Peter Scheirich

The 2029 close approach between asteroid 99942 Apophis and Earth represents a unique opportunity to study the influence of tidal encounters on the deformation and rotational evolution of an S-complex near-Earth object in the 0.1–1 km size range—a class of object of particular interest to the planetary defense community. The encounter may give insight into the internal structure of the body, presenting an opportunity for our first direct probe of an asteroid’s interior. We present results from numerical simulations of the Apophis–Earth tidal encounter, modeling Apophis as a self-gravitating granular aggregate of meter-sized spherical particles, with the aim of constraining measurable rotation change, deformation, and seismicity induced in Apophis during the encounter, in support of potential rendezvous missions. We vary particle size distributions, particle shapes, packing configurations, and interior structures (with large embedded cores) to examine the effects that internal structure has on these measurable encounter outcomes. We find that the packing configuration with the lowest shear strength exhibits the most deformation by a factor of 4 over the most compact configuration, and the presence of internal cores in a low shear strength configuration can reduce the stress felt by the asteroid. We also find that short-period seismic events are likely to occur in the shallow subsurface layers of the body in the hours just following the time of perigee. These events have amplitudes on the order of 10 ⁻⁴ m s ⁻² Hz ^−1/2 in the 0.1–1 Hz range, which should be measurable with the latest seismic instruments developed for asteroid interior characterization.

Using thorough photometric observations taken from 1994 to 2024, we derived physical and dynamical properties of two binary near-Earth asteroids (175706) 1996 FG3 and (385186) 1994 AW1.For 1996 FG3, we obtained a unique solution with a quadratic drift of the mean anomaly of the satellite of 0.018 ± 0.040 deg/yr2 (all quoted uncertainties correspond to 3 σ), i.e., consistent with zero. This means that the drift of the semimajor axis of the mutual orbit is also consistent with zero, specifically, it is −0.026 ± 0.06 cm/yr. The zero drift for this system was already observed by Scheirich et al. (2015), but our new data constrain it with less uncertainty.For 1994 AW1, we determined that the quadratic drift of the mean anomaly is 0.034 ± 0.021 deg/yr2, implying a small, but non-zero inward drift in semimajor axis of −0.063 ± 0.05 cm/yr.The zero drift of 1996 FG3 suggests that the system is in a state of stable equilibrium between the BYORP effect (Jacobson & Scheeres, 2011), binary Yarkovsky effect (Zhou, 2024) and mutual tides.The inward drift of 1994 AW1 indicates that either BYORP or the binary Yarkovsky effect must be acting on the system, as this cannot be attributed to tidal expansion. The small value of the drift with a near-zero 3σ uncertainty suggests that if the BYORP is causing the drift, the system is very close to the tidal-BYORP equilibrium.We will present the implications for the two systems dynamics and the BYORP and binary Yarkovsky effects.Figure: Time evolution of the mean anomaly difference ΔM with respect to the reference solution of 1994 AW1 with no drift in mean anomaly. Black symbols correspond to lightcurve mutual events covered by the observed data. Vertical error bars represent estimated 1σ uncertainties of the event times, expressed in the mean anomaly. A quadratic fit to the data points, represented by the black curve, gives the quadratic term of 0.034 deg/yr2. The plot was constructed as follows. We generated a synthetic lightcurve using a model with mean anomaly drift fixed at zero. Then, for each light-curve event separately, we fitted the mean anomaly of the model in order to obtain the best match between its synthetic lightcurve and the observed data. ΔM is the difference between the mean anomaly of the original model and the fitted one. AcknowledgmentsThe work by the authors from [1] was supported by the Grant Agency of the Czech Republic, Grant 23-04946S.ReferencesScheirich, P., et al., 2015. Icarus 245, 56–63.Jacobson, S.A., Scheeres, D.J., 2011. The Astrophysical Journal Letters 736, L19.Zhou, W.H., 2024. A&A 692, L2.

