Strategic Considerations for Cislunar Space Infrastructure

Wendell W. Mendell
Solar System Exploration Div.
NASA Johnson Space Center
Houston, TX 77058 USA

Steven Hoffman
Science Applications International Corporation
Houston, TX 77058 USA


Abstract

The international low Earth orbit (LEO) space station ultimately derives from the vision of Wernher von Braun (as part of a Mars exploration strategy) via the 1969 NASA Space Task Group. In its contemporary NASA incarnation, the space station has been touted as a research laboratory with a broad agenda. Unfortunately, the LEO environment is inhospitable for the many of the advertised research objectives. More importantly, space debris in LEO and in GEO is a long-term threat. We argue that a crewed space station at the libration point between the Earth and the Moon provides an environment that is more suitable for almost every research objective, from microgravity to planetary exploration. An important corollary of the existence of an L1 space station is the development of a reliable, low-cost heavy lift vehicle by an international consortium for an international market. Therefore, an international decision to support what we call Space Station ˝ will have broad implications for space development beyond the objectives of the program itself. This example illustrates how strategic goals associated with human exploration will be best served by careful reconsideration of appropriate infrastructure for human presence in cislunar space.

Introduction

Only the Soviet and the American space programs have pursued human spaceflight. Competition between the programs during the Cold War gave rise to a race to put humans in orbit, a race to land humans on the Moon, and a commitment to permanent human presence in Earth orbit. Only Americans have walked on the Moon, but only the Soviet program has maintained long-term human presence in orbit.

Although the societal investment in human spaceflight was driven by political perceptions of national security, the programmatic goals in both societies were shaped by visionaries who saw human exploration of the solar system as the ultimate goal. Cold War politics provided a means to that end.

The vision of Wernher von Braun was reflected strongly in the plans drawn up by the 1969 Space Task Group in NASA in its report to President Nixon on the future of the U.S. space program. The STG report spoke of a 100-person orbiting station, a permanent base on the Moon, and exploration of Mars. Space transportation elements included a reusable chemically-fueled shuttle moving between the surface of the Earth and Earth orbit, a space tug for moving among different Earth orbits or ferrying payload from the lunar surface to and from lunar orbit, and a nuclear vehicle for moving between the Earth and the Moon or Mars. The space station - or space base in its expanded form - was envisioned as a research laboratory and as a prototype for piloted interplanetary spacecraft. The STG plan was rejected by the Nixon Administration for fiscal reasons, and in the ensuing budget battles NASA was allowed to develop a partially reusable form of the shuttle described in the report.

Although no Soviet long-range plan was ever published, spokesmen alluded to human missions to Mars. Many analysts in the U.S. saw the long duration stays aboard Salyut and Mir as practice runs for flights to Mars.

During the Carter and Reagan Administrations NASA lost the vision of human exploration; and the Space Shuttle and the Space Station became ends in themselves. The Space Station was touted as a laboratory for discovering commercial microgravity processes, for making astronomical discoveries, and for observing the Earth. The Space Station Evolution Workshop in 1986 also considered future support of communications satellites (servicing, launching to geostationary orbit) and support of lunar and planetary missions (launch, return).

Unfortunately, many of these proposed functions are incompatible with each other or with the Space Station orbit. Microgravity research scientists were not happy with jolts to the station from spacecraft arriving and leaving. They barely tolerated human beings onboard and preferred "co-orbiting" free flyers. Astronomers were not happy with the contamination of optics by effluents traveling in orbit with the station. They preferred putting their telescopes in high orbits where the Earth did not cover so much of the sky. Their connection to the station was through servicing of observatories by space-based orbital transfer vehicles. Earth scientists did not fly over enough of their favorite planet in the 28ű inclination orbit and wanted to put their instruments in polar orbit. The orbital transfer vehicles would service their spacecraft, too. Lunar and planetary explorers were unclear whether a stopover at the Space Station added value to their mission profile.

The speech by President Bush on the 20th anniversary of the Apollo 11 landing temporarily returned long range vision to human exploration of space. However, NASA did not take the presidential declaration as an opportunity to examine the entire human program for consistency. Rather, it interpreted the speech as permission to plan two more programs - lunar base and Mars flights - and make the Space Station more elaborate in the process. Congress treated that strategy with the budgetary equivalent of Agent Orange (pecunicide?), and the Space Exploration Initiative died aborning. One residuum of the brief SEI flash was emphasis of life science research on the Space Station when the previous favorite, microgravity, failed to live up to its advertised potential.

