This morning saw the end of an era for United Launch Alliance (ULA). At dawn, the defense contractor’s Atlas V rocket launched its final national security mission for the United States Space Force (USSF). USSF-51 placed a classified satellite into an undisclosed orbit. It was the 58th and final national security mission for the Atlas V, the ultimate member of a family of rockets which has served the United States since the dawn of the Space Age. While the Atlas will continue launching Starliner crew capsules and Amazon Kuiper satellites through the end of the decade, the military is transitioning all of its payloads to a new generation of launchers.
USSF-51 was not originally supposed to fly on an Atlas V. The Space Force awarded the contract to ULA, a joint venture between Boeing and Lockheed Martin, in 2020 as its first mission under the National Security Space Launch (NSSL) Phase 2 program. The company intended to launch the satellite on its next-generation Vulcan rocket. However, the mission shifted to an Atlas V in 2021, largely due to delays in the development of the BE-4 main engines which Blue Origin is building for Vulcan [1]. This decision ultimately led to a fittingly spectacular sendoff for the final military Atlas V.
Today’s launch utilized the “551” configuration of the Atlas V. This variant of the rocket, nicknamed “The Bruiser” by ULA CEO Tory Bruno, is the most powerful Atlas V in service. It features five strap-on solid rocket boosters to maximize thrust at liftoff, a fairing with a five-meter diameter to accommodate large payloads, and a Centaur upper stage with a single engine. Assembly of the rocket began on July 3rd when ULA teams secured the bronze-colored Atlas booster onto its mobile launch platform. The five smaller side boosters were then bolted into place, followed by the Centaur and the already-encapsulated classified satellite. The flight-ready rocket was rolled out to Space Launch Complex 41, the home of four generations of Boeing, Lockheed, and ULA rockets, on July 27th.
The Space Force’s final Atlas V mission began less than half an hour after sunrise. At 6:45 AM, the rocket ignited its two RD-180 main engines. Two seconds later, the five strap-on boosters simultaneously ignited, providing the thrust required to lift the rocket off the launch pad. The soft light and stark shadows of “golden hour” produced a beautiful sight for the Space Coast residents who witnessed the launch. Almost immediately, the Atlas V began pitching downrange to accelerate towards orbit. The rocket rapidly accelerated under the thrust of the five boosters, leaving an extensive plume of exhaust in its wake.
Just 35 seconds after liftoff, USSF-51 passed through Mach 1. One minute and 45 seconds after liftoff, the boosters depleted the last of their fuel and were jettisoned, tumbling away from the rest of the rocket. Just over three minutes after the Atlas V departed from SLC-41, the payload fairing followed suit. The next milestone in the flight was scheduled to be the separation of the first stage. However, ULA terminated its broadcast at the moment of fairing separation in order to protect the identity of the Space Force’s payload. The timing of the milestones for the remainder of the mission, including the number of burns which the Centaur will conduct and their durations, are classified in order to maximize the tactical advantage which USSF-51 will provide for military servicemembers.
The precise identity of USSF-51 is a closely-kept secret. Some outlets wrote that the satellite was bound for geosynchronous orbit [e.g. 2]. This is a popular destination for military spacecraft, as it allows them to stand vigil over a single point on Earth; leveraging Centaur’s 8-hour endurance to place satellites directly into these orbits is one of ULA’s specialties. However, even this has not been confirmed by ULA or the Department of Defense (DoD).
We know that USSF-51 is likely not a spy satellite. ULA has launched both Orion signal intelligence spacecraftand KENNAN high-resolution imaging missions in the past, but these launches are typically given National Reconnaissance Office (NRO) designations rather than USSF identifiers. Other Space Force Atlas V missions have launched Global Space Situational Awareness Program (GSSAP) satellites which inspect adversary spacecraft, Space-Base Infrared System (SBIRS) satellites which provide early warning of ballistic missile launches, and Advanced Extremely High Frequency (AEHF) satellites which facilitate communications between large networks of ships, aircraft, and ground forces.
These three classes of spacecraft are the most likely candidates for USSF-51’s payload. According to Space Operations Command (SpOC), the next GSSAP satellite was scheduled to be deployed this year [3]. This might be the most plausible identity for the newest member of the Space Force’s fleet based on current data. However, USSF-51’s true identity will likely remain mysterious for some time to come. “It’s one of the most critical payloads we’ve put up to date,” said ULA Vice President Gary Wentz [4].
Bruno confirmed that Atlas V successfully reached orbit [5]. This brought its 58th and final military mission to a close. USSF-51 was also ULA’s 100th mission for the DoD, its anchor customer. The Atlas family of rockets has a long and proud history of defending the United States’ national security. The initial Atlas variant, the SM-65, was America’s first intercontinental ballistic missile. It established nuclear deterrence with the Soviet Union at the height of the Cold War. Subsequent Atlas configurations were paired with the lightweight, efficient Centaur upper stage to enable orbital launches. Numerous upgrades have allowed the Atlas family to place a steady string of military payloads into orbit for over six decades, beginning with the MIDAS missile warning satellites in 1960.
Ultimately, the Atlas V was priced out of the market by SpaceX’s less expensive Falcon family. Beginning in 2015, ULA began developing the Vulcan rocket, a more affordable successor to the heritage Atlas and Delta vehicles which will ultimately become partially reusable. Thanks to Vulcan, ULA won the lion’s share of the NSSL Phase 2 contract and is well-positioned to play a keystone role in the upcoming Phase 3. The new rocket is currently scheduled to take over the Atlas V’s military workload at the end of this calendar year. ULA hopes that Vulcan will continue the company’s enviable 100% mission success rate for years to come.
As for the Atlas V, it still has 15 launches left on its manifest. These remaining rockets will deploy one satellite for ViaSat and the first eight clutches of satellites in Amazon’s Kuiper megaconstellation. Other Atlas boosters will launch Boeing’s Starliner capsule to the ISS on six crew rotation missions for NASA. Therefore, ULA’s workhorse rocket will occasionally grace the Space Coast through the end of the decade.
