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.

How to cite: Knight, M. M., Scheirich, P., Pravec, P., Thomas-Osip, J., Chandler, C. O., Frissell, M., Agrusa, H., Chesley, S. R., Farnham, T. L., de Leon, J., Fatka, P., Kokotanekova, R., Kueppers, M., Lister, T. A., Moskovitz, N. A., Oldroyd, W. J., Opitom, C., Rozek, A., Snodgrass, C., and Thomas, C. and the additional authors: Ground-based observations of (65803) Didymos 2.5 years after the DART impact: searching for ongoing evolution, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-866, https://doi.org/10.5194/epsc-dps2025-866, 2025.

Summary

Asteroid 1998 KY₂₆ is a decametre-sized, rapidly rotating body that has been selected as a target for the extended Hayabusa2# mission. Understanding its physical properties is crucial for optimizing mission planning, as it presents both challenges and opportunities for asteroid exploration. This study[1] provides a detailed characterization of 1998 KY₂₆ using a combination of new optical photometric data and radar observations, extending previous analyses of the asteroid’s size, spin state, shape, and surface composition[2]. Our results offer new insights into the asteroid's structure and dynamics and highlight the importance of integrating diverse observational techniques to refine the models for small asteroid behavior and non-gravitational forces.

Observational Data and Methodology

Our new observations were gathered during the 2024 apparition with the GTC, Gemini South, VLT and Blanco telescopes. For our study, we incorporated the photometric and radar data collected in 1998. Optical data covered a wide range of phase angles to allow for accurate determination of the asteroid's albedo and surface properties. Additionally, the 2024 close approach offered an opportunity to examine potential non-gravitational forces, such as outgassing, which could influence the asteroid's trajectory.

Lightcurve inversion techniques were applied to the optical data to refine the asteroid's spin period and shape. The combination of radar data and lightcurve inversion models allowed us to generate a three-dimensional model of 1998 KY₂₆ and derive its size, surface properties, and taxonomic classification.

Results

Spin Period One of the primary results of this study is a revised spin period for 1998 KY₂₆ of 5.35 minutes, significantly shorter than the previously reported value of 10.7 minutes (Figure 1). The reduced spin period is consistent with the asteroid's small size and relatively high rotational stress, which is expected for bodies of this scale.

Figure 1: Lightcurve analysis of 1998 KY₂₆ showing the revised spin period of 5.35 minutes, with a comparison to the previous model from Ostro et al. 1999[2].

Size and Shape

Using the revised spin period and lightcurve data, we derived convex and non-convex shape models which indicates that 1998 KY₂₆ is an irregular, slightly elongated body.

We used the new models to fit the 1998 radar data (Figure 2), leading to an updated diameter estimate of 11±2 meters (see Figure 2), which is significantly smaller than the previously reported value of approximately 30 meters. The asteroid’s small size presents unique challenges for spacecraft exploration, particularly in terms of navigation and surface operations.

Figure 2: Rebinned Doppler-only (cw) observations of 1998 KY26 collected on 8 June 1998, compared with synthetic echoes generated using models derived with lightcurve-inversion methods.

Surface Albedo and Composition

From the combination of optical photometric data at multiple phase angles and radar observations, we derived a geometric albedo of 0.52±0.08. This high albedo is consistent with that observed in enstatite-rich, Xe-type asteroids, which are known for their bright surfaces.

Impact on Mission Planning

The new spin period, size, and surface properties of 1998 KY₂₆ have significant implications for the extended Hayabusa2# mission. The smaller size of the asteroid will necessitate adjustments to the mission's approach and landing strategies. Additionally, the revised spin period and irregular shape mean that precise navigation will be critical to ensure safe operations near the asteroid.

Conclusion

This study provides a comprehensive physical characterization of asteroid 1998 KY₂₆ using a variety of observational techniques. Our results indicate that 1998 KY₂₆ is a small, rapidly rotating body with an irregular shape and a high albedo, consistent with an enstatite-rich composition. We revised its spin period to 5.35 minutes and determined its diameter to be 11±2 meters, which is significantly smaller than previous estimates. We also found no evidence of non-gravitational forces acting on the asteroid, such as outgassing, during the 2024 close approach. These findings provide critical insights into the physical properties of 1998 KY₂₆ and will inform mission planning for the Hayabusa2# spacecraft. Further studies, particularly with regard to non-gravitational forces and surface properties, will be essential for refining our understanding of small, fast-rotating asteroids.