As of this writing, debates over the purpose of the Space Station have been eclipsed by concerns over its survival as a program, by uncertainty over its configuration, and by controversy over the degree of Russian participation. It is now called "Space Station a" to emphasize its change from "Space Station Freedom".

Here, we wish to reexamine the most fundamental concepts of a space station, how it fits into a program of human exploration of space, and how it can function as a multifaceted research laboratory. We will conclude that Space Station a is simply not in the best location in space. Since our concept is consistent with discussions of functionality of the station but far removed from any current conceptualization of it, we will refer to our new concept as Space Station ˝.

Problems with Low Earth Orbit (LEO)

Environmental Issues

Space Debris. Mendell and Kessler[1], in a paper entitled "Limits to Growth in Low Earth Orbit", discussed the implications of the proliferation of world launch capability and projections of traffic to low Earth orbit over the next two decades. In particular, Yamanaka and Nagatomo[2] had suggested that cargo traffic and passenger traffic could grow by two orders of magnitude. Although this estimate seems extreme, the numbers are derived from economic assumptions that are not unreasonable. Mendell and Kessler conclude that uncertainties in traffic projections are large and that substantial growth is possible. For example, a new type of growth scenario which has emerged in the last few years is proposals to orbit tens or hundreds of small satellites as commercial communication constellations.

The authors then review model predictions of the growth of debris as a function of traffic, paying particular attention to the work of Su[3], who considers the effects of large structures in orbit. Placement of large (>100m diam.) structures in LEO shortens the time to production of the critical flux density which brings on a runaway condition in the debris population. Space Station a by itself does not create an emergency, but the trend is in the wrong direction. Mendell and Kessler conclude that there is a limit to the growth of mass in LEO - implying that launches will have to be controlled internationally - and that LEO is an unsafe environment in the long term for crewed facilities.

Currently the management of the international space station program is not highly concerned with the debris hazard because the Russian program has not had a problem. Nevertheless, the (developing) requirements for Space Station a include capability for maneuvers to avoid large objects and shielding against small objects. The shielding requirement will eventually impose a serious mass penalty[4].

Reboost. The space station orbit decays with time as a result of drag from the outer reaches of the Earth's atmosphere. The rate of decay can vary with solar activity as the atmosphere expands under heating from solar particle storms. Atmospheric drag imposes design constraints on the station configuration to minimize the effective cross-section along the velocity vector. Fuel must be shipped to the station to periodically raise the orbit.

Materials Degradation. The plasma environment in LEO is rich in reactive free radicals, particularly atomic oxygen. The Long Duration Exposure Facility (LDEF) illustrated the chemical damage to spacecraft materials. In addition, the station collides with atoms at the orbital velocity causing sputtering of materials[5]. The spacecraft glow seen on the Shuttle is a chemical reaction in the ambient plasma with surfaces and degrades optical experiments onboard.

Microgravity Experiments. The orbiting station is in freefall, and the gravitational force is balanced at the center of mass of the structure. As one moves away from the center of mass, the effective gravitational potential increases, making the location less suitable for microgravity research. In regions of low microgravity, the gradient in the Earth's field becomes important. Since it lies along the radial direction, it changes direction relative to an inertial platform moving along the orbit. The Earth's field is not spherically symmetrical, and the station will experience small accelerations along the orbit.

Other accelerations occur whenever the station must fire reaction control engines to maneuver or to reboost. Accelerations due to movement of the human crew can be noticeable to microgravity researchers.

Many experiments require a higher vacuum than is available in LEO. The Wake Shield Facility being flown on the Space Shuttle is one attempt to create a volume of very high vacuum for materials research.

Orbital Inclination and Altitude. As mentioned in the previous section, the low inclination of the space station orbit make it unattractive for Earth observation. Astronomers are unhappy with the low altitude where the orbital period limits the length of time for any one observation. The alternation between sunlight and shadow causes thermal shocks to telescopes.

Co-orbiting Facilities. One solution to the conflicts in environmental requirements is to put different experiments on different platforms in the same orbit or different orbits from the crewed facility. Unfortunately, station-keeping in similar orbits can be fuel-intensive. Servicing platforms in orbits with other inclinations and altitudes can require large delta-v capability in an orbital maneuvering vehicle.

Space Transportation Node

The NASA Report of the 90-Day Study on Human Exploration of the Moon and Mars[6] depicted an enhanced LEO space station as a transportation node and assembly point for massive interplanetary vehicles. Alignment between the space station orbit and the plane of the interplanetary trajectory limited launch windows. The duration of the launch window to Mars was constrained by the energy penalties paid to change the phase plane of the trans-Mars trajectory. Phasing is particularly complicated in translunar trajectories where the station orbit, the orbit of the Moon, and a spacecraft orbit around the Moon must be properly aligned.

