The Earth-Moon L1 point, illustrated above, may be the logical starting point for any “flexible path” strategy of human space exploration. (credit: NASA)

First stop for Flexible Path?

by Dan Lester
Monday, November 30, 2009

While the ink is barely dry on the Review of US Human Space Flight Plans (aka Augustine) Committee’s final report, and the White House and NASA are pondering the future of human space flight for the United States, one strategy for doing it that was proposed by that committee has garnered a lot of attention. The flexible path—or flexpath strategy, as it is more compactly known—is less a detailed plan and more a guiding philosophy for future work. Guided by this philosophy, humans would visit sites never visited before and extend our knowledge of how to operate in space, while traveling greater and greater distances from Earth, as opposed to efforts directed at a single solar system body, such as the Moon or Mars.

The flexibility is in response to technological advances. Such flexpath destinations could well include these individual bodies, but also opportunistic sites bereft of deep gravity wells such as NEOs, Lagrange (libration) points in the vicinity of the Earth, and eventually a Martian moon to be used as a teleoperations center for work on the Red Planet. It is recognized that both the flexpath strategy and a strategy directed wholly towards the Moon, as is the baseline plan, were both considered by the Augustine committee to be exploration strategies that were viable.

While the arguments for and against these strategies have been vociferous, it is not the purpose of this essay to consider the relative value of the two. Instead, we ask a more focused question. If the flexible path strategy is going to guide our future efforts in deep space, where do we go first?

If not a return to the surface of the Moon, a trip to a NEO has been raised as a potential near-term opportunity that would be implementable with planned elements of the Constellation architecture. Such a voyage would exercise many capabilities for operations in deep space, and bring back information about and presumably even samples from at least one such object that might someday constitute a serious threat to humanity. Such an object might also even be a source for future resource development. The relative scientific advantages of such a human journey to a NEO versus a robotic mission are not entirely clear, though putting humans on such an object would certainly exercise our abilities in deep space with regard to propulsion, navigation, and proximity operations, as well as human factors, and be more generally a monumental challenge for our nation that would bring the kind of pride that the Apollo missions brought. Such a mission would allow humans to look at, as well as touch, a new world that is highly relevant to our aspirations.

A trip to a NEO is, however, a major step for a space program that has not ventured beyond LEO for almost forty years. The trip would involve many weeks of travel in a small and poorly shielded capsule. It would furthermore be to an object that can be considered unfriendly, in many respects, especially with regard to what little we might know about it beforehand, if not the almost certain complexity of proximity operations in a highly non-isotropic gravitational field. The schedule windows for launch and return are quite small as well. All of these factors add mission risk.

It has also been suggested that a loop around the Moon would be a straightforward way to break out of LEO, though the technological sophistication of that is not substantially different than when we first did it with Apollo 8. Such a misson would be strongly perceived by the American public as a “been-there-done-that” one.

Another possibility, which we address here, is a human trip to an Earth-Moon Lagrange point. Of course such a voyage would not be to a specific physical point, but to an orbit around a locale that is defined by gravitational potential energy contours. Nevertheless, such orbits require a precision in navigation, propulsive insertion, and stationkeeping that rivals that of rendezvous and landing on a rocky body. We could consider such a destination a precise location, not in Cartesian space, but in Lagrangian phase space in the restricted three-body problem! The Earth-Moon first Lagrange point (L1) is a highly accessible venue for a lunar-capable space program, and has rich promise for a number of important activities. Many of these opportunities were mapped out in detail by the NASA Decadal Planning Team about a decade ago (see “Forging a vision: NASA’s Decadal Planning Team and the origins of the Vision for Space Exploration”, The Space Review, December 19, 2005), and were also considered in the ESAS study.

