Dholmes wrote:

What did I miss in my quick list?

Development of a communications relay satellite in a halo orbit around the
L2 Lagrange point in the Earth-Moon-system, providing a communications link
to the far side of the Moon.

From http://www.tsgc.utexas.edu/archive/design/farside.html

"During the construction phase, a satellite in an L2 halo orbit will relay
data from the lunar surface to a geostationary satellite in Earth orbit to
the Earth's surface. When the base becomes fully operational, however, a
radio-free sky is desired to take accurate astronomical readings. Therefore,
a fiber optic cable will be used as a communication link from the base to a
transmitter/receiver station on the near side of the Moon. It will be laid
out by a robotic rover from the base to the limb of the Moon. From there,
the signal can be broadcasted directly to Earth without interfering with
astronomical observations."

From http://www.tsgc.utexas.edu/archive/design/farside2.html

"To provide constant communication with the lunar nearside base as well as
Earth, a communication system was devised where the relaying point is a
satellite in a halo orbit about the L2 unstable libration point. This would
permit full coverage of the lunar farside and the constant monitoring of the
construction activities being conducted. Since L2 is an unstable libration
point, stationkeeping burns totaling 93.3 fps/yr and period control burns
totaling 240 fps/yr will have to be conducted in order to maintain the
orbit. After three years, the satellite at L2 will be refueled and moved to
the stable libration point L5, where it will remain throughout the lifetime
of the observatory."

From http://adsabs.harvard.edu/cgi-bin/np...TIN...9512644B

"Project ECHO: Electronic Communications from Halo Orbit - Abstract: The
design of a communications relay to provide constant access between the
Earth and the far side of the Moon is presented. Placement of the relay in a
halo orbit about the L2 Earth-Moon Lagrange point allows the satellite to
maintain constant simultaneous communication between Earth and scientific
payloads on the far side of the Moon. The requirements of NASA's
Discovery-class missions adopted and modified for this design a total
project cost should not exceed $150 million excluding launch costs, launch
must be provided by Delta-class vehicle, and the satellite should maintain
an operational lifetime of 10 to 15 years. The spacecraft will follow a
transfer trajectory to the L2 point, after launch by a Delta II 7925 vehicle
in 1999. Low-level thrust is used for injection into a stationkeeping-free
halo orbit once the spacecraft reaches the L2 point. The shape of this halo
orbit is highly elliptical with the maximum excursion from the L2 point
being 35000 km. A spun section and despun section connected through a
bearing and power transfer assembly (BAPTA) compose the structure of the
spacecraft. Communications equipment is placed on the despun section to
provide for a stationary dual parabolic offset-feed array antenna system.
The dual system is necessary to provide communications coverage during
portions of maximum excursion on the halo orbit. Transmissions to the NASA
Deep Space Network 34 m antenna include six channels (color video, two
voice, scientific data from lunar payloads, satellite housekeeping and
telemetry and uplinked commands) using the S- and X-bands. Four radioisotope
thermoelectric generators (RTG's) provide a total of 1360 W to power onboard
systems and any two of the four Hughes 13 cm ion thrusters at once. Output
of the ion thrusters is approximately 17.8 mN each with xenon as the
propellant. Presence of torques generated by solar pressure on the antenna
dish require the addition of a 'skirt' extending from the spun section of
the satellite for balance. Total mass of the satellite is approximately 900
kg at a cost of $130 million FY99."

I wasn't able to find a suitable illustration on the web, so I scanned an
illustration (it's a very rough sketch!) from a book I have ("Voyage through
the Universe - Spacefarers", Time-Life Books 1989, Danish ed., pp. 58-59):
http://www.dsri.dk/~sej/pics/lunar_halo_orbit.jpg

--
Steen Eiler Jørgensen
"Time has resumed its shape. All is as it was before.
Many such journeys are possible. Let me be your gateway."