The SpaceX Dragon 2 is a class of reusable spacecraft developed and manufactured by American aerospace manufacturer SpaceX as the successor to Dragon 1, a reusable cargo spacecraft. There are two variants: Crew Dragon, a space capsule capable of ferrying up to seven astronauts, and Cargo Dragon, an updated replacement for the original Dragon spacecraft. The spacecraft launches atop a Falcon 9 Block 5 rocket and returns to Earth via an ocean splashdown. Unlike its predecessor, the spacecraft can autonomously dock to the International Space Station (ISS) instead of being berthed.
Crew Dragon is equipped with an integrated launch escape system (LES) consisting of eight SuperDraco engines, capable of accelerating the vehicle away from the rocket in an emergency. The spacecraft features redesigned solar arrays and a modified outer mold line compared to the original Dragon, and possesses new flight computers and avionics. As of March 2020, four operational Dragon 2 spacecraft have been manufactured, as well as several unflown test articles.
There are two variants: Crew Dragon and Cargo Dragon.[4] Crew Dragon was initially called "DragonRider"[7][8] and it was intended from the beginning to support a crew of seven or a combination of crew and cargo.[9][10] It is able to perform fully autonomous rendezvous and docking with manual override ability, using the NASA Docking System (NDS).[11][12] For typical missions, Crew Dragon will remain docked to the ISS for a period of 180 days, but is designed to remain on the station for up to 210 days, matching the Russian Soyuz spacecraft.[13][14][15] From the beginning of the development process, SpaceX planned to use an integrated pusher launch escape system for the Dragon spacecraft.[16][17][18]
SpaceX originally intended to land Crew Dragon on land using the LES engines, with parachutes and an ocean splashdown available in the case of an aborted launch. Precision water landing under parachutes was proposed to NASA as "the baseline return and recovery approach for the first few flights" of Crew Dragon.[19] Propulsive landing was later cancelled, leaving ocean splashdown under parachutes as the only option.[20] As of 2011[update], the Paragon Space Development Corporation was assisting in developing Crew Dragon's life-support system.[21]
In 2012, SpaceX was in talks with Orbital Outfitters about developing space suits to wear during launch and re-entry.[22] Each crew member wears a custom space suit fitted for them. The suit is primarily designed for use inside the Dragon (IVA type suit): however, in the case of a rapid cabin depressurization, the suit can protect the crew members. The suit can also provide cooling for astronauts during normal flight.[23][24] For the Demo-1 mission, a test dummy nicknamed Ripley was fitted with the spacesuit and sensors. The spacesuit "is made from Nomex", a fire retardant fabric similar to Kevlar.
At a NASA news conference on 18 May 2012, SpaceX confirmed their target launch price for crewed Dragon flights of US$160 million, or about US$23 million per seat if the maximum crew of seven is aboard and NASA orders at least four Crew Dragon flights per year.[25] This contrasts with the 2014 Soyuz launch price of US$76 million per seat for NASA astronauts.[26] The spacecraft's design was unveiled on 29 May 2014, during a press event at SpaceX headquarters in Hawthorne, California.[27][28][29] In October 2014, NASA selected the Dragon spacecraft as one of the candidates to fly American astronauts to the International Space Station, under the Commercial Crew Program.[30][31][32] SpaceX is using the Falcon 9 Block 5 launch vehicle to launch Dragon 2.[3]
Cargo Dragon
Although Dragon 2 was intended from the earliest design concept to carry crew, or with fewer seats, both crew and cargo, a second round of multi-year cargo supply contracts (also known as CRS-2) was solicited by NASA in 2014, to supply the ISS in 2020–2024. This led to SpaceX proposing a separately-named model, Cargo Dragon, for the NASA flights.[33] SpaceX won a contract award for Cargo Dragon as a result of the CRS-2 bid competition, with contracts awarded in January 2016 for six flights.[34]
Cargo Dragons are different from the crewed variant by launching without seats, cockpit controls, astronaut life support systems, or SuperDraco abort engines.[35][36] Cargo Dragon improves on many aspects of the original Dragon design, including the recovery and refurbishment process.[37] SpaceX plans to reuse each Cargo Dragon capsule up to five times.
