#  Lunar Limb Observatory: 2006 Supplement

This Technical Memorandum was published in 1996, and is being re-issued
for possible use in NASA\'s return to the Moon as outlined in the 2005
Exploration System Architecture Study (ESAS) and the 2006 NASA Strategic
plan. These envision a series of robotic landing missions leading up to
a human return, and establishment of a permanent outpost, by 2018. These
have been formalized as the Constellation Program in June, 2006.

The Lunar Limb Observatory plan is in principle still relevant to
current programs. It has been re-printed without change, with the
intention of providing a documented review of previous studies.
Obviously, there have been major advances in several fields, summarized
as follows. No new references will be supplied, but the reader can
easily catch up with these fast-moving fields through the Internet.

## New Launch Vehicles and Spacecraft

The Shuttle is scheduled to be phased out by 2010, and replaced by new
launch vehicles, partly Shuttle-derived, and spacecraft for Space
Station servicing and later for manned lunar missions. The latter
include the Crew Exploration Vehicle (CEV), Lunar Surface Access Module
(LSAM) and Cargo Launch Vehicle (CLV). Mission modes will be basically
similar to those of Apollo, earth-orbital rendezvous and lunar orbital
rendezvous in various combinations. All are still under development and
will not be further discussed here.

## Robotic Vehicles

Telerobotic vehicles (two Lunokhods) were operated successfully on the
Moon by the Soviets in the 1970s. Since then, and in particular since
1996, there has been enormous progress in telerobotic and autonomous
vehicles. Those operated on Mars have been the most spectacular and
productive, and their achievements are well-known. However, comparable
progress has been made in terrestrial robotic vehicles, especially
autonomous ones, demonstrated by recent desert trials. This progress is
analogous to the development and operational use of autonomous robotic
aircraft, used in combat in southwest Asia. Similar combat use has been
made of robotic vehicles for mine-clearing and similar functions. In
summary, for 21^st^ century technology, lunar surface exploration by
robots is hardly a challenge; the lunar surface environment is well
known and the Moon's closeness and continual accessibility make programs
such as those on Mars almost trivial by comparison.

## Optical Interferometry

Optical interferometry was recommended in the 1996 TM (Appendix B) as
the most promising astronomical use of the Moon, based on early
feasibility demonstrations. Since then, optical interferometry has
become an operational technique for many terrestrial observatories. The
Keck telescopes on Mauna Kea, for example, are now referred to as the
Keck Interferometer. The Mauna Kea telescopes have been successfully
linked by fiber optics, forming the OHANA network, with 800 meter
baselines as of 2005. The European Southern Observatory Very Large
Telescope(VLT)at Cerro Paranal, in Chile, consists of four 8.2 m
telescopes that can be used as an interferometer, the Very Large
Telescope Interferometer (VLTI). In brief, optical inteferometry has
more than fulfilled its promise of 1996.

### Submillimeter Interferometry

This technique was given high priority in the 1996 TM, second only to
optical interferometry. Submm interferometry too has developed as hoped.
The 8 dish Submillimeter Array (SMA) on Mauna Kea went into operation in
November, 2003, and has since been extremely successful. The European
Southern Observatory Atacama Large Millimeter Array, ALMA, is under
construction, and should be extraordinarily productive when completed
because of its large baseline and high altitude (15,000 feet). A related
development since 1996 was the successful launch and operation of the
Submillimter Wave Astronomy Satellite (SWAS) in 1998. Although not an
interferometer, SWAS demonstrated the potential of Moon-based
submillimeter astronomy provided by the unlimited spectral window.

### Millimeter Wave Radar

Since 1996, there has been great progress in the technology and
application of millimeter wave radar. Mm-wave radar is now in wide use,
both civil and military, chiefly because of its inherent higher range
accuracy. This progress has important implications for the feasibility
of a Moon-based submillimeter telescope or interferometer. Terrestrial
submm telescopes now in use, such as the SMA, have dish diameters of
several meters for increased gain. However, the success of millimeter
wave radar points to the possibility of much smaller dishes with
diameters as low as 15 centimeters, which are now commercially available
for ground-based use. Wave-guides and oscillators are small in
proportion, about 2-3 cm in size. Such small components should make a
Moon-based robotic submm telescope possible with a very small landed
payload.

### Knowledge of the Lunar Surface and Surface Environment

Probably the most important discoveries since 1996 have been those of
the Lunar Prospector Mission of 1998. The neutron spectrometer of the LP
found large quantities of hydrogen at both north and south lunar poles.
This has been interpreted as support for the 1994 Clementine bistatic
radar experiment indicating ice at the south pole in permanently-shaded
areas. However, the LP results imply that implanted solar wind hydrogen
might occur in the same areas. The conflicting interpretations of the
Clementine radar have not been resolved, but it is clear that the south
pole has become an area of great strategic importance for its hydrogen,
in whatever form, and NASA plans are now centered on a south pole
location for in situ resource utilization.