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The Lunar Orbital Platform-Gateway (LOP-G) is an American-led international project led by NASA proposed to create a lunar-orbit space station. It is intended to serve as a solar-powered communications hub, science laboratory, short-term habitation module, and holding area for rovers and other robots.[1]

Lunar Orbital Platform-Gateway
Lunar Orbital Platform-Gateway.jpg
Artist's concept of Lunar Orbital Platform-Gateway orbiting the Moon. The Orion MPCV is docked on the left.
Station statistics
Crew4 (proposed)
Carrier rocketSpace Launch System
Commercial vehicle
Mass75,000 kg
Pressurised volume125.00 m3 (4,414.33 cu ft)

The science disciplines to be studied on the Gateway are expected to include planetary science, astrophysics, Earth observations, heliophysics, fundamental space biology and human health and performance.[2]

The Gateway is meant to be developed, serviced, and utilized in collaboration with commercial and international partners. It will also serve as the staging point for crewed and robotic lunar exploration and a staging point for NASA's proposed Deep Space Transport craft to perform a 300-400 day shakedown mission prior to NASA's first crewed Mars mission.[3] Deep Space Transport is a concept of a reusable vehicle that uses electric and chemical propulsion and would be specifically designed for crewed missions to destinations such as Mars.[4][5]

The development is led by the International Space Station partners: ESA, NASA, Roscosmos, JAXA and CSA for construction in the 2020s.[4][6][7] The International Space Exploration Coordination Group (ISECG), which comprises 14 space agencies participating with NASA, have concluded that LOP-G will be critical in expanding human presence to the Moon, Mars and deeper into the Solar System.[8] Formerly known as the Deep Space Gateway, the station was renamed in NASA's proposal for the 2019 United States federal budget.[9][10] The omnibus spending bill passed by Congress in March 2018 provided NASA with $504 million for preliminary studies during the 2019 fiscal year.[11] The final funding amount enacted by Congress was slightly lower at $450 million.[12]



The Gateway 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.
Inside the LOP-G space station mock-up module at the Space Station Processing Facility, with four astronauts.
NASA and Lockheed Martin employees group photo with one of the LOP-G space station modules training mock-up inside the SSPF

Originally, NASA had intended to build the Gateway as part of the now cancelled Asteroid Redirect Mission.[13][14] An informal joint statement on cooperation between NASA and Russia's Roscosmos was announced on 27 September 2017.[7] 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 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.[15] 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. Orbital station-keeping would require less than 10 m/s of delta-v per year.[16]

The Gateway could conceivably also support in-situ resource utilization (ISRU) development and testing from lunar and asteroid resources,[17] and would offer the opportunity for gradual buildup of capabilities for more complex missions over time.[18] Various components of the Gateway would be launched on commercial launch vehicles and on the Space Launch System as Orion co-manifested payloads on the Artemis 3 through Artemis 8 missions.[19] According to Roscosmos, they may also use Proton-M and Angara-A5M heavy launchers to fly payloads or crew.[7]

The Power and Propulsion Element (PPE) for the LOP-G would have a mass of 8-9 t and be capable of generating 50 kW[14] of solar electric power for its ion thrusters system for maneuverability, which can be supported by chemical propulsion.[20] Patrick Troutman serves as the lead for strategic assessments for the Deep Space Transport and the Lunar Orbital Platform-Gateway.[21]


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.[2] The Deep Space Gateway Concept Science Workshop was held in Denver, Colorado from February 27 to March 1, 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.[22]

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.[23] 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.[24]

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 in June 2019 at the RASC-AL Forum in Cocoa Beach, Florida and will receive a $6,000 stipend to participate in the Forum.[1]

Power and propulsionEdit

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.[25][14] These awards are in addition to 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,[5] so the LOP-G is likely to incorporate components developed under NextSTEP as well.[14][26]

NASA officials 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) which has an Isp of up to 2,600 s. The engine is being developed by Glenn Research Center, the Jet Propulsion Laboratory, and Aerojet Rocketdyne.[27] Four identical AEPS engines would consume the 50 kW generated.[27]

The contract to manufacture the PPE was given to a division of Maxar Technologies (formerly SSL).[28] After a one-year demonstration period, NASA would then "exercise a contract option to take over control of the spacecraft."[29]

Proposed modulesEdit

The Gateway will serve as an all-in-one solar-powered communications hub, science laboratory, short-term habitation module, refueling depot, and holding area for landers and other robots.

