the surface must be accomplished by chemical propulsion with low
specific impulse (unless you've got your complex mass driver infrastructure
or tether infrastructure already built).

Thus, the required delta-V for a NEO mission could be much larger than for
a Moon mission but yet have more cargo capacity because of the high
efficiency engines that are usable for NEO missions but not for Moon
missions.

Ron Menich