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arxiv: 2605.29124 · v1 · pith:5RUGIVFInew · submitted 2026-05-27 · ⚛️ physics.space-ph

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

Juha Vierinen , Dabrowka Knach , Jorge L. Chau , Gerd Baumgarten , Devin Huyghebaert , Matthias Clahsen , Nico Pfeffer , Toralf Renkwitz
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Robin Wing Kenneth S. Obenberger Bj\"orn Gustavsson Daniel Kastinen
This is my paper

Pith reviewed 2026-06-29 08:56 UTC · model grok-4.3

classification ⚛️ physics.space-ph
keywords Falcon 9 re-entrymultistatic radarmeteor radarre-entry plasmaoptical observationsspacecraft re-entryradar cross-sectionStarlink
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The pith

Multistatic meteor radar systems detect Falcon 9 upper stage re-entry plasma consistent with optical fragment tracks.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper reports combined optical observations from 43 meteor cameras across central Europe and radar detections from the 32.55 MHz SIMONe Germany multistatic system of the February 2025 Falcon 9 upper stage re-entry. Optical data reconstructed 30 fragment trajectories between 85 and 36 km altitude, identified two main fragment families, and showed peak detections near 60 km where kinetic energy loss is greatest. Radar echoes appeared at 55-75 km with positions matching the optical results and revealed two echo types from re-entry plasma: specular trails up to 60 dBsm and short-lived non-specular trails of 20-30 dBsm. The authors conclude that comparable multistatic meteor radar systems deployed globally can detect re-entries of other spacecraft, including smaller objects such as Starlink satellites.

Core claim

Optical observations from 43 meteor cameras reconstructed 30 fragment trajectories and identified two main fragment families, with the optical detection-height distribution peaking near 60 km. Radar echoes at 55-75 km from the SIMONe Germany system matched the optical positions, showing specular trail echoes from overdense wake plasma with RCS up to 60 dBsm and short-lived non-specular trail echoes with RCS of 20-30 dBsm delayed by 1-2 s. The characteristic decay time of both echo types is about 1 s, and Knudsen numbers indicate continuum-flow conditions with shock-driven plasma production. These observations demonstrate that multistatic meteor radar systems can detect spacecraft re-entries.

What carries the argument

Multistatic radar detection of specular and non-specular trail echoes from re-entry plasma at 55-75 km altitude, cross-checked against optically reconstructed fragment trajectories.

If this is right

  • Re-entries of objects smaller than the Falcon 9 upper stage, such as Starlink satellites, may produce detectable radar echoes with similar systems.
  • Multistatic meteor radar systems deployed globally can monitor spacecraft re-entries in addition to natural meteors.
  • Radar data in the 55-75 km range captures the altitude of maximum kinetic energy loss during re-entry.
  • The two identified echo types provide distinct signatures of overdense wake plasma and delayed non-specular trails.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Existing global networks of multistatic meteor radars could be repurposed for routine tracking of re-entering space debris without new hardware.
  • The technique might enable statistical studies of re-entry plasma production across a range of object sizes and entry angles.
  • Expanded coverage could support real-time alerts for the final stages of uncontrolled re-entries.

Load-bearing premise

The radar echoes at 55-75 km are produced by the Falcon 9 re-entry plasma and are correctly associated with the optically observed fragments based solely on broad altitude overlap and positional consistency.

What would settle it

A quantitative analysis showing that radar-derived positions deviate from optically reconstructed fragment locations by more than the combined measurement uncertainties, or independent evidence that the echoes arise from unrelated atmospheric phenomena, would falsify the association.

Figures

Figures reproduced from arXiv: 2605.29124 by Bj\"orn Gustavsson, Dabrowka Knach, Daniel Kastinen, Devin Huyghebaert, Gerd Baumgarten, Jorge L. Chau, Juha Vierinen, Kenneth S. Obenberger, Matthias Clahsen, Nico Pfeffer, Robin Wing, Toralf Renkwitz.