The early concept for the lunar Gateway is still evolving, and may include the following component modules:[30]

  • 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.[31][32] Maxar Technologies was contracted by NASA to manufacture this module.[33]
  • 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.[34] It would have a mass of approximately 4 t (8,800 lb), and a length of 3.91 m (12.8 ft).[35]
  • 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 within the Artemis 3 mission.[34]
  • The International Partner Habitat and the U.S. Habitat are the two habitation modules. These will be launched on Artemis 4 and Artemis 5 and together will provide a minimum of 125 m3 (4,400 cu ft) of habitable volume to the station.[34]
  • 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.[36][37]
  • The Gateway Airlock Module will be used for performing extravehicular activities outside the space station and would be the berth for the proposed Deep Space Transport.

Conceptual landersEdit

The work on crewed landers would be supported under a new budget line called "Advanced Cislunar and Surface Capabilities" included in the fiscal year 2019 budget proposal, which seeks $116.5 million for the program. Congress has yet to pass a final fiscal year 2019 appropriations bill for NASA.[38] Some lander architectures have already been suggested and are listed below.

  • 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 Gateway station.[39] It involves dispatching an 11-ton lunar lander from Earth aboard an Ariane 64[40]:slides 7, 9 and 10 which would land on the Moon before an ascent module heads to the Gateway station. The ascent modules 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.[41] 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 (1,100 lb) 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.
  • The Lockheed Martin Lunar Lander concept, presented in October 2018, proposes a reusable crewed lunar lander with a mass of 22 tons and capable of carrying up to 1 ton of payload, and a crew of four, for a duration of two weeks before returning to the Gateway for servicing and refueling.[42][43] A drawback is that not even the future Block 1B version of Space Launch System can place more than 45 tons onto a trajectory to the Moon, so additional launches would be required to transport fuel depots for the lander.[38][44]
  • Advanced Exploration Lander is a lander concept by a NASA team that is studying a three-stage vehicle that would allow departure from the Gateway and take the crew to a low lunar orbit and then separate, after which the descent module would handle the rest of the journey to the lunar surface.[38] A crew of up to four would spend up to two weeks on the surface before boarding the ascent module, which would take them back to the Gateway.[44] The three stages are: the transfer module (a space tug), the descent module and the ascent module. Each module would have a mass of approximately 12 to 15 metric tons[38] that would be delivered separately by commercial launchers and integrated there. The astronauts would board the lander at the Gateway and its transfer module would take the lander from the Gateway's near-rectilinear halo orbit that goes between about 1,000 and 70,000 kilometres (620 and 43,500 mi) above the Moon, to a circular low orbit about 100 kilometres (62 mi) high. The ascent and descent stages would then go down together to the lunar surface, and at the end of the mission the ascent stage would fire its engines to go back directly the Gateway while leaving the descent stage behind. Both the ascent and transfer modules could be designed to be reused.[38][44] If selected and funded, it could be tested as a stand-alone robotic mission in 2024[38] and begin crewed landings in 2028 departing from the Gateway.[44]

Proposed timelineEdit

Year Vehicle assembly objective Mission name Launch vehicle Human/robotic elements
Q4 2022[45] Start of the Lunar Orbital Platform-Gateway assembly by launching the Power and Propulsion Element (PPE)[46] TBA Commercial launch vehicle[31][47] Uncrewed
2023 Small pressurized compartment with docks[48] TBA Commercial vehicle Uncrewed
2024 Orion docking to the Gateway, followed by a lunar landing and return to the Gateway[48] Artemis 3 Space Launch System, Block 1B Crewed
2025 Crewed flight to the Gateway to deliver the U.S. Habitation module; lunar landing. Artemis 4 Space Launch System, Block 1B Crewed
2026 Delivery of U.S. Habitat; lunar landing Artemis 5 Space Launch System, Block 1B Crewed
2027 Delivery of the first logistics module and the robotic arm[34] Artemis 6 Space Launch System, Block 1B Crewed
2028 Deliver a logistics module Artemis 8 Space Launch System, Block 1B Uncrewed


