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The UltraViolet Imaging Spectrograph (UVIS), a box of 4 telescopes, creates pictures by observing ultraviolet light. In ultraviolet wavelengths of light, gases are observable and UVIS determines what type it is by splitting the light into its component wavelengths, or colors.

With its 5150-km diameter, Titan is the largest moon of Saturn and the 2nd-largest planetary satellite in Milky Way after Ganymede (5276 km). But, it is the only natural satellite known to have a dense atmosphere with clear evidence of stable bodies of surface liquid.


By the Space Policy Directive-1 (SPD-1) signed by the President Donald J. Trump, in December 2017, NASA have to lead an innovative and sustainable program of exploration. With commercial and international partners, this program will enable human expansion across the solar system and bring back new knowledge and opportunities. Then, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations.

As reflected in the NASA's Exploration Campaign, the next step in the human spaceflight is the establishment of U.S. pre-eminence in the cislunar space through the operations and the deployment of a U.S.-led Lunar Orbital Platform, “Gateway,” (LOP-G).
The LOP-G can evolve depending on mission needs. Basically, the initial functionality will include four main elements, such as a Power and Propulsion Element (PPE), the Habitat, the Airlock to enable Docking and Extra-Vehicular Activities (EVA), and the Logistics for thecargo delivery, science utilization, exploration technology demonstrations, and potential commercial utilization.
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To respond at this request and at the Human Exploration Roadmap (NASA Transition Authorization Act of 2017-P.L. 115-10), NASA have laid out the National Space Exploration Campaign. This Campaign addresses five core national drivers, such as the Scientific Knowledge, the Global Engagement, the Economic Development, the Societal Improvement and, the Leadership and Inspiration.

Credit: Nathan Kaga for

Slide from one of the presentations to the NAC HEO committee showing currently defined Gateway modules. The SLS launch date for the Early Operational Capability was said in the meeting to be 2024. Credit: NASA.


The spacecraft “Gateway” will serve as a reusable command module for the lunar vicinity and the surface exploration. It will evolve to serve as a way station for the development of refueling depots, servicing platforms, and a sample return facility from the surface of the Moon and other bodies in support of science and commerce.
The Gateway will be constructed in orbit, incrementally, with the uses of the American-built Orionspacecraft and the Space Launch System (SLS), as well as commercial launch vehicles.
In fact, NASA plans to build the Gateway with just five or six rocket launches, compared to the 34 launches it took to build the space station. Large parts will be set up by automatic assembly, mean robotically. Learn More @ Space Economy


The PPE is the core element of the Gateway. It is by him that the transportation of the LOP-G between cislunar orbits will be provided, with the option to perform any needed orbital maintenance. It can do these operations with its attitude control in multiple configurations, its communication to and from Earth, as well as space-to-space communication, space-to-lunar communication, and in support of astronaut EVA activities. At the end, it will also carry systems necessary for the deep space navigation, docking, and refueling.

Artist concept of PPE in lunar orbit. Credit: NASA.

The High-level PPE for the spacecraft needs to have a 15-year on-orbit operational lifetime, beginning at theseparation from the launch vehicle. At the end of the LOP-G operational life, PPE will move the integrated LOP-G stack to a disposal orbit.

