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Telescopes and space missions

Telescopes and space missions

Another giant leap for mankind

02 Mar 2009

The Moon has been neglected by space scientists and astronomers alike since the Apollo days, but now we want to go back. Paul D Spudis explains what motivates the new vision of lunar exploration

Vision of the future

Three spacecraft are currently orbiting the Moon, Chang’e-1 from China, Kaguya from Japan and Chandrayaan-1 from India. The American Lunar Reconnaissance Orbiter will join them later this year. Russia is developing lunar rover hardware, for itself and for other countries. In Europe, both Germany and the UK are contemplating their own lunar missions, both outside the boundaries of the European Space Agency, to which they both belong. China and India are discussing the idea of sending manned missions to the Moon within the next decade or so. The Moon has once again risen to the top of the space agenda worldwide.

What is going on? Why has the Moon suddenly become the destination for spacefaring nations? After the Apollo programme ended in 1975, relatively little attention was paid to the Moon. Yet a small group of lunar scientists, enthusiasts and space visionaries have continued to think seriously about both the scientific questions the Moon poses as well as the opportunities it offers. We know a lot more about the Moon that we did only a couple of decades ago. And we now understand that the Moon has a key role to play in humanity’s exploration of the solar system.

Return to the Moon

Our spacecraft have reached the breadth of the solar system, probed the nature of the Sun, and examined our galaxy. For almost 50 years, we have tentatively explored the edges of the cosmos, examining the physics and history of our universe.

We have accomplished much with this model of space exploration but are limited in what we can send into space. Launch vehicles are costly and not always reliable. The high cost of spaceflight makes the fate of such programmes inevitably tied to political winds that may change at a moment’s notice. Failures occur, and when they do, it can take years to recover and obtain the information originally sought.

The current paradigm of space exploration has developed largely because we must lift everything we need for our study out of Earth’s deep gravity well. Because launch costs are so high, satellites must be built to last for long periods of time, thus making individual missions costly and rare. The logistical train to the various levels of Earth orbit where our space assets reside is long, tenuous and difficult to maintain.

The US’s new Vision for Space Exploration (VSE), proposed by President George Bush in 2004 and endorsed by Congress in 2005, outlines a different approach to the fundamentally limiting problem of spaceflight: what if we were no longer limited only to what we can lift from the Earth’s surface? Suppose that we were able to “live off the land” in space? What would the advent of this scenario mean for the future exploration of space?

The human part of the space programme has been trapped in low Earth orbit with no plans to go further, even though robotic space exploration passed that horizon years ago. The International Space Station (ISS) could have served as a test bed for farther destinations, but did not, largely as a result of conscious policy decision. The tragic loss of the Space Shuttle Columbia in 2003 only drew attention to the hollowness and lack of direction in space policy.

The VSE proposes that a new vehicle be designed and built for human spaceflight beyond the confines of low Earth orbit, one that can adapt to different kinds of missions going to varying destinations. We would conduct robotic exploration of the Moon in preparation for the resumption of human exploration of our satellite by the next decade. On the Moon, we would learn how to live and work productively on another world and use the knowledge and capabilities created from these activities to venture beyond it to the planets.

Water from outer space

One of the most interesting and unusual aspects of the VSE is the idea of using the abundant resources found at the Moon and elsewhere in space to create new capabilities. Although widely discussed in space-advocacy circles, the use of space resources has been dismissed by many in the spaceflight community, with development considered only likely in the far distant future. Yet, we have been using one cosmic resource since the very first flights into space — the conversion of abundant solar energy into electricity to power the spacecraft sent to various destinations.

Space resources consist not only of energy, but of materials as well. We know that the bodies closest to us in space offer usable resources that can be harvested — water (bound in minerals or as condensates in specialized environments) and the bound oxygen found in common rock-forming minerals. The Moon and near-Earth asteroids also contain metals and ceramics that can be used in the construction of new rockets and spacecraft.

