1. Overview

LOP-G advances NASA's goals of sustaining human space exploration and serves as a platform to further cislunar operations, lunar surface access and missions to Mars. https://handwiki.org/wiki/index.php?curid=1603615

Originally, NASA had intended to build the Gateway as part of the now cancelled Asteroid Redirect Mission.^[1][2] An informal joint statement on cooperation between NASA and Roscosmos was announced on 27 September 2017.^[3] Traveling to and from cislunar space (lunar orbit) will help gain the knowledge and experience necessary to venture beyond the Moon and into deep space. The LOP-G would be initially placed in a highly elliptical near-rectilinear halo orbit (NRHO) around the Moon which will bring the station within 1,500 km (930 mi) of the lunar surface at closest approach and as far away as 70,000 km (43,000 mi) on a six-day orbit.^[4] This orbit would allow lunar expeditions from the Gateway to reach a polar low lunar orbit using 730 m/s of delta-v in half a day and also allow less than 10 m/s of delta-v to be required per year in stationkeeping.^[5] The Gateway could conceivably also support in-situ resource utilization (ISRU) development and testing from lunar and asteroid resources.^[6] The Gateway would offer the opportunity for gradual buildup of capabilities for more complex missions over time.^[7] Various components of the Gateway are to be launched on a commercial launch vehicle and on the Space Launch System as Orion co-manifested payloads on the flights EM-3 through EM-8.^[8] According to Roscosmos, they may also use Proton-M and Angara-A5M heavy launchers to fly payloads or crew.^[3]

The Power and Propulsion Element (PPE) for the LOP-G will have a mass of 8-9 t and be capable of generating 50 kW^[2] of solar electric power for its ion thrusters system for maneuverability, which can be supported by chemical propulsion.^[9] NASA stated that the most likely ion engine to be used on the PPE is the 14 kW Hall thruster called Advanced Electric Propulsion System (AEPS) still being developed by Glenn Research Center, the Jet Propulsion Laboratory, and Aerojet Rocketdyne.^[10] Four identical AEPS engines would consume the 50 kW generated.^[10]

Patrick Troutman serves as the lead for strategic assessments for the Deep Space Transport and the Lunar Orbital Platform-Gateway.^[11]

1.1. Studies

On 1 November 2017, NASA commissioned 5 studies lasting four months into affordable ways to develop the Power and Propulsion Element (PPE), hopefully leveraging private companies' plans. These studies had a combined budget of $2.4 million. The companies performing the PPE studies are Boeing, Lockheed Martin, Orbital ATK, Sierra Nevada and Space Systems/Loral.^[2][12] These awards are in addition of the ongoing set of NextSTEP-2 awards made in 2016 to study development and make ground prototypes of habitat modules that could be used on the Lunar Orbital Platform-Gateway as well as other commercial applications,^[13] so the LOP-G is likely to incorporate components developed under NextSTEP as well.^[2][14]

On 7 November 2017, NASA asked the global science community to submit concepts for scientific studies that could take advantage of the Gateway's location in cislunar space.^[15] The Deep Space Gateway Concept Science Workshop was held in Denver, Colorado from February 27th to March 1st 2018. This three-day conference was a workshop where 196 presentations were given for possible scientific studies that could be advanced through the use of the Gateway.^[16]

An earlier NASA proposal for a cislunar station had been made public in 2012 and was dubbed the Deep Space Habitat. That proposal had led to funding in 2015 under the NextSTEP program to study the requirements of deep space habitats.^[17] In February 2018 it was announced that those NextSTEP studies and other ISS partner studies would help to guide the capabilities required of the Gateway's habitation modules.^[18]

NASA has also initiated a Revolutionary Aerospace Systems Concepts Academic Linkage (RASC-AL) competition for universities to develop concepts and capabilities for the Gateway. The competitors are asked to employ original engineering and analysis in one of the following areas:

• Gateway Uncrewed Utilization & Operations
• Gateway-Based Human Lunar Surface Access
• Gateway Logistics as a Science Platform
• Design of a Gateway-Based Cislunar Tug

Teams of undergraduate and graduate students were asked to submit a response by January 17, 2019 addressing one of these four themes. NASA will select 20 teams to continue developing proposed concepts. Fourteen of the teams will be invited to present their projects in person on June 2019 at the RASC-AL Forum in Cocoa Beach, Florida and will receive a $6,000 stipend to participate in the Forum.^[19]

2. Proposed Components

The Gateway will serve as an all-in-one solar-powered communications hub, science laboratory, short-term habitation module, and holding area for rovers and other robots. https://handwiki.org/wiki/index.php?curid=1603998

The early concept for the LOP-G is still evolving, and may include the following component modules:^[20]