Lower mass interplanetary vehicles could be designed around low thrust technologies with high specific impulse. However, these fuel-efficient spacecraft spend a very long time spiraling outward from LEO to the edge of the Earth's gravity well. Exposure to radiation from the Van Allen belts is high. In the 1991 International Space University study[7], a chemically fueled crew vehicle was used to rendezvous with the nuclear electric interplanetary vehicle after it had traversed the radiation belts under automatic pilot.

As the SEI foundered politically, later studies for lunar[8] and martian[9] transport ignored the space station altogether, unless orbital assembly was required.

International Access

Orbital inclination is a signature of national ownership. The 28° inclination of Space Station Freedom indicated that it was a U.S. facility, supported from Cape Canaveral. The Space Station a inclination is a measure of the degree of Russian participation. Making a LEO station "orbit-neutral", while politically nice, is not efficient from utilization of launch capability.

An Alternative to Low Earth Orbit

Cislunar Infrastructure for Human Presence

Let's assume that we have been given the mandate to plan for permanent human presence in space for the long term. The scope includes human exploration of the planets as well as permanent human presence in Earth orbit. The crew will conduct life science research on human response to the space environment and will operate a space transportation node for planetary exploration. The node will also support astronomy and astrophysics research, microgravity research, and Earth observations. Where should this node be located?

It should be near the edge of the Earth's gravity well so that reusable interplanetary vehicles can come and go with a minimum of propellant. It should be accessible from the Earth and the Moon with a minimum of constraints on launch windows. There should be no hazard from artificial space debris. The region of space in which it sits should support colocated unmanned scientific platforms which can be reached from the space station by small delta-v transfers. The fuel requirements for station-keeping should be small. A good view of the Earth and the Moon would be useful for scientific observations and would provide the crew with a sense of place. International character would be enhanced by a location independent of political boundaries on the Earth.

These criteria are satisfied by volumes of space surrounding the unstable Earth-Moon libration (or Lagrangian) points. The Lagrangian point L1 between the Earth and the Moon is best for planetary observations and the sense of place. The far side of the Moon has been set aside by international agreement as a radio-quiet zone for scientific research since it is the only environment in the solar system completely shielded from terrestrial electronic emissions. Therefore, the Lagrangian point L2, on the far side of the Moon from the Earth, is unsuitable for a space station even though the location is probably better for astronomy.

The Lagrangian Points

On a line connecting the center of mass of two bodies, there is a point where the gravitational attraction by one mass is exactly balanced the gravitational attraction from the other mass. If a small mass is placed on that point, it (in theory) will not move. If is shifted a little one way or the other, it will fall toward one of the bodies.

If the two bodies are in a circular orbit about their center of mass with a constant angular velocity, a small mass on the (rotating) line connecting them will feel not only gravitational attraction from both but also a "centrifugal force". The small mass in theory will not move in the rotating system if it is placed on the line so that the three accelerations balance. Unlike the single balanced location in the stationary situation, there exist three balanced points for the rotating system. One is between the two bodies and the other two lie on either side of the two bodies. These three points are called Lagrangian points, and any movement from the balancing point will cause a small mass to fall toward one of the bodies or fly out into space.

Specifically, uncontrolled objects originating in the L1 region will leave it, making an artificial debris belt impossible. This characteristic is a major consideration in the suggestion by Mendell and Kessler[1] for a station there.

Lagrangian Point Orbits

The point between the Earth and the Moon is traditionally called the L1 point, and the point beyond the Moon is L2. In Moulton's book10 is presented the theory for periodic oscillating orbits about these points in the Earth-Moon system. Farquhar11 presents a comprehensive analysis of control theory for satellites stationed at the collinear libration points of any two-body system with particular development for the Earth-Moon L2 point and the Sun-Earth L1 point. He demonstrated in extraordinary fashion the power of his work with the diversion of the ISEE satellite at the Sun-Earth L1 point in 1983 to an intercept with the comet Giacobini-Zinner in 1986 under the name ICE.

Farquhar points out that the orbits about L1 are unstable and require station-keeping for maintenance. However, the propellant needs are modest - approximately the same as maintaining the east-west station-keeping of synchronous satellites. From a theory developed for controlling a satellite at the collinear point using only a (perfectly reflecting) solar sail, he performs a sample calculation to estimate that the sail area would be 0.374 m2 per kg of satellite mass.