On 2022 September 26, NASA's DART mission intentionally impacted Dimorphos, the moon of near-Earth asteroid (65803) Didymos, changing the binary system’s orbital period. The system was studied intensively from the ground over the ensuing months until February 2023 when its brightness and low solar elongation precluded further observations. These observations revealed a clear period change of 33 minutes due to the DART impact (Thomas et al. 2023). However, there is uncertainty on whether there was a single impulsive change in the orbital period resulting in a constant post-impact value, or if there is (or was) an evolution in the period with time as the system settled into a new configuration. Careful analysis of the full post-DART lightcurve dataset (through February 2023) showed that there was insufficient data to distinguish between constant or changing post-impact orbital periods (Naidu et al. 2024, Scheirich et al. 2024). In order to understand the final outcome of the DART mission, it is therefore critical to monitor the evolution of the Didymos system prior to the European Space Agency’s Hera mission arrival in late 2026. Observations from Hera will be too far removed from the time of impact to reveal whether or not the orbit period was changing over months as the asteroid system reached a new equilibrium. Our team obtained data during 2024 June to August on Magellan (6.5-m, PI: Thomas), SOAR (4.1-m, Program ID 2024A-120042), NTT (3.6-m, PI: Snodgrass), and Faulkes Telescope South (2-m, PI: Lister), but poor weather at all sites plus Didymos’s positioning in front of the galactic plane throughout the visibility window prevented our analyses from attaining the needed sensitivity. The next visibility window occurred from late January through early March 2025, and our team was awarded time on Faulkes Telescope North (FTN, 2-m, PI: Lister), Lowell Discovery Telescope (LDT, 4.3-m, PI: Moskovitz), Palomar (5.1-m, PI: Chesley), Gemini-N (8.1-m, Program ID GN-2025A-Q-142), and Gran Telescopio Canarias (GTC, 10.4-m, PI: de Leon). Here we report on Gemini-N observations obtained on 2025 February 24, 26, and 28 when Didymos was at an apparent V magnitude about 20.3. The GTC observing run was weathered out. Observations acquired with FTN, LDT, and Palomar were successfully acquired, but are still being analyzed. Our Gemini observations were timed to optimize coverage of the Didymos binary orbit within the run and to provide complementary observations with neighboring runs scheduled on Palomar and GTC. We observed Didymos for 3.7 hr at the start of each night using the SDSS-i filter. We tracked at the sidereal rate and limited individual exposure times to 20 sec to keep the asteroid’s trailing from significantly exceeding the stellar point spread function. Images were processed using the Gemini DRAGONS pipeline (Labrie et al. 2023) and photometry was measured using Photometry Pipeline (Mommert 2017). After manually removing frames that were contaminated by nearby stars or other image artifacts, lightcurve deconvolution was performed using the binary asteroid lightcurve decomposition method as earlier Didymos datasets (Pravec et al. 2022, 2024). Our preliminary analysis finds that mutual events occurred about 70 min earlier than the nominal prediction (see Figure 1), a difference of 1.6-sigma. This yields a binary orbital period of 11.3667 +/- 0.0002 hr (3-sigma), assuming that the period has been constant since 2022/2023. With the much longer time baseline, these observations reduce the uncertainty in the binary period by about a factor of 6 compared to the previously published 3-sigma measurement of 11.3675 +/- 0.0012 hr (Scheirich et al. 2024). As expected, the data quality was insufficient to constrain Dimorphos’ rotation (which needed rms residuals of 0.004-0.007 mag, while residuals of 0.017 mag were obtained). Deep stacking of all images collected on a night did not reveal evidence of a tail or any remaining large fragments, though we have not yet quantified these non-detections. We will provide updated results on the full 2025 dataset. If ongoing analyses of the FTN, LDT, and/or Palomar datasets yield sufficiently small rms residuals, the lightcurve deconvolution will be re-run on the larger dataset, though the solution is not expected to change appreciably. The question of whether or not the orbital period has changed or if the apparently shorter period is just a statistical fluke is unlikely to be resolved from these data since they were all acquired relatively close to the Gemini observations. We plan to propose for similar observations during Didymos’s next apparition in 2026 July to attempt to resolve this question.Acknowledgements: The work at Ondřejov has been supported by the "Praemium Academiae" award by the Academy of Sciences of the Czech Republic, grant AP2401.References Labrie et al., RNAAS 7, id.214 (2023) Mommert, M. Astronomy & Computing 18, 47 (2017) Naidu et al., PSJ 5, 74 (2024) Pravec et al., PSJ 3, 175 (2022) Pravec et al., Icarus 418, id.116138 (2024) Scheirich et al., PSJ 5, 17 (2024) Thomas et al., Nature 616, 448 (2023) Figure 1: Decomposition of Didymos system lightcurve from 2025 February 24-28 (a) into signals from mutual events (b), and primary rotation (c). The top panel (a) shows the combined lightcurve over 11.37 hr. The full lightcurve can be decomposed into a contribution from the 2.260-hr rotation of Didymos (bottom panel c) and a contribution due to mutual events. The mutual events are indicated by horizontal lines underneath the lightcurve in panel (b). The blue lines are the observed mutual events (PE = primary eclipse, PO = primary occultation, SE = secondary eclipse, SO = secondary occultation), while the red lines are the nominal predictions of these same events from Scheirich et al. (2024), which occurred ~70 minutes later than observed.

