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Return to the Moon - Pursuing Our Destiny in the Sky -
“A Stepping-Stone Into the Solar System”
Paul D. Spudis
Principal Professional Staff, Johns Hopkins University Applied Physics Laboratory

Q. Can you tell us about the United States' past lunar exploration missions? What did you learn from those missions?

Basically, as part of the effort to send people to the Moon in the 1960s, we flew a series of robotic missions first. There was a hard lander called Ranger, which impacted into the Moon and took pictures as it came in. Then there was Surveyor, which soft-landed on the Moon and took pictures of the surface after it had landed. And finally, there was a mission called Lunar Orbiter, which orbited the Moon and mapped it with photographs. From that information, we found out what the Moon was like up close, at human scale. We wanted to be sure that it was safe to send people there. Once we had that data, we were convinced that it was safe to send humans to the Moon. And we landed on the Moon six times with Apollo. Apollo explored the Moon, sampled it, took a lot of photographs, and laid out experiment packages that lasted long after the astronauts had visited. We also collected about 350 kg of lunar samples, which are still being analyzed to this day to unravel the secrets of the Moon.
After that was done - the last Apollo mission was in 1972 - we sent two more missions to the Moon, two orbiters. One was Clementine. The Clementine mission went in 1994, and then a few years later, in 1998, was the Lunar Prospector mission. Those two missions mapped the global composition of the Moon. So using all the data from all those missions, from the Apollo precursors, to the Apollo missions, to the Clementine and Lunar Prospector missions, we were able for the first time to get a global map of the geology and composition of the Moon. And from that, we pieced together lunar history. Basically, what we know about the origin and evolution of the Moon comes from that data set.
In fact, I was just an author on a paper that we just submitted, that's looking at the distribution of the element thorium on the Moon. It's a very interesting element because it's radioactive. And if you can map it, you can understand the igneous history, the melting history of parts of the lunar crust. So there are all kinds of projects that are still ongoing using this data. It's a very active field of research.

Apollo 17 astronaut collecting lunar rocks (Courtesy of NASA)

Q. What kind of information do we get from Moon rocks?

Geologists study rocks for a variety of reasons, but basically they give you information on two things: composition and process. When you look at a rock, you understand what it's made of. You understand its chemical composition - what elements are in there and how much of each type. And then its mineralogy, which is how the elements are arranged with respect to each other. Minerals are crystals, and different crystals contain different kinds of elements. So measuring the chemistry and mineralogy of a rock tells you about its composition, and that is a clue to how it formed. So when you study composition first, you then get into process. For example, if I pick up a sample of mare basalt from one of the landing machines that went to the dark, smooth part of the Moon, the maria, I find out that it's made up of lava that was erupted as a liquid onto the surface billions of years ago, and then cooled very close to the surface in space. So studying the composition and the origin of these rocks tells us about the process.
Because we know where it came from on the Moon, that data will tell us about the history of a given area of the Moon. So it's a process that's very complicated. You collect a lot of different data, studying the rock's age, the rock's composition, and the processes that went into forming it. And then you try to put it into a context by figuring out where it came from on the Moon and what that's telling you about that particular area of the Moon and its history.

Q. How much do we know about the Moon?

We know quite a bit, and we have a good idea of the history of the Moon from the Apollo samples and from the missions that followed. In a nutshell, we know that the Moon is the same age as the Earth. It formed about four and a half billion years ago. We also know that it's likely that immediately after the Moon formed, it melted globally - that the entire surface of the Moon was covered in an ocean of liquid rock called the magma ocean. That's how the crust was created. In this ocean of liquid rock, the light, low-density minerals floated to the top, and the high-density minerals sank. And that segregated them into two different units, the crust and the mantle. That happened very early in lunar history. After the crust solidified, we had a very long period of bombardment, where you formed all the big basins and craters of the Moon. It was a period of great violence -- there were many impacts in a very short period of time. And that's been called the cataclysm, meaning a very intensely violent, very, very short period. After the cataclysm was over, the Moon was partly flooded by lava, but that largely ended about three billion years ago. So ever since then, the only thing that's happened to the Moon is the occasional large impact, and the constant impact of the very small micrometeorites that have ground up the surface rocks into a very powdery layer. The history of the Moon that I just recounted is what we learned from the previous missions.

Q. What can you tell us about future American lunar missions?

NASA announced the new plan in December 2006, at a meeting in Houston. We now have a directive to return to the Moon. We're going to return to the Moon for a lot of different reasons: to study it, to understand how it formed and evolved, but also to learn how to live and work off-planet. We're going back to the Moon both to fill in the missing gaps in our knowledge from Apollo, but also to learn new things, and make new discoveries.

