|Interplanetary space showing the positions of the Sun, Earth, Earth's orbit about the Sun, the Moon, the Moon's orbit about the Earth, and the five Earth-Sun Libration Points. Image credit: NASA.|
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, some space exploration planners perceive them to be unsatisfying destinations. The L points have, however, long been proposed as space transportation way stations and as radio relay and scientific instrument sites.
In 1999, the Decadal Planning Team (DPT), a secretive 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 observe 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 20 December 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 radio contact between the two planets.
Drake hastened to add that the Sun-Earth L points would not be good staging places for Mars expeditions. He explained that the trip to and from a Sun-Earth L point would add almost two months to the typical duration of a roundtrip Mars voyage that started from low-Earth orbit (LEO).
Piloted missions to Sun-Earth L points might, however, serve as experience-building intermediate steps between piloted LEO and Mars missions. Drake suggested that L point missions could enable astronauts to experience interplanetary conditions (for example, solar radiation undiminished by Earth's magnetic field), yet would have one-way trip times as short as 25 days.
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" crew module, a 4271-kilogram Apollo Command Module-shaped Earth Return Vehicle (ERV), and a 14,164-kilogram two-stage chemical propulsion module. The spacecraft would reach LEO within the streamlined shroud of a next-generation expendable rocket called an Evolved Expendable Launch Vehicle-Heavy (EELV-H).
A Space Shuttle Orbiter would then rendezvous with the telescope-servicing spacecraft in LEO. Shuttle astronauts would oversee inflation of its donut-shaped single-deck mini-Transhab, install equipment, extend its 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 maintenance following the SEP Boost Phase. A Shuttle Orbiter would deliver to LEO a maintenance crew, a small lifting-body Crew Taxi, and a chemical-propulsion rocket stage. The stage would push the Taxi out of LEO.
The maintenance crew would 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, the Crew Taxi would deliver a four-person crew to the telescope-servicing spacecraft in elliptical Earth orbit. 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. They would then cast off the stage.
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 might 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 other components of the telescope-servicing spacecraft would burn up in Earth's atmosphere.
Even as Drake presented his Earth-Sun L1 servicing mission concept, NASA engineers conceived of a Gateway space station in halo orbit about Earth-Moon L1 as a base for observatory servicing and as a stepping stone to points all over the lunar surface. They envisioned that observatories needing servicing would ignite small thrusters to begin a slow transfer from their Earth-Sun L1 and L2 halo orbits to the vicinity of the Gateway. Once at Earth-Moon L1, they would be serviced by spacewalking astronauts, "cherry picker" booms, and teleoperated systems.
|Cislunar space showing the positions of Earth, the Moon, the Moon's orbit about Earth, and the five Earth-Moon Libration Points. Image credit: NASA|
In January 2004, in the aftermath of the STS-107 Columbia Space Shuttle accident and at the start of the 2004 election cycle, President George W. Bush called for a new NASA program to take humans to the Moon and Mars. At first, the Vision for Space Exploration (VSE), as it became known, incorporated many elements of DPT/NExT. Soon after Michael Griffin became NASA Administrator on 13 April 2005, however, the VSE veered away from DPT/NExT and toward the Constellation Program, which Griffin called "Apollo on steroids." Bush showed little interest in the VSE after he announced it, so did not intervene to keep his program on track.
Never popular, Constellation and the VSE were mostly abandoned in 2009-2010 under President Barack Obama. The global economy was in crisis following the collapse of the U.S. housing market in 2008 and the near-collapse of the global financial system. Spaceflight, rarely a high priority, took a back seat to repairing the U.S. economy.
When Obama unveiled a new space plan in 2010, it resembled DPT/NExT more than Constellation. The Bush Administration's decision to cancel the Space Shuttle led to the most significant deviation from the DPT/NExT architecture: retention of Constellation's large rocket under the name Space Launch System. Resembling an oversized EELV-H, SLS replaced the Shuttle Orbiter and the solar-electric tug of the DPT/NExT plan. The Orion Crew Exploration Vehicle (CEV) replaced the lifting-body taxi.
Meanwhile, China launched its program to explore the Moon using robots. Chang'e 1 orbited the Moon in 2007-2009; Chang'e 2 orbited the Moon in 2010-2012 before leaving lunar orbit for a flyby of the Near-Earth Asteroid 4179 Toutatis; and Chang'e 3 landed on the Moon in late 2013.
Chang'e 4, targeted for the lunar farside hemisphere, landed successfully in January 2019. It transmits radio signals to Earth via the Queqiao satellite, which reached a halo orbit around Earth-Moon L2 in June 2018. In addition to relaying signals from Chang'e 4 and its rover to Earth, Queqiao also serves as a radio observatory remote from the radio noise of Earth.
|A radio-relay satellite in Earth-Moon L2 halo orbit enables communication with spacecraft out of line-of-sight radio contact on the hidden farside hemisphere of the Moon. Image credit: NASA|
Solar Flares and Moondust: The 1962 Proposal for an Interdisciplinary Science Satellite at Earth-Moon L4 - The product of a collaboration between an orbital dynamicist and a lunar geologist.
Lunar GAS (1987) - From a Space Shuttle payload bay to lunar polar orbit by electric propulsion.
"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, 20 December 1999.