Optical and Radar Observations of the February 2025 Falcon 9 Upper-Stage Re-entry

This paper presents a multi-instrumental analysis of the February 2025 Falcon 9 upper-stage re-entry, combining optical and radar data to characterize fragment trajectories, plasma dynamics, and echo types, thereby demonstrating the feasibility of using global multistatic meteor radar systems to detect the atmospheric re-entry of various spacecraft.

The Gist

Imagine a giant, empty rocket stage (the top part of a SpaceX Falcon 9) coming back down to Earth from space. It's like a heavy, empty soda can falling from the sky. On February 19, 2025, this "can" broke apart over central Europe.

This paper is like a detective story where scientists used two different sets of "eyes" to watch this breakup: cameras that saw the glowing pieces of metal, and radars that "heard" the invisible electric clouds (plasma) created by the heat.

Here is the simple breakdown of what they found:

1. The Two Sets of "Eyes"

• The Cameras (The Visuals): Scientists used 43 different cameras across Europe (like a giant security camera network) to take pictures of the glowing fragments. By looking at the same object from different angles, they could build a 3D map of where every piece was flying. They tracked 30 different fragments as they fell from 85 km down to 36 km high.
• The Radar (The Invisible Clouds): They also used a special radar system in Germany. This radar doesn't just bounce off solid metal; it bounces off the super-hot, electric "soup" (plasma) that forms around the pieces as they burn up in the atmosphere.

2. The "Family" of Fragments

As the rocket fell, it didn't just break into random pieces; it split into two main "families" of debris:

• Family F1 (The Heavy Engine): This was the brighter, hotter, and heavier piece. The scientists think this was the rocket's vacuum engine. It stayed together longer and fell deeper.
• Family F2 (The Fuel Tank): This was the lighter, thinner piece. The scientists think this was the fuel tank. It broke apart more easily, and the pieces they found on the ground in Poland (like thin metal sheets and tank parts) came from this family.

The Analogy: Imagine dropping a heavy, dense rock and a thin, hollow cardboard box from a plane. The rock (F1) stays together and falls fast. The box (F2) rips apart easily into many small pieces that flutter down. That's what happened here.

3. The "Ghost" Trail (The Radar Mystery)

This is the most interesting part. The radar saw two types of signals:

• The "Specular" Echo (The Mirror): When the radar beam hit the plasma cloud at just the right angle (like a mirror reflecting a flashlight), it got a huge, bright signal. This happened when the fragments were about 60 km high.
• The "Non-Specular" Echo (The Wake): The radar also saw a fainter signal that appeared 1 to 2 seconds after the cameras saw the bright piece.

The Analogy: Think of a speedboat on a lake.

• The cameras see the boat itself.
• The radar sees the boat and the wake (the choppy water) trailing behind it.
• The "wake" (the plasma turbulence) takes a second or two to form and then fades away quickly (in about 1 second). The radar was catching this "wake" of electric gas, not just the metal piece itself.

4. Why Did It Glow? (The Physics)

Usually, meteors (space rocks) glow because they hit air molecules so hard they knock electrons off (like rubbing a balloon on your hair). But this rocket was falling slower than a typical meteor.

The scientists found that the rocket pieces were large enough (about the size of a small car or a room) and falling fast enough that they created a shockwave.

• The Analogy: Imagine a supersonic jet breaking the sound barrier. It creates a shockwave. This rocket created a similar "shockwave" in the air, but because it was so hot, the air turned into a super-heated electric soup (plasma) before it even hit the ground. This plasma is what the radar detected.

5. Why Does This Matter?

The paper explains that as space gets more crowded with satellites and rockets, more of this "space trash" is burning up in our atmosphere.

• The "Ash" Analogy: When a rocket burns up, it leaves behind "ash" (metal particles) in the sky. We don't know exactly how much "ash" is falling or where it lands.
• The Solution: This study shows that we can use existing weather radars and camera networks (which are already everywhere) to track exactly where this "ash" is being deposited. It's like using a smoke detector to figure out where a fire is burning, even if we can't see the fire directly.

Summary

The scientists watched a SpaceX rocket stage break apart. They used cameras to see the glowing metal and radar to see the invisible electric clouds trailing behind it. They learned that the heavy engine part stayed together longer, while the fuel tank broke apart early. Most importantly, they proved that we can use standard radar systems to track the "electric wake" of falling space junk, which helps us understand how space debris affects our atmosphere.

