Just days after the Progress MS-32 cargo ship launched to the International Space Station from Kazakhstan, SpaceX launched the first flight of a stretched Northrop Grumman Cygnus XL cargo ship to the Station for an additional delivery of supplies.
The Cygnus XL cargo ship, named S.S. William C. “Willie” McCool after the pilot of Columbia on STS-107, successfully launched atop a Falcon 9 on the NG-23 mission on Sunday, Sept. 14, at 6:11 PM EDT (22:11 UTC).
Falcon 9 lifted off from Space Launch Complex 40 (SLC-40) at the Cape Canaveral Space Force Station (CCSFS) in Florida. The launch window was instantaneous, as is typically the case with many flights to the International Space Station (ISS). The U.S. Space Force’s 45th Weather Squadron’s forecast showed a 70% chance of favorable weather the day prior to launch.
Primary concerns were the cumulus cloud rule, the surface electric fields rule, and the lightning rule. Forecast conditions improved to 80% favorable for a 24-hour launch delay, with the same primary concerns remaining. There were low risks for solar activity and upper-level winds on both days.
Falcon booster B1094 flew on a northeast trajectory that sent the NG-23 Cygnus XL to an initial 245 km altitude circular parking orbit inclined 51.64 degrees to the equator, the same orbital inclination of the ISS. After stage separation, the booster safely conducted a return-to-launch-site landing at Landing Zone 2 (LZ-2).
B1094 is one of the youngest boosters in the fleet, flying its first mission, Starlink Group 12-10, from Launch Complex 39A (LC-39A) in late April. Its next mission, also from LC-39A, was the Axiom-4 crewed flight with Crew Dragon Grace. Its most recent flight prior to NG-23 was another crewed mission from LC-39A, Crew-11, with Crew Dragon Endeavour. This was B1094’s first flight from SLC-40 and the CCSFS.
Cygnus NG-21 during berthing operations in August 2024. (Credit: NASA)
The S.S. William C. McCool was scheduled to take roughly 60 hours to rendezvous with the ISS after launch, where the Canadarm2 robotic arm will grapple it before being berthed to the Station’s Unity node module nadir port on Wednesday, Sept. 17. The berthing was set for 10:35 UTC that day, but was postponed and Cygnus XL did not arrive on Sept. 17 as scheduled.
The spacecraft’s main engine stopped early during two orbit adjustment burns and controllers assessed the situation. A new engine burn plan was devised and the Cygnus XL was successfully berthed to the ISS on Thursday, Sept. 18 at 14:10 UTC.
The berthing process differs from docking in that the Cygnus XL and other spacecraft that are berthed do not connect to the Station under their own power. The Station’s Canadarm2 grapples the Cygnus XL at one of several grapple fixtures on the cargo ship, before an astronaut — in this case, NASA astronaut Jonny Kim — moves Cygnus XL to the common berthing mechanism on Unity via remote controls.
All versions of Cygnus are attached to the ISS using the same process that was used to add modules to the Station during its assembly. Other cargo ships like Progress and Cargo Dragon use their own thrusters to dock to the Station at ports designed for this purpose.
Infographic showing the Cygnus XL compared to an earlier version of the spacecraft. (Credit: Northrop Grumman)
The Cygnus XL spacecraft, capable of carrying up to 5,000 kg to ISS, can remain berthed to the Station for up to 200 days. Once unberthed and released, the spacecraft will deorbit and perform a destructive reentry in Earth’s atmosphere.
After unberthing and before reentry, however, the spacecraft can launch CubeSats and host experiments that require a free-flying environment. NG-23’s Cygnus XL is carrying several CubeSats for deployment after unberthing from the Station.
Cygnus XL is the latest development of the Cygnus cargo ship, which first flew in 2013. The Cygnus XL is a cylindrical spacecraft measuring eight meters in height and carrying 33% more cargo than earlier versions of the spacecraft.
The Cygnus XL’s cargo module arrival in the U.S. for the NG-23 mission. (Credit: Northrop Grumman)
Before this flight, NASA and Northrop Grumman studied how the latest stretched configuration of the Cygnus would interact with the Station and its systems to ensure its compatibility and certify the spacecraft for ISS operations.
The S.S. William C. McCool will be carrying spares for several systems to the Station, along with equipment to be installed during future spacewalks to replace failed devices. In addition, the spacecraft will bring a number of experiments for the Expedition 73 crew to work on, including 15 payloads sponsored by the ISS National Lab.
Spare station hardware onboard NG-23 includes a urine processing assembly, a distillation assembly, a power control module, a charcoal bed, two air selector valves, a mass spectrometer, sample distribution assemblies, treadmill isolator assemblies, and other spare parts.
An IDA planar reflector is being flown to the ISS as a replacement for a degraded unit and will be installed during a future spacewalk, while other parts are also being launched as replacements or upgrades. A pressure management device for vestibules between an ISS hatch and visiting vehicles or airlocks is also among the equipment onboard NG-23.
The Cygnus XL NG-23 spacecraft is named after STS-107 pilot William “Willie” McCool. (Credit: Northrop Grumman)
One experiment aboard NG-23 is an inflatable bag designed to capture space debris developed by TransAstra. This bag, known as the Capture Bag, will be tested in the Bishop Airlock at the end of the Station’s Tranquility module, and the testing will measure how the Capture Bag inflates in microgravity.
Besides the Capture Bag, there is also an experiment from Arizona State University and BioServe Space Technologies that will measure how germicidal ultraviolet light affects biofilms forming in space-based water systems. An experiment to crystallize small organic molecules, an experiment involving in-space fuel storage, and others are also aboard NG-23.
The NG-23 mission is scheduled to last until March 2026. Cygnus XL will be temporarily unberthed from Unity in November to support the Soyuz MS-28 docking to the Rassvet module on the Russian segment. There are contingency plans to end the NG-23 mission early if the spacecraft cannot be held safely by the Canadarm2 during the docking.
The Capture Bag shown during a fit check on a ground test version of the Bishop Airlock. (Credit: TransAstra Corporation)
Northrop Grumman plans to launch the next Cygnus spacecraft aboard another Falcon 9 while the Antares 330 is still under development, with “great progress” being reported. NG-23 is the first Cygnus spacecraft to fly since the NG-21 mission in August 2024, as the NG-22 spacecraft was damaged during shipping to the launch site earlier this year.
The next cargo flight to the Station after the NG-23 mission will be the first launch of Japan’s HTV-X cargo ship, which is currently scheduled for Oct. 21. The damaged NG-22 spacecraft will be manifested on a future mission, with the extent of its damage still being investigated. NG-24 will fly aboard a Falcon 9, and NG-25 is expected to be the first mission to launch atop the Antares 330.
(Lead image: The CRS NG-23 mission launching from Cape Canaveral on Sept. 14, 2025. Credit: Jerry Pike for NSF)
