The two innermost moons of Neptune, Naiad and Thalassa, are currently in a 73:69 mean-motion resonance. This resonance relies on the large inclination of Naiad, and we estimate that Naiad requires multiple Gyr to reach its 4 .degrees 7 inclination through this resonance. However, we find through direct numerical simulations that the current Naiad-Thalassa resonance is unstable on Myr timescales due to perturbations from the neighboring moon Despina. As this instability is a product of convergent tidal evolution predicted by equilibrium tidal theory, we propose that the innermost moons of Neptune may migrate through resonant-lock tides. If both Despina and Thalassa are locked to two resonant oscillation modes within Neptune, the frequencies of which evolve approximately in parallel, Naiad-Thalassa resonance can be stable for much longer. We find that Lindblad resonances with low-order l = m = 1, n = 1 g-modes at Neptune may be suitable candidates for driving the resonant-lock evolution of Thalassa and Despina, and possibly even Galatea.

Near-Earth asteroid (275677) 2000 RS11 was observed over 5 d in March 2014 with both the Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) planetary radar systems. The continuous-wave spectra and delay-Doppler images collected revealed a sub-km-sized object with a strongly bifurcated shape. We used these radar observations, in combination with seven optical light curves collected in 2014 and one light curve from 2023, to create a comprehensive shape and spin-state model for RS11. We find a rotation period of P = (4 . 445 +/- 0 . 001) h around a pole of lambda = (225 +/- 80) degrees and beta = (-80 +/- 9) degrees relative to the plane of the ecliptic. The shape of RS11 is unusual in that it does not resemble many of the other near-Earth asteroids modelled with ground-based radar. Whilst RS11 consists of a largely spherical, smaller lobe attached to an elongated, larger lobe via a narrow neck, the smaller lobe is not aligned with the long axis of the larger lobe, but is closer to the larger lobe's shortest principal axis. In combination with a large concavity observed on the outer face of the larger lobe, this may point to an unusual formation or event in the object's past. We estimate that RS11 has a geometric albedo of pv = (0 . 16 +/- 0 . 06) and a radar albedo 0 . 08 < r7OC < 0 . 16 . Analysis of its gravitational environment reveals that for standard S-type asteroid densities, we would not expect rotational instability and it is possible for RS11 to be a low tensile strength rubble-pile asteroid.

We report on the new ephemerides (MAR099) for the satellites of Mars derived from a large set of ground-based and spacecraft astrometric measurements. Our integrated orbit fit yielded latest estimates for the Martian tidal parameter tau = k2/Q = 1.816 +/- 0.084 x 10-3 and half the tidal acceleration of Phobos of s=12n=1.258 +/- 0.058x10-3 deg yr-2. We used the spacecraft-measured physical libration amplitude of Phobos, A=1.14 +/- 0.03 deg, to fit J2 and C22 and found that the result is consistent with the value derived from its shape and the uniform density assumption. The data show no sensitivity for the physical libration of Deimos. We report on the statistics of the observation residuals, the satellites' orbital elements, and projected uncertainties of the satellites' positions in the next few decades.

Contact binaries are found throughout the Solar System. The recent discovery of Selam, the satellite of main-belt asteroid (152830) Dinkinesh, by the NASA Lucy mission has made it clear that the term 'contact binary' covers a variety of different types of bimodal mass distributions and formation mechanisms. Only by modelling more contact binaries can this population be properly understood. We determined a spin state and shape model for the Apollo group contact binary asteroid (388188) 2006 DP14 using ground-based optical and radar observations collected between 2014 and 2023. Radar delay-Doppler images and continuous-wave spectra were collected over 2 d in February 2014, while 16 light curves in the Cousins R and SDSS-r filters were collected in 2014, 2022, and 2023. We modelled the spin state using convex inversion before using the shape modelling software to include the radar observations in modelling concavities and the distinctive neck structure connecting the two lobes. We find a spin state with a period of (5.7860 +/- 0.0001) h and pole solution of lambda=(180 +/- 121)degrees and beta=(-80 +/- 7)degrees with morphology indicating a 520 m long bilobed shape. The model's asymmetrical bimodal mass distribution resembles other small near-Earth asteroid contact binaries such as (85990) 1999 JV6 or (8567) 1996 HW1, which also feature a smaller 'head' attached to a larger 'body'. The final model features a crater on the larger lobe, similar to several other modelled contact binaries. The model's resolution is 25 m, comparable to that of the radar images used