Design
Dragon 2 includes the following features:[27][28][38]
Dragon 2 is partially reusable, potentially resulting in a significant cost reduction. After earlier plans of SpaceX to use new capsules for every crewed flight for NASA[39] both agreed to reuse Crew Dragon capsules for NASA flights.[40][41] Cargo Dragon can carry 3,307 kg (7,291 lb) to the ISS; Crew Dragon has a capacity of seven astronauts (only four seats are used for NASA missions). Above the seats, there is a three-screen control panel, a toilet (with privacy curtain), and the docking hatch. Ocean landings are accomplished with four main parachutes in both variants. The parachute system was fully redesigned from the one used in the prior Dragon capsule, due to the need to deploy the parachutes under a variety of launch abort scenarios.[42]
Crew Dragon has eight side-mounted SuperDraco engines, clustered in redundant pairs in four engine pods, with each engine able to produce 71 kN (16,000 lbf) of thrust to be used for launch aborts.[27] Each pod also contains four Draco thrusters that can be used for attitude control and orbital maneuvers. The SuperDraco engine combustion chamber is printed of Inconel, an alloy of nickel and iron, using a process of direct metal laser sintering. Engines are contained in a protective nacelle to prevent fault propagation if an engine fails.
Once in orbit, Dragon 2 is able to autonomouslydock to the ISS. Dragon used berthing, a non-autonomous means to attach to the ISS that was completed by use of the Canadarm2 robotic arm. Pilots of Crew Dragon retain the ability to dock the spacecraft using manual controls interfaced with a static tablet-like computer. The spacecraft can be operated in full vacuum, and "the crew will wear SpaceX-designed space suits to protect them from a rapid cabin depressurization emergency event". Also, the spacecraft will be able to return safely if a leak occurs "of up to an equivalent orifice of 6.35 mm [0.25 in] in diameter".[19]
Propellant and helium pressurant for both launch aborts and on-orbit maneuvering is contained in composite-carbon-overwrap titanium spherical tanks. A PICA-X heat shield protects the capsule during reentry, while a movable ballast sled allows more precise attitude control of the spacecraft during the atmospheric entry phase of the return to Earth and more accurate control of the landing ellipse location.[19] A reusable nose cone "protects the vessel and the docking adaptor during ascent and reentry",[19] pivoting on a hinge to enable in-space docking and returning to the covered position for reentry and future launches.[29]
The previous Cargo Dragon's deployable solar arrays have been eliminated and are now built into the trunk itself. This increases volume space, reduces the number of mechanisms on the vehicle and further increases reliability.
Crewed flights
Launch of Demo-1, Crew Dragon's maiden spaceflight
Dragon is intended to fulfill a set of requirements that will make the capsule useful to both commercial and government customers. SpaceX and Bigelow Aerospace were working together to support round-trip transport of commercial passengers to low Earth orbit (LEO) destinations, but the plan was canceled. Instead Axiom plans to launch tourists to the Space Station and eventually their own private space station. NASA flights to the ISS will only have four astronauts, with the added payload mass and volume used to carry pressurized cargo.[42]
On 16 September 2014, NASA announced that SpaceX and Boeing had been selected to provide crew transportation to the ISS. SpaceX will receive US$2.6 billion under this contract.[43] Dragon was the less expensive proposal,[31] but NASA's William H. Gerstenmaier considered the CST-100 proposal the stronger of the two.
In a departure from the prior NASA practice, where construction contracts with commercial firms led to direct NASA operation of the spacecraft, NASA is purchasing space transport services from SpaceX, including construction, launch, and operation of the Dragon 2.[44]
In August 2018, NASA and SpaceX agreed on the loading procedures for propellants, vehicle fluids and crew. High-pressure helium will be loaded first, followed by the passengers approximately two hours prior to scheduled launch; the ground crew will then depart the launch pad and move to a safe distance. The launch escape system will be activated approximately 40 minutes prior to launch, with propellant loading commencing several minutes later.[45] The first automated test mission launched to the International Space Station (ISS) on 2 March 2019.[46]
In early 2019, crewed flights were expected to begin no earlier than July 2019.[47] They were later planned to begin no earlier than on 30 May 2020. The first crewed flight launched on 30 May 2020[48] with the launch of the Demo-2 mission.
In June 2019, Bigelow Space Operations announced it had reserved with SpaceX up to four missions of four passengers each to ISS as early as 2020 and planned to sell them for around US$52 million per seat.[49] These plans were canceled by September 2019.