American Moon Explorer Lunar Reconnaissance Orbiter (Courtesy of NASA)

The first step in our return to the Moon is a lunar orbital mission called Lunar Reconnaissance Orbiter (LRO), which will be launched in 2008. Its mission is to fill in some critical gaps in our data. For example, we discovered from the Clementine and Prospector missions that the poles of the Moon are very interesting. And yet, because we haven't been there - we didn't send a landing there, and they were poorly explored by orbital spacecraft - there's a lot about the poles of the Moon that we don't know. So this mission will try to fill in some of those gaps by measuring the shape of the pole, the topography (the highs and lows), what areas are in permanent shadow, and what the surface characteristics are in those areas. Also, it will carry instrumentation designed to look for evidence of ice in the dark areas of the poles.
That's the first step in the return to the Moon. After that, there will probably be a lander to the polar areas, to make measurements on the surface, to characterize the environment of the polar regions from the ground. It's quite different than what we experienced during the Apollo missions. And then, after that, the plan is to go to the Moon with people, and begin living and working on the Moon.

Q. Do you want to go to the Moon if you can?

Oh, I would love to. But I don't think that we're going to be back there in time for me to go. The current plan has us calling for the first landings in 2020. That's about 13 years from now, and at that time I will be in my late 60s. So I'll probably be too old to go to the Moon when we get back there.

Q. What attracts you to lunar exploration? And what do you want to know about the Moon?

Well, everything. I want to know everything about the Moon that we don't know. The sketch I outlined for you of what we know about the Moon is only a fraction of the Moon's true story. I suspect that there's a lot more to it. And in fact, I suspect there are probably parts that we think we know that we've gotten wrong, and other parts that we don't know anything about, that will be quite a surprise when we find out about them. I'm a planetary scientist, and I'm interested in the Moon from a scientific point of view. I want to know how it formed, how it evolved, and what its history has been. And I also think that, from a slightly different perspective, the better we know the history and evolution of the Moon and how it formed, the better we'll be able to actually use it.
People often ask me, what are we going to do on the Moon differently this time than we did last time? Well, I think we're going to learn to live on the Moon. We're going to be able to live off what's there, to use the resources of the Moon to live in space. And the better we understand how it's put together and what it's made of, the better we'll be able to do that.

Q. What do you think about the Japanese lunar explorer SELENE?

I think that SELENE will provide a very high-quality set of data to globally map the Moon. In particular, I believe that for the first time SELENE will carry two sub-satellites, Relay Satellite and VRAD Satellite. They're small satellites that will be launched from the main satellite, so that we'll be able to follow the main satellite's progress on the far side of the Moon using the Relay Satellite as a relay. And through that, we'll be able to map the gravity field of the far side, which is something that we don't have right now. That is a critical data set in man's return to the Moon - to understand the gravity field on a global basis. And SELENE should get that.

Q. Do you expect to cooperate with the Japanese team in the future?

I certainly hope to. Unfortunately, although I see them at conferences, I've never actually collaborated with them on any projects. In the next few years there's a series of missions being sent to the Moon by a variety of countries. Of course Japan is launching SELENE, the big satellite to map the Moon. China is launching a satellite next year as well, called Chang'e. And the Indians are launching a satellite, Chandrayaan-1, an orbiter to the Moon, in 2008. I am ac building an imaging radar instrument to fly on the Indian mission. So I would say the world is going to the Moon.
I think international collaboration is very important to achieve the goals of the Moon mission, because everyone is collecting a variety of high-quality data. If we share it, then we have access to all the data, and that makes it much easier to make comparative studies and breakthroughs. My hope is that all the data that's collected by all these missions will be put into a common databank that we can all share and draw on.

Q. Why people are interested, and why we are going back to the Moon now?

The main reason we're going back to the Moon is that it is the next logical step for humans in space beyond low-Earth orbit. We learned a lot by going into low-Earth orbit for these last 30 years. And we've learned a great deal by building the International Space Station. It's taught us how to assemble spacecraft in orbit and make them work. It's taught us how to live and work productively in space for long periods of time. But it's time to take the next step. It's time to go beyond low-Earth orbit, deeper into the solar system. And the Moon is the first step, where we'll learn how to explore space. We'll learn how to live and work on another planet, and we'll learn how to use the resources of space to create new spacefaring capabilities. So that's the principal objective, the mission of going back to the Moon. In addition, since we're going to be there, we'll have a whole new planet to explore. We'll be able to explore and understand how the Moon formed and evolved in a much greater degree of detail than we ever could have hoped to before. So fundamentally, that's the reason we're going back to the Moon. It's to use the Moon as a stepping-stone into the bigger solar system beyond.

Paul D. Spudis
Principal Professional Staff, Johns Hopkins University Applied Physics Laboratory
A planetary geologist, Dr. Spudis received his education at Arizona State University (B.S., 1976; Ph.D., 1982) and at Brown University (Sc.M., 1977). He has specialized in research on the processes of impact and volcanism on the planets and studies of the requirements for sustainable human presence on the Moon. He was formerly with the Branch of Astrogeology, U.S. Geological Survey in Flagstaff, Arizona and the Lunar and Planetary Institute. He is a former member of the Committee for Planetary and Lunar Exploration (COMPLEX), an advisory committee of the National Academy of Sciences; and the Synthesis Group, a White House panel that analyzed a return to the Moon to establish a base and the first human mission to Mars. Dr. Spudis was Deputy Leader of the Science Team for the Department of Defense Clementine mission to the Moon in 1994, and currently is the Principal Investigator of an imaging radar experiment on the Indian Chandrayaan-1 mission, to be launched to the Moon in 2008.