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

Vierinen, Juha; Knach, Dabrowka; Chau, Jorge Luis; Baumgarten, Gerd; Huyghebaert, Devin; Clahsen, Matthias; Pfeffer, Nico; Renkwitz, Toralf; Wing, Robin; Obenberger, Kenneth; Gustavsson, Björn; Kastinen, Daniel

doi:10.5194/egusphere-2026-2857

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https://doi.org/10.5194/egusphere-2026-2857

Preprints

03 Jun 2026

| 03 Jun 2026

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

Juha Vierinen, Dabrowka Knach, Jorge Luis Chau, Gerd Baumgarten, Devin Huyghebaert, Matthias Clahsen, Nico Pfeffer, Toralf Renkwitz, Robin Wing, Kenneth Obenberger, Björn Gustavsson, and Daniel Kastinen

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.

Received: 18 May 2026 – Discussion started: 03 Jun 2026

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Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-2857', David Holdsworth, 26 Jun 2026
Review of Optical and Radar Observations of the February 2025 Falcon 9 Upper-Stage Re-entry" by Juha Vierinen et al.

This is a very interesting paper of interest to both the atmospheric physics and hypersonic vehicle detection communities, especially given the dearth of similar observations published for hypersonic vehicle detection owing to the classified nature of this topic.

The science described in the article appears to be sound. However, the authors assume the reader is as familiar with the topic as they are, and more effort has to be made to make their assertions clearer.

Due to the large number of comments below, I feel the paper needs revision before it is ready for publication.

Main concerns:

It is not clear what features or the radar echoes shown in Figure 6 are attributable to “specular” and “non-specular” echoes. There is some discussion on the discrimination of specular and non-specular echoes around line 264 but this does not make the distinction much clearer. It may be best to indicate this discrimination in the Figure.

Discussion about prior re-entry studies is limited to the paragraph starting on line 42, and focuses on plasma sheath studies. It may be worth the authors mentioning other work in re-entry detection, such as the use of weather radars (e.g. https://ui.adsabs.harvard.edu/abs/2025amos.conf...98J/abstract).

Line 3 of the abstract refers to “Optical observations of fragment emissions”. However, I believe the luminescence during spacecraft re‑entry between ~80 km and 30 km results primarily from the atmosphere constituents, not from the spacecraft fragments themselves. I therefore suggest that these words are corrected.

The paper often uses the terminology “optical fragments” - e.g. line 210. This is misleading and should probably be rephrased along the lines of “optimal signatures produced by the fragments”. Alternatively, if the authors want to continue using this terminology they should define the term “optical fragments” on its first use.

Line 269 states “The distribution indicates that fragment creation was most active between about 40 and 65 km altitude.” It is not clear how Panel a) shows this – it merely shows the distribution of the optical and radar detection altitudes, and neither distribution corresponds with the 40 and 65 km altitudes. I suspect the information regarding fragmentation altitudes actually comes from Figure 4?

Recommended revisions:

I feel that all references to the “Falcon 9 upper stage” should say “Falcon 9 upper stage rocket”, but I’ll leave that up to authors to judge.

Line 24 and 29, “0.36 kt out of 1.59 kt” and “1.6 kt corresponds to only about 400 ppt”. Suggest authors expand the acronyms kt and ppt for clarity.

Figure 1 caption – after “radar receivers”, add “green” before “circles”.

Line 75, “horizontally oriented” - I assume this means the longest (85 deg) dimension of the field of view? Please clarify.

Line 84, “an unaliased Doppler shift range”. May be clearer to say “a Doppler shift ambiguity” or “ambiguous Doppler shift”.

Line 101, “was” should be “were”.

Line 102, refers to “accurate plate solving and lens modeling”. A brief description of what these processes entail would be welcome.

Line 110, expand acronym “fps”.

Line 110, “Videos recorded at 25 fps were sampled at 1 s intervals (every 25th frame) for the triangulation runs.” 1) “sampled” should be “resampled”, 2), how was this resampling done? 25 sample decimation? Sum? Median?