Understanding the physical characteristics of small bodies in the Solar System is crucial for refining models of their formation and evolution. Although several kilometre-sized asteroids have already been visited by spacecraft, 1998 KY26 will be the first asteroid in the decametre range to be explored in situ. Its diameter and spin period place it above the spin barrier, suggesting that its formation and properties may differ from those of previously visited bodies. However, the key physical characteristics of 1998 KY26 remain poorly understood. We conducted a photometric observing campaign during 1998 KY26’s close approach to Earth in 2024. Our observations revealed that 1998 KY26 has a high optical albedo and colours consistent with an E-type taxonomy. We also determined its spin period to be (5.3516 ± 0.0001) minutes—twice as fast as previously reported. Using lightcurve inversion methods, we derived a retrograde pole solution and constructed convex and non-convex shape models. By integrating these models with Goldstone radar data from 1998, we estimated 1998 KY26’s diameter to be (11 ± 2) m. The most likely structure for 1998 KY26 is monolithic, yet a fine grained rubble-pile structure is theoretically possible given its required small strength level. We found that it will be possible to validate these results with future JWST observations. Our comprehensive characterisation of 1998 KY26 can inform the planning of the Hayabusa2# spacecraft’s rendezvous encounter, scheduled for July 2031. Additionally, it provides valuable insights into the non-gravitational forces acting on small Solar System bodies, enhancing our understanding of their orbital evolution which could shed light on the nature of dark comets.

IntroductionBilobed objects (contact binaries) appear across the solar system in both asteroid and comet populations. Radar imaging reveals that at least 15% of near-Earth asteroids (NEAs) are contact binaries [1] but bilobed objects have also been detected in the Asteroid Belt and the Kuiper Belt. Notable examples include Itokawa (NEA), Selam (main-belt asteroid), and Arrokoth (trans-Neptunian object). It is currently unknown whether contact binaries form predominantly as two separate objects or are the natural evolution of a single body. Theories for contact binary formation in asteroids include the reaccumulation of fragments resulting from a catastrophic collision [2]. A second possibility is that the two lobes could be formed from a rotational instability as a result of YORP [3]. To date, the sample size of modelled contact binaries stands at thirteen, with only six from the NEA population. We present the model of Apollo group asteroid (388188) 2006 DP14 (henceforth DP14) which demonstrates a contact binary like structure in publicly available radar data from 2014 (Figure 2).ObservationsWe collected optical photometry of DP14 during over two nights in 2014, one from a 0.36-m telescope at the Perth Observatory in Australia, and one from the PROMPT1 0.41-m telescope in Cerro Tololo, Chile. We have 9 nights of data from 2022, 5 of which are semi-sparse lightcurves obtained from the Danish 1.54-m telescope on La Silla, while the other 4 are complete lightcurves obtained from the 2.54-m Isaac Newton Telescope on La Palma, Spain. Finally, we have 3 nights of densely populated lightcurves from the Danish telescope from 2023. The INT and 2023 lightcurves were collected in the SDSS-R filter, reduced with standard procedures and calibrated to ATLAS REFCAT-2 stars in the field after PSF photometry. The 2022 data from the Danish telescope was calibrated in the Cousins R system with the Landolt standards with absolute accuracy of 0.01 mag. In our modelling, we also used published data spanning two nights in 2014, collected from a 0.6-m telescope at the CS3-Palmer Divide Station [4].The radar data used was recorded in 2014 at the Goldstone DSN antenna in the United States and contains two nights of delay-Doppler images and 3 sets of echo power spectra.ModellingTo model DP14 we first performed convex inversion [5,6] in order to refine the spin-state solution and generate a convex hull of the asteroid's shape. While these results were inconclusive for the rotational pole it allowed us to gain a good estimate of the period (see Figure 1), and allowed us to confirm the rotational pole was in the southern hemisphere when creating our initial conditions for further modelling with radar.Figure 1: Periodogram of the convex inversion performed on all the lightcurves. It is known from radar imaging that the object is elongated, so although the best solution is a fit for a single-peaked lightcurve, we instead used the two-peaked solution that agreed with Warner (2014) [1] as initial conditions for the radar modelling.We used the SHAPE modelling software [7] to combine the optical lightcurves with radar echo spectra and delay-Doppler images from 2014 from the Goldstone DSN antenna. We first used a simple single ellipsoid estimation of shape in order to refine the spin-state solution, before moving on to a bi-ellipsoid model. The inherent degeneracy of pole solutions in radar modelling resulted in minima at both the south and north pole but, given the results of the convex inversion and measurement of Yarkovsky effect (negative A2 in the most recent orbital solution) from optical and radar astrometry, we used the southern pole solution. We then switched to a vertex shape model fit in order to refine the shape and allow for better fitting to the narrow neck and the crater visible on the elongated body of the target seen in the delay-Doppler images.As a preliminary result, we have determined that DP14 has a distinctive narrow neck connecting two lobes of unequal size. Results suggest a period of 5.77 ± 0.05 hours and a rotational pole of approximately β = -80 and λ = 50. The radar model can be seen in Figure 2.Figure 2: Left column: Delay-Doppler images of DP14 taken with the Goldstone DSN antenna in the United States on 12/02/2014. The second image was taken approximately 50 minutes after the first, with the third another 30 minutes later. Centre column: The simulated delay-Doppler images created from the preliminary model. Right: The plane of sky views of the model with a pink axis representing the rotational pole.Summary & ConclusionWe have reached a spin-state solution with a rotational pole close to the south pole of the ecliptic, as is observed in the majority of NEAs. Modelling reveals results expected from a visual inspection of the data, with an elongated body with a large crater on its side and smaller more spherical head connected by a narrow neck. The model of DP14 will allow better comparisons to theory based dynamical simulations to investigate its likely origins, while an additional spin-state solution and shape model give vital hints into not only the object's own formation history, but allow us to place the results in the wider context of other similarly shaped solar system objects.Bibliography[1] Benner, L. A. M. et al. In Asteroids IV ; University of Arizona Press: 2015, pp 165–182.[2] Campo Bagatin, A. et al. Icarus 2020, 339, 113603.[3] Jacobson, S. A. et al. Icarus 2016, 277, 381–394.[4] Warner, B. D. Minor Planet Bulletin 2014, 41, 157–168[5] Kaasalainen, M.; Torppa, J. Icarus 2001, 153, 24–36.[6] Kaasalainen, M.; Torppa, J.; Muinonen, K. Icarus 2001, 153, 37–51.[7] Magri, C. et al. Icarus 2007, 186, 152–177.

