Like the International Space Station, the Gateway will be a permanent and changeable human outpost. Instead of circling our planet however, it will orbit the Moon, acting as a base for astronauts and robots exploring the lunar surface.
Like a mountain refuge, it will also provide shelter and a place to stock up on supplies for astronauts en route to more distant destinations, as well as providing a place to relay communications and a laboratory for scientific research.
Mission analysis teams at ESOC are continuing to work closely with international partners to understand how this choice of orbit affects vital aspects of the mission – including landing, rendezvous with future spacecraft and contingency scenarios needed to keep people and infrastructure safe.
The angelic halo orbit
The Gateway, it has recently been decided, will follow a ‘near-rectilinear halo orbit’, or NRHO. Instead of orbiting around the Moon in a low lunar orbit like Apollo, the Gateway will follow a highly ‘eccentric’ path. At is closest, it will pass 3000 km from the lunar surface and at its furthest, 70 000 km. The orbit will actually rotate together with the moon, and as seen from the Earth will appear a little like a lunar halo.
Orbits like this are possible because of the interplay between the Earth and Moon’s gravitational forces. 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.
Such locations are perfect for planning long-term missions, and to some extent dictate the design of the spacecraft, what it can carry to and from orbit, and how much energy it needs to get – and stay – there.
Travelling on the NRHO path, one revolution of the Gateway in its orbit about the Moon would take approximately seven days. This period was chosen to limit the number of eclipses, when the gateway would be shrouded by the Earth or Moon’s shadow.
“Finding a lunar orbit for the gateway is no trivial thing.” says Markus Landgraf, Architecture Analyst working with ESA’s Human and Robotic Exploration activities.
“If you want to stay there for several years, the near rectilinear halo orbit is slightly unstable and objects in this orbit do have a tendency of drifting away”.
To keep the Gateway in position, regular small station-keeping manoeuvres will be required.