Line 179, define omega

Line 188, define adrag.

Line 197 refers to “the specific heat ratio of the gas”. What gas, or mixture of gasses, has been assumed?

Line 206 states “The last triangulated fragment was observed at an altitude of 40 km at 3:47:25.” I assume this is from Fragment 1? This should be made explicit.

Figure 4a, I notice there are a few fragment labels (e.g. F6, Fb) missing? It may be worth explaining why this is the case. I assume these are fragments that could not be unambiguously attributed to F1 or F2? Or not associated with F1 or F2 at all?

FIgure 4 caption “Panel c) shows the positions of optical fragment and

radar detections of the re-entry plasma (time vs height)”. I’d argue that position implies lat/lon/altitude, whereas this figure only shows altitude.

Figure 5: subplots b and d look like fragment 1 family results are also plotted. However, the caption only references fragment 2 family?

Figure 7c, the ordinate axis should be “Doppler shift” rather than “Doppler”

Line 254, “The peak RCS is 61.5 dBsm”. This agrees well with the value quoted in the Lin reference (10^6 m^2), so I suggest the authors note this to validate their estimate.

Line 255 notes “An exponential decay fit” is applied in Figure 7. This corresponds to a linear fit in log coordinates, and the figure indeed shows RCS in log coordinates and what appears to be a linear fit. The authors should acknowledge this “apparent” inconsistency clearer to the reader.

Figure 8h. The symbols obscure the range-time intensity plot. I recommend the authors replace the symbols with lines which may cause less obscuration. It may also be worthwhile indicating the bistatic link on each plot or in the caption.

Line 266, suggest modify “with the exception of Bornholm” with something like “with the exception of bistatic links where Bornholm is the receiving node”.

FIgure 9 a) top axis -  suggest “Number of radar detections” is printed in red to make it clearer this corresponds with the top axis tick marks. Also, I assume the distributions are calculated using all cameras and bistatic radar links? This should be made more explicit.

Line 289 states “which is consistent with the fragments of the F2 family decaying faster than fragments of the F1 family, as seen in Figure 9c.” However, the fragment families are not labelled on Figure 9c so it is not clear this is the case.

Line 291 states “the Merlin Vacuum (MVac) engine, which is expected to be more heat-tolerant". Why is this expected to be the case?

Line 294 states “They were derived by converting the fitted ballistic coefficients...” This information should be moved to where Table 1 is first referenced.

Line 309 states “A small fraction of the re-entry object’s motion may still be retained, as a slight motion along the trajectory is detectable in the Doppler shift”. While it is true that the Doppler shifts in Figure 7b have the same sign as in Figure 7c, and therefore may justify the claim that “small fraction of the re-entry object’s motion may still be retained”, is it possible the Doppler shifts in Figure 7b may be due to the drift of the plasma trail due to atmospheric winds?

Line 339 “The enhancement of RCS at specular geometry...has a resemblance with overdense specular meteor trail echoes”. It’s not clear why this specifically resembles overdense echoes. Doesn’t it also resemble underdense echoes? I make this point as Figure 8 (between 3:46:05 and 3:46:08) looks very much like an underdense echo, albeit one with more pronounced Fresnel “oscillations” than typically seen for underdense echoes.

Line 355, “Knudsen number, which is the ratio of the mean free path to the object scale size Kn = λMFP/L”. This definition of the Knudsen number should be moved to where the Knudsen number is first introduces.

Line 359, should “Equation 9” be “(9)”?. Similarly, line *, “equation 5”

Line 374, “the re-entry behaviour of meteors”. This should be “the entry behaviour of meteors”. A similar change could also be made on line 349.