The main technology development to be integrated into the spacecraft is a Solar Electric Propulsion (SEP) system. The requirements will be a capability to operate over a thrust-to-power ratio range of at least 43 – 52 mN/kW (millinewtons per kilowatt). The PPE will include SEP and chemical propulsion systems based on monopropellants hydrazine.
The PPE is the initial element of the LOP-G, and it will be placed around the Moon by 2022, through a competitive commercial launch contract with U.S. commercial companies. At that time, the contractor will demonstrate more powerful solar-electric power (SEP) and propulsion bus systems that will support both NASA and commercial applications. 
PPE works with the U.S. industry to utilize deliverables from Space Technology Mission Directorate (STMD), which will be part of the new Exploration Research & Technology [ER&T] organization in Advanced Electric Propulsion Systems (AEPS). It will demonstrate an advanced SEP system, including ER&T- developed 12.5 kW AEPS thrusters, power processing units, and advanced high power solar array systems. 
PPE will use SEP technology to insert itself into cislunar orbit and provide overall station-keeping for the LOP-G. Also, it will move the LOP-G into different low gravity lunar orbits depending on mission requirements and will provide high-bandwidth communications. The targeted launch readiness is 2022.
The PPE is intended to supply power and propulsion for additional elements and systems on the LOP-G as well as communication to and from Earth, space-to-space, space-to-lunar, and in support of astronaut EVA.  Refuellable, the PPE needs to support cislunar operations for more than a decade.
Once the PPE, habitation, airlock capabilities and sufficient logistics have been delivered to cislunar space, a crew of four - launched on Orion - will visit the LOP-G on missions initially lasting 30 days. As additional habitation capabilities are incorporated (e.g., volume, life support systems, and logistics resupply), crew visits may increase in duration.
NASA transitioned away from the Asteroid Redirect Mission (ARM) and all relevant work previously conducted on power and propulsion was continued to advance toward a demonstration of that capability in deep space. Activities transitioned from ARM include low-thrust mission design, advanced SEP-based spacecraft bus requirements development, industry spacecraft design studies, and human/robotic mission integration. These activities provide important ground work for the LOP-G PPE and other future deep space exploration objectives.
In 2017, PPE has selected five proposals for further study from inputs received by U.S. industry. The final contract will be awarded in 2018 and a mature design in early 2019. The most important thing about this selection process is the realization of a deep space operational power and propulsion capability that will be directly applicable to wide range of commercial, robotic, and human spaceflight missions.
It is by the Broad Agency Announcement (BAA) that all the process has been done. Then, the final solicitation for the PPE, "the Spacecraft Demonstration of a Power and Propulsion Element", has been released by NASA on September 6, 2018. No matter that this solicitation defines a set of unique requirements that the spacecraft must meet, the BAA is intended to allow bidders to use as much off the shelf technology as possible. So, it is possible to use a simple communication satellite bus augmented by an electric propulsion.

Ion propulsion systems generate a tiny amount of thrust. Hold nine quarters in your hand, feel Earth's gravity pull on them, and you have an idea how little thrust they generate. They can't be used for launching spacecraft from bodies with strong gravity. Their strength lies in continuing to generate thrust over time. This means that they can achieve very high top speeds. Ion thrusters can propel spacecraft to speeds over 320,000 kp/h (200,000 mph), but they must be in operation for a long time to achieve that speed.
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The European Space Agency (ESA) module ESPRIT, European System Providing Refuelling, Infrastructure and Telecommunications will actually increase the overall capacity as high as doubling that before the need of a refuel. In the requirements of the BAA, the PPE Flight System shall conduct orbit mainteance maneuvers in the NRHO with a maximum magnitude of les than 10 m/s of delta-v per year.
At the end of the demonstration period, NASA could exercise the option to assume ownership of the spacecraft. At this time, the PPE mass is required not to exceed 8000 kg, including a minimum of 1050 kg of Xenon for the Solar Electric Propulsion (SEP) and 800 kg of hydrazine for the Reaction Control System (RCS). The Xenon tank in the PPE is required to have a minimum capacity of 2000 kg, and both the Xenon and hydrazine propellant tanks will have the capability to be refueled.


NASA is currently assessing specific configurations and strategy for unpressurized logistics delivery, refueling, commercial launch vehicle pressurized logistics, and the Space Launch System (SLS) pressurized logistics. In fact, all potential concepts deliver cargo to the LOP-G to enable extended crew mission durations, science utilization, exploration technology demonstrations, potential commercial utilization, and other supplies.

NASA's Astronaut Akihiko Hoshide near the airlock in the Kibo lab in the International Space Station. Credit: NASA
Depending on the development and/or the of other Gateway modules, the Logistics Module must deliver pressurized and/or unpressurized cargo to the Lunar Gateway located in a Near Rectilinear Halo Orbit no earlier than 2024.
As showing in the below illustration, It is expected that the initial requirement will be for three missions, with a single mission expected to deliver up to 5 metric tons of pressurized cargo and 2.6 metric tons of unpressurized cargo. The first Logistics Module may be required to transport a Robotic Arm as unpressurized cargo. The Logistics Module must include guidance and navigation, power generation, and propulsion to enable autonomous docking to a port on the Utilization Module or Habitat Modules via an International Docking System Standard (IDSS) compliant docking port.
Once docked, the module will be used by crew primarily for stowage volume, trash stowage, and trash disposal. In addition, the module will depart the Gateway and perform self-disposal without assistance after a period of no more than three years of cislunar space operations.
It is anticipated that the first two logistics missions will launch the Logistics Module using commercial launch vehicles, but after Gateway assembly, the Space Launch System (SLS) may be available for co-manifested logistics delivery.


