arxiv: 2604.15807 · v1 · submitted 2026-04-17 · 🌌 astro-ph.EP · astro-ph.IM

Recognition: unknown

Asteroid modelling by starlight diffraction: The shape of Dimorphos, the satellite of (65803) Didymos

P. Tanga , K. Tsiganis , D. Souami , R. Anderson , E. Barbotin , A. Cazaux , F. Colas , J. Hanu\v{s}

show 12 more authors

F. Marchis J-L. Dauvergne G. Langin A. Leroy B. Lott A. Manna L. Rousselot A. Siakas S. Sposetti Ch. Vigna F. Weber A. W\"unche

Authors on Pith no claims yet

Pith reviewed 2026-05-10 07:57 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.IM

keywords Dimorphosstellar occultationdiffraction modelingasteroid shapeDidymosDART impactellipsoidal fitpost-impact reshaping

0 comments

The pith

Modeling starlight diffraction on multiple occultation chords constrains the projected shape of Dimorphos to a consistent post-impact ellipsoid.

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

The paper shows how diffraction patterns from a single stellar occultation event can be used to determine the silhouette of Dimorphos, the small moon of Didymos, by fitting an ellipsoidal model to the observed light signatures across several chords. This yields a tightly bounded projected shape at the January 2023 epoch that aligns with independent measurements taken after the DART spacecraft impact. Extending the fit to a full three-dimensional triaxial ellipsoid produces axis ratios compatible with earlier analyses and points to an equatorially elongated form. The approach marks the first use of simultaneous multi-chord diffraction data for asteroid shape recovery and demonstrates that ground-based occultation records can supply useful size and shape information even for bodies too small for direct resolution.

Core claim

We modelled the diffraction signatures recorded on multiple occultation chords to constrain the projected shape and size of Dimorphos, assuming an ellipsoidal model. This is the first time diffraction observed simultaneously on several chords of a single event has been used for such a purpose. The projected shape at the epoch of the event is well constrained and consistent with recent post-DART determinations. When extended to a full three-dimensional ellipsoidal solution, the result remains compatible with previous studies, suggesting an equatorially elongated post-impact shape.

What carries the argument

The fitting of an ellipsoidal asteroid model to diffraction signatures recorded simultaneously on multiple stellar occultation chords.

If this is right

The projected dimensions at the 2023 occultation epoch match independent post-impact determinations from other techniques.
A full 3D ellipsoidal solution remains consistent with prior studies while indicating equatorial elongation after the DART impact.
Diffraction modeling on multi-chord occultations offers an independent ground-based route to size and shape constraints for small solar-system bodies.
Repeated occultations could track further evolution of the shape if reshaping continues.

Where Pith is reading between the lines

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

The method could be tested on other binary asteroid systems where one component is too faint for direct imaging.
If the equatorial elongation is confirmed, it would constrain models of how rubble-pile bodies redistribute mass after impulsive impacts.
Combining such occultation results with light-curve inversion might reduce the need for dedicated spacecraft visits to characterize small satellites.

Load-bearing premise

Dimorphos can be represented as a smooth triaxial ellipsoid whose occultation signal arises primarily from the projected silhouette rather than from unresolved topography, albedo patches, or other effects.

What would settle it

Spacecraft or radar measurements that yield principal axes differing by more than a few percent from the derived ellipsoidal dimensions at the corresponding epoch.

Figures

Figures reproduced from arXiv: 2604.15807 by A. Cazaux, A. Leroy, A. Manna, A. Siakas, A. W\"unche, B. Lott, Ch. Vigna, D. Souami, E. Barbotin, F. Colas, F. Marchis, F. Weber, G. Langin, J. Hanu\v{s}, J-L. Dauvergne, K. Tsiganis, L. Rousselot, P. Tanga, R. Anderson, S. Sposetti.

**Figure 1.** Figure 1: Photometry of the occultation chords exploited to model the shape of Dimorphos. Note that none of the chords show a total extinction of light, and that local increases in the star brightness are also present. The blue dots represent the photometry and its error bars, while the green crosses indicate the best model fit (Sect. 4). lengths. The resulting flux map was then sampled along the observed chords, t… view at source ↗

**Figure 2.** Figure 2: Geometry of the occultation on the fundamental plane. The chords that produced positive detections are reported for Didymos, along with their error bars (grey segments). The two profiles for Didymos represent the ellipsoidal model by Naidu et al. (2024) (blue) and the topographic model by DRACO (orange) . The chord segments to the left mark the approximate location and length of occultation features seen … view at source ↗