Line 421, “Objects of this size are already tracked by the U.S. Space Surveillance Network, so radar observations are not expected to provide substantial additional information about the number of re-entering objects”. I think this line undersells the utility of all-sky meteor radars for re-entry observations. The US SSN is comprised of a few large, exquisite (and expensive) systems that do not look everywhere simultaneously. Networks of all-sky meteor radars may be able to “fill in the gaps” of the US SSN.
Citation: https://doi.org/10.5194/egusphere-2026-2857-RC1
- AC1: 'Reply on RC1', Juha Vierinen, 13 Aug 2026
  
  We thank the referee for the careful and constructive review. We will address all the issues raised in the revised manuscript. In particular, we will clarify the distinction between specular and non-specular radar echoes and mark these features in the relevant figure; broaden the discussion of previous radar observations of re-entry; replace ambiguous terms such as “optical fragments”; define technical terms, symbols, and assumptions at first use; and correct and clarify the figure captions, labels, and references.
  Figure 9a is the intended reference for the statement concerning the 40–65 km altitude range. We agree, however, that its detection-altitude distributions alone do not demonstrate that fragment creation was most active in this interval. We will explain that the optical distribution in Figure 9a is used only as an observational proxy for the altitude range of fragmentation and mass deposition. As the referee points out, Figure 4 provides important complementary evidence by showing the appearance and branching of individual fragment trajectories. We will discuss how Figures 4 and 9a, together with the modeled kinetic-energy loss, indicate enhanced fragmentation, ablation, and likely mass deposition within this altitude range. We will also emphasize that this is not a direct measurement of deposited mass and is subject to observational biases related to camera coverage, sensitivity, fragment brightness, visibility duration, and fragment identification.
  We will also qualify the physical interpretation of the radar observations. In particular, we will acknowledge that the small Doppler shifts may include wind-driven plasma motion and that the observations do not uniquely classify the specular echoes as either underdense or overdense. Finally, we will revise the conclusion to better describe how distributed meteor-radar networks can complement dedicated space-surveillance systems during atmospheric re-entry.
  
  Citation: https://doi.org/10.5194/egusphere-2026-2857-AC1
RC2:
'Comment on egusphere-2026-2857', Anonymous Referee #2, 05 Jul 2026
This manuscript presents an analysis of the 19 February 2025 uncontrolled re-entry of a Falcon 9 upper stage, combining optical and radar observations to characterise the fragmentation and dynamics of the break-up. It derives altitudes, trajectories, energy-loss rates, fragment velocity distributions, and associated post-shock temperatures. These are currently very poorly known properties that are needed for accurate ablation modelling, which is in turn needed for studies of the atmospheric impact of space debris. I am not aware of any earlier study that has derived these quantities directly from a re-entry event, which makes this paper worthy of prompt publication.
The paper further shows that the production of plasma around re-entering orbital objects differs from that of meteors. This is perhaps to be expected, but to my knowledge it has not been experimentally verified before, and any implications of this for ion production in the atmosphere are of high importance. The result that radar observations are constrained to a much narrower altitude interval than optical observations is also valuable.
However, given the papers importance for atmospheric modelling and impact, I believe the authors could do a better job of making the paper easier to read for readers who are not experts in radar techniques.
Specific (minor) comments
Line 9 (Abstract): The radar cross-section (RCS) values are given in dBsm. Please briefly explain this unit, when it is first used, as readers from neighbouring fields may not be familiar with it.

Line 142: Please explain what the bistatic radar cross-section is more conceptually, rather than presenting only the formula, so that a reader can understand its physical meaning and why it is used here, not only the mathematics. Please also introduce/define σ_b explicitly at the point where the bistatic RCS is first introduced.

Figure 9c: The line colours used to distinguish the child fragments of the F1 and F2 families are difficult to tell apart; please use a more distinct colour scheme.

Lines 289-290: The text states "these components have relatively high area-to-mass ratios, which is consistent with the fragments of the F2 family decaying faster than fragments of the F1 family, as seen in Figure 9c". This is not visible from the figure as currently presented (perhaps due to comment #3). Please explain better or make the feature more visible in Figure 9c.

Lines 352-353: "This implies that atoms ablated from the heated spacecraft would not ionize efficiently through collisions with atmospheric molecules alone". Does this imply that space debris re-entry should produce a relatively higher proportion of neutral atoms compared to ions than natural meteor ablation does. If so, I think it is worth stressing this, since it would be relevant, for example, to studies seeking to identify space debris re-entry using spectroscopic signatures.