A supply of water on the Moon would make the establishment of a self-sustaining lunar presence come about sooner and easier. The samples returned by the Apollo missions revealed that the lunar interior is essentially devoid of water. However, the surface is regularly bombarded with water-rich objects such as comets, and scientists suspect that some of that water might have accumulated to usable quantities. Where would this water end up? Most of it would be split by sunlight into its constituent atoms of hydrogen and oxygen, and lost into space, but some would migrate by literally hopping along to places where it is very cold. As the Moon’s axis of rotation is nearly perpendicular to the plane of its orbit around the Sun, the Sun always appears close to the horizon at the poles of the Moon. If you are on a topographic high, you may be in permanent sunlight. If you are in a hole, you may be in both permanent darkness and in extreme cold, with temperatures as low as 40–50 K. Moreover, these “cold traps” have existed on the Moon for at least the last two to three billion years — plenty of time for water to accumulate from impacting comets.

Two NASA missions sent to the Moon in the 1990s looked for evidence of water at the poles. In 1994 Clementine thoroughly mapped the poles of the Moon, revealing areas of near-permanent sunlight and permanent darkness. Although the spacecraft did not carry instruments designed to look for lunar ice, during the mission an improvised experiment obtained information on some properties of the polar surface. Radio waves are reflected from planetary surfaces differently depending on the compositional make up of the surfaces. Specifically, radio waves are scattered in all directions when they are reflected from surfaces consisting of ground-up rock (as exists on most of the Moon, Mercury, Venus, Mars and the asteroids). However, radio waves are reflected more coherently from ice surfaces (the polar caps of Mercury and Mars, and the icy surfaces of Jupiter’s satellites Europa, Ganymede and Callisto). When radio waves encounter ice, they are partly absorbed and reflected multiple times by internal flaws in the ice then reflected back out into space. A consequence of multiple reflections is that some of the radio reflections come back in the same sense as they were transmitted (think of the reflection of light from two mirrors — reflection from a single mirror makes text unreadable, but double reflection makes the text “normal” again). Thus, ice reflects the radio waves back partly in the same sense as incident waves.

Analysis of the data returned from the radio-wave experiment on Clementine reveal that ice deposits might exist in permanently dark regions near the south pole of the Moon. Initial estimates suggest that a small ice lake (more than 109 m3 in volume) exists at the south pole. This amount of water would be equivalent to the fuel (hydrogen and oxygen) used for more than a 100,000 Space Shuttle launches.

The Lunar Prospector (LP), launched in 1998, orbited the Moon in a 100 km orbit for over 18 months. It carried a variety of instruments that, in many ways, complemented the instruments of the earlier Clementine mission. The LP’s neutron spectrometer detected high concentrations of hydrogen at both poles. In the form of water ice, results from the LP show an amount of hydrogen equivalent to about 10 m3 of ice, with the south pole having slightly more than the north pole. Moreover, the low-altitude (high-resolution) neutron data show that these high concentrations of hydrogen are correlated with the large areas of darkness seen in the Clementine images. This result almost certainly means that water ice exists in the dark areas, thus confirming Clementine’s earlier result.

The discovery of ice has enormous implications for a permanent human return to the Moon. Water ice is made up of hydrogen and oxygen, two elements vital to human life and space operations. Lunar ice could be mined and disassociated into hydrogen and oxygen by electric power provided by solar panels or a nuclear generator. Hydrogen and oxygen are prime rocket fuels, giving us the ability to refuel rockets at a lunar “filling station” and making transport to and from the Moon more economical by at least a factor of 10. Additionally, both the water from lunar polar ice and the oxygen generated from the ice could support a permanent outpost on the Moon. The extraction and use of this material, rare on the Moon but so vital to human life and operations in space, will make our expansion into the solar system easier and reaffirms the immense value of the Moon as a stepping stone to the wider universe.