The lunar Gateway has received numerous criticisms from several space professionals for lacking a proper scientific goal, while NASA officials promote the Gateway as a "reusable command module" that could direct activities on the lunar surface.[44]

Michael Griffin, a former NASA administrator, said that in his opinion, the Gateway can be useful only after there are facilities on the Moon producing propellant that could be transported to the Gateway. Griffin thinks that after that is achieved, the Gateway would then best serve as a fuel depot.[44] He said that "putting a Gateway before boots on the Moon is, from a space-systems engineer's standpoint, a stupid architecture".[49]

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 lunar Gateway would "shackle human exploration, not enable it". Terry stated that there is no concrete human spaceflight goal with the 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.[50]

Mars Society founder Robert Zubrin, who has been one of the staunchest advocates for a human mission to Mars, called the lunar Gateway "NASA's worst plan yet" in an article on the National Review. Zubrin 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". Zubrin 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."[51]

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.[52]

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."[53] Pei said the Chinese plan is to focus on a research station on the surface.[54]

Former Apollo 11 astronaut Buzz Aldrin stated that he is "quite opposed to the Gateway" and that "using the Gateway as a staging area for robotic or human missions to the lunar surface is absurd." Aldrin also questioned as to "why would you want to send a crew to an intermediate point in space, pick up a lander there and go down?" On the other hand, Aldrin expressed support for Robert Zubrin's Moon Direct concept which involves lunar landers traveling from Earth orbit to the lunar surface and back.[55]

Former NASA Astronauts Eileen Collins, who was a Space Shuttle pilot and commander, and Harrison Schmitt, who was Lunar Module pilot aboard Apollo 17, although did not mention the Gateway directly, criticized NASA's plans for not being ambitious enough. Collins stated that "2028 for humans on the moon seems like it's so far off" and that "we can do it sooner" while Schmitt stated that "the pace of the proposed program didn't match what took place under Apollo."[55]

Mark Whittington, who is a contributor to the Hill Newspaper and an author of several space exploration studies stated in an article that "NASA’s unnecessary $504 million lunar orbit project doesn’t help us get back to the Moon". Whittington also pointed out "that a lunar orbiting space station was not necessary for men to go to the moon and back during the Apollo program" and that a "reusable lunar lander could be refueled from a depot on the lunar surface and left in a parking orbit between missions without the need for a big, complex space station."[56]

Ethan Siegel, an astrophysicist wrote in an article in Forbes that "NASA's Idea For A Space Station In Lunar Orbit Takes Humanity Nowhere". Siegel stated that "Orbiting the Moon represents barely incremental progress; the only scientific "advantages" to being in lunar orbit as opposed to low-Earth orbit are twofold: 1. You're outside of the Van Allen belts. 2. You're closer to the lunar surface, allowing you to control landers and rovers there with less of a time delay than you experience on Earth. By approximately one second." and that the Lunar Gateway is "a great way to spend a great deal of money, advancing science and humanity in no appreciable way."[57]