A number of orbits are possible in the volume about L1. One attractive aspect is that different platforms could be placed on orbits which require very small Ćelta-v for orbital transfer even though they may be separated by considerable distances. Thus, separation of functions serviced by a crewed space station becomes quite reasonable. A microgravity free flyer can be left alone for long periods of time. Astronomy satellites can view targets for long, uninterrupted integration times. Earth observation platforms can survey a terrestrial hemisphere to understand the global interrelationships of environmental processes. The Earth's magnetosphere can be imaged. Transportation to and from any point on the lunar surface can take place at any time; launch windows are unconstrained by orbit phasing relationships. In the L1 environment, the space station becomes a true hub of multidisciplinary research.

Transportation Node at L1

Since the L1 sits at a "hill" in the pseudopotential of the rotating frame of the Earth-Moon system, all other locations in cislunar space are "downhill". A small delta-v is required to exit cislunar space, but it becomes possible to plummet toward the Earth of the Moon for gravity assists to change orbit planes to interplanetary destinations or to pick up energy. Low-thrust interplanetary spacecraft can be docked and reused at an L1 station.

Unfortunately, the scenic view from the top of the hill comes at a price. The Space Shuttle cannot be used to transport crew to L1, and the cargo delivery capability of other launchers is considerably diminished compared to delivery at LEO. We can describe the transportation requirements by comparing Ćelta-v for various points to and from LEO, L1, and geostationary orbit (GEO). (GEO has been suggested in the past as a space station location even though manmade debris exists there.)

We assume that all materials destined for the space station, regardless of its location, must start from the surface of the Earth. Thus a station in LEO has the advantage, from the transportation perspective, of being the least expensive; all other locations will require additional propellant to deliver the same mass, be that crew or supplies. Three mission scenarios will be discussed for each of the candidate locations - simple round trip missions, a mission to the lunar surface, and a mission departing for Mars. Some basic assumptions that are used include the following:

A common measure of performance demanded of a transportation system to get to a particular destination is delta-v. This quantity is the summation of velocity changes required leave one orbit and to enter another. Each velocity change requires usage of propellant. Therefore, the initial mass launched into LEO must include not only the payload to be delivered but also propellant needed to perform the maneuvers along the way. The initial mass into LEO is a convenient measure to describe the launch vehicle required for the task. Table 1 presents the additional propellant mass in the form of a "surcharge" per kilogram placed on the mission to deliver the payload to the given destination. For example, in the first entry, we see that delivery of a kilogram of payload to GEO requires a surcharge of 2.4 kg. Thus, delivery of 1 kg to GEO requires delivery of 3.4 kg first to LEO.

We note in passing that getting to the L2 point on the far side of the Moon is actually cheaper than getting to L1 by using a gravity assist when flying by the Moon. The difference in delta-v is of the order of 300 m/s, or about 8% of the cost to get to L1. However, the cost to get from L1 to L2 is only 140 m/s, making travel between the two locations quite feasible.

Missions to the Moon or to interplanetary locations become less costly from higher orbits. However, one of the original assumptions made was that all materials must come from the surface of the Earth. Therefore, the total cost must include not only departing from the station, but to cost to get to the station as well. Table 1 illustrates these additional costs to the system. While the additional costs for using a GEO station loom quite large, the cost for using an L1 station are more reasonable, particularly for access to the lunar surface.

Table 1: "Surcharge" to Move Each Kilogram From LEO to Other Destinations

PathDelta-V (km/sec)Propellant Mass "Surcharge"
LEO to GEO4.332.44
LEO to L13.771.87
LEO to Lunar Surface5.915.24
LEO to Mars Escape3.711.81
LEO to GEO to Lunar Surface8.2527.84
LEO to GEO to Mars Escape6.778.40
LEO to L1 to Lunar Surface6.296.40
LEO to L1 to Mars Escape4.262.80

The discussion thus far has examined one-way delta-Vs and associated propellant masses. This is appropriate for logistics flights or robotic missions. Round trip missions will also occur and the impact on them must be considered as well. Table 2 illustrates these costs. The values shown here assume that a different propulsion system is used for the "up" leg and for the "down" leg. These values also assume that aerobraking is used to slow the payload on the "down" leg.