Figure 1
Figure 1. Figure 1 [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Visibility intervals of the fragments as seen from the available AllSky7 stations, where the underscored number indicate the per station camera id. This overview helped identify compatible observation pairs for triangulation and ensured coverage of all time segments. same technique but with the SIMONe Peru system. Similar processing was also used to estimate the range and Doppler shift for a head echo asso… view at source ↗
Figure 3
Figure 3. Figure 3: Six representative AllSky7 frames of the re-entry debris, ordered by time. Panels a–f show AMS95 at 03:44:30, AMS21 at 03:45:30, AMS35 at 03:46:02, AMS16 at 03:46:20, AMS88 at 03:47:00, and AMS22 at 03:47:15. Fragment identifiers for triangulated fragments are shown with colored numbers and letters. Panels c) and d) are during the time when radar echoes of the re-entry plasma were observed (3:46:00-3:46:30… view at source ↗
Figure 4
Figure 4. Figure 4: Overview of the optical, dynamical, and radar results in time-altitude and longitude–altitude coordinates. Panel a) shows the triangulated optical fragment detections with the fragment identifier as a function of time and height. Panel b) shows a representative subset of ballistic fits colored by velocity relative to the atmosphere (longitude vs height). Panel c) shows the positions of optical fragment and… view at source ↗
Figure 5
Figure 5. Figure 5: Example ballistic fit for the fragment chain F2,F7,Fa,Fk. Gray markers show all triangulated optical positions, darker gray markers show the measurements used in this fit, the solid red curve is the best-fit trajectory, the light red curves show 100 uncertainty samples, and the dashed red curve shows the extrapolated trajectory toward the nominal impact point. Panel a) shows the latitude-longitude ground p… view at source ↗
Figure 6
Figure 6. Figure 6: (S + N)/N in dB with 100 Hz noise bandwidth as a function of time and propagation range for the eight SIMONe transmitter￾receiver links that detected the event. Each panel shows one bistatic link with a panel-specific propagation-range interval chosen to show the echo region. Optical fragment positions are overlaid only in panel h): open circles show the propagation ranges computed from triangulated optica… view at source ↗
Figure 7
Figure 7. Figure 7: Radar signatures for the Kühlungsborn-Hagenow link. Panel a) shows the estimated RCS as a function of time and propagation range. Panel b) shows the Doppler shift. Panel c) shows the Doppler shift predicted from the fitted trajectories of fragments F1 and F2. 4.4 Radar detections relative to the optical fragments Panel c) of [PITH_FULL_IMAGE:figures/full_fig_p015_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Estimated RCS as a function of time for the Kühlungsborn-Hagenow range gate with the highest (S+N)/N. The conversion from measured power to RCS uses the transmitter-to-target and target-to-receiver ranges estimated from the fitted trajectory of fragment F1. The dashed line shows an exponential fit over the interval 03:46:05–03:46:07 UTC [PITH_FULL_IMAGE:figures/full_fig_p016_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Summary of the altitude distribution and fitted dynamical evolution of the re-entry fragments. Panel a) shows the altitude histogram of all optical fragment detections from the triangulations together with the altitude histogram of radar detection heights from SIMONe. In panel a), the dashed Gaussian fit to the optical distribution indicates a broad maximum centered at 60 km with a standard deviation of 10… view at source ↗
read the original abstract