On 18 February 2020, building on development for NASA's commercial crew program, Space Adventures announced an agreement with SpaceX to fly up to four paying space tourists on a standalone mission aboard a Crew Dragon spacecraft in late 2021 or 2022 that could reach an altitude two-to-three times higher than the International Space Station.[50]
Testing
SpaceX planned a series of four flight tests for the Crew Dragon - a "pad abort" test, an uncrewed orbital flight to the ISS, an in-flight abort test, and finally a 14-day crewed demonstration mission to the ISS,[51] which was initially planned for July 2019,[47] but after a Dragon capsule explosion, was delayed to May 2020.[52]
Pad abort test of a Dragon 2 article on 6 May 2015 at CCAFS, SLC-40
The pad abort test was conducted successfully on 6 May 2015 at SpaceX's leased SLC-40.[42] Dragon landed safely in the ocean to the east of the launchpad 99 seconds after ignition of the SuperDraco engines.[53] While a flight-like Dragon 2 and trunk were used for the pad abort test, they rested atop a truss structure for the test rather than a full Falcon 9 rocket. A crash test dummy embedded with a suite of sensors was placed inside the test vehicle to record acceleration loads and forces at the crew seat, while the remaining six seats were loaded with weights to simulate full-passenger-load weight.[44][54] The test objective was to demonstrate sufficient total impulse, thrust and controllability to conduct a safe pad abort. A fuel mixture ratio issue was detected after the flight in one of the eight SuperDraco engines causing it to under perform, but did not materially affect the flight.[55][56][57]
On 24 November 2015, SpaceX conducted a test of Dragon 2's hovering abilities at the firm's rocket development facility in McGregor, Texas. In a video, the spacecraft is shown suspended by a hoisting cable and igniting its SuperDraco engines to hover for about 5 seconds, balancing on its 8 engines firing at reduced thrust to compensate exactly for gravity.[58] The test vehicle was the same capsule that performed the pad abort test earlier in 2015; it was nicknamed DragonFly.[59]
The Crew Dragon mockup (background) and four of the astronauts of its first two crewed missions (foreground), from left to right: Douglas Hurley, Robert Behnken, Michael Hopkins, and Victor Glover
In 2015, NASA named its first Commercial Crew astronaut cadre of four veteran astronauts to work with SpaceX and Boeing – Robert Behnken, Eric Boe, Sunita Williams, and Douglas Hurley.[60] The Demo-1 mission completed the last milestone of the Commercial Crew Development program, paving the way to starting commercial services under an upcoming ISS Crew Transportation Services contract.[44][61] On 3 August 2018, NASA announced the crew for the DM-2 mission.[62] The crew of two will be formed by NASA astronauts Bob Behnken and Doug Hurley. Behnken previously flew as mission specialist on the STS-123 and the STS-130 missions. Hurley previously flew as a pilot on the STS-127 mission and on the final Space Shuttle mission, the STS-135 mission.
The first orbital test of Crew Dragon was an uncrewed mission, officially designated "Crew Demo-1" and launched on 2 March 2019.[63][64] The spacecraft tested the approach and automated docking procedures with the ISS,[65] remained docked until 8 March 2019, then conducted the full re-entry, splashdown and recovery steps to qualify for a crewed mission.[66][67] Life-support systems were monitored all along the test flight. The same capsule was planned to be re-used in June 2019 for an in-flight abort test before it exploded on 20 April 2019.[63][68]
Explosion during testing
On 20 April 2019, the Crew Dragon capsule used in the Demo-1 mission was destroyed in an explosion during static fire testing at the Landing Zone 1 facility.[69][70] On the day of the explosion, the initial testing of the Crew Dragon's Draco thrusters was successful, with the anomaly occurring during the test of the SuperDraco abort system.[71]
Telemetry, high-speed camera footage, and analysis of recovered debris indicate the problem occurred when a small amount of dinitrogen tetroxide leaked into a helium line used to pressurize the propellant tanks. The leakage apparently occurred during pre-test processing. As a result, the pressurization of the system 100 ms before firing damaged a check valve and resulted in the explosion.[71][72]
Since the destroyed capsule had been slated for use in the upcoming in-flight abort test, the explosion and investigation delayed that test and the subsequent crewed orbital test.[73]
The SuperDraco engine test that failed on 20 April 2019 was repeated successfully on 13 November 2019. The full duration static fire test of Crew Dragon's launch escape system took place at Cape Canaveral Air Force Station at SpaceX's Landing Zone 1 at 20:08 UTC. The test was successful, showing that the modifications made to the vehicle to prevent a failure like the one that happened 20 April 2019 were successful. The vehicle used for this ground test would also be used for the following in-flight abort test.[74]
Some of the modifications are:
Replacement of the valves with burst discs: Unlike valves, burst discs are designed for single use.