We report on the ephemeris development for Menoetius, the satellite of Patroclus. Our data set consisted of ground–based and Hubble Space Telescope relative astrometry, as well as 42 lightcurves from the mutual events seasons in 2007, 2012, and 2017/2018. Our dynamical model included the effects of oblate, nonspherical shapes of the components, and we assumed that Menoetius contained ∼22% of the system’s mass. We numerically integrated the equations of motion and obtained a set of dynamical parameters that fit the data. We report the fit results in terms of residuals, state vectors, orbital elements and their 1 σ uncertainties. The mean osculating semimajor axis is a = 692.5 ± 4.0 km, the mean eccentricity is e = 0.004 ± 0.004, and the International Celestial Reference Frame pole direction in R.A. and decl. is R. A. = 178.0 ± 0.5 deg, δ = −74.1 ± 0.2 deg. We determined the siderial orbital period of P = 4.282753 ± 0.000023 days. The fit yielded the system GM = 0.0950 ± 0.0012 km ^3 s ^−2 , which, in combination with the system volume determined from the stellar occultation and the assumed volume uncertainty of 20%, suggests a system bulk density of 1.05 ± 0.21 g cm ^−3 (1 σ ). The next season of mutual events starts in February of 2024 and lasts until January of 2025. The Patroclus system is in opposition for the observers on Earth in late September and is suitable for observations of the mutual events with an edge-on geometry in October.