Technical corrections
line 300: "atmopsheric" → "atmospheric".

line 417: "many similar sytems around the world" → "many similar systems around the world".
Citation: https://doi.org/10.5194/egusphere-2026-2857-RC2
- AC2: 'Reply on RC2', Juha Vierinen, 13 Aug 2026
  
  We thank the referee for the positive assessment of the manuscript and for emphasizing the value of these observations for atmospheric-entry and ablation modeling. We agree that the paper should be more accessible to readers who are not specialists in radar techniques. In the revised manuscript, we will define dBsm at first use as decibels relative to one square metre and explain how logarithmic RCS values relate to values in square metres. We will also introduce σ_b explicitly and add a conceptual explanation of bistatic radar cross-section as a measure of the effective scattering strength observed when the transmitter and receiver are at different locations.
  We will improve Figure 9c by using more readily distinguishable colors for the F1 and F2 fragment families. We will also revise the associated discussion to explain more clearly how differences in area-to-mass ratio affect the fitted deceleration of the two families and make this behavior easier to identify in the figure.
  We thank the referee for raising the important question concerning the relative proportions of neutral and ionized species. Unfortunately, the present observations do not allow us to determine whether spacecraft re-entry produces a higher proportion of neutral ablated species relative to ions than natural meteor ablation. Earlier modeling studies suggest that the ionized species are produced primarily through ionization within the shock front. However, quantifying the relative abundances of ionized and neutral species originating from the ablated spacecraft material would require detailed thermochemical modeling together with spectroscopic or in situ observations. We will clarify this limitation in the revised manuscript and identify it as an important topic for future modeling and observational work. We will improve referencing to modeling work in the discussion when it comes to this topic.
  
  Finally, we will correct the typographical errors “atmopsheric” and “sytems.”
  
  Citation: https://doi.org/10.5194/egusphere-2026-2857-AC2
- AC3: 'Reply on RC2', Juha Vierinen, 13 Aug 2026
  
  We thank the referee for the positive assessment of the manuscript and for emphasizing the value of these observations for atmospheric-entry and ablation modeling. We agree that the paper should be more accessible to readers who are not specialists in radar techniques. In the revised manuscript, we will define dBsm at first use as decibels relative to one square metre and explain how logarithmic RCS values relate to values in square metres. We will also introduce σ_b explicitly and add a conceptual explanation of bistatic radar cross-section as a measure of the effective scattering strength observed when the transmitter and receiver are at different locations.
  We will improve Figure 9c by using more readily distinguishable colors for the F1 and F2 fragment families. We will also revise the associated discussion to explain more clearly how differences in area-to-mass ratio affect the fitted deceleration of the two families and make this behavior easier to identify in the figure.
  We thank the referee for raising the important question concerning the relative proportions of neutral and ionized species. Unfortunately, the present observations do not allow us to determine whether spacecraft re-entry produces a higher proportion of neutral ablated species relative to ions than natural meteor ablation. Earlier modeling studies suggest that the ionized species are produced primarily through ionization within the shock front. However, quantifying the relative abundances of ionized and neutral species originating from the ablated spacecraft material would require detailed thermochemical modeling together with spectroscopic or in situ observations. We will clarify this limitation in the revised manuscript and identify it as an important topic for future modeling and observational work. We will improve referencing to modeling work in the discussion when it comes to this topic.
  
  Finally, we will correct the typographical errors “atmopsheric” and “sytems.”
  
  Citation: https://doi.org/10.5194/egusphere-2026-2857-AC3

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Short summary

We observed the re-entry of a Falcon 9 rocket upper stage over Europe using a network of cameras and radars normally used to study meteors. The measurements showed how hot ionized gas formed around the debris during hypervelocity entry, producing strong radio echoes high in the atmosphere. As the number of satellites and rocket launches increases, these observations can help improve monitoring of space waste re-entering the atmosphere and its possible environmental effects.


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