References

[1] Toni Santana-Ros, Przemyslaw Bartczak, Karri Muinonen et al., "Hayabusa2# mission target 1998 KY₂₆ preview: a small optically bright rapid rotator", 21 January 2025, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-5821856/v1]

[2] Ostro, S. J., “Radar and Optical Observations of Asteroid 1998 KY26”, Science, vol. 285, pp. 557–559, 1999. doi:10.1126/science.285.5427.557.

How to cite: Santana-Ros, T., Bartczak, P., Muinonen, K., Rożek, A., Müller, T., Hirabayashi, M., Farnocchia, D., Micheli, M., Cannon, R., Brozović, M., Hainaut, O., Oszkiewicz, D., Virkki, A., Benner, L., Campo Bagatin, A., Benavidez, P., Cabrera-Lavers, A., Martínez-Vázquez, C., and Vivas, K.: Hayabusa2# mission target 1998 KY26 preview: a small optically bright rapid rotator, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1454, https://doi.org/10.5194/epsc-dps2025-1454, 2025.

Amateur astronomers make useful contributions to many areas of astronomy.  For comets, they can add temporal coverage and multi-scale observations which aid the study of fast-changing, and large-scale features. 

The Halley, Deep Impact and Rosetta comet Missions all included amateur campaigns as part of their wider ground-based observing campaigns.  Amateur observations can particularly add value when the viewing geometry is unfavourable for larger telescopes eg at small solar elongation, and (due to the worldwide spread of amateurs) when observing windows are small.

We will present the results of an analysis of the effectiveness of these campaigns, particularly the Rosetta Amateur Observing Campaign, and lessons and recommendations for action for future campaigns (Usher et al 2022). 

The analysis of the Rosetta professional and amateur campaign data shows that around the critical period of the perihelion of comet 67P there were 15 days when the only observations available were from amateur observers.

The lessons for future campaigns include the need for: clarity of objectives; recognising the wider impact campaigns can have on increasing science capital; clear, consistent, timely and tailored guidance; easy upload procedures with in-built quality control; and, regular communication, feedback and recognition.

We have been working with the Pro-Am comet community to implement these recommendations and we will report progress and outstanding work (Usher et al 2022).

Astronomy is recognised as an effective point of engagement for students of all ages, inspiring curiosity and providing a stimulus to learn and exciting context for developing skills (Salimpour et al. 2021).  Linking student learning with space missions can add an extra dimension.

We will present our experience of working with educators, supported by professional and amateur astronomers, to use observing campaigns to engage, educate, inspire and raise the aspirations of students.  This will include a case study of observations and activities linked to the DART Mission (Usher et al .  2023, Lister et al 2024).

Salimpour, S., Bartlett, S., Fitzgerald, M.T. et al. The Gateway Science: a Review of Astronomy in the OECD School Curricula, Including China and South Africa. Res Sci Educ 51, 975–996 (2021)

Usher, H., Snodgrass, C., Biver, N., Kargl, G., Tautvaišienė, G., James, N., Walter, F., and Černý, J.: Strengthening Pro-Am Comet Community Cooperation: Report on Europlanet Pro-Am Workshop (10-12 June 2022)  , Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1135, https://doi.org/10.5194/epsc2022-1135, 2022

Usher, H., Snodgrass, C., Green, S.F., Norton, A., Roche, P.,   Seeing the Bigger Picture: Rosetta Mission Amateur Observing Campaign and Lessons for the Future 2020 Planet. Sci. J. 1 84 DOI 10.3847/PSJ/abca46

Usher, H., Stoddard-Jones, I.C. , Roche, P.D., Snodgrass, C., Wooding, B., and Lister, T.,  Using Killer Asteroids to Engage Children in Astronomy and Science (Planetary Defence Conference, April 2023)

Lister, T., Constantinescu, C., Ryan William, Ryan, E., Gomez,E., Phillips, L., Rożek, A., Usher, H., Murphy, B. P., Chatelain J., Greenstreet, S., 2024 Planet. Sci. J. 5 127

How to cite: Usher, H., Snodgrass, C., Norton, A., Green, S., Roche, P., Jordan, S., Angel, T., Miles, R., and Wooding, B.: Pro-Am-Schools Observing Campaigns in Support of Space Missions and the  Broader Study of Small Bodies, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1683, https://doi.org/10.5194/epsc-dps2025-1683, 2025.

How to cite: Kokori, A.: Integrating data from ground-based and space telescopes to support future space missions in the new exoplanet era: The case of the ExoClock project , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1789, https://doi.org/10.5194/epsc-dps2025-1789, 2025.