Although NASA has been studying Lagrange-point missions for at least a decade, the idea gained new momentum in August 2011, when William Gerstenmaier, NASA’s associate administrator for human exploration and operations, created a special working group to examine NASA’s human spaceflight options. Affordability was a primary requirement, and for space mission planners, a sure way to lower launch costs is to cut down on the consumable supplies, especially fuel, a crew needs to take along. A principal advantage of any Lagrange point is that once a spacecraft arrives, very little energy is required to keep it there. “I’m using the gravity properties to minimize the fuel requirements,” Gerstenmaier says.
The L2 point is about 15 percent farther than the Apollo astronauts traveled. The quickest way to get there—in only three to five days—is to use the Apollo approach: take aim and fire. This, however, requires extra fuel. So NASA would prefer to take a slower and more circuitous route, using a gravity assist from the moon to hurl Orion to its destination.
“This is a trick that NASA has been using in robotic missions for the last 40 years, but it would be the first time for human spaceflight,” says University of Colorado space scientist Jack Burns, director of the NASA-funded Lunar University Network for Astrophysics Research, who has been studying options for L2 missions. “We’re talking a week to get to L2 because of the gravity assist and the resulting trajectory.”
By early last year, Gerstenmaier’s in-house NASA group had worked up a PowerPoint prospectus on a Gateway mission, emphasizing its value as a training ground for deep space. Astronauts would gain critical experience operating their spacecraft in a new and unfamiliar environment beyond Earth orbit, would use dosimeters to monitor radiation levels, and would work more independently of Earth-based controllers. All this could be done from Orion, which will be about two and a half times roomier than the Apollo spacecraft and weigh nearly 11 tons, almost twice Apollo’s weight. With its European-built service module, it will carry four astronauts instead of Apollo’s three.
For an L2 mission, Orion wouldn’t simply fly to the far side of the moon and park at the Lagrange point. Instead, the spacecraft would be placed in a halo orbit around L2. “You set it up so you can always see the Earth,” says Josh Hopkins, Lockheed Martin’s space exploration architect and a close collaborator with the Burns team in Boulder. Hopkins describes the preferred orbit as asymmetrical, its dimensions and shape dictated by the changing gravitational forces at L2. The orbit would have a radius of 22,000 miles east to west, and 3,000 to 6,000 miles north to south. It would not quite lie in a flat plane, Hopkins adds. “It looks like a Pringles potato chip.”
A spacecraft in this orbit requires very little fuel compared with the space station, which needs regular burns to overcome the drag of Earth’s atmosphere, and occasional tweaks to steer it past space junk. At L2, the only real problem would be a tendency for the spacecraft to drift. “If you start out a little behind L2, the moon pulls you back,” Hopkins says. “You need very small bursts of energy to keep station.”
This, Gerstenmaier says, “may be more complicated than it looks.” An L2 orbit “is a little bit of a trade-off,” he explains. “From a propellant standpoint, it’s easy, but we don’t have a lot of experience navigating around a gravity point.”
Besides mastering the orbital mechanics of deep space, the Gateway astronauts would have work to do. In a recent paper for the journal Advances in Space Research, the Burns group described some possibilities, beginning with a first-ever comprehensive robotic exploration of the far side of the moon. This mission would begin with the landing of an unmanned rover in the South Pole–Aitken basin, a favored target for geologic exploration because it offers a broad swatch of lunar history. The basin includes some of the oldest rocks on the moon, but there also is evidence of more recent volcanism.
Orion, with astronauts on board, would arrive at L2 after the rover lands. With a signal delay of only 0.2 second (it takes about two seconds for a signal to travel from Earth to the moon), astronauts could steer the vehicle in near-real time. “We’re talking about a surface rover that can move kilometers in a day instead of meters,” Burns says.