**Figure 3.** Figure 3: Diffraction pattern created by Dimorphos, with the geometry of the chords (dotted lines). The horizontal and vertical axis are, respectively, the along- and cross- track directions. The white ellipse represents the geometric projection of the shape of Dimorphos. The background shows the diffraction pattern sampled by the four observers, with a large Poisson spot at the centre. Only one chord (F2) is a g… view at source ↗

read the original abstract

The DART spacecraft impacted Dimorphos, the satellite of (65803) Didymos, in September 2022. Evidence of crater formation and possible global reshaping has been obtained indirectly from spacecraft and ground-based data. Since the impact, several stellar occultations by Didymos have been observed, but only one in particular, on January 21, 2023, can provide useful constraints on the size and shape of Dimorphos. We modelled the diffraction signatures recorded on multiple occultation chords to constrain the projected shape and size of Dimorphos, assuming an ellipsoidal model. This is the first time diffraction observed simultaneously on several chords of a single event has been used for such a purpose. The projected shape at the epoch of the event is well constrained and consistent with recent post-DART determinations. When extended to a full three-dimensional ellipsoidal solution, the result remains compatible with previous studies, suggesting an equatorially elongated post-impact shape.

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.

Desk Editor's Note private letter to a colleague

They used multi-chord diffraction from one occultation to constrain Dimorphos's projected shape and got consistency with post-DART results.

read the letter

The paper applies simultaneous diffraction signatures from several chords of a January 2023 stellar occultation to fit an ellipsoidal silhouette for Dimorphos. This is the first reported use of that approach for an asteroid shape, and the projected dimensions line up with independent post-impact measurements from DART and other sources. Extending the fit to a 3D ellipsoid suggests equatorial elongation, which matches the reshaping picture from the mission data. The consistency across techniques is the concrete takeaway here. The work is narrow but direct: one event, one body, one modeling choice. The soft spots are the usual ones for a single-occultation result. Everything depends on the assumption that a smooth triaxial ellipsoid captures the silhouette without major interference from topography or other effects, and the abstract gives no error budgets, exclusion rules, or synthetic-data tests. If the full paper shows those checks and they hold, the constraint is useful; if not, the numbers stay provisional. This is for readers who follow planetary-defense follow-up or ground-based asteroid techniques. Someone already working on Dimorphos or occultation modeling will get a new data point and a method to consider. It deserves peer review because the application is new enough and the result is timely enough to warrant referee scrutiny on the uncertainties and assumptions.

Referee Report

2 major / 3 minor

Summary. The paper models diffraction signatures from a stellar occultation by Dimorphos on 2023 January 21 using multiple chords. Assuming a triaxial ellipsoidal silhouette, the authors fit the observed patterns to constrain the projected shape and size at that epoch. They report the result is well constrained and consistent with post-DART determinations; extending the fit to a full 3D ellipsoid yields an equatorially elongated post-impact shape.

Significance. If the central result holds, the work provides the first demonstration of simultaneous multi-chord diffraction modeling for asteroid shape recovery. This technique supplies an independent, ground-based constraint on Dimorphos that complements spacecraft and radar data, and it supports inferences of global reshaping following the DART impact. The approach is falsifiable and could be applied to future occultations.

major comments (2)

[§3] §3 (diffraction modeling and fitting procedure): the manuscript asserts that the projected shape is 'well constrained' yet provides neither formal uncertainty estimates on the fitted semi-axes nor a covariance analysis of the multi-chord data. This omission is load-bearing for the primary claim.
[§3.1] §3.1 (model assumptions): no synthetic-data injection tests are described to verify that the diffraction-fitting pipeline recovers known ellipsoidal parameters when the input silhouettes are perturbed by realistic noise or unresolved topography. Such validation is required to substantiate that the observed signatures are dominated by the projected outline rather than by the weakest-assumption effects listed in the reader's note.

minor comments (3)

[Figures] Figure 2 and 3 captions should explicitly list the number of chords, the wavelength, and the adopted Fresnel scale so readers can assess the diffraction regime without returning to the text.
[§4] The 3D extrapolation paragraph would benefit from a short table comparing the derived axis ratios with the three most recent post-DART shape solutions cited in the discussion.
[§2] A sentence clarifying the data-exclusion criteria (e.g., which chords were discarded and why) is missing from the methods.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive report. The comments highlight important aspects of the analysis that require clarification and strengthening. We address each major comment below and will revise the manuscript accordingly to improve the rigor of the presentation.

read point-by-point responses

Referee: [§3] §3 (diffraction modeling and fitting procedure): the manuscript asserts that the projected shape is 'well constrained' yet provides neither formal uncertainty estimates on the fitted semi-axes nor a covariance analysis of the multi-chord data. This omission is load-bearing for the primary claim.

Authors: We agree that the claim of a 'well constrained' projected shape would benefit from quantitative support. The original manuscript presented the fit results through direct comparison of modeled and observed diffraction patterns across the chords but did not report formal uncertainties or covariances. In the revised version we will derive and include 1σ uncertainties on the fitted semi-axes from the multi-chord least-squares procedure and provide the associated covariance matrix (or correlation coefficients) to allow readers to assess the robustness of the solution. revision: yes
Referee: [§3.1] §3.1 (model assumptions): no synthetic-data injection tests are described to verify that the diffraction-fitting pipeline recovers known ellipsoidal parameters when the input silhouettes are perturbed by realistic noise or unresolved topography. Such validation is required to substantiate that the observed signatures are dominated by the projected outline rather than by the weakest-assumption effects listed in the reader's note.

