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Near-Earth asteroid 2014 AA entered the Earth’s atmosphere on 2014 January 2, only 21 h after being discovered by the Catalina Sky Survey. In this paper we compute the trajectory of 2014 AA by combining the available optical astrometry, seven ground-based observations over 69 min, and the International Monitoring System detection of the atmospheric impact infrasonic airwaves in a least-squares orbit estimation filter. The combination of these two sources of observations results in a tremendous improvement in the orbit uncertainties. The impact time is 3:05 UT with a 1 σ uncertainty of 6 min, while the impact location corresponds to a west longitude of 44.2° and a latitude of 13.1° with a 1 σ uncertainty of 140 km. The minimum impact energy estimated from the infrasound data and the impact velocity result in an estimated minimum mass of 22.6 t. By propagating the trajectory of 2014 AA backwards we find that the only window for finding precovery observations is for the three days before its discovery. [ABSTRACT FROM AUTHOR]

Highlights: [•] Asteroid Bennu has a well-determined orbit due primarily to 12years of radar ranging. [•] The Yarkovsky effect on Bennu causes a semimajor axis drift of 284±1.5m/year. [•] We estimate Bennu’s bulk density at 1260±70kg/m3 and macroporosity 40±10%. [•] Bennu has a 1 in 2700 chance of an Earth impact late in the 22nd century. [ABSTRACT FROM AUTHOR]

Abstract: In calculating the orbit of a minor planet with a least-squares algorithm, current practice is to assume that all observations of a given era have the same uncertainty, and that the errors in these observations are uncorrelated. These assumptions are unrealistic; and they lead to sub-optimal orbits. Our objective is to develop and validate an observational error model that provides realistic estimates of the uncertainties and correlations in asteroid observations. When used to populate the covariance matrix of the least-squares algorithm, the resulting orbits are shown to more accurately and precisely represent asteroid trajectories. [Copyright &y& Elsevier]

Abstract: In this paper, we discuss the detection of systematic biases in star positions of the USNO A1.0, A2.0, and B1.0 catalogs, as deduced from the residuals of numbered asteroid observations. We present a technique for the removal of these biases, and validate this technique by illustrating the resulting improvements in numbered asteroid residuals, and by establishing that debiased orbits predict omitted observations more accurately than do orbits derived from non-debiased observations. We also illustrate the benefits of debiasing to high-precision astrometric applications such as asteroid mass determination and collision analysis, including a refined prediction of the impact probability of 99942 Apophis. Specifically, we find the IP of Apophis to be lowered by nearly an order of magnitude to 4.5×10−6 for the 2036 close approach. [ABSTRACT FROM AUTHOR]

Abstract: The potentially hazardous Asteroid (101955) 1999 has a possibility of colliding with the Earth in the latter half of the 22nd century, well beyond the traditional 100-year time horizon for routine impact monitoring. The probabilities accumulate to a total impact probability of approximately , with a pair of closely related routes to impact in 2182 comprising more than half of the total. The analysis of impact possibilities so far in the future is strongly dependent on the action of the Yarkovsky effect, which raises new challenges in the careful assessment of longer term impact hazards. Even for asteroids with very precisely determined orbits, a future close approach to Earth can scatter the possible trajectories to the point that the problem becomes like that of a newly discovered asteroid with a weakly determined orbit. If the scattering takes place late enough so that the target plane uncertainty is dominated by Yarkovsky accelerations then the thermal properties of the asteroid, which are typically unknown, play a major role in the impact assessment. In contrast, if the strong planetary interaction takes place sooner, while the Yarkovsky dispersion is still relatively small compared to that derived from the measurements, then precise modeling of the nongravitational acceleration may be unnecessary. [Copyright &y& Elsevier]