Context. The very wide binary asteroid (VWBA) population is a small subset of the population of known binary and multiple asteroids made of systems with very widely orbiting satellites and long orbital periods, on the order of tens to hundreds of days. The origin of these systems is debatable, and most members of this population are poorly characterized. Aims. We aim to develop orbital solutions for some members of the VWBA population, allowing us to constrain possible formation pathways for this unusual population. Methods. We compiled all available high-angular-resolution imaging archival data of VWBA systems from large ground- and space-based telescopes. We measured the astrometric positions of the satellite relative to the primary at each epoch and analyzed the dynamics of the satellites using the Genoid genetic algorithm. Additionally, we used a NEATM thermal model to estimate the diameters of two systems, and we modeled the orbit of Litva’s inner satellite using photometric light curve observations. Results. We determine the effective diameters of binary systems (17246) Christophedumas and (22899) Alconrad to be 4.7 ± 0.4 km and 5.2 ± 0.3 km, respectively. We determine new orbital solutions for five systems, (379) Huenna, (2577) Litva, (3548) Eurybates, (4674) Pauling, and (22899) Alconrad. We find a significantly eccentric (e = 0.30) best-fit orbital solution for the outer satellite of (2577) Litva, moderately eccentric (e = 0.13) solutions for (22899) Alconrad, and a nearly circular solution for (4674) Pauling (e = 0.04). We also confirm previously reported orbital solutions for (379) Huenna and (3548) Eurybates. Conclusions. It is unlikely that BYORP expansion could be solely responsible for the formation of VWBAs, as only (4674) Pauling matches the necessary requirements for active BYORP expansion. It is possible that the satellites of these systems were formed through YORP spin-up and then later scattered onto very wide orbits. Additionally, we find that some members of the population are unlikely to have formed satellites through YORP spin-up, and a collisional formation history is favored. In particular, this applies to VWBAs within large dynamical families, such as (22899) Alconrad and (2577) Litva, or large VWBA systems such as (379) Huenna and NASA’s Lucy mission target (3548) Eurybates.

Context. The discovery rate of near-Earth asteroids (NEAs) has steadily increased over the past three decades, yet the physical characterization of these objects has not kept pace. Aims. In an effort to help address this gap, we combined targeted photometric observations, archival data, and sparse photometric data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey to extract as much information as possible about NEAs’ rotation rates, spin-axis orientations, and shapes. Methods. We selected 17 NEAs with a potential for shape reconstruction and applied the light curve inversion method to derive their sidereal rotation periods, spin-axis directions, and convex shape models. Results. We successfully determined unique spin and shape models for seven NEAs: (5189) 1990 UQ, (6569) Ondaatje, (7025) 1993 QA, (8566) 1996 EN, (86450) 2000 CK₃₃, the Hayabusa2# flyby target (98943) 2001 CC₂₁, and (512245) 2016 AU₈. For an additional four asteroids – (66251) 1999GJ₂, (137199) 1999KX₄, (276786) 2004 KD₁, and (495615) 2015 PQ₂₉₁ – we constrained their sidereal periods, spin-axis orientations, and in some cases, their shapes. Conclusions. This study highlights the importance of integrating new photometric data with archival dense light curves and sparse observations to improve the physical characterization of NEAs, even when working with suboptimal datasets. We constructed 11 NEA models, contributing to the limited set of a few dozen models derived from space missions, radar observations, and light curve inversions.

