Like the International Space Station (ISS), Gateway is modular. And like ISS, it is designed to expand over time, with each module adding function and capacity. The first two elements, the Power and Propulsion Element (PPE) and the Habitation and Logistics Outpost (HALO), will launch together on a commercial vehicle.
The PPE is a sixty-kilowatt solar electric ion propulsion spacecraft that will furnish power, communications, attitude control, and orbital transfer capabilities. This technology ionizes a noble gas to propel the vessel. Instead of generating thrust with expanding gases from exothermic chemical reactions, xenon, which is like neon or helium but heavier, is ionized. It is then electrically accelerated to a speed of about thirty km/second, providing thrust.
The module will also provide high-rate and reliable communications for the Gateway including space-to-Earth and space-to-lunar uplinks and downlinks, spacecraft-to-spacecraft crosslinks, and support for spacewalks. Finally, it can accommodate an optical communications demonstration, using lasers to transfer large data packages at faster rates than traditional radio frequency technology.
HALO is where astronauts will live and conduct research. The pressurized living quarters will house command and control systems for the outpost, and three docking ports for visiting supply or personnel spacecraft, or attachment points for additional modules (PPE, ERM, I-HAB and HLS). The module will distribute power across Gateway, will host science experiments, and communicate with lunar surface expeditions. The Orion crew delivery capsule will dock first to HALO, then to I-HAB after its attachment at a later date.
Batteries provided by the Japan Aerospace Exploration Agency (JAXA) will power HALO until PPE solar arrays are deployed, then during eclipse periods. Interfaces supplied by the Canadian Space Agency will handle payloads and provide basepoints for a robotic arm. The European Space Agency (ESA) will supply a communications system to enable high-data-rate transmissions between the lunar surface and Gateway.
The first three science instruments to be housed in the module have already been selected. Two of them, the Heliophysics Environmental and Radiation Measurement Experiment Suite (HERMES) and the European Radiation Sensors Array (ERSA), will be attached to Gateway’s exterior. HERMES will monitor lower energy solar particles, including the solar winds. ERSA, led by ESA, will monitor high energy particles with a focus on solar storms.
The Internal Dosimeter Array (IDA) will study radiation shielding effects inside HALO and improve radiation physics models for cancer, cardiovascular and central nervous systems, assessing crew risk on exploration missions. IDA is being built by ESA, with additional science instruments from JAXA.
NASA selected Maxar Technologies of Westminster, Colorado, to develop and build the PPE. Northrop Grumman of Dulles, Virginia was awarded contracts for the preliminary and final HALO designs. NASA contracted SpaceX to provide launch services for the integrated PPE and HALO spacecraft. After integration on Earth, launch will occur no earlier than November 2025 on a Falcon Heavy rocket from Launch Complex 39A at Kennedy Space Center.
NASA has let two Human Landing System (HLS) contracts to Blue Origin and SpaceX, Blue Moon and Starship, respectively. These craft will dock directly with HALO before their descents to the surface, then re-dock prior to their return flights.
ESA signed an agreement with NASA to contribute habitation and refueling modules, enhanced lunar communications to the Gateway and two more Orion Service Modules. The International Habitat (I-HAB) will arrive at Gateway with pre-occupation supplies and equipment, then provide expanded living quarters once inhabited. After connection to HALO, I-HAB will have three docking ports for Orion, the Logistics Module (LM), and one spare.
Japan will provide several capabilities to NASA for I-HAB. JAXA’s planned contributions include I-HAB’s environmental control and life support system, batteries, thermal regulation, and imagery components, which will be integrated into the module by ESA prior to launch. These systems are critical for sustained outpost operations during crewed and uncrewed time periods.
The Esprit Refueling Module (ERM) will also be provided by ESA. It houses the fuel tanks used to refuel PPE. The ERM will feature crew observation windows.
Canada signed an agreement with NASA to provide advanced robotics. The Gateway External Robotics System (GERS) is a next-generation smart robotic arm, Canadarm3. It will move end-over-end the entire length of Gateway’s exterior, where its anchoring hand will plug into combined power, data, and video interfaces.
GERS will deploy in 2028. It will repair and inspect the space station, capture visiting vehicles, reconfigure the space station by relocating modules, and help astronauts during spacewalks. Canadarm3 will be able to maintain itself in space and swap out parts. Its AI will allow it to plan its missions, optimize resources and monitor system performance. Designed to work autonomously, the arm could also be operated remotely from Canada, or by onboard crew.
The final planned module for Gateway is an Airlock. No contract has been let yet for a supplier. However, it will be similar to the Nanoracks Bishop Airlock on ISS, allowing transfer of internal personnel/materiel into space and visa-versa. Material transfers in or out of the airlock will be accomplished by the GERS, reducing the need and complexity of astronaut spacewalks.
At present, NASA has defined Gateway missions for assembling modules or to directly support Artemis missions in 2024, 2025, and 2028, then yearly thereafter. But don’t be too surprised to see other Artemis Accord space agencies or countries purchase time aboard the space outpost for their own missions. I expect the same will happen for private companies with commercial and/or space tourism flights either to the station or the lunar surface. Once a permanent lunar base is established, shuttles between the Moon and the space station will become commonplace.
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Want a deeper dive? Check out these sources.
https://www.nasa.gov/feature/nasa-s-lunar-outpost-will-extend-human-presence-in-deep-spacehttps://www.nasa.gov/gatewayhttps://www.nasa.gov/gateway/overviewhttps://www.nasa.gov/specials/artemis-accords/img/Artemis-Accords-signed-13Oct2020.pdf 