Small bodies such as the near-Earth asteroid Bennu drift in their orbit due to thermal radiation forces (the Yarkovsky effect). Ground-based observations have indicated a nonzero probability of Bennu impacting Earth, depending on how its orbit evolves. Thus, among the goals of the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission to Bennu were to precisely measure the Yarkovsky effect and refine the impact hazard assessment for this body. Here we address these objectives. Using OSIRIS-REx spacecraft tracking data, we derive meter-level constraints on the distance between Earth and Bennu from January 2019 to October 2020. While these data greatly improve the knowledge of the trajectory of Bennu, they also require an unprecedented fidelity for the modeling of an asteroid's trajectory. In particular, special care is needed to take into account the contribution of 343 small-body perturbers and the uncertainty in their masses. Radiation effects such as the Poynting–Robertson drag, so far only considered for interplanetary dust dynamics, now become a consideration for modeling the trajectory of a 500-m asteroid such as Bennu. By employing a thermophysical model based on OSIRIS-REx's characterization of Bennu, we estimate a semimajor axis drift of − 284. 6 ± 0. 2 m/yr (signal-to-noise ratio ∼ 1400) at epoch 2011 January 1 caused by the Yarkovsky effect. The largest source of modeling error is solar wind drag, which may lower the magnitude of the semimajor axis drift from the Yarkovsky effect by up to 0.16 m/yr. The Yarkovsky-related semimajor axis drift varies by roughly ± 1 m/yr as the orbit of Bennu evolves due to planetary perturbations from 1900 to 2135. The Yarkovsky thermophysical model proves to be extremely accurate by predicting a bulk density estimate within 0.1% of that estimated through gravity science analysis. Compared to the information available before the OSIRIS-REx mission, the knowledge of the circumstances of the scattering Earth encounter that will occur in 2135 improves by a factor of 20, thus allowing us to rule out many previously possible impact trajectories. However, there remain some impact trajectories compatible with the data. Prior to the spacecraft encounter, the overall impact probability through 2200 was 3. 7 × 1 0 − 4 (1 in 2700). As a result of our analysis, the cumulative impact probability through 2300 becomes 5. 7 × 1 0 − 4 (1 in 1750) and the most significant individual impact solution is for September 2182, with an impact probability of 3. 7 × 1 0 − 4 (1 in 2700). Both Bennu and (29075) 1950 DA have a Palermo scale value of − 1. 42 and share the distinction as the currently most hazardous object in the asteroid catalog. • Tracking data from the OSIRIS-REx mission tightly constrain the orbit of Bennu. • The data require an unprecedented fidelity in modeling an asteroid's trajectory. • The Yarkovsky semimajor axis drift is measured with a signal-to-noise ratio ¿1400. • Some impact trajectories after 2175 persist with a cumulative probability of 0.06%. • Bennu and 29075 are the most hazardous known asteroids in terms of Palermo scale. [ABSTRACT FROM AUTHOR]

Abstract: When a new near Earth asteroid is discovered, it is important to know whether or not there is the possibility of an impact with the Earth in the near future. In this paper, we describe the technical approaches employed by the two operational second-generation asteroid impact monitoring systems, CLOMON2 and Sentry, paying particular attention to the similarities and differences between these independent systems. The detection and characterization of a potential impact requires the propagation of the orbital probability density function from the time of discovery to the time of hypothetical impact. Since the N-body problem is not integrable, this can be done only by sampling the orbital elements space with a finite number of Virtual Asteroids (VAs), the orbit of each one being propagated numerically. Our methods, illustrated in this paper, use the Line Of Variation (LOV), a unidimensional subspace, to perform this sampling. The primary goal is to detect Virtual Impactors (VIs), which are regions in the initial conditions space leading to dynamically distinct collision solutions; then a probability integral needs to be computed on the volume of the VI. An important issue is how to assure completeness of such a search down to some impact probability threshold. This problem cannot be efficiently solved just by computing more VAs, but requires a geometric description of the behavior of the LOV in order to identify the critical segments of this curve. We have studied these behaviors on the Target Plane (TP) through our analytical theory and the output of many numerical tests. Assuming that the geometry is the simplest compatible with the data available from the sampling, we obtain a classification which allows us to use iterative methods, appropriate for each case, to find the closest approach distance possible along the LOV. After an LOV minimum has been identified, it is possible to use a probability density linearized at this point. However, when the cross section of the Earth is not crossed by the LOV, there is no guarantee that nonlinearity would be negligible in the direction on the TP transversal to the LOV. We describe how to test for such nonlinearity, and thus reduce or eliminate the possibility of spurious VIs. In this way, the performance of our impact monitoring systems has been significantly increased in comparison to the earlier and simpler solitary system. These more advanced systems have identified and then eliminated (through additional observations) more than one hundred cases of asteroids with VIs in the years 2002–2003. [Copyright &y& Elsevier]