Addition of flaps on each SuperDraco in order to reseal the thrusters prior to splashdown in the ocean, preventing water intrusion.[75]
The Crew Dragon in-flight abort test was launched on 19 January 2020 at 15:30 UTC from LC-39A on a suborbital trajectory to conduct a separation and abort scenario in the troposphere at transonic velocities shortly after passing through max Q, where the vehicle experiences maximum aerodynamic pressure. The Dragon 2 used its SuperDraco abort engines to push itself away from the Falcon 9 after an intentional premature engine cutoff. Ten seconds after Dragon 2 was jettisoned, the Falcon 9 exploded and was destroyed. The spacecraft followed its suborbital trajectory to apogee, at which point the spacecraft's trunk was jettisoned. The smaller Draco engines were then used to orient the vehicle for the descent. All major functions were executed, including separation, engine firings, parachute deployment, and landing. Dragon 2 splashed down at 15:38:54 UTC just off the Florida coast in the Atlantic Ocean.[76] The test objective was to demonstrate the ability to safely move away from the ascending rocket under the most challenging atmospheric conditions of the flight trajectory, imposing the worst structural stress of a real flight on the rocket and spacecraft.[42] The abort test was performed using a Falcon 9 Block 5 rocket with a fully fueled second stage with a mass simulator replacing the Merlin engine.[77]
Earlier, this test had been scheduled before the uncrewed orbital test,[78] however, SpaceX and NASA considered it safer to use a flight representative capsule rather than the test article from the pad abort test.[79]
This test was previously planned to use the capsule C204 from Demo-1, however, C204 was destroyed in an explosion during a static fire testing on 20 April 2019.[80]Capsule C205, originally planned for Demo-2 was used for the In-Flight Abort Test[81] with C206 being planned for use during Demo-2. This was the final flight test of the spacecraft before it began carrying astronauts to the International Space Station under NASA's Commercial Crew Program.
Prior to the flight test, teams completed launch day procedures for the first crewed flight test, from suit-up to launch pad operations. The joint teams conducted full data reviews that needed to be completed prior to NASA astronauts flying on the system during SpaceX's Demo-2 mission.[82]
SpaceX Crew Dragon Endeavour as it approached the International Space StationEndeavour being recovered after splashdown
On 17 April 2020, NASA Administrator Jim Bridenstine announced the first crewed Crew Dragon Demo-2 to the International Space Station would launch on 27 May 2020.[83] Astronauts Bob Behnken and Doug Hurley crewed the mission, marking the first crewed launch to the International Space Station from U.S. soil since STS-135 in July 2011. The original launch was postponed to 30 May 2020 due to weather conditions at the launch site.[84] The second launch attempt was successful, with capsule C206, later named Endeavour by the crew, launching on 30 May 2020 19:22 UTC.[85][86] The capsule successfully docked with the International Space Station on 31 May 2020 at 14:27 UTC.[87] On 2 August 2020, Crew Dragon undocked and splashed-down successfully in the ocean.