The airlock element provides the LOP-G with the capability to enable astronaut EVAs as well as the potential to accommodate docking additional elements, observation ports, or a science utilization airlock.
Multiple concepts are currently being assessed and feasibility studies have identified several different approaches ranging from small, single chamber airlocks to large, dual chamber airlocks with inflatable crew locks.

Above: Astronaut ingresses the Airlock-hatch of the Quest Module on the ISS. Right: View of the Quest exterior. Credit: NASA




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NASA Ties SpaceX to Lunar Gateway Supply Contract NASA. Built on the foundations of the successful agreement to resupply the ISS, SpaceX will employ its Falcon Heavy rocket to launch a new spacecraft called the Dragon XL to the lunar outpost under a Gateway Logistics Services (GLS) contract.

As part of Artemis program, the Gateway is going to need supplies and science experiments from time to time, and SpaceX is the first commercial company charged with making that happen. Followed this announcement of March 27 of NASA, SpaceX is guaranteed a minimum of two supply missions under the Gateway Logistics Services contract. The deal gives SpaceX its first major role in NASA’s Artemis program, which aims to land astronauts on the moon before the end of 2024. 

The SpaceX Dragon XL, a large cargo vehicle - like a large Cygnus XL, it will carry pressurized and unpressurized cargo, experiments and other supplies to the Gateway. The equipment delivered could include sample collection materials, spacesuits and other items astronauts may need on the station and on the moon’s surface.


In August 2016, NASA selected six U.S. companies to help expand knowledge, commercial capabilities and opportunities in space by developing full-sized ground prototypes and concepts for deep space habitats under the second Next Space Technologies for Exploration Partnerships(NextSTEP) Broad Agency Announcement, or NextSTEP-2. NextSTEP establishes unique public-private partnerships that seek to advance commercial development of space while advancing deep space exploration capabilities to support more extensive human space flight missions in the area of space near the moon that will be the proving ground for Mars.

The NextSTEP Phase 2 Habitation contracts, funded in Exploration Advanced Systems (EAS), to develop prototype deep space habitats are directly applicable to the Lunar Orbital Transport - Gateway. NASA is formulating the LOP-G by defining system requirements, developing design and interoperability standards, establishing program and system-level control boards, developing strategy and execution mechanisms to acquire a PPE, and developing an integrated ground test plan for prototype habitats.
At the end of NextSTEP Phase 2 study contracts, the industry partners will provide the functional habitat ground prototype units to NASA for testing. The intended outcome of these activities is a complete set of long-duration deep space architecture designs (including standards, common interfaces, and testing approaches) from the awarded contractors as well as development and test of full-size ground prototypes.
The LOP-G habitation element provides a livable section and short-duration life support functions for the crew in cislunar space. The docking ports allow for attachment to the PPE, other elements and visiting vehicles. The habitat also provides attach points for external robotics, external payloads or rendezvous sensors; thermal radiators provide heat rejection and micro-meteoroid protection; and additional habitat systems provide accommodations for crew exercise, science/utilization and stowage. Some functions may be outfitted via future logistics flights.

Initially, NASA's requirements was for a minimum of 55 cubic meters of habitable volume, providing a small place to work for astronauts. After the analysis of the U.S. industry and international partners, the Gateway is now expected to need two habitation modules.

Of the two Hab modules, one is provided by the United States and the other internationally. they would provide at least 125 cubic meters of habitable volume. After the ship of the ESPRIT and Utilization modules on the EM-3 trip, those habitations would be launched with the orion spacecraft and the SLS rocket.

Even without crew present, cutting-edge robotics and computers will operate experiments inside and outside the spaceship, automatically returning data back to Earth.


NanoRacks envisions a future in space where commercial outposts – today known as space stations and habitats – populate the solar system. The company is the only commercial space station company with existing customers with a pathway to be operating in space in the near term, with realistic price points. This is the NanoRacks Space Outpost Program.

The Ixion module, an upper stage left in orbit and refitted as a space station module, being attached to the ISS. 