We investigate the February 19, 2025, re-entry of a Falcon 9 upper stage using optical observations from 43 meteor cameras across central Europe together with radar detections of re-entry plasma obtained with the 32.55 MHz SIMONe Germany multistatic radar system. Optical observations of fragment emissions between 85 and 36 km altitude were used to reconstruct 30 fragment trajectories, identify two main fragment families, and fit ballistic trajectories to estimate kinetic energy loss per unit mass. The optical detection-height distribution peaks near 60 km with a standard deviation of 10 km, and both optical and radar signatures occur in the same broad altitude region as the maximum kinetic-energy loss. Radar echoes were detected at altitudes between 55 and 75 km, and the radar-derived positions are consistent with those obtained from optical observations. Two distinct radar echo types associated with the re-entry plasma were identified: (1) specular trail echoes from overdense wake plasma, with radar cross-sections (RCS) of up to 60 dBsm, and (2) short-lived non-specular trail echoes with RCS values of 20--30 dBsm, exhibiting a delay of 1--2 s compared to optical signatures. The characteristic decay time of both echo types is approximately 1 s. In the radar-echo altitude range, the estimated Knudsen numbers for meter-scale fragments are well below unity, consistent with continuum-flow conditions and shock-driven plasma production rather than ordinary meteor-like impact ionization. These serendipitous radar observations demonstrate that the atmospheric re-entry of other spacecraft, including objects smaller than the Falcon 9 upper stage such as Starlink satellites, may likewise be detectable using comparable multistatic meteor radar systems deployed globally.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 1 minor

Summary. The paper reports optical observations from 43 meteor cameras across central Europe of the February 19, 2025 Falcon 9 upper-stage re-entry, reconstructing 30 fragment trajectories and identifying two main families with ballistic fits to estimate kinetic energy loss. These are combined with radar detections from the 32.55 MHz SIMONe Germany multistatic system, which recorded echoes at 55-75 km altitudes whose positions are stated to be consistent with the optical data. Two echo types are described (specular overdense trails with RCS up to 60 dBsm and non-specular trails of 20-30 dBsm with 1-2 s delay), both with ~1 s decay times. Knudsen numbers for meter-scale fragments in this altitude range are reported as well below unity, supporting continuum-flow plasma production. The work concludes that comparable global multistatic meteor radars can detect re-entries of smaller objects such as Starlink satellites.

Significance. If the radar-optical association is robust, the result demonstrates that existing multistatic meteor radar networks can serendipitously detect spacecraft re-entry plasma, extending observational capability to smaller objects without new dedicated hardware. The use of real multi-instrument data and the explicit link to continuum-flow conditions at 55-75 km provide a concrete observational benchmark for re-entry plasma signatures.

major comments (3)
  1. [Abstract] Abstract: The association of the 55-75 km radar echoes with the optically observed fragments rests only on broad altitude overlap (optical peak ~60 km) and an unspecified claim of 'positional consistency,' with no reported trajectory-fit residuals, timing offsets, covariance matrices, or background-rate statistics. This is load-bearing for the headline claim that the echoes originate from re-entry plasma rather than ordinary meteor trails or ionospheric structures.
  2. [Abstract] Abstract: No error bars, uncertainty estimates, or quantitative validation metrics are supplied for the RCS values (up to 60 dBsm and 20-30 dBsm), the ~1 s decay times, the 1-2 s delay of non-specular echoes, or the Knudsen-number calculations used to argue for continuum-flow conditions. These omissions leave the central observational claims only partially supported.
  3. [Abstract] Abstract: Data exclusion criteria for both optical trajectories and radar echoes are not described, nor is the procedure for matching the two datasets to the 30 reconstructed fragments. Without these, it is not possible to assess whether the reported consistency is statistically significant or could arise from chance overlap at the relevant altitudes.
minor comments (1)
  1. [Abstract] Abstract: The phrase 'radar-derived positions are consistent with those obtained from optical observations' should be replaced by a quantitative statement (e.g., mean offset and standard deviation) once the supporting analysis is added.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and valuable comments on our manuscript. We address each of the major comments below and indicate the revisions we will make to strengthen the paper.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The association of the 55-75 km radar echoes with the optically observed fragments rests only on broad altitude overlap (optical peak ~60 km) and an unspecified claim of 'positional consistency,' with no reported trajectory-fit residuals, timing offsets, covariance matrices, or background-rate statistics. This is load-bearing for the headline claim that the echoes originate from re-entry plasma rather than ordinary meteor trails or ionospheric structures.