NASA’s Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the natural satellite of (65803) Didymos, on 2022 September 26, as a first successful test of kinetic impactor technology for deflecting a potentially hazardous object in space. The experiment resulted in a small change to the dynamical state of the Didymos system consistent with expectations and Level 1 mission requirements. In the preencounter paper, predictions were put forward regarding the pre- and postimpact dynamical state of the Didymos system. Here we assess these predictions, update preliminary findings published after the impact, report on new findings related to dynamics, and provide implications for ESA’s Hera mission to Didymos, scheduled for launch in 2024 October with arrival in 2026 December. Preencounter predictions tested to date are largely in line with observations, despite the unexpected, flattened appearance of Didymos compared to the radar model and the apparent preimpact oblate shape of Dimorphos (with implications for the origin of the system that remain under investigation). New findings include that Dimorphos likely became prolate due to the impact and may have entered a tumbling rotation state. A possible detection of a postimpact transient secular decrease in the binary orbital period suggests possible dynamical coupling with persistent ejecta. Timescales for damping of any tumbling and clearing of any debris are uncertain. The largest uncertainty in the momentum transfer enhancement factor of the DART impact remains the mass of Dimorphos, which will be resolved by the Hera mission.

On 2022 September 26 (UTC), NASA's Double Asteroid Redirection Test (DART) mission achieved a successful impact on Dimorphos, the secondary component of the near-Earth binary asteroid system (65803) Didymos. Subsequent ground-based observations suggest a significant reshaping of Dimorphos, with its equatorial axis ratio changing from 1.06 to ∼1.3. Here we report the effects of this reshaping event on Dimorphos's orbit and attitude. Given the reported reshaping magnitude, our mutual dynamics simulations show that approximately 125 s of the observed 33 minute orbit period change after the DART impact may have resulted from reshaping. This value, however, is sensitive to the precise values of Dimorphos's post-impact axis ratios and may vary by up to 2 times that amount, reaching approximately 250 s within the current uncertainty range. While the rotational state of the body is stable at the currently estimated axis ratios, even minor changes in these ratios or the introduction of shape asymmetry can render its attitude unstable. The perturbation to Dimorphos’s orbital and rotational state delivered by the impact directly, combined with any reshaping, leads to a strong possibility for a tumbling rotation state. To accurately determine the momentum enhancement factor (β) through measurements by the European Space Agency’s Hera spacecraft and to evaluate the effectiveness of the kinetic deflection technique for future planetary defense initiatives, the effects of reshaping should not be overlooked.

The Double Asteroid Redirection Test (DART) mission impacted Dimorphos, the satellite of binary near-Earth asteroid (65803) Didymos, on 2022 September 26 UTC. We estimate the changes in the orbital and physical properties of the system due to the impact using ground-based photometric and radar observations, as well as DART camera observations. Under the assumption that Didymos is an oblate spheroid, we estimate that its equatorial and polar radii are 394 ± 11 m and 290 ± 16 m, respectively. We estimate that the DART impact instantaneously changed the along-track velocity of Dimorphos by −2.63 ± 0.06 mm s⁻¹. Initially, after the impact, Dimorphos’s orbital period had changed by −32.7 minutes ± 16 s to 11.377 ± 0.004 hr. We find that over the subsequent several weeks the orbital period changed by an additional 34 ± 15 s, eventually stabilizing at 11.3674 ± 0.0004 hr. The total change in the orbital period was −33.25 minutes ±1.5 s. The postimpact orbit exhibits an apsidal precession rate of 6.7 ± 0.°2 day⁻¹. Under our model, this rate is driven by the oblateness parameter of Didymos, J ₂, as well as the spherical harmonics coefficients, C ₂₀ and C ₂₂, of Dimorphos’s gravity. Under the assumption that Dimorphos is a triaxial ellipsoid with a uniform density, its C ₂₀ and C ₂₂ estimates imply axial ratios, a/b and a/c, of about 1.3 and 1.6, respectively. Preimpact images from DART indicate Dimorphos’s shape was close to that of an oblate spheroid, and thus our results indicate that the DART impact significantly altered the shape of Dimorphos.