Repurposing spent upper stages of launch vehicles and converting them into commercial space stations in orbit. Credit: NanoRacks
Above: The NanoRacks Airlock Module (Bishop) is the first-ever commercial Airlock that will operate on the International Space Station. Owned by NanoRacks, Bishop will be both a permanent commercial un-crewed module onboard the International Space Station, and also a module capable of being removed from the Space Station and used on a future commercial platform, such as Independence-1. Bishop will offer five times the satellite deployment volume than current opportunities available on the Space Station today. Credit: NanoRacks, Published on Sep 13, 2018.
At Right: Featuring a simple cut-away view of the B2100 "Olympus" to show the interior, this video is a compilation of previously uploaded Bigelow habitat clips as well as some new ones. B330 and B2100 models by fragomatik & ISS model by NASA. Credit: Fragomatik, published on June 28, 2018


Lockheed Martin will repurpose a space shuttle-era ISS cargo container into a deep space habitat prototype for NASA. Credit: Lockheed Martin

Full animation of Lockheed Martin's concept to support NASA's Gateway at the Moon. Credit: MDx media, published on October 3, 2018



Artist concept of a Cygnus-derived deep space habitat and logistics modules. Credit: Northrop Grumman Corporation
Orbital ATK’s vision for the next step toward human space missions to Mars employs our flight-proven Cygnus advanced maneuvering spacecraft as a human habitat in cislunar space, the region between the Moon and Earth. In the early 2020s we would launch the initial habitat on NASA’s SLS rocket. Featuring a modular design, the habitat would serve both as a destination for crewed missions and as an unmanned testbed to prove-out the technologies needed for long-duration human space missions. The habitat is also envisioned as a base for lunar missions by international partners or commercial ventures. With additional habitation and propulsion modules, the habitat could be outfitted for a Mars pathfinder mission. Credit: Northrop Grumman, published on May 9, 2018

NASA Awards Northrop Grumman Artemis Contract for Gateway Crew Cabin 

Orbital Science Corporation of Dulles, Virginia, a wholly owned subsidiary of Northrop Grumman Space, has been awarded $187 million to design the HAbitation and LOgistics outpost (HALO) for the Gateway, which is part of NASA’s Artemis program and will help the agency build a sustainable presence at the Moon. This award funds HALO’s design through its preliminary design review, expected by the end of 2020.

The HALO will be the pressurized living quarters where astronauts will spend their time while visiting the Gateway. About the size of a small studio apartment, it will provide augmented life support in tandem with NASA’s Orion spacecraft.

The preliminary design review is one of a series of checkpoints in the design life cycle of a complex engineering project before hardware manufacturing can begin. As the review process progresses, details of the vehicle’s design are assessed to ensure the overall system is safe and reliable for flight and meets all NASA mission requirements. LEARN MORE @ NASA

Artist's concept of the Gateway power and propulsion and Habitation and Logistics Outpost, or HALO, in orbit around the Moon. Credits: NASA


Boeing today (April 3, 2017) unveiled concepts for the deep space gateway and transport systems that could help achieve NASA's goal of having robust human space exploration from the Moon to Mars. Video credit: Boeing.


Sierra Nevada Corporation (SNC) is developing a Lunar Orbital Platform - Gateway architecture under NASA’s Next Space Technologies for Exploration Partnerships-2 (NextSTEP-2) Habitat Systems program. Our design supports both crewed and autonomous lunar orbit activities and surface operations.
Sierra Nevada Corporation (SNC) is developing a Lunar Orbital Platform - Gateway architecture under NASA’s Next Space Technologies for Exploration Partnerships-2 (NextSTEP-2) Habitat Systems program. Our design supports both crewed and autonomous lunar orbit activities and surface operations. Credit: Sierra Nevada Corporation. published on May 16, 2018.

Credits: Left: NASA Leading Human Space Exploration, August 2018, Mack McElroy, PhD  & Right: Explore: exptending human presence into the solar system, William H. Gerstenmaior, NASA   

The Space Launch System (SLS) Evolution. Credit: NASA
The SLS is designed for deep space missions. It will send the Orion or other cargo to the Moon, which is nearly 1,000 times farther than where the space station resides in low-Earth orbit. The rocket have the power to help the Orion spacecraft to reach the speed of at least 24,500 mph needed to break out of low-Earth orbit gravity and travel to the Moon. 
Every SLS configuration uses the core stage with four RS-25 engines. The first SLS vehicle, called Block 1, can send more than 26 metric tons (t) or 57,000 pounds (lbs.) to orbits beyond the Moon. It will be powered by twin five-segment solid rocket boosters and four RS-25 liquid propellant engines. After reaching the space, the Interim Cryogenic Propulsion Stage (ICPS) will sends the Orion spacecraft to the Moon.
The Block 1B crew vehicle is the second planned evolution of the SLS, and it will use a new and more powerful Exploration Upper Stage (EUS) to enable more ambitious missions. It can, in a single launch, carrying the Orion crew vehicle along with exploration systems like a deep space habitat module. Then, it can send approximately 37 t (81,571 lbs.) in the deep space including Orion and its crew.
Launching with cargo only, SLS has a large volume payload fairing to send larger exploration systems or science spacecraft on solar system exploration missions.
The last and third configuration is the Block 2, which will provide 11.9 million lbs. of thrust. This version is the workhorse vehicle for sending cargo to the Moon, Mars and other deep space destinations. With its high level of thrust, it can lift more than 45 t (99,000 lbs.) to deep space.