    Authors: We agree that the current description in the abstract relies on a qualitative statement of consistency. The full manuscript contains detailed trajectory reconstructions from the optical data and position comparisons with radar echoes. To address this, we will revise the abstract and add a new paragraph in the results section providing quantitative measures including trajectory-fit residuals, timing offsets, and a statistical assessment of the association to demonstrate it is not due to chance overlap. revision: yes

  2. Referee: [Abstract] Abstract: No error bars, uncertainty estimates, or quantitative validation metrics are supplied for the RCS values (up to 60 dBsm and 20-30 dBsm), the ~1 s decay times, the 1-2 s delay of non-specular echoes, or the Knudsen-number calculations used to argue for continuum-flow conditions. These omissions leave the central observational claims only partially supported.

    Authors: We acknowledge that the abstract presents approximate values without uncertainties. The observations allow for estimation of these quantities, and we will include error bars and uncertainty estimates derived from the data variability and measurement precision in the revised abstract and main text. A brief description of the calculation methods for Knudsen numbers will also be added. revision: yes

  3. Referee: [Abstract] Abstract: Data exclusion criteria for both optical trajectories and radar echoes are not described, nor is the procedure for matching the two datasets to the 30 reconstructed fragments. Without these, it is not possible to assess whether the reported consistency is statistically significant or could arise from chance overlap at the relevant altitudes.

    Authors: The manuscript details the use of 43 meteor cameras for optical trajectories and the SIMONe radar system for echoes, but we agree that explicit criteria and matching procedures are needed for clarity. We will add a methods subsection describing the data exclusion criteria, quality control for trajectories, and the procedure used to associate radar echoes with the 30 optical fragments, including any statistical tests for significance. revision: yes

Circularity Check

0 steps flagged

Purely observational study; no derivation chain or self-referential elements present

full rationale

The manuscript reports serendipitous optical and radar observations of a known Falcon 9 re-entry event. Trajectory reconstruction and ballistic fitting are standard data-reduction steps applied to measured positions and times; they do not generate a 'prediction' that is then compared back to the same inputs. Radar-echo association rests on altitude overlap and positional consistency, which is an interpretive judgment rather than a mathematical reduction to fitted parameters or self-citations. No equations, uniqueness theorems, or ansatzes are invoked that could create any of the enumerated circularity patterns. The central claim is an empirical demonstration, not a derived result.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract was available; no explicit free parameters, axioms, or invented entities can be extracted beyond the implicit assumption that radar echoes originate from re-entry plasma.

pith-pipeline@v0.9.1-grok · 5904 in / 1208 out tokens · 26325 ms · 2026-06-29T08:56:18.443793+00:00 · methodology

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Works this paper leans on

24 extracted references · 17 canonical work pages

  1. [1]

    Chau, J. L. and Clahsen, M.: Empirical Phase Calibration for Multistatic Specular Meteor Radars Using a Beamforming Approach, Radio Science, 54, 60–71, https://doi.org/10.1029/2018RS006741,

    work page doi:10.1029/2018rs006741

  2. [2]

    L., Urco, J

    Chau, J. L., Urco, J. M., Vierinen, J. P., V olz, R. A., Clahsen, M., Pfeffer, N., and Trautner, J.: Novel specular meteor radar systems using coherent MIMO techniques to study the mesosphere and lower thermosphere, Atmospheric Measurement Techniques, 12, 2113–2127, https://doi.org/10.5194/amt-12-2113-2019,

    work page doi:10.5194/amt-12-2113-2019 2019

  3. [3]

    L., Urco, J

    Chau, J. L., Urco, J. M., Vierinen, J., Harding, B. J., Clahsen, M., Pfeffer, N., Kuyeng, K., Milla, M., and Erickson, P. J.: Multi- static specular meteor radar network in Peru: System description and initial results, Earth and Space Science, 8, e2020EA001 293, https://doi.org/10.1029/2020EA001293,

    work page doi:10.1029/2020ea001293

  4. [4]