See alsoEdit


  1. ^ a b Jackson, Shanessa (September 11, 2018). "Competition Seeks University Concepts for Gateway and Deep Space Exploration Capabilities". NASA. Retrieved September 19, 2018.
  2. ^ a b Mahoney, Erin (August 24, 2018). "NASA Seeks Ideas for Scientific Activities Near the Moon". NASA. Retrieved September 19, 2018.
  3. ^ Gebhardt, Chris (April 6, 2017). "NASA finally sets goals, missions for SLS – eyes multi-step plan to Mars". NASA Spaceflight. Retrieved September 19, 2018.
  4. ^ a b Kathryn Hambleton. "Deep Space Gateway to Open Opportunities for Distant Destinations". NASA. Retrieved April 5, 2017.
  5. ^ a b Robyn Gatens, Jason Crusan. "Cislunar Habitation & Environmental Control & Life Support System" (PDF). NASA. Retrieved March 31, 2017.
  7. ^ a b c Weitering, Hanneke (September 27, 2017). "NASA and Russia Partner Up for Crewed Deep-Space Missions". Retrieved November 5, 2017.
  8. ^ NASA (May 2, 2018). "Gateway Memorandum for the Record" (PDF). NASA. Retrieved September 19, 2018.
  9. ^ Davis, Jason (February 26, 2018). "Some snark (and details!) about NASA's proposed lunar space station". The Planetary Society. Archived from the original on February 26, 2018. Retrieved February 26, 2018.
  10. ^ Yuhas, Alan (February 12, 2018). "Trump's Nasa budget: flying 'Jetson cars' and a return to the moon". the Guardian. Retrieved February 25, 2018.
  11. ^ Foust, Jeff (June 12, 2018). "Senate bill restores funding for NASA science and technology demonstration missions". Space News. Retrieved September 16, 2018.
  12. ^ "NASA just got its best budget in a decade". Retrieved February 27, 2019.
  13. ^ NASA Seeks Information on Developing Deep Space Gateway Module. Jeff Foust, Space. 29 July 2017.
  14. ^ a b c d NASA issues study contracts for Deep Space Gateway element. Jeff Foust, Space News. 3 November 2017.
  15. ^ Mike Wall, 10 September 2018.
  16. ^ Whitley, Ryan; Martinez, Roland (October 21, 2015). "Options for Staging Orbits in Cis-Lunar Space" (PDF). NASA. Retrieved September 19, 2018.
  17. ^ Research Possibilities Beyond Deep Space Gateway. David Smitherman, Debra Needham, Ruthan Lewis. NASA. February 28, 2018.
  18. ^ Human Exploration and Operations Mission Directorate - Architecture Status. (PDF) Jim Free. NASA. 28 March 2017.
  19. ^ Godwin, Curt (April 1, 2017). "NASA's human spaceflight plans come into focus with announcement of Deep Space Gateway". Spaceflight Insider. Retrieved April 2, 2017.
  20. ^ Chris Gebhardt. "NASA finally sets goals, missions for SLS – eyes multi-step plan to Mars". NASA Spaceflight. Retrieved April 9, 2017.
  21. ^ NASA Langley Talk to Highlight Sending Humans to the Deep Space Gateway. April 25, 2018.
  22. ^ "Program and Presenter Information". Lunar and Planetary Institute. Universities Space Research Association. Retrieved September 19, 2018.
  23. ^ Doug Messier on (August 11, 2016). "A Closer Look at NextSTEP-2 Deep Space Habitat Concepts". Parabolic Arc. Retrieved September 19, 2018.
  24. ^ Warner, Cheryl (May 2, 2018). "NASA's Lunar Outpost will Extend Human Presence in Deep Space". NASA. Retrieved September 19, 2018.
  25. ^ Jimi Russell. "NASA Selects Studies for Gateway Power and Propulsion Element". NASA.GOV. Retrieved November 2, 2017.
  26. ^ Erin Mahoney. "NextSTEP Partners Develop Ground Prototypes to Expand our Knowledge of Deep Space Habitats". NASA.GOV. NASA. Retrieved November 6, 2017.
  27. ^ a b Overview of the Development and Mission Application of the Advanced Electric Propulsion System (AEPS). (PDF). Daniel A. Herman, Todd A. Tofil, Walter Santiago, Hani Kamhawi, James E. Polk, John S. Snyder, Richard R. Hofer, Frank Q. Picha, Jerry Jackson and May Allen. NASA; NASA/TM—2018-219761. 35th International Electric Propulsion Conference. Atlanta, Georgia, October 8–12, 2017. Accessed: 27 July 2018.