New missions to the Moon

The initial steps in a return to the Moon involve robotic orbiters. Chang’e-1, Kaguya (SELENE) and Chandrayaan-1 are all currently in lunar orbit. These missions are making maps of the Moon at unprecedented levels of detail and quality. Soon we will know the global topography, composition and structure of the Moon to a degree never before attempted for any planet, including the Earth. The basic data acquired by these missions will let us select future landing sites for both scientific and resource purposes.

These missions should be followed by others, including both orbiters and landers. A series of small spacecraft in lunar orbit could create a communications and navigation infrastructure for the Moon, providing continuous communication with areas out of sight from the Earth (such as the far side and deep craters near the poles) and positional information for both orbital and surface navigation around the Moon (a lunar GPS). With landers, we can explore the surface using rovers, as shown by the recent experience with the Mars Exploration Rovers, and deliver robotic payloads to begin developing the surface infrastructure near a future outpost site. Rovers can access the dark floors of polar craters, gathering detailed chemical and physical information on the ice deposits — necessary precursor information for the extraction of water.

In parallel with this programme of robotic exploration, a new human spacecraft (the Crew Exploration Vehicle, a replacement for the Space Shuttle) will need to be developed and tested. Humans will return to the Moon using both the knowledge gained and equipment emplaced by the robotic precursors. Using the Moon’s resources will enable us to build a space-transportation infrastructure in “cislunar space” (between the Earth’s atmosphere and the Moon). Such a system — allowing routine access to the Moon and all points in between — is a fundamental step towards creating true spacefaring capability. A system that can routinely land on the Moon, refuel and return to Earth orbit, bringing with it fuel and consumables produced on the lunar surface, also gives us the ability to journey to the planets.

The ability to routinely access cislunar space would also bring about a new capability, one of surprising significance. All current satellite assets, commercial and strategic, reside in the volume of space between the Earth and the Moon. Currently, we have no way of accessing these satellites — if one breaks down or becomes obsolete, it is written off and must be replaced. If we had the ability to travel between the various energy levels of cislunar space, carrying out servicing and upgrading missions, we could maintain a more robust, more capable and more extensible set of satellite assets. Thus, cislunar space would become as accessible as low Earth orbit is today and we could use this lunar-based transport system for a variety of commercial missions as well as exploration.

The meaning behind the vision

Rather than being simply a “new human space programme” or a “manned Mars mission”, the entire solar system is the goal of this new vision. Existing launch vehicles, spacecraft, instrumentation and supporting infrastructure are too limited — in mass, power, bandwidth and computational ability. The goals of the VSE are nothing less than revolutionary: to exploit our existing capabilities by developing and using new technology, but also to create new capabilities by using space resources and building spacefaring infrastructure. Thus, the vision is not a zero-sum game, with some winners and some losers — the goal is for all to win through the creation of new capabilities.

It is important to emphasize that the VSE does not call for the use of space resources to lower the costs of the US space programme, although that is a long-term goal of such use. The real goals are to understand how difficult it is to use lunar and space resources, to develop the technologies needed to do so, and to experiment with different processes in a real space environment. It may turn out that using space resources is more trouble than it is worth; if so, we can then devote our efforts to a space programme that does not feature an extensive human presence. In other words, can we make what we need from what we find in space?

However, it is my view that a new scientific opportunities will become available through the presence of people on lunar and planetary surfaces. A key goal of the VSE is to break down the false dichotomy between human and robotic exploration. To maximize the return, both techniques are needed. We can use a return to the Moon to learn how to best explore planetary surfaces and to decide the optimum mix of human and robotic capability.

We face a clear choice in our future direction in space. We can continue on our existing path, limited in space by what we can launch from the Earth, or we can embrace a model that creates new capabilities by using the unlimited resources of space to build a transportation infrastructure that can routinely access cislunar space and beyond. We can generate new wealth by extracting these resources for the use in space and back on Earth. Using the combined power of people and machines, we can robustly explore planetary surfaces and build scientific instruments of extraordinary power and capability. The first step in this direction will represent another giant leap for mankind.

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