We have conducted extremely ultra-deep pencil beam observations for new satellites around both Uranus and Neptune. Tens of images on several different nights in 2021, 2022 and 2023 were obtained and shifted and added together to reach as faint as 26.9 and 27.2 magnitudes in the r-band around Uranus and Neptune, respectively. One new moon of Uranus, S/2023 U1, and two new moons of Neptune, S/2021 N1 and S/2002 N5, were found. S/2023 U1 was 26.6 mags, is about 7 km in diameter and has a distant, eccentric and inclined retrograde orbit similar to Caliban and Stephano, implying these satellites are fragments from a once larger parent satellite. S/2023 U1 almost completely overlaps Stephano in orbital phase space. S/2021 N1 was 26.9 mags, about 14 km in size and has a retrograde orbit similar to Neso and Psamathe, indicating they are a dynamical family. We find S/2021 N1 is in a Kozai-Lidov orbital resonance. S/2002 N5 was 25.9 mags, is about 23 km in size and it makes a family of distant prograde satellites with Sao and Laomedeia. All three new moons show for the first time dynamical groups of moons exist around both Uranus and Neptune. The creation of these groups likely produced dust that could be the source of red material seen on the leading hemispheres of some larger inner satellites like Titania, Oberon and Umbriel. We also detected all known outer moons of Uranus and Neptune on multiple nights. This survey mostly completes the outer satellites of Uranus to about 8 km and Neptune to about 14 km in diameter. The size distributions of satellite dynamical families around the giant planets shows a strong steepening in the power law size distribution smaller than 5 km in diameter. The satellites of a family become much more common smaller than 5 km and their size distribution is consistent with a collisional break-up of a once larger parent satellite.

AbstractWe are conducting an observing campaign with a sample of near-Earth asteroids (NEAs) to detect YORP-induced acceleration. Photometric lightcurves from small to medium optical telescopes are used to detect changes to spin state. Where available, radar and thermal-IR observations are used to develop physical models that are used to determine expected YORP strengths. We will present the results of an analysis of 2000 PN9 (hereafter PN9) using optical and radar observations between 2001 and 2016. A detailed physical model has been developed to describe the shape and spin-state of the asteroid. PN9 is top-shaped with an equatorial ridge, and is rotating close to the spin-breakup limit. PN9 is the largest asteroid known to have this distinctive and highly symmetrical 'YORPoid' shape, and is the fastest-rotating top-shaped body that is not part of a multiple system. Due to the size and shape of PN9, an observational detection of YORP acceleration has not been possible with the available data. The shape and rotation period indicate that PN9 is a YORP-evolved body similar to 1999 KW4 [1], and that it is a good candidate for future detection of mass-lofting events.IntroductionThe YORP effect is a thermal torque caused by the reflection, absorption and anisotropic re-emission of Solar radiation [2,3]. YORP is the main driver of physical and dynamical evolution for small bodies in the inner Solar System [4]. There are seven confirmed detections of YORP to date, all of which are in the 'spin-up' mode.The observing campaign aims to increase the number of YORP detections such that theoretical understanding of YORP can be improved. Optical and IR observations were carried out on 42 NEAs through our ESO Large Programme from 2010 to 2014, with continued monitoring conducted through associated programmes. In addition to photometric observations, thermal-IR and radar observations have been obtained for the NEA sample.PN9 was observed in 2001, 2006, 2010, 2011, 2015 and 2016 with various optical telescopes resulting in 29 lightcurves. Radar observations were also conducted at the Arecibo and Goldstone radar facilities in 2001 and 2006. Previously published lightcurves report rotational periods around 2.53 h [5,6] and initial diameter estimates range from 1.6 - 2.0 km [5,7]. PN9 has been previously classified as an Sq-type asteroid [8].Results & ConclusionsDetecting YORP requires well constrained measurement of the evolving rotation period and pole orientation. These parameters can be determined using lightcurve data alone, although it is beneficial to have a detailed radar shape model. This greatly assists with the computation of rotational phase offsets between observations and synthetic lightcurves derived from a constant-period shape model. YORP causes a linear change in rotation period, which in turn causes a quadratic increase in rotational phase offset against time. A radar shape model greatly assists in the detection of this quadratic trend, which is highly desirable for constraining YORP strength.Figure 1: Shape model for asteroid 2000 PN9 developed using both radar and lightcurve observations. The top row shows the model from the positive end of the Z, Y and X axes in the body-centric co-ordinate system. The bottom row shows the model from negative end of the Z, Y and X axes. Lightcurves of PN9 have very low amplitudes due to the highly symmetrical nature of the object. Lightcurve data alone was not sufficient to constrain the orientation of the rotational axis, so the spin-state was determined using a combination of lightcurve and radar data. Figure (2) shows the χ2 fit of pole solutions across the whole sky, with the pole-scan converging to two regions that are opposite each other on the celestial sphere. This pole solution degeneracy implies uncertainty as to whether the asteroid is a prograde or retrograde rotator. Physical models were generated for all possible pole solutions, with all solutions within 1% of the best solution having a shape and period consistent with the best solution. For the best-fit pole solution at λ=228° β=-30°, we have determined the sidereal rotation period to be 2.532972±0.000015 hours. In the body-centric coordinate system where the z-axis is aligned with the rotation axis, the maximum extents of the X, Y and Z axes are 1.885x1.888x1.780 km. As shown in Figure (1), the shape is highly symmetrical with an equatorial ridge. This shape is characteristic for a rapidly rotating rubble pile, where material is being migrated towards the equator [9] and closely resembles the convex hull model of 1917 Cuyo, a candidate for rotationally-induced mass lofting [10]. We have measured the circular polarisation ratio (SC/OC) to be 0.225±0.004, which is consistent with PN9's taxonomic type. We have been unable to measure YORP acceleration; the size and shape of PN9 results in a low expected YORP strength, and phase offsets cannot be precisely measured with relatively featureless lightcurves.Further work that will be presented at the meeting includes an updated physical model developed with newly obtained lightcurves, and an update on Spitzer thermal-IR lightcurves of PN9.Figure 2: The results of a search for the rotational pole of 2000 PN9 using radar and lightcurve data. The best solution is marked with a yellow '+'. The yellow and white lines enclose regions where χ2 is within 1% and 5% of the best solution respectively.AcknowledgementsThe authors would like to thank A. Fitzsimmons, R. Ya. Inasaridze, V. A. Ayvazian, V. V. Rumyantsev, V. G. Chiorny, I. V. Reva, M. A. Krugov, I. E. Molotov, M. W. Busch, M. C. Nolan, A. A. Hine, V. Negrón, L. A. M. Benner and M. D. Hicks for their contributions to this work.References[1] Ostro, S. J. et al. (2006) Science 314, 1276. [2] Rubincam, D. P. (2000). Icarus 148, 2. [3] Lowry, S. C. et al. (2007). Science 316, 272. [4] Bottke, W. F. et al. (2006). AREPS 34. [5] Belskaya, I. N. et al. (2009). Icarus 201, 167. [6] Warner, B. D. (2016). Minor Planet Bulletin 43, 240. [7] Busch, M. W. et al. (2006). Society for Astronomical Sciences Annual Symposium 25, 169. [8] Thomas, C. A. et al. (2014). Icarus 228, 217. [9] Walsh, K. J. et al. (2008). Nature 454, 188. [10] Rożek, A. et al. (2019) A&A 627, A172.

