Mission planners for the lunar Gateway project have decided how the lunar outpost should orbit the Moon—and it’s actually quite brilliant.
They’ve chosen a near-rectilinear halo orbit.
This highly elliptical orbit should solve a bunch of problems, making it easy for astronauts to embark on missions to the lunar surface and for the outpost to receive supplies from Earth, among other things, according to mission planners from NASA and the European Space Agency (ESA), which announced the decision in a press release today.
The team “spent months debating the pros and cons of different orbits,” noted ESA, with the near-rectilinear halo orbit, or NRHA, getting the final and definitive thumbs up. Indeed, it was a critically important decision given the requirements of the Gateway project and the demands that will be placed on the lunar outpost, a collaborative project involving NASA, ESA, the Canadian Space Agency (CSA), Roscosmos, the Japanese Space Agency (JAXA), and other international partners.
Slated for completion by the mid 2020s, the lunar Gateway will serve as a staging post for crewed missions to the Moon, namely NASA’s upcoming Artemis program, which aims to place a man and woman on the lunar surface by 2024. The orbital station will provide a short-term place for astronauts to stay, a laboratory to conduct scientific research, a depot to stock up on supplies and fuel, a hub for relaying communications, and a base to dispatch astronauts, robots, and other supplies to the lunar surface. Eventually, the base could be used as a staging post for a crewed mission to Mars.
The demands of the Artemis project will require Gateway to be close enough to the lunar surface for quick and easy access, but not so far from Earth to make transportation to and from the base a protracted ordeal, among many other practical and logistical concerns.
And that’s where a near-rectilinear halo orbit will help. Once in NRHA, Gateway will follow a highly elliptical path that will carry it to within 3,000 kilometers (1,865 miles) of the Moon during its closest approach (perilune), and to within 70,000 kilometers (43,500 miles) of the lunar surface when it reaches its farthest point, or apolune.
The gravitational interplay between Earth and the Moon makes this orbital configuration possible. As “the two large bodies dance through space, a smaller object can be ‘caught’ in a variety of stable or near-stable positions in relation to the orbiting masses, also known as libration or Lagrange points,” according to ESA. That said, NRHO does present some long-term challenges; in this type of orbit, objects will drift away from their hosts over time. Gateway will have to conduct regular adjustment maneuvers to prevent this from happening, according to ESA.
One complete orbit of the Moon will take about seven days. Seven happens to be a particularly happy number because the Artemis mission is aiming to keep astronauts on the lunar surface for one full week (by comparison, Apollo 17 astronauts, the longest of the Apollo missions, stayed on the Moon for three days). This orbital length will also minimize the number of eclipses experienced by Gateway, reducing the amount of darkness produced by shadows from Earth and the Moon—an important consideration, given that Gateway will run on solar power.
In this orbital configuration, it should take about five days for spacecraft from Earth to reach Gateway. That’s a bit longer than the three-day journeys taken by Apollo astronauts, but a near-rectilinear halo orbit will be a good way to conserve energy. A moderate maneuver will be required to decelerate spacecraft rendezvousing with Gateway, providing a more cost-effective and energy-efficient way of delivering equipment, supplies, parts, and personnel to the outpost.
“In human spaceflight we don’t fly one single, monolithic spacecraft,” explained Florian Renk, a mission analyst at NASA and ESA’s Operations Centre (ESOC), in a statement. “Instead we fly bits and pieces, putting parts together in space and soon on the surface of the Moon.”
Gateway will slowly come together over the course of several years. NASA will use a combination of private rockets and its own Space Launch System (SLS) rocket to deliver the components required to build the lunar outpost. As we learned yesterday, however, NASA will likely push the inaugural launch of SLS from 2020 to 2021, as Ars Technica reported.
That’s not great news for a program that’s slated to put people on the Moon a scant three years later, but that’s the timeline NASA has been ordered to deal with. Sadly, politics is becoming a large part of the equation, as NASA struggles to deal with aggressive due dates and budgetary ambiguities. Hopefully sense will prevail such that the 2020s will prove to be as exciting for human space exploration as we’re expecting.
Correction: A previous version of this post incorrectly used the terms perihelion and aphelion, which are specific to objects orbiting around the Sun. The correct terms in this case are perilune and apolune. We regret the error. Thanks to commenter Brianorca for pointing this out.