Technical Summary

Technical Summary: Optical and Radar Observations of the February 2025 Falcon 9 Upper-Stage Re-entry

Problem Statement
The rapid commercialization of space has significantly increased the mass and surface area of objects in Low Earth Orbit (LEO), leading to a projected 2–3 fold increase in annual atmospheric re-entries over the next decade. A significant fraction of this mass influx originates from spent upper stages of launch vehicles, such as the Falcon 9. These re-entries introduce anthropogenic metals into the middle atmosphere, potentially influencing aerosol composition, stratospheric chemistry, and radiative balance. While theoretical work exists on radar scattering from hypervelocity entry plasma, observational data on re-entering orbital spacecraft remains scarce compared to meteor studies. Furthermore, understanding the specific dynamics of fragmentation, plasma production mechanisms (shock-driven vs. impact ionization), and the altitude of mass deposition is critical for assessing ground safety risks and atmospheric impacts. This study addresses the need for a multi-instrument characterization of a specific Falcon 9 upper-stage re-entry to constrain these processes.

Methodology
The study utilizes a synergistic dataset combining optical and radar observations of the February 19, 2025, re-entry of a Falcon 9 upper stage (Starlink 11-4 mission) over central Europe.

• Optical Observations: Data were acquired from the European AllSky7 Fireball Network, comprising 43 meteor cameras across central Europe and the UK. The network uses NetSurveillance cameras with Sony Starvis sensors recording at 25 fps. The authors performed stereoscopic triangulation of 30 fragment trajectories using dual-camera observations. A ballistic trajectory model was fitted to these optical positions to estimate kinematic parameters, including velocity, ballistic coefficient, and specific kinetic-energy loss rates.
• Radar Observations: Simultaneous radar detections were obtained using the SIMONe Germany multistatic radar system (32.55 MHz). This network utilizes interferometric direction finding and range-Doppler matched filtering to detect plasma signatures. The system processed data from eight transmit-receive links to determine scattering locations, Radar Cross-Sections (RCS), and Doppler shifts.
• Analysis: The study correlated optical fragment trajectories with radar echo positions. It employed a point-mass dynamic model incorporating atmospheric drag (NRLMSIS 2.1) and wind corrections (ERA5) to reconstruct the re-entry path. The authors also estimated post-shock temperatures and Knudsen numbers to evaluate the flow regime and ionization mechanisms.

Key Contributions and Results

1. Fragment Trajectory Reconstruction: The authors successfully reconstructed 30 fragment trajectories, identifying two primary fragment families:
   • Family F1: A higher-altitude, optically brighter fragment family, hypothesized to be the Merlin Vacuum (MVac) engine.
   • Family F2: A lower-altitude family associated with the propellant tank structure. This family is linked to the nine ground-recovered fragments (carbon fiber tank parts and metallic sheets) found in Poland.
2. Dynamics and Energy Loss: Ballistic fits indicate that the maximum specific kinetic-energy loss occurs between 30 and 70 km altitude. The optical detection-height distribution peaks at approximately 60 km (standard deviation of 10 km), which aligns with the altitude of maximum energy dissipation and the region where radar echoes were detected.
3. Radar Signature Characterization: Two distinct radar echo types were identified, both exhibiting a characteristic decay time of approximately 1 second:
   • Specular Trail Echoes: Overdense wake plasma echoes with RCS values up to 60 dBsm, occurring near the specular aspect angle.
   • Non-Specular Trail Echoes: Short-lived echoes with RCS values of 20–30 dBsm. These echoes appear 1–2 seconds after the optical signatures, suggesting they originate from turbulent wake plasma that takes time to develop.
4. Ionization Mechanism: The study concludes that the observed plasma is generated by a high-temperature shock front rather than ordinary meteor-like impact ionization. This is supported by the low velocities (6–7 km/s) of the fragments, which fall below the threshold for impact ionization, and the low Knudsen numbers ($Kn < 0.01$) for meter-scale fragments, indicating continuum-flow conditions necessary for shock-driven ionization.
5. Size Threshold: The radar observations suggest a lower size threshold for detectability. Only fragments estimated to be in the 0.5–5 meter range produced detectable plasma echoes, implying that smaller objects may not generate sufficient plasma density for detection by current multistatic meteor radars.

Significance and Claims
The paper claims that these serendipitous observations demonstrate the capability of global multistatic meteor radar systems to detect the atmospheric re-entry of spacecraft, including objects smaller than the Falcon 9 upper stage (such as Starlink satellites). The study provides an observational benchmark for the altitude of fragmentation and mass deposition (peaking near 60 km), which offers a useful constraint for models of atmospheric material deposition.

The authors emphasize that while the radar system did not detect a "meteor head echo" (plasma surrounding the ablating object moving at the same velocity), it successfully detected wake turbulence and specular trail echoes. This distinction is crucial for understanding the physics of re-entry plasma, which differs from meteoric plasma due to the larger size and different velocity regimes of spacecraft. The study concludes that combining optical and radar data provides independent information on the altitude range where plasma production and mass loss occur, which is valuable for improving ground-impact hazard assessments and understanding the impact of space debris on the middle atmosphere. The authors note that further modeling, such as using SCARAB, is required to precisely determine mass deposition distributions, but the current dataset serves as a foundational observational constraint.