    L., Urco, J

    Chau, J. L., Urco, J. M., Weber, T., and Aweda, J. O.: Atmospheric Radar Imaging Improvements Using Compressed Sensing and MIMO, pp. 369–400, Springer International Publishing, Cham, ISBN 978-3-031-09745-4, https://doi.org/10.1007/978-3-031-09745-4_12,

    work page doi:10.1007/978-3-031-09745-4_12

  5. [5]

    Close, S., Brown, P., Campbell-Brown, M., Oppenheim, M., and Colestock, P.: Meteor head echo radar data: Mass–velocity selection effects, Icarus, 186, 547–556, https://doi.org/10.1016/j.icarus.2006.09.007,

    work page doi:10.1016/j.icarus.2006.09.007 2006

  6. [6]

    Cong, Z., Chen, R., and He, Z.: Numerical modeling of EM scattering from plasma sheath: A review, Engineering Analysis with Boundary Elements, 135, 73–92, https://doi.org/10.1016/j.enganabound.2021.11.013,

    work page doi:10.1016/j.enganabound.2021.11.013 2021

  7. [7]

    T., Jones Jr, M., Siskind, D

    Emmert, J. T., Jones Jr, M., Siskind, D. E., Drob, D. P., Picone, J. M., Stevens, M. H., Bailey, S. M., Bender, S., Bernath, P. F., Funke, B., Hervig, M. E., and Pérot, K.: NRLMSIS 2.1: An Empirical Model of Nitric Oxide Incorporated Into MSIS, Journal of Geophysical Research: Space Physics, 127, e2022JA030 896, https://doi.org/10.1029/2022JA030896,

    work page doi:10.1029/2022ja030896

  8. [8]

    Evans, J. S. and Huber, P. W.: Calculated Radio Attenuation Due to Plasma Sheath on Hypersonic Blunt-Nosed Cone, Technical Note NASA TN D-2043, NASA,

    2043

  9. [9]

    Gustavsson, B.: AIDA_tools, https://github.com/BjornG-son/AIDA_tools, GitHub repository, accessed 2026-04-23,

    2026

  10. [10]

    He, X., Gao, C., and Jiang, T.: A Simplified Method for Calculating Spectral Emission of Nonequilibrium Air Plasmas in Hypersonic Shock-Layers, Advances in Aerodynamics, 3, 15, https://doi.org/10.1186/s42774-021-00070-1,

    work page doi:10.1186/s42774-021-00070-1

  11. [11]

    Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., Muñoz-Sabater, J., Nicolas, J., Peubey, C., Radu, R., Schepers, D., et al.: The ERA5 global reanalysis, Quarterly journal of the royal meteorological society, 146, 1999–2049,

    1999

  12. [12]

    Hersbach, H., Bell, B., Berrisford, P., Biavati, G., Horányi, A., Muñoz Sabater, J., Nicolas, J., Peubey, C., Radu, R., Rozum, I., Schepers, D., Simmons, A., Soci, C., Dee, D., and Thépaut, J.-N.: ERA5 hourly data on pressure levels from 1940 to present, https://doi.org/10.24381/cds.bd0915c6, accessed on 17-05-2026,

    work page doi:10.24381/cds.bd0915c6 1940

  13. [13]

    P., Grumer, J., Vierinen, J

    Hupe, P., Pilger, C., Assink, J., Schneider, S., Näsholm, S. P., Grumer, J., Vierinen, J. P., Knach, D., Morfa, Y ., Wing, R., Gerding, M., and Baumgarten, G.: Seismoacoustic analysis of a Falcon 9 rocket stage re-entry over central Europe on 19 February 2025, manuscript submitted toPure and Applied Geophysics,

    2025

  14. [14]

    C., Conte, J

    Huyghebaert, D., Vierinen, J., Gustavsson, B., Latteck, R., Renkwitz, T., Zecha, M., Stephan, C. C., Conte, J. F., Kastinen, D., Kero, J., and Chau, J. L.: Monitoring of Lower Thermospheric Neutral Density Variations Using Meteor Head Echoes, EGUsphere, 2025, 1–16, https://doi.org/10.5194/egusphere-2025-2323,

    work page doi:10.5194/egusphere-2025-2323 2025

  15. [15]