We report radar, photometric, and visible-wavelength spectrophotometry observations of NEA 2018 EB obtained in 2018. The radar campaign started at Goldstone (8560 MHz, 3.5 cm) on April 7, and it was followed by more extensive observations from October 5 to 9 by both Arecibo (2380 MHz, 12.6 cm) and Goldstone. 2018 EB was observed optically on April 5, 8, and 9 and again on October 18. Spectrophotometry was obtained on October 19 with the SOAR telescope, and the data suggest that 2018 EB is an Xk-class object. The echo power spectra and delay-Doppler radar images revealed that 2018 EB is a binary system. Radar images constrained the satellite's diameter to 0.15 - 0.05 + 0.02 km, but the data were not sufficient for shape modeling. Shape modeling of lightcurves and radar data yielded an oblate primary with an effective diameter D = 0.30 +/- 0.04 km and a sidereal rotation period of 4.3 - 0.5 + 0.6 hr. Measurements of delay-Doppler separations between the centers of mass of the primary and the satellite, along with the timing of a radar eclipse observed on October 9, resulted in an orbit fit for the satellite with a semimajor axis of 0.50 - 0.01 + 0.04 km, an eccentricity of 0.15 +/- 0.04, a period of 16.85 - 0.26 + 0.33 hr, and an orbit pole constrained to the ecliptic longitudes and latitudes of lambda = 93 - 43 degrees + 27 degrees and beta = 48 - 18 degrees + 7 degrees . The system mass was estimated to be 2.03 - 0.08 + 0.52 x 10 10 kg, which yielded a bulk density of 1.4 - 0.5 + 0.6 g cm-3. Our analysis suggests that 2018 EB has a low optical albedo of p V = 0.028 +/- 0.016 and a relatively high radar albedo of eta OC = 0.29 +/- 0.11 at Arecibo and eta = 0.22 +/- 0.10 at Goldstone.