  28. ^ NASA Awards Artemis Contract for Lunar Gateway Power, Propulsion Nasa, 23 May 2019
  29. ^ NASA updates Lunar Gateway plans. Philip Sloss, NASA 11 September 2018.
  30. ^ Cursan, Jason (March 27, 2018). "Future Human Exploration Planning:Lunar Orbital Platform-Gateway and Science Workshop Findings" (PDF). Retrieved April 13, 2018.
  31. ^ a b "NASA FY 2019 Budget Overview" (PDF). Quote: "Supports launch of the Power and PropulsionElement on a commercial launch vehicle as the first component of the LOP - Gateway, (page 14)
  32. ^ NASA considers acquiring more than one gateway propulsion module. Joe Faust, Space News. 30 March 2018.
  33. ^ Jeff Foust (May 23, 2019). "NASA selects Maxar to build first Gateway element". SpaceNews. Retrieved May 23, 2019.
  34. ^ a b c d Sloss, Philip (September 11, 2017). "NASA updates Lunar Gateway plans". Retrieved September 15, 2017.
  35. ^ ESA develops logistics vehicle for cis-lunar outpost. Anatoly Zak, Russian Space Web. September 8, 2018.
  36. ^ Mortillaro, Nicole (February 28, 2019). "Canada's heading to the moon: A look at the Lunar Gateway". CBC News. Retrieved March 2, 2019.
  37. ^ "Canadian Space Agency to build robotic arms for lunar space station". Global News. Retrieved September 29, 2017.
  38. ^ a b c d e f NASA Studying Three-Stage Approach to Human-Class Lunar Landers. Jeff Foust, 21 November 2018.
  39. ^
  40. ^
  41. ^
  42. ^ Lockheed Martin Reveals New Human Lunar Lander Concept. Lockheed Martin press release on 3 October 2018.
  43. ^ Concept for a Crewed Lunar Lander Operating from the Lunar Orbiting Platform-Gateway. (PDF) Timothy Cichan, Stephen A. Bailey, Adam Burch, Nickolas W. Kirby. 69th International Astronautical Congress (IAC), Bremen, Germany, 1–5 October 2018.
  44. ^ a b c d e f Is the Gateway the right way to the moon? Jeff Foust, Space News. 25 December 2018.
  45. ^ Foust, Jeff (February 27, 2019). "Shutdown to delay first element of NASA's lunar Gateway". Spacenews. Retrieved February 28, 2019. NASA stated in the procurement filing that it expects “a corresponding shift in the target launch date from September 2022 to no later than December 31, 2022.”
  46. ^ Daines, Gary (December 1, 2016). "Crew Will Mark Important Step on Journey to Mars". Retrieved January 2, 2018.
  47. ^ Status of Power and Propulsion Element (PPE) for Gateway. (PDF) Michele Gates, NASA's NAC HEO Committee Meeting August 27, 2018.
  48. ^ a b "NASA's roadmap for Artemis". May 23, 2019. Retrieved May 15, 2019.
  49. ^ Berger, Eric. "Former NASA administrator says Lunar Gateway is "a stupid architecture"". Ars Technica. Retrieved November 23, 2018.
  50. ^ "Op-ed: The Deep Space Gateway would shackle human exploration, not enable it". Ars Technica. Retrieved May 20, 2018.
  51. ^ "NASA's Worst Plan Yet". National Review. Retrieved May 20, 2018.
  52. ^ The Lunar Orbital Platform-Gateway: an unneeded and costly diversion. Gerald Black, The Space Review. 14 May 2018.
  53. ^ Berger, Eric. "Chinese space official seems unimpressed with NASA's lunar gateway". Ars Technica. Retrieved July 17, 2018.
  54. ^ Kapoglou, Angeliki. "". Twitter. Retrieved July 17, 2018.
  55. ^ a b Foust, Jeff. "Advisory group skeptical of NASA lunar exploration plans". Ars Technica. Retrieved December 20, 2018.
  56. ^ Whittington, Mark. "NASA's unnecessary $504 million lunar orbit project doesn't help us get back to the Moon". The Hill. Retrieved December 20, 2018.
  57. ^ Siegel, Ethan. "NASA's Idea For A Space Station In Lunar Orbit Takes Humanity Nowhere". Forbes. Retrieved February 15, 2019.

External linksEdit