Table 2: LEO to Lunar Surface Round Trip Delta-V's and "Surcharge" Different Station Locations

PathDelta-V "up" (km/sec)Delta-V "down" (km/sec)Total Mass "Surcharge"
LEO to GEO to LEO4.332.065.00
LEO to L1 to LEO3.770.772.69
LEO to GEO to Lunar Surface to GEO to LEO8.255.9877.09
LEO to GEO to Lunar Surface to LEO8.252.7429.06
LEO to L1 to Lunar Surface to L1 to LEO6.293.2914.47
LEO to L1 to Lunar Surface to LEO6.292.7410.59

To place this in one final perspective, one can consider the cost to move a crew capsule from LEO to one of these other station locations. NASA recently considered a four person capsule for use in lunar exploration. This capsule, without its required propulsion systems, has an estimated mass of 7.5 metric tons. ESA is also examining a capsule for crew transfers that has an estimated mass of 10 metric tons. If the L1 station is used as the example, the 7.5 metric ton NASA capsule would require an additional 20.2 metric tons of propellant, tankage, and engines to reach this station and return; a total of 27.7 metric tons delivered to LEO. The 10 metric ton ESA capsule would require an additional 26.9 metric tons of propellant, tankage, and engines to reach this station and return; a total of 36.9 metric tons delivered to LEO. Table 3 provides payload to LEO capability for several different launch vehicles.

As can be seen, at least two launches of currently available launch vehicles would be required to lift either of these capsules. The three heavy lift launch vehicles at the bottom of the table would not have any difficulty with either spacecraft. Mounting a lunar surface mission, assumed to consist of the capsule mentioned above plus a habitat and/or some amount of additional cargo, will require two or more of these heavy lift launch vehicles or many more launches of the small launch vehicles.

Table 3: Payload to Orbit Capability of Several Launch Vehicles

Launch VehiclePayload to 300 km Circular Orbit
(metric tons)
Zenit14
Ariane V15
Proton19
Space Shuttle22
Shuttle "C"71
Energiya95
Saturn V100

Other Considerations

Launch Window. There are two launch windows associated with the L1 station that must be considered: launch from the Earth's surface and departure from the L1 station for an interplanetary trajectory. For the first situation there is roughly a once-per-day launch opportunity to the L1 point from any launch site on the Earth, if no other constraints are placed on the trajectory. Launches to interplanetary departure trajectories will not be any worse than current constraints placed on these missions.

Accessibility of Lunar Surface Sites. Once at the L1 point, any site on the lunar surface becomes accessible for roughly the same delta-V cost. This implies that sites at all latitudes are accessible to vehicles departing from the L1 point. Depending on the trajectory, one-way transit times are approximately one day in length. There is also the great advantage of no launch window or return window constraints resulting from trajectory considerations. That is, a vehicle can depart from the L1 point at any time to reach any desired location on the surface and returning vehicles can leave the surface at any time. (If lighting constraints are imposed on landing, such as was the case for Apollo, then a twice per month opportunity results.)

Aborts and Rescues. Because the location of the L1 point is outside of the Moon's sphere of influence, the basic trajectory used to reach it and return will be a simple ellipse. Thus in case of an engine failure or other situation causing an abort, the trajectory will return the vehicle to the vicinity of Earth. If an Earth atmosphere entry was the planned mode of return, the vehicle can be targeted for this entry despite a main propulsion system failure. The round trip time will be on the order of a week, implying that critical systems must function for at least that period of time.

As mentioned above, there are no launch window or lunar landing site latitude constraints on a trajectory between the L1 point and the Moon. This implies that if a serious problem should develop for a crew on the surface, they are no more than a day's flight from the L1 station. Conversely, the lunar surface may be a better location to find shelter in the event of a solar flare, again being only approximately one day's travel from the L1 station as compared to the three to five days travel to Earth.

Space Station ˝

Lemke[12] has looked at a transportation node at L1. His views are heavily weighted toward support of passengers and traffic during the buildup of lunar surface operations and human planetary exploration. By the year 2050, he envisions a facility of mass 1180 metric tons supporting a flow of 90 passengers per month. Our current concept is more modest in scope and could be an early stage of Lemke's large operations center.

We see Space Station ˝ beginning as a research base and evolving into a transportation node. Most of all, we want to emphasize that any space station ought to be an integral part of a strategic plan for human exploration. To explore what that plan might be, we note that a commitment to a Space Station ˝ will immediately change some important parameters of the human exploration capability.