We present the results of a fourth planetary defense exercise, focused this time on the small near-Earth asteroid (NEA) 2023 DZ2 and conducted during its close approach to the Earth in 2023 March. The International Asteroid Warning Network (IAWN), with support from NASA's Planetary Defense Coordination Office (PDCO), has been coordinating planetary defense observational campaigns since 2017 to test the operational readiness of the global planetary defense capabilities. The last campaign focused on the NEA Apophis, and an outcome of that exercise was the need for a short burst campaign to replicate a real-life near-Earth object impact hazard scenario. The goal of the 2023 DZ2 campaign was to characterize the small NEA as a potential impactor and exercise the planetary defense system including observations, hypothetical risk assessment and risk prediction, and hazard communication with a short notice of just 24 hr. The entire campaign lasted about 10 days. The campaign team was divided into several working groups based on the characterization method: photometry, spectroscopy, thermal IR photometry and optical polarimetry, radar, and risk assessment. Science results from the campaign show that 2023 DZ2 has a rotation period of 6.2745 ± 0.0030 minutes; visible wavelength color photometry/spectroscopy/polarimetry and near-IR spectroscopy all point to an E-type taxonomic classification with surface composition analogous to aubrite meteorites; and radar observations show that the object has a diameter of 30 ± 10 m, consistent with the high albedo (0.49) derived from polarimetric and thermal IR observations.

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.degrees 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.

The Double Asteroid Redirection Test (DART) spacecraft successfully performed the first test of a kinetic impactor for asteroid deflection by impacting Dimorphos, the secondary of near-Earth binary asteroid (65803) Didymos, and changing the orbital period of Dimorphos. A change in orbital period of approximately 7 min was expected if the incident momentum from the DART spacecraft was directly transferred to the asteroid target in a perfectly inelastic collision 1 , but studies of the probable impact conditions and asteroid properties indicated that a considerable momentum enhancement ( β ) was possible 2 , 3 . In the years before impact, we used lightcurve observations to accurately determine the pre-impact orbit parameters of Dimorphos with respect to Didymos 4 – 6 . Here we report the change in the orbital period of Dimorphos as a result of the DART kinetic impact to be −33.0 ± 1.0 (3 σ ) min. Using new Earth-based lightcurve and radar observations, two independent approaches determined identical values for the change in the orbital period. This large orbit period change suggests that ejecta contributed a substantial amount of momentum to the asteroid beyond what the DART spacecraft carried.

ABSTRACT We present a physical model and spin-state analysis of the potentially hazardous asteroid (23187) 2000 PN9. As part of a long-term campaign to make direct detections of the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, we collected optical light curves of the asteroid between 2006 and 2020. These observations were combined with planetary radar data to develop a detailed shape model, which was used to search for YORP acceleration. We report that 2000 PN9 is a relatively large top-shaped body with a sidereal rotation period of 2.53216 ± 0.00015 h. Although we find no evidence for rotational acceleration, YORP torques smaller than $\sim 10^{-8}\, \rm rad\,{d}^{-2}$ cannot be ruled out. It is likely that 2000 PN9 is a YORP-evolved object, and may be an example of YORP equilibrium or self-limitation.

We explore the effects of the 2029 Earth encounter on asteroid (99942) Apophis' non-principal axis spin state, leveraging refined orbit, spin state, and inertia information provided by more recent optical and radar observations. Propagating the asteroids' coupled orbit and rigid body attitude dynamics through the flyby, we present the range of possible post-flyby spin states. These spin state distributions will be valuable for planning Apophis observation campaigns and spacecraft missions, most notably OSIRIS-APEX. The simulations indicate that gravitationally induced changes to the asteroid's tumbling periods and rotational angular momentum direction (pole) will likely be significant and measurable. For the current spin state and inertia estimates and their uncertainties, Apophis is likely to remain in a short axis mode (SAM) tumbling state but its effective spin rate could halve or double. Its pole is likely to shift by 10 degrees or more and increase in longitude while moving closer to the ecliptic plane. These spin state changes are very sensitive to the asteroid's close approach attitude and mass distribution. With ground-based tracking of the asteroid's spin state through the encounter, this sensitivity will help refine mass distribution knowledge. We also discuss the implications of this abrupt spin state alteration for Apophis' Yarkovsky acceleration and geophysical properties, identifying possible pathways for surface and internal changes, most notably if Apophis is a contact binary. Comparison of the pre and post-flyby inertia estimates obtained from the ground-based observations will help assess the extent of possible geophysical changes.

