The Earth-moon and Sun-Earth Libration (L) points are not places in the sense that one can land on them and pick up rocks. Because of this, many space exploration planners have perceived them to be unsatisfying destinations. The L points have, however, long been proposed as space transportation way stations and scientific instrument sites.
In 1999, the Decadal Planning Team (DPT), a NASA-wide study group chartered by President William Clinton's Office of Management and Budget, identified astronomical observatories in "halo orbits" around the Sun-Earth L points as a key NASA goal for the early 21st century. These large and complex instruments would, among other tasks, seek to image Earth-like worlds around other stars. The NASA Exploration Team (NExT), the DPT's immediate successor, subsequently sought to incorporate the Sun-Earth L point emphasis into its piloted spaceflight planning. In a December 20, 1999 presentation to the NeXT, for example, NASA Johnson Space Center exploration planner Bret Drake examined ways that the Sun-Earth L points might aid future piloted Mars missions.
An automated solar observatory orbiting the Sun-Earth L1 point, 1.5 million kilometers from Earth, could provide Mars crews with early warning of solar flares, Drake explained. Radio relays in halo orbit about Sun-Earth L4, 60° ahead of the Earth along its Sun-centered orbit, and Sun-Earth L5, 60° behind the Earth along its orbit, could enable continuous radio communication between Earth and crews exploring Mars during superior conjunctions, when the Sun blocks line-of-sight contact between the two planets.
Drake hastened to add that the Sun-Earth L points are not good staging places for Mars expeditions. This is, he explained, because a trip to Mars via a Sun-Earth L point would need more propulsive energy and last about two months longer than one launched from low-Earth orbit (LEO).
Piloted missions to Sun-Earth L points might, however, serve as Mars mission precursors, he suggested. Such missions could provide experience with interplanetary conditions (for example, solar radiation undiminished by Earth's magnetic field), yet would have one-way trip times as short as 25 days. They might thus serve as experience-building intermediate steps between flights in LEO and Mars voyages.
Drake proposed that NASA conduct a 100-day telescope-servicing mission to Sun-Earth L2, 1.5 million miles from Earth, that would employ Solar-Electric Propulsion (SEP) technologies and techniques first proposed in 1998 for NASA's Mars Design Reference Mission. The Sun-Earth L2 mission would begin with the unmanned launch to LEO of a 32,975-kilogram telescope-servicing spacecraft comprising a 14,450-kilogram inflatable "mini-Transhab," a 4271-kilogram Apollo Command Module-shaped Earth Return Vehicle (ERV), and a 14,164-kilogram two-stage chemical propulsion module. It would reach LEO under the streamlined shroud of a next-generation expendable rocket, an Evolved Expendable Launch Vehicle-Heavy (EELV-H).
A Space Shuttle Orbiter would then rendezvous with the telescope-servicing spacecraft in LEO. The astronauts would oversee inflation of its donut-shaped single-deck mini-Transhab, then would install equipment, extend the spacecraft's twin electricity-generating solar arrays, and load it with supplies. They would then return to Earth.
A second EELV-H would place a 33,000-kilogram automated SEP Vehicle into LEO, where it would deploy solar array wings and dock with the uncrewed telescope-servicing spacecraft. Over the next seven months, it would operate its thrusters at perigee (the low point in its orbit about the Earth) to raise its apogee (the high point in its orbit). The result would be a highly elliptical orbit loosely bound to the Earth. The SEP Vehicle would then detach from the telescope-servicing spacecraft and return to LEO for refurbishment and reuse.
Drake inserted into his telescope-servicing mission sequence an optional piloted mission that would fly only if the telescope-servicing spacecraft needed repairs following the seven-month SEP Boost Phase. A Shuttle Orbiter would deliver to LEO a servicing crew, a small lifting-body Crew Taxi, and a chemical-propulsion rocket stage. The stage would push the Taxi out of LEO. The crew would then rendezvous and dock with the telescope-servicing spacecraft. After completing the needed repairs, they would undock in the Taxi, fire its rocket motors at apogee to lower its perigee into Earth's atmosphere, perform reentry, and glide to a landing.
If no repairs were needed, however, then the Crew Taxi would deliver the four-person telescope-servicing crew. After casting off the Taxi, they would ignite the telescope-servicing spacecraft's first chemical-propulsion stage at perigee to boost their apogee and begin the 25-day voyage to Sun-Earth L2.
In the Sun-Earth L2 Operations Phase, the telescope-servicing spacecraft would enter a "halo parking orbit" centered on Sun-Earth L2. For 50 days the astronauts would service large next-generation telescopes in halo orbits around Sun-Earth L2, much as Space Shuttle crews in 1993, 1997, 1999, 2002, and 2009 serviced the Hubble Space Telescope in LEO. Drake suggested that during their down time between servicing calls they could conduct unspecified scientific research.
Their mission completed, the astronauts would ignite the telescope-servicing spacecraft's second chemical-propulsion stage to begin their return to Earth. About 25 days later, they would strap into the ERV capsule, undock from their home of the previous 100 days, reenter Earth's atmosphere, and parachute to a landing. The remaining components of the telescope-servicing spacecraft would burn up in Earth's atmosphere.
Reference:
Representative Human Missions to the Sun-Earth Libration Point (L2) "100" Day Class Mission, SEL2 Ver. R, Bret G. Drake, NASA Johnson Space Center, presentation materials, December 20, 1999.
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