Authors: We acknowledge that explicit end-to-end validation with synthetic data would strengthen confidence in the pipeline. The submitted manuscript did not contain such injection-recovery tests. We will add a dedicated subsection describing synthetic occultation datasets generated from known ellipsoids, perturbed with realistic noise levels and minor topographic deviations, and will demonstrate that the fitting procedure recovers the input parameters within the reported uncertainties. This will directly address the concern that the observed signatures could be influenced by unmodeled effects. revision: yes

Circularity Check

0 steps flagged

No significant circularity: shape parameters fitted directly to diffraction data

full rationale

The derivation chain consists of modeling diffraction signatures on multiple occultation chords by fitting an ellipsoidal silhouette to the observed data. This produces a projected shape constraint that is then compared (not derived from) independent post-DART results. No self-definitional loops, fitted inputs renamed as predictions, or load-bearing self-citations appear in the provided abstract or method description. The central result remains an independent data fit under the stated ellipsoidal assumption.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on fitting an ellipsoidal model to diffraction data from one occultation event; no independent verification of the model assumptions is provided in the abstract.

free parameters (1)

ellipsoid semi-axes
Three axis lengths fitted to match the observed diffraction chords.

axioms (1)

domain assumption Dimorphos is adequately described by a triaxial ellipsoid at the epoch of observation
Explicitly stated as the modeling assumption in the abstract.

pith-pipeline@v0.9.0 · 5574 in / 1078 out tokens · 21554 ms · 2026-05-10T07:57:52.020899+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

16 extracted references

[1]

Arimatsu, K., Tsumura, K., Usui, F., et al. 2019, 1

2019
[2]

2024, PASJ, 76, 940

Arimatsu, K., Yoshida, F., Hayamizu, T., et al. 2024, PASJ, 76, 940

2024
[3]

S., Ballouz, R

Barnouin, O. S., Ballouz, R. L., Marchi, S., et al. 2023, 54th LPSC, 2806, 2605

2023
[4]

T., Ernst, C

Daly, R. T., Ernst, C. M., Barnouin, O. S., et al. 2024, Planeary Science Journal, 5, 24 Gomes-Júnior, A. R., Morgado, B. E., Benedetti-Rossi, G., et al. 2022, MNRAS, 511, 1167

2024
[5]

Goodman, J. W. 1968, Introduction to Fourier optics | BibSonomy (New York, NY: McGraw-Hill)

1968
[6]

2020, MNRAS, 499, 4570–4590

Herald, D., Gault, D., Anderson, R., et al. 2020, MNRAS, 499, 4570–4590

2020
[7]

P., Chesley, S

Naidu, S. P., Chesley, S. R., Moskovitz, N., et al. 2024, The Planetary Sc. Journal, V olume 5, Issue 3, id.74, 11 pp., 5, 74

2024
[8]

J., Scheirich, P., et al

Pravec, P., Meyer, A. J., Scheirich, P., et al. 2024, Icarus, 418, 116138

2024
[9]

P., Fors, O., & Wang, P

Richichi, A., Chen, W. P., Fors, O., & Wang, P. F. 2011, A&A, 532, A101

2011
[10]

2008, The Kuiper Belt Ex- plored by Serendipitous Stellar Occultations (Barucci, M

Roques, F., Georgevits, G., & Doressoundiram, A. 2008, The Kuiper Belt Ex- plored by Serendipitous Stellar Occultations (Barucci, M. A. and Boehnhardt, H. and Cruikshank, D. P. and Morbidelli, A. and Dotson, R.), 545–556

2008
[11]

1987, AJ, 93, 1549

Roques, F., Moncuquet, M., & Sicardy, B. 1987, AJ, 93, 1549

1987
[12]

J., et al

Scheirich, P., Pravec, P., Meyer, A. J., et al. 2024, The Planetary Sc. Journal, 5, 17

2024
[13]

E., Ofek, E

Schlichting, H. E., Ofek, E. O., Wenz, M., et al. 2009, Nature, 462, 895–897

2009
[14]

& Dettwiller, L

Sicardy, B. & Dettwiller, L. 2026, A&A, 707, A208

2026
[15]

1999, Computing in Science and Engineering, 1, 77–83

Trester, S. 1999, Computing in Science and Engineering, 1, 77–83

1999
[16]

Zinzi, A., Hasselmann, P. H. A., Della Corte, V ., et al. 2024, The Planetary Sc. Journal, 5, 103 Article number, page 4 of 7 Tanga et al.: Diff raction modelling of the shape of Dimorphos Appendix A: Complementary tables and plots. Table A.1. Main properties of the occultation event. Closest geocentric approach to the star 2023-01-21 23:33:17.620 UT Star...

2024