Updated launch sequence

Credit: Cislunar and Gateway Overview, William Gerstenmaier, HEOMD AA / Jason Crusan, AES Director and Gateway Formulation Lead, NASA HQ

The Exploration Mission 1 (EM-1) is an uncrewed mission to test Orion’s capabilities in deep space. EM-1 is the first flight of the European Service Module (ESM) and also Orion’s first flight on the new SLS. During the mission, the Crew Module guidance navigation and control system will command the ESM propulsion system to place the spacecraft into a Distant Retrograde Orbit around the moon. The nominal mission duration is 25 days, but will be adjusted between 21 and 43 days in order to ensure a landing under daylight conditions.
The above presentations show that the Exploration Mission-1 (EM-1) is to be launching in 2020. This first mission will fly without crew in a Distant Retrograde Orbit (DRO). For the Exploration Mission-2 (EM-2) in 2022, the Orion spacecraft will take first astronauts on a lunar flyby. 
The SLS Block 1 cargo variant will be capable of delivering Orion beyond LEO in an Trans-Lunar Injection (TLI). The evolved Block 1B will be capable of delivering 9-10 metric tons to TLI, co-manifested with Orion. These SLS/Orion missions will demonstrate the capability to operate safely and productively around the Moon.
Altogether the first three SLS launches would fly on the Block 1 vehicle, the two Orion test mission launches and the launch of the Europa Clipper probe directly to Jupiter.

The SLS co-manifested payloads are delivered with the Orion through the Trans-Lunar Injection (TLI) burn by the Exploration Upper Stage (EUS). With the payload still attached to the EUS, the Universal Stage Adapter (USA-2) fairing is jettisoned and the Orion rotates to dock with the payload. After docking, the EUS releases the payload and the Orion Service Module delivers the payload to the Lunar DRO destination

Credit: Cislunar and Gateway Overview, William Gerstenmaier, HEOMD AA / Jason Crusan, AES Director and Gateway Formulation Lead, NASA HQ

The Exploration Mission 2 (EM-2) is the first crewed mission for the Orion spacecraft. This mission will be historical for the Europe because the European Service Module 2 (ESM-2) will be the first European spacecraft to be part of a human transportation system beyond the low Earth orbit. After the launch, the SLS and the Interim Cryogenic Propulsion Stage (ICPS) will place the Orion spacecraft into a high Earth orbit, where the astronauts will remain for 24 hours to check out the spacecraft’s systems. When Mission Control gives the approval to initiate trans-lunar injection, the ESM-2’s Orion Orbital Manoeuvring System (OMS) will fire and send the spacecraft on a free-return trajectory out toward and around the Moon. The ESM-2’s Auxiliary (AUX) thrusters will be used to make any needed trajectory corrections along the way.
NASA and Lockheed Martin will conduct the EM-2 Critical Design Review (CDR) in late 2018. In 2015, NASA and LM completed the Orion CDR evaluating the common aspects of the spacecraft for EM-1 and the spacecraft for EM-2. The EM-2 CDR will confirm that the EM-2 unique systems also meet the necessary requirements and that the upgrades to the ESM design from ESM-1 to ESM-2 are appropriately integrated into the overall spacecraft design.
The European System Providing Refuelling, Infrastructure and Telecommunications, (ESPRIT), and the American-provided Utilization Module would fly with the Orion and crew on the third Exploration Mission (EM-3) in 2024. This latter trip would be the debut of ML-2 and the SLS Block 1B vehicle with the larger Exploration Upper Stage (EUS). Although NASA indicated there weren’t any notional masses for the modules, the initial SLS Block 1B launch is expected to be able to place both the 25-metric ton Orion spacecraft and several metric tons of secondary, “co-manifested” payloads on a trans lunar injection (TLI) trajectory. The habitation modules would then follow one at a time on EM-4 and EM-5.