Launching in the Dragon 2 spacecraft was described by astronaut Bob Behnken as "smooth off the pad" but "we were definitely driving and riding a dragon all the way up... a little bit less g's [than the Space Shuttle] but more 'alive' is probably the best way I would describe it".[88]
Regarding descent in the spacecraft, Behnken stated, "Once we descended a little bit into the atmosphere, Dragon really came alive. It started to fire thrusters and keep us pointed in the appropriate direction. The atmosphere starts to make noise - you can hear that rumble outside the vehicle. And as the vehicle tries to control, you feel a little bit of that shimmy in your body. ... We could feel those small rolls and pitches and yaws - all those little motions were things we picked up on inside the vehicle. ... All the separation events, from the trunk separation through the parachute firings, were very much like getting hit in the back of the chair with a baseball bat ... pretty light for the trunk separation but with the parachutes it was a pretty significant jolt."[89]
One of two Cargo Dragons in production as noted by Gwynne Shotwell after the Crew-1 launch on 15 November 2020.[98][99]
C2XX
N/A
Cargo
Under construction
0
TBD
One of two Cargo Dragons in production as noted by Gwynne Shotwell after the Crew-1 launch on 15 November 2020.[98][99]
C2XX
TBA
Crew
Under construction
0
TBD
One of three Crew Dragons in production as noted by Gwynne Shotwell after the Crew-1 launch on 15 November 2020.[98][99]
C2XX
TBA
Crew
Under construction
0
TBD
One of three Crew Dragons in production as noted by Gwynne Shotwell after the Crew-1 launch on 15 November 2020.[98][99]
C2XX
TBA
Crew
Under construction
0
TBD
One of three Crew Dragons in production as noted by Gwynne Shotwell after the Crew-1 launch on 15 November 2020.[98][99]
List of flights
As of October 2020, there are nine Crew Dragon flights contracted to carry crew; seven are contracted by NASA (one crewed test flight and six operational crew rotation missions),[100] and two (one each) contracted by private companies Axiom Space[101] and Space Adventures.[102]
First crewed flight test of Dragon capsule, flight extended from two weeks in order to allow the crew to bolster activity on the ISS ahead of Crew-1, including partaking in four spacewalks. The mission lasted 63 days, 23 hours and 25 minutes, while being docked for 62 days, 9 hours and 8 minutes.
Currently transporting four astronauts to the ISS for a six-month mission. Did not fly a Russian cosmonaut on board as originally planned, as Roscosmos had not yet certified the Crew Dragon vehicle as flight proven, so a third NASA astronaut was added instead of a Russian cosmonaut.[111]
The first all-civilian mission to orbit. The launch from Launch Complex 39A, carrying Isaacman, Proctor, Arceneaux and Sembroski, will reach a 540 km orbit and conduct science experiments and public outreach activities for 3 days before reentering the atmosphere for a water landing off the Florida coast.
Up to four space tourists in a flight between 3 and 5 days, on an elliptical orbit with the apogee three times that of the ISS, higher than the Earth orbital altitude record set by Gemini 11 in 1966.[50][118]
First SpaceX mission performed under the CRS-2 contract with NASA and the first flight of Cargo Dragon 2. Delivered the NanoRacksBishop Airlock Module to the ISS along with 6,400 pounds of cargo.
^Alongside the 3 other crew members, Megan McArthur is using the same seat of the SpaceXCrew DragonEndeavour in this mission which her husband, Bob Behnken, used in SpaceX Demo-2 mission, the first mission of the Endeavour capsule.[113]
^The European Portion of SpaceX Crew-2 is called Mission Alpha, which is headed by Thomas Pesquet shown by the logo, File:European Section Logo of SpaceX Crew-2 Mission Alpha.jpg
^The European Portion of SpaceX Crew-3 is called Mission Cosmic Kiss, which is headed by Matthias Maurer shown by the logo, File:Cosmic Kiss mission patch for European portion of SpaceX Crew-3.jpg
^ abcdefSpaceX (1 March 2019). "Dragon". SpaceX. Archived from the original on 2 March 2019. Retrieved 3 March 2019.
^ abAudit of Commercial Resupply Services to the International Space StationArchived 30 August 2018 at the Wayback Machine NASA 26 April 2018 Report No. IG-18-016 Quote: "For SpaceX, certification of the company's unproven cargo version of its Dragon 2 spacecraft for CRS-2 missions carries risk while the company works to resolve ongoing concerns related to software traceability and systems engineering processes" This article incorporates text from this source, which is in the public domain.
^Clark, Stephen (7 December 2019). "After redesigns, the finish line is in sight for SpaceX's Crew Dragon spaceship". Spaceflight Now. Archived from the original on 6 June 2020. Retrieved 1 June 2020. 'With [the addition of parachutes] and the angle of the seats, we could not get seven anymore', Shotwell said. "So now we only have four seats. That was kind of a big change for us".
^Gwynne Shotwell (21 March 2014). Broadcast 2212: Special Edition, interview with Gwynne Shotwell (audio file). The Space Show. Event occurs at 24:05–24:45 and 28:15–28:35. 2212. Archived from the original(mp3) on 22 March 2014. Retrieved 22 March 2014. we call it v2 for Dragon. That is the primary vehicle for crew, and we will retrofit it back to cargo.