    Kruzynski, M., Zubowicz, T., Arminski, K., Karawacki, M., Teofilewicz, M., Wnuk, E., Aniol, Z., and Trocki, T.: The February 2025 re-entry event over Poland, in: The Advanced Maui Optical and Space Surveillance (AMOS) Technologies Conference, p. 109,

    2025

  16. [16]

    Lin, S.-C.: Radio echoes from a manned satellite during re-entry, Journal of Geophysical Research, 67, 3851–3870, https://doi.org/10.1029/JZ067i010p03851,

    work page doi:10.1029/jz067i010p03851

  17. [17]

    M., Portmann, R

    Maloney, C. M., Portmann, R. W., Ross, M. N., and Rosenlof, K. H.: Investigating the Potential Atmospheric Accumulation and Radiative Impact of the Coming Increase in Satellite Reentry Frequency, Journal of Geophysical Research: Atmospheres, 130, e2024JD042 442, https://doi.org/10.1029/2024JD042442,

    work page doi:10.1029/2024jd042442

  18. [18]

    M., Abou-Ghanem, M., Cziczo, D

    Murphy, D. M., Abou-Ghanem, M., Cziczo, D. J., Froyd, K. D., Jacquot, J., Lawler, M. J., Maloney, C., Plane, J. M., Ross, M. N., Schill, G. P., et al.: Metals from spacecraft reentry in stratospheric aerosol particles, Proceedings of the National Academy of Sciences, 120, e2313374 120, https://doi.org/10.1073/pnas.23133741,

    work page doi:10.1073/pnas.23133741

  19. [19]

    Qian, J.-W., Zhang, H.-L., and Xia, M.-Y .: Modelling of Electromagnetic Scattering by a Hypersonic Cone-Like Body in Near Space, International journal of antennas and propagation, 2017, 3049 532,

    2017

  20. [20]

    469–473, IEEE,

    Rubin, G., Carney, T., Floyd, J., Henderson, H., Mitchell, J., Siegel, A., and Vila, J.: Airborne radar measurements of the reentry of the Ariane 5 EPC, in: IEEE International Radar Conference, 2005., pp. 469–473, IEEE,

    2005

  21. [21]

    Advances in Space Research , author =

    Schulz, L., Glassmeier, K.-H., Herberhold, M., Mitchell, A., Murphy, D. M., Plane, J. M. C., and Plaschke, F.: Space waste: An update of the anthropogenic matter injection into Earth’s atmosphere, Advances in Space Research, 77, 9589–9616, https://doi.org/10.1016/j.asr.2026.03.026,

    work page doi:10.1016/j.asr.2026.03.026 2026

  22. [22]

    Turunen, E.: Incoherent scatter radar contributions to high latitude D-region aeronomy, Journal of Atmospheric and Terrestrial Physics, 58, 707–725, https://doi.org/10.1016/0021-9169(95)00069-0,

    work page doi:10.1016/0021-9169(95)00069-0

  23. [23]

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

    Vierinen, J.: Falcon9 data processing, https://github.com/jvierine/falcon9, GitHub repository, accessed 2026-05-25, 2026a. Vierinen, J.: Replication Data for "Optical and VHF Radar Observations of the February 2025 Falcon 9 Upper- Stage Re-entry", https://doi.org/10.5281/zenodo.20070800, 2026b. Vierinen, J., Chau, J. L., Pfeffer, N., Clahsen, M., and Stob...

    work page doi:10.5281/zenodo.20070800 2026

  24. [24]

    25 Wing, R., Gerding, M., Plane, J., Morfa, Y ., Urco, J. M., Yamazaki, Y ., Schulz, L., Höffner, J., Mielich, J., Renkwitz, T., et al.: Mea- surement of a Lithium Plume from the Uncontrolled Re-entry of a Falcon 9 Rocket, Communications Earth & Environment, 7, 161, https://doi.org/10.1038/s43247-025-03154-8,

    work page doi:10.1038/s43247-025-03154-8