Support of the L1 operation will require a new vehicle capable of supporting a crew capsule similar to the ones described in the previous section. From the tables we can see that the new vehicle must have a lift capacity of at least 35 Mg into LEO. Most scenarios for buildup of a space station or a lunar base assume that the transportation system can deliver payloads on the order of 20 Mg, approximately the mass of a space station habitat module. Such a requirement boosts our new launch vehicle to a capacity on the order of 70 to 80 Mg to 300 km LEO, placing it at the lower end of the Energiya or Saturn V class.

The most important innovation in the vehicle must be low operating cost. The support teams must number no more than a few hundred rather than many thousands. Undoubtedly, some efficiencies of performance will be sacrificed for high reliability. Even conceding some loss of performance, the missions described here fall comfortably within the known Saturn V or Energiya technology.

Since Space Station ˝ sits in a politically neutral location in space, it can be supported from many countries. The launch vehicle development can take place under an international consortium of aerospace companies for an international market. With Energiya-class launch capabilities routinely available in several countries, the possibilities for space exploration and development multiply. In other words, the existence of Space Station ˝ will be a catalyst for many new ventures unrelated to the space station program itself.

We suggest consideration of a design which controls the station-keeping and the attitude with solar radiation pressure as discussed by Farquhar[11]. This strategy has implications for radiation protection, propellant usage, and the power environment.

Consider a 100 Mg station continuously positioned with a sail. From Farquhar's calculation of the specific area at L1 of 0.374 m2/kg, the sail would be approximately 200 m on a side. The sail could be composed of solar arrays, generating power for the station while being used to control position and attitude. With proper configuration design the sail would serve as a sunshade for thermal control of the station. If the sail were covered by inexpensive, radiation-resistant alpha-Si solar cells, it would generate approximately 4 MW and have a mass of 6.5 Mg (92 W/m2, 613 W/kg)[13]. The solar array/sail would cover almost 2š steradians of the sky from the point of view of the station structure and would be part of the radiation shielding strategy from solar particles.

The configuration of the station would not be constrained by aerodynamic considerations because drag is not a problem in the L1 environment. A grouping of facilities roughly radially about a central core might be appropriate. An extended structure such as Space Station Freedom would have varying moments of inertia which might complicate the orbital station-keeping and attitude control with a solar sail. A central core surrounded by structural mass would also be part of a safe haven philosophy for solar particle events.

A space-based orbital servicing vehicle would transport crew to and from co-orbiting telescopes, microgravity research facilities, and solar electric interplanetary vehicles. Telescopes orbiting the L2 point would also be accessible. As Lemke[12] points out, the L1 station eliminates the need for an lunar orbit station in a lunar transportation system. The high energy cost associated with supply of the station would encourage development of lunar resources for propellant and shielding mass.

For people on the Earth, Space Station ˝ would stand out as a small star near the full Moon. It would symbolize international cooperation in space and the expansion of life into the cosmos.

Conclusions

We are becoming more aware that low Earth orbit is a fragile environment, threatened by the growing population of artificial space debris. The high velocity orbiting particles add one more hazard to human presence in an already hostile space environment. The large structures associated with human presence also exacerbate the debris problem in the long term.

Low Earth orbit is less than ideal for many proposed research objectives of a space station. The reactive plasma environment chemically attacks spacecraft materials, produces aerodynamic drag which forces continual reboost of a station, and interferes with research requiring very high vacuum free of contamination. The nearby Earth makes astronomical studies difficult. The low orbital inclination required for optimum launch support frustrates global Earth observation. Efficient interplanetary transportation with low-thrust spacecraft is not really possible from a location so deep in the terrestrial gravity well.

Artificially generated debris cannot accumulate in the vicinity of the colinear libration points of the Earth-Moon system. The L1 point can support large scale human activity with no environmental damage. The location is also more suitable than LEO for many major research objectives associated with a staffed space station.

A major obstacle to establishing Space Station ˝ at L1 is the lack of an operating launch vehicle to support it. However, routine and reliable access to space is an international problem suitable for an international collaborative solution in support of a truly international human space program. The required lift capacity has been developed in two separate vehicles in two countries. The real challenge is to refine the technology to create a vehicle which requires only 100 people for launch support rather than 10,000. An international vehicle developed to support Space Station ˝ could be operated out of several countries because the L1 location is accessible from everywhere on Earth. Such a vehicle would transform the whole face of space development as a side effect.

The most important facet of planning a strategic approach to human space exploration is to ensure that the space infrastructure elements are fully integrated into a long term vision. A space station is an important statement about establishing permanent human presence in space and should fit naturally into future efforts to settle the Moon and explore Mars. Space Station ˝ is a logical and integral part of the human conquest of space.


References

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