SpaceX Falcon 9 Upper Stage

History

The SpaceX Falcon 9 upper stage, also called the second stage, is the expendable upper component of the Falcon 9 two-stage orbital launch vehicle. It uses a single vacuum-optimized Merlin engine to accelerate payloads after separation from the first stage and to place them into their required orbits.

The Falcon 9 made its first successful launch on June 4, 2010. SpaceX developed the vehicle in the United States, with its structures, engines, avionics, and ground systems designed, manufactured, and tested domestically. The upper stage formed part of the vehicle from its initial operational configuration.

One discarded Falcon 9 upper stage, cataloged as 2025-010D, remained in a distant Earth orbit after supporting a lunar mission in January 2025. A later report in the supplied material stated that a discarded Falcon 9 upper stage subsequently struck the Moon in an uncontrolled impact at about 8,700 km/h.

Development

Falcon 9 was developed by SpaceX as a two-stage rocket using liquid oxygen and rocket-grade kerosene, or RP-1. According to Spaceflight Now, the Falcon 9 second-stage tank was designed as a shorter version of the first-stage tank. It used much of the same tooling, materials, and manufacturing techniques, reducing the number of distinct production processes required for the vehicle.

The first Falcon 9 successfully launched on June 4, 2010, followed by another successful flight on December 8, 2010. The upper stage was powered by one Merlin engine, in contrast to the nine Merlin engines installed on the first stage.

2025-010D lunar trajectory

On January 15, 2025, a Falcon 9 launched the Blue Ghost and Hakuto-R lunar landers. The Falcon 9 upper stage from that launch received the catalog designation 2025-010D. It had propelled the payload combination from a low Earth orbit onto a trajectory capable of reaching lunar distance before the hardware separated.

Unlike most Falcon 9 upper stages that later re-enter the atmosphere or remain in other trajectories, 2025-010D continued in a highly elongated orbit around Earth. According to the tracking analysis published by Project Pluto, observations accumulated during 2025 and 2026 showed that the stage would encounter the Moon. Its trajectory was affected primarily by gravity, while solar radiation pressure introduced smaller uncertainties because the discarded stage was tumbling.

Tracking calculations made in September 2025 projected a lunar impact on August 5, 2026. By August 1, 2026, the predicted impact time had been refined to 06:35:37.5 UTC, with an estimated impact point near lunar latitude 19.461 degrees north and longitude 266.707 degrees east. The supplied later report stated that a discarded Falcon 9 upper stage did strike the lunar surface earlier in August 2026.

Design

Structure

The Falcon 9 upper stage is the smaller of the launch vehicle's two stages. Its propellant tank is based on the first-stage tank architecture but is shorter. Falcon 9 tank structures use aluminum-lithium alloy and friction-stir welding. Sharing manufacturing methods between the stages simplifies production compared with developing an entirely separate upper-stage tank architecture.

The upper and lower stages are connected during ascent by the Falcon 9 interstage. The interstage is a composite structure and contains the pneumatic mechanisms used to separate the stages. After separation, the upper-stage engine ignites and continues the mission independently.

Propulsion

The current Falcon 9 second stage is powered by one Merlin Vacuum engine. SpaceX lists a thrust of 981 kN and a burn time of 397 seconds for the second-stage engine. The engine uses liquid oxygen and RP-1 propellants.

The Merlin family uses a single-shaft, dual-impeller turbopump and a gas-generator cycle. Earlier Falcon 9 documentation also described dual redundant pyrophoric igniters using triethylaluminum-triethylborane, or TEA-TEB, to improve restart reliability. The upper-stage engine can be restarted multiple times when a mission requires separate burns to place payloads into different orbits.

Mission role

The upper stage takes over propulsion after first-stage separation. Its primary function is to deliver the payload to the desired orbit or departure trajectory. While the Falcon 9 first stage is designed for recovery and reuse on many missions, the upper stage is not recovered for reuse.

After payload deployment, the fate of an upper stage depends on its trajectory. Many stages operate in orbits from which they later re-enter Earth's atmosphere. Stages used for missions reaching lunar or interplanetary distances can remain in high-energy Earth orbits or enter heliocentric trajectories. The 2025-010D stage remained in an Earth orbit extending to approximately lunar distances before its lunar impact.

Operational history

Falcon 9 upper stages have supported the launch vehicle's missions since Falcon 9 entered flight testing in 2010. They provide the final powered phase needed to carry satellites, spacecraft, and other payloads from the trajectory established by the first stage to their intended orbital or departure conditions.

The 2025-010D stage illustrates the behavior of an expended upper stage following a high-energy lunar mission. Project Pluto reported that its orbit had a period of about 26 days, with a perigee of approximately 220,000 km and an apogee of approximately 510,000 km. The Moon's average distance from Earth lies within that broad orbital range, allowing repeated gravitational encounters.

The stage had a reported mass of about 4,900 kg during its later orbital phase. Its lunar impact velocity was calculated at approximately 2.43 km/s, equivalent to about 8,700 km/h. The impact was uncontrolled rather than a planned landing or deliberate terminal maneuver.

The event was not the first artificial-object impact on the Moon. Earlier space programs deliberately sent rocket stages and other hardware into the lunar surface, and another discarded upper stage created lunar craters in 2022. The Falcon 9 event nevertheless provided another opportunity to observe the trajectory and impact of a large piece of human-made space hardware at lunar distance.

Specifications (Falcon 9 second stage)

General characteristics

• Type: Orbital launch vehicle upper stage
• Manufacturer: SpaceX
• Propellants: Liquid oxygen and rocket-grade kerosene (RP-1)
• Tank construction: Aluminum-lithium alloy structure using manufacturing techniques shared with the Falcon 9 first-stage tank
• Number of engines: One Merlin Vacuum engine

Performance

• Engine thrust: 981 kN (220,500 lbf)
• Engine burn time: 397 seconds
• Restart capability: Multiple engine restarts are supported for missions requiring separate orbital insertion burns

2025-010D impact-stage data

• Catalog designation: 2025-010D
• Launch date: January 15, 2025
• Reported mass: About 4,900 kg
• Lunar impact speed: Approximately 2.43 km/s, or 8,700 km/h
• Impact nature: Uncontrolled

Related equipment

• SpaceX Falcon 9: The two-stage orbital launch vehicle of which the upper stage is the second powered stage.