^Parma, George (20 March 2011). "Overview of the NASA Docking System and the International Docking System Standard"(PDF). NASA. Archived from the original(PDF) on 15 October 2011. Retrieved 30 March 2012. iLIDS was later renamed the NASA Docking System (NDS), and will be NASA's implementation of an IDSS compatible docking system for all future U.S. vehicles This article incorporates text from this source, which is in the public domain.
^Bolden, Charles (9 May 2012). "2012-05-09_NASA_Response"(PDF). NASA. Archived from the original(PDF) on 15 February 2013. Retrieved 20 June 2012. This article incorporates text from this source, which is in the public domain.
^Clark, Stephen (9 October 2014). "NASA clears commercial crew contractors to resume work". Spaceflight Now. Archived from the original on 11 October 2014. Retrieved 10 October 2014. a highly-modified second-generation Dragon capsule fitted with myriad upgrades and changes – including new rocket thrusters, computers, a different outer mold line, and redesigned solar arrays – from the company's Dragon cargo delivery vehicle already flying to the space station.
^Richardson, Derek (30 July 2016). "Second SpaceX Crew Flight Ordered by NASA". Spaceflight Insider. Archived from the original on 6 August 2016. Retrieved 9 August 2016. Currently, the first uncrewed test of the spacecraft is expected to launch in May 2017. Sometime after that, SpaceX plans to conduct an in-flight abort to test the SuperDraco thrusters while the rocket is traveling through the area of maximum dynamic pressure – Max Q.
^Foust, Jeff (4 February 2016). "SpaceX seeks to accelerate Falcon 9 production and launch rates this year". SpaceNews. Archived from the original on 3 July 2020. Retrieved 6 February 2016. Shotwell said the company is planning an in-flight abort test of the Crew Dragon spacecraft before the end of this year, where the vehicle uses its thrusters to separate from a Falcon 9 rocket during ascent. That will be followed in 2017 by two demonstration flights to the International Space Station, the first without a crew and the second with astronauts on board, and then the first operational mission.
^Siceloff, Steven (1 July 2015). "More Fidelity for SpaceX In-Flight Abort Reduces Risk". NASA. Archived from the original on 16 June 2016. Retrieved 19 June 2016. In the updated plan, SpaceX would launch its uncrewed flight test (DM-1), refurbish the flight test vehicle, then conduct the in-flight abort test prior to the crew flight test. Using the same vehicle for the in-flight abort test will improve the realism of the ascent abort test and reduce risk. This article incorporates text from this source, which is in the public domain.
^"We were surprised a little bit at how smooth things were off the pad ... and our expectation was as we continued with the flight into second stage that things would basically get a lot smoother than the Space Shuttle did, but Dragon was huffing and puffing all the way into orbit, and we were definitely driving and riding a dragon all the way up, and so it was not quite the same ride, the smooth ride as the Space Shuttle was up to MECO. A little bit less g's but a little bit more "alive" is probably the best way I would describe it". NASA Astronauts Arrive at the International Space Station on SpaceX Spacecraft. 31 May 2020. Retrieved 31 May 2020. at 03:46:02
^ abcde"NASA SpaceX Post-Launch News Conference". SpaceX NASA Crew-1 News Conferences. NASA. 15 November 2020. Stephen Clark of SpaceflightNow:"How many SpaceX Dragon 2 capsules in production?" Gwynne Shotwell answers:"There's also the CRS-21 capsule. I think there's Two more Cargo capsules after that. And we'll have three additional Crew Dragon capsules. I believe, and most — they're all to some extent in Production, right now."
^"Megan to reuse Bob's demo-2 seat in crew-2 mission". aljazeera.com. 20 April 2020. File:Megan McArther of Crew-2 Mission seen using same seat that Bob Behnken used in Demo-2 mission.gifMegan McArthur of Crew-2 Mission seen using same seat that Bob Behnken used in Demo-2 mission
^"Launch Schedule". Spaceflight Now. 9 November 2020. Retrieved 11 November 2020.
^ abc"Microgravity Research Flights". Glenn Research Center. NASA. 25 February 2021. Retrieved 27 February 2021. This article incorporates text from this source, which is in the public domain.
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