WHY THE MOON?
We know more about many aspects of the Moon than we know about any world beyond our own, and yet we have barely begun to solve its countless mysteries. In the decades since the Apollo missions there has been a widespread misperception that the Moon has already told us all the important things it has to tell, that scientifically it is a “been there, done that” world. Nothing could be further from the truth.
The Moon is, above all, a witness to 4.5 billion years of solar system history, and it has recorded that history more completely and more clearly than any other planetary body. Nowhere else can we see back with such clarity to the time when Earth and the other terrestrial planets were formed.
Planetary scientists have long understood the Moon’s unique significance as the starting point in the continuum of the evolution of rocky worlds. Many of the processes that have modified the terrestrial planets have been absent on the Moon. The lunar interior retains a record of the initial stages of planetary evolution. Its crust has never been altered by plate tectonics, which continually recycle Earth’s crust, or by planetwide volcanism, which resurfaced Venus only half a billion years ago, or by the action of wind and water, which have transformed the surfaces of both Earth and Mars. The Moon today presents a record of geologic processes of early planetary evolution in the purest form. Its airless surface also provides a continuous record of solar-terrestrial processes.
For these reasons, the Moon is priceless to planetary scientists: It remains a cornerstone for deciphering the histories of those more complex worlds. But because of the limitations of current data, researchers cannot be sure that they have translated the message correctly. Now, thanks to the legacy of the Apollo program, it is possible to pose sophisticated questions that are more relevant and focused than those that could be asked more than three decades ago. Only by returning to the Moon to carry out new scientific explorations can we hope to close the gaps in our understanding and learn the secrets that the Moon alone has kept for eons.
INTERNATIONAL LUNAR EXPLORATION
The lunar exploration activities of the recent past and the near future are pervasively international in scope. The European Space Agency launched SMART-1 to the Moon in September 2003 on a technology-demonstration mission to validate solar-electric propulsion systems. After entering orbit around the Moon in November 2004, SMART-1 began limited studies of the lunar surface with a suite of small, innovative instruments. SMART-1 has scheduled an end-of-mission impact on the lunar nearside along with coordinated observations during fall 2006.
The Japanese Aerospace Exploration Agency has planned two missions for near-term implementation, Lunar A and SELENE. Lunar A is designed to study the lunar interior using seismometers and heat-flow probes deployed by penetrators, but technical difficulties during testing have put the mission on hold. On the other hand, SELENE is a mature orbiter prepared for launch in 2007 for a 1-year nominal mission. The goals of SELENE are to study lunar origin and evolution and to develop technology for future lunar exploration. It carries an array of modern remote sensing instruments for global assessment of surface morphology and composition. SELENE also carries two sub-satellites that will enable the gravitational field of the farside to be measured accurately.
The Chinese National Space Administration formally announced its Chang’e lunar program in March 2003. Chang’e 1, a lunar orbiter with a broad complement of modern instruments, is prepared for launch in fall 2007. Chang’e carries several remote sensing instruments to study surface topography and composition as well as the particle environment near the Moon. In addition, Chang’e carries a four-
wavelength microwave sounder to probe the regolith structure. Future elements being planned for the Chang’e program include a lander/rover and a later sample-return mission.
The Indian Space Research Organization will launch its Chandrayaan-1 spacecraft in early 2008 on a 2-year orbital mission to perform simultaneous composition and terrain mapping using high-resolution remote sensing observations at visible, near-infrared, x-ray, and low-energy gamma-ray wavelengths. This spacecraft will carry two sophisticated instruments from the United States to characterize and map mineralogy using near-infrared spectroscopy and to map the shadowed polar regions by radar. In addition to the remote sensing instruments, the Chandrayaan-1 spacecraft will also carry an instrumented probe that will be released and targeted for a hard surface landing.
NASA’s Lunar Reconnaissance Orbiter (LRO) is scheduled for launch in fall 2008. LRO’s goals are to improve the lunar geodetic net, evaluate the polar areas, and study the lunar radiation environment. A secondary payload, LCROSS, launched with LRO will result in an impact into a polar region target with coordinated analysis.
All of these participants in lunar exploration have expressed their intention of releasing data returned in a compatible format that will allow fruitful comparisons. A variety of coordinated and cooperative international lunar analyses have been proposed.