We describe results of a planetary defense exercise conducted during the close approach to Earth by the near-Earth asteroid (99942) Apophis during 2020 December–2021 March. The planetary defense community has been conducting observational campaigns since 2017 to test the operational readiness of the global planetary defense capabilities. These community-led global exercises were carried out with the support of NASA’s Planetary Defense Coordination Office and the International Asteroid Warning Network. The Apophis campaign is the third in our series of planetary defense exercises. The goal of this campaign was to recover, track, and characterize Apophis as a potential impactor to exercise the planetary defense system including observations, hypothetical risk assessment and risk prediction, and hazard communication. Based on the campaign results, we present lessons learned about our ability to observe and model a potential impactor. Data products derived from astrometric observations were available for inclusion in our risk assessment model almost immediately, allowing real-time updates to the impact probability calculation and possible impact locations. An early NEOWISE diameter measurement provided a significant improvement in the uncertainty on the range of hypothetical impact outcomes. The availability of different characterization methods such as photometry, spectroscopy, and radar provided robustness to our ability to assess the potential impact risk.

We report on an ephemerides update for the irregular satellites of Uranus and Neptune, including Nereid. Our data set contains ground-based and Voyager 2 position measurements of the satellites. Some of the new observations were reduced with respect to the GAIA star catalog and represent high-quality, modern astrometry. The orbital fits are based on the numerically integrated equations of motion. We summarized results in terms of state vectors, orbital uncertainties, and mean elements. A large number of the irregulars have not been followed up in over a decade, and our study shows that their orbits still contain significant uncertainties.

We report observations of the Apollo-class potentially hazardous asteroid 1981 Midas, which passed 0.090 au from Earth (35 lunar distances) on 2018 March 21. During this close approach, Midas was observed by radar both from the Arecibo Observatory on March 21 through 25 (five nights) and from NASA’s Goldstone Deep Space Communications Complex on March 19 and 21. Optical lightcurves were obtained by other observers during four apparitions (1987, 1992, 2004, and 2018), which showed a rotation period of 5.22 hr. By combining the lightcurves and radar data, we have constructed a shape model for Midas. This model shows that Midas has two lobes separated by a neck, which, at its thinnest point, is about 60% of the width of the largest lobe. We also confirm the lightcurve-derived rotation period and show that Midas has a pole direction within 6° of ecliptic longitude and latitude (λ, β) = (39°, −60°) and dimensions of (3.41 ± 9%) × (1.90 ± 11%) × (1.27 ± 29%) km. Analysis of gravitational slopes on Midas indicates that nearly all of the surface has a slope less than the typical angle of repose for granular materials, so it does not require cohesion to maintain its shape. In addition, we measured a circular polarization ratio of 0.83 ± 0.04 at Arecibo’s 13 cm wavelength, which is the highest seen to date for any near-Earth asteroid with visible and near-infrared spectral type V.

We report a new determination of the orbits of the irregular Saturnian satellites. We fit their numerically integrated orbits to a data set containing Earth-based observations and imaging data from the Cassini spacecraft. We include the statistics of the observation residuals, the satellites’ orbital elements, and projected accuracies of the satellites’ positions. We also provide astrometric positions derived from the Cassini imaging. Two of the satellites are considered lost because they have not been observed for more than one epoch and have indeterminate uncertainties in their positions. Three of the satellites appear to be in a Kozai resonance, with one being the first irregular satellite of any planet found to be in a 270° rather than 90° resonance.

Assess the joint capabilities of emerging telescopes for near-Earth objects (NEOs) survey and characterization, and what they will add to the current capabilities or replace. NASA telescopes in prime mission, in development, or under study, and requested for this assessment, include: - The Transiting Exoplanet Survey Satellite (TESS) - The James Webb Space Telescope (JWST) - The Wide Field Infrared Survey Telescope (WFIRST) - The Near-Earth Object Camera (NEOCam). Also requested for this assessment is the Large Synoptic Survey Telescope (LSST), an 8.4-meter ground-based telescope in development by the National Science Foundation and Department of Energy (DOE), with the capability to discover and catalogue NEOs.