United Launch Alliance has closed a major chapter in its launch history with the final Atlas 5 flight in a 551 configuration. The mission lifted off from Cape Canaveral Space Force Station early Thursday and delivered 29 Amazon Leo broadband satellites to orbit.
The launch, known as Amazon Leo 8 or Leo Atlas 8, began at 12:30:15 a.m. EDT from Space Launch Complex 41. ULA said the satellites were successfully deployed, marking another clean mission for a rocket family that has carried Amazon’s constellation through its early buildup.
Atlas 5 bows out after 22 flights in the 551 configuration
Atlas 5 551 has now flown 22 times, starting with NASA’s New Horizons mission to Pluto in 2006. Thursday’s rocket carried ULA’s internal designation AV-114 and became the 110th Atlas 5 launch overall.
The configuration combines a five-meter fairing, five solid rocket boosters, and a single-engine Centaur upper stage. ULA said only six Atlas 5 rockets remain, and all of them are reserved for Boeing’s CST-100 Starliner.
| Atlas 5 551 Fact | Detail |
|---|---|
| Final flight | Launched Amazon Leo 8 from Cape Canaveral |
| Total 551 launches | 22 |
| First 551 mission | New Horizons, launched Jan. 19, 2006 |
| Remaining Atlas 5 rockets | 6, all for Starliner |
Amazon Leo’s fleet keeps growing across multiple rockets
Amazon Leo Director of Launch Systems Melissa Wuerl said Atlas 5 “has played a critical role in the early deployment phase for Amazon Leo, launching 224 satellites with a 100 percent success rate across all eight missions.” She added that the company plans to continue expanding its network with ULA as it transitions to Vulcan.
With the latest deployment, Amazon Leo now has 396 satellites in low Earth orbit. The company says it is aiming for an initial commercial service rollout later this year, though it has not said how many satellites must be in orbit before that starts.
| Amazon Leo Launch Partners | Rockets Used | Status |
|---|---|---|
| ULA | Atlas 5, Vulcan | Atlas 5 active; Vulcan still preparing |
| Arianespace | Ariane 6 | Three missions flown |
| SpaceX | Falcon 9 | Three missions flown |
| Blue Origin | New Glenn | Ordered 27 launches, but grounded |
Weather, rollout and the move to Vulcan
The 45th Weather Squadron predicted an 85 percent chance of favorable weather in the 29-minute window, with only a small risk of cumulus cloud interference. Countdown activities began Wednesday after a launch readiness review, followed by rollout from the Vertical Integration Facility-Government to the pad.
ULA moved the 205-foot rocket on a Mobile Launch Platform along train tracks to Space Launch Complex 41, then began propellant loading after the vehicle was hard down on the pad. RP-1 fueling on the booster started at about 2:30 p.m. EDT and finished about an hour later.
Amazon Leo is not relying on ULA alone for its constellation buildout. The company has also purchased 38 Vulcan launches and 27 New Glenn launches, but both vehicles are currently grounded amid anomaly investigations.
Before the launch, Amazon Leo’s Steven Metayer said one more Ariane 6 mission is expected this year, while the first Vulcan flight carrying Amazon Leo satellites should happen sometime in the third quarter of 2026. ULA has already stacked Vulcan in its new VIF-A hangar and plans a wet dress rehearsal after the Atlas 5 farewell flight.
Spaceflight Now reported that the launch closed out the Atlas 5 551 line even as ULA prepares to shift more of its future work to Vulcan. For Amazon Leo, the mission brings the constellation closer to its planned service launch and keeps the satellite deployment pace moving despite delays around other rockets in its launch fleet.