• The Power and Propulsion Element (PPE) will be used to generate electricity for the space station and its solar electric propulsion. It is targeting launch on a commercial vehicle in 2022.^[21][22]
• The European System Providing Refuelling, Infrastructure and Telecommunications (ESPRIT) module will provide additional xenon and hydrazine capacity, additional communications equipment, and an airlock for science packages.^[23] It would have a mass of approximately 4 tons, and a length of 3.91 m.^[24]
• The U.S. Utilization Module is a small pressurized space that would enable a crew ingress on the very first mission to the Gateway assembly sequence. It will initially store additional food and will be launched along with ESPRIT on EM-3.^[23]
• The International Partner Habitat and the U.S. Habitat are the two habitation modules. These will be launched on EM-4 and EM-5 and together will provide a minimum of 125 m³ of habitable volume to the station.^[23]
• The Gateway Logistics Modules will be used to refuel, resupply and provide logistics on board the space station. The first logistics module sent to LOP-G will also arrive with a robotic arm, which will be built by the Canadian Space Agency.^[25]
• The Gateway Airlock Module will be used for performing extravehicular activities outside the space station and will be the berth for the Deep Space Transport.

3. Surface Missions

• HERACLES (Human-Enhanced Robotic Architecture and Capability for Lunar Exploration and Science) is a proposed ESA-JAXA-CSA robotic lander and sample-return mission utilising the station.^[26] It involves dispatching a 11,000 kg lunar lander from Earth aboard an Ariane 64^{[27]:slides 7, 9 and 10} which would land on the Moon before an ascent module returns to the Gateway station. The ascent modules (labelled I on the diagram above) are reusable and would be paired at Gateway with a fresh lander module dispatched from Earth. The rovers would land on the first and fourth lander missions collecting samples and loading them on the ascent module then traversing the hundreds of kilometres between landing sites on the lunar surface to rendezvous and load the next lander.^[28] The ascent module would return each time to the Gateway where it would be captured by the Canadian robotic arm and samples transferred to an Orion craft for transport to Earth with returning astronauts while the 2nd and 3rd landings would each have 500 kg payload available for alternate uses. The aim of the project is the development by ESA of a reusable lunar ascent engine, four of which could be clustered to power a wholly reusable crewed or robotic lander in the future, alongside the development of Gateway telecommunication command and control technology. ESA envisages that HERACLES would be subject to ministerial approval in 2019, and could allow a sample-return on the fourth or fifth Orion flight in the 2026-2030 timeframe, generating an early scientific return for the station and robotic surveying of the conditions that will be encountered at future crewed landing sites several years in advance.

4. Proposed Timeline

5. Criticisms

The Deep Space Gateway has received numerous criticisms from several space professionals for lacking a proper scientific goal. Former NASA Astronaut Terry Virts, who was a pilot of STS-130 aboard Space Shuttle Endeavour and Commander of the International Space Station on Expedition 43 wrote in an Op-ed on Ars Technica that the Deep Space Gateway would "shackle human exploration, not enable it". Terry stated that there is no concrete human spaceflight goal with the Deep Space Gateway and that he cannot envision a new technology that would be developed or validated by building another modular space station. Terry further criticized NASA for abandoning its safety dictum of separating the crew from the cargo which was put in place following the Space Shuttle Columbia accident in 2003.^[32]

Mars Society founder Robert Zubrin, who has been one of the staunchest advocates for a human mission to Mars, called the Deep Space Gateway "NASA's worst plan yet" in an article on the National Review. Robert went on to say "We do not need a lunar-orbiting station to go to the Moon. We do not need such a station to go to Mars. We do not need it to go to near-Earth asteroids. We do not need it to go anywhere. Nor can we accomplish anything in such a station that we cannot do in the Earth-orbiting International Space Station" and that "there is nothing at all in lunar orbit: nothing to use, nothing to explore, nothing to do". Robert also stated that "If the goal is to build a Moon base, it should be built on the surface of the Moon. That is where the science is, that is where the shielding material is, and that is where the resources to make propellant and other useful things are to be found."^[33]

Retired aerospace engineer Gerald Black stated that the "LOP-G is useless for supporting human return to the lunar surface and a lunar base." He added that it is not even planned to be used as a rocket fuel depot and that stopping at LOP-G on the way to or from the Moon would serve no useful purpose and it would actually waste rocket fuel.^[34]

Pei Zhaoyu, deputy director of the Lunar Exploration and Space Program Center of the China National Space Administration, concludes that, from a cost-benefit standpoint, the gateway would have "lost cost-effectiveness."^[35] Pei said the Chinese plan is to focus on a research station on the surface.^[36]