Earth-Moon L1 is about 85 percent of the way to the Moon along the Earth-Moon line. From this location, the Moon appears about seven times larger than it does from the Earth, and the Earth appears about 50 percent smaller than that. This Lagrange point is not a potential well, so small stationkeeping maneuvers are required to keep from drifting away. But such maneuvers are relatively economical of propellant. We have actually never sent a spacecraft to an Earth-Moon Lagrange point orbit, though many vehicles have been sent to Earth-Sun L1 and L2, such as ACE, SOHO, and WIND, which now operate around the former; and WMAP, Herschel, and Planck, which now operate around the latter. The first mission to be deployed to a Lagrange point was the pioneering ISEE-3, which, after residing at Earth-Sun L1 for several years, was redeployed to intercept several comets.

The goal of the flexpath strategy, to learn how to operate in space, would be well served by such a mission to Earth-Moon L1. This locale (and L2, on the other side) have been widely understood to have important potential as materiel depot sites for lunar exploration, whether materiel from the Earth that would be equally available to all possible lunar exploration sites (unlike a depot in low lunar orbit), or in situ resource utilization (ISRU) material from the Moon that would be made available for voyages beyond the Moon. It is noteworthy that properly shielded from the Sun, with the solid angles of Earth and Moon being so small, the Earth-Moon Lagrange points are pretty cold places, so long-term storage of cryogenic propellants would be especially tractable there.

Such Lagrange points are connected with other Lagrange points in the solar system by extraordinarily small delta-V’s (the “interplanetary superhighway”, as termed by Martin Lo and Shane Ross), such that moving at least cargo from Earth-Moon L1 to anywhere else is pretty easy, though it may be time consuming. In particular, movement from Earth-Moon L1 to the scientific “high ground” at Earth-Sun L1 and L2 is just a matter of several tens of meters per second! That’s vastly less than what it would take to move into and out of LEO from these distant sites. In this respect, Earth-Moon Lagrange points can be considered jumping off points for solar system exploration. Earth-Moon L1 has been suggested as an optimal place to do whole-disk observations of the Earth and Moon, and even as a depot for Earth surveillance microsats that, from this location, can be easily deployed into any orbital inclination. Such a venue has also been noted as a secure quarantine site for planetary sample return.

For science spacecraft operating at the less accessible but more scientifically useful Earth-Sun Lagrange points, about four lunar distances away, the easy redeployment to Earth-Moon L1 can enable maintenance and servicing. In this picture, Earth-Sun Lagrange points are the distant “ops sites”, while Earth-Moon L1 is the more accessible “job site”. All of these uses for Earth-Moon L1 depend on competence in cis-lunar navigation, formation flying, proxops, and rendezvous and docking, skills that we’ll certainly need for a later NEO visit.

So let’s posit Earth-Moon L1 as the first destination for the flexpath strategy. One scenario would be to meet up with a target spacecraft preplaced there that is friendly by design, perhaps one that needs some work done on it. This would exercise many of the competencies that would be required to take full advantage of these Lagrange points. As for NEOs, in many respects, robotic missions could be considered for this destination as well.

Now, some might argue that the true spirit of “exploration” is not fulfilled by a visit to a Lagrange point. As was pointed out earlier in The Space Review, it unfortunately often takes rocks and at least a bit of gravity to validate exploration in the popular vernacular (see “Destinations for exploration: more than just rocks?”, The Space Review, July 16, 2007). It’s not easy to leave flags and footprints in free space. I would argue that the spirit of flexpath is well served by such a trip, however, and it may be time for the American public to develop a more mature attitude about real destinations for human space flight. There are only so many rocks around! While L1 is not a site where resources can be excavated, it is most definitely a site where such resources can be managed and distributed. The awesome dance of the Earth, Moon, and a spacecraft in a delicate orbit precisely rendezvousing with a target vehicle at L1 may communicate a conquest of gravity far better than would a vehicle landing on some large rock and kicking up dust as it fights that gravity.

Dan Lester is an infrared astronomer at the University of Texas and, most recently, headed the SAFIR Vision Mission Study. He has been active in NASA strategic planning activities, and writes regularly on space science policy.

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