Radar ranging from Arecibo, Puerto Rico, to the 0.5-kilometer near-Earth asteroid 6489 Golevka unambiguously reveals a small nongravitational acceleration caused by the anisotropic thermal emission of absorbed sunlight. The magnitude of this perturbation, known as the Yarkovsky effect, is a function of the asteroid's mass and surface thermal characteristics. Direct detection of the Yarkovsky effect on asteroids will help constrain their physical properties, such as bulk density, and refine their orbital paths. Based on the strength of the detected perturbation, we estimate the bulk density of Colevka to be 2.7[sup+0.4sub-0.6] grams per cubic centimeter. [ABSTRACT FROM AUTHOR]

Predictions of future potential Earth impacts by near-Earth objects (NEOs) have become commonplace in recent years, and the rate of these detections is likely to accelerate as asteroid survey efforts continue to mature. In order to conveniently compare and categorize the numerous potential impact solutions being discovered we propose a new hazard scale that will describe the risk posed by a particular potential impact in both absolute and relative terms. To this end, we measure each event in two ways, first without any consideration of the event''s time proximity or its significance relative to the so-called background threat, and then in the context of the expected risk from other objects over the intervening years until the impact. This approach is designed principally to facilitate communication among astronomers, and it is not intended for public communication of impact risks. The scale characterizes impacts across all impact energies, probabilities and dates, and it is useful, in particular, when dealing with those cases which fall below the threshold of public interest. The scale also reflects the urgency of the situation in a natural way and thus can guide specialists in assessing the computational and observational effort appropriate for a given situation. In this paper we describe the metrics introduced, and we give numerous examples of their application. This enables us to establish in rough terms the levels at which events become interesting to various parties. [Copyright &y& Elsevier]

We performed a statistical analysis of the astrometric errors for the major asteroid surveys. We analyzed the astrometric residuals as a function of observation epoch, observed brightness and rate of motion, finding that astrometric errors are larger for faint observations and some stations improved their astrometric quality over time. Based on this statistical analysis we develop a new weighting scheme to be used when performing asteroid orbit determination. The proposed weights result in ephemeris predictions that can be conservative by a factor as large as 1.5. However, the new scheme is robust with respect to outliers and handles the larger errors for faint detections. [ABSTRACT FROM AUTHOR]

Comet C/2013 A1 (Siding Spring) will experience a high velocity encounter with Mars on 2014 October 19 at a distance of 135,000 km ± 5000 km from the planet center. We present a comprehensive analysis of the trajectory of both the comet nucleus and the dust tail. The nucleus of C/2013 A1 cannot impact on Mars even in the case of unexpectedly large nongravitational perturbations. Furthermore, we compute the required ejection velocities for the dust grains of the tail to reach Mars as a function of particle radius and density and heliocentric distance of the ejection. A comparison between our results and the most current modeling of the ejection velocities suggests that impacts are possible only for millimeter to centimeter size particles released more than 13 AU from the Sun. However, this level of cometary activity that far from the Sun is considered extremely unlikely. The arrival time of these particles spans a 20-minute time interval centered at 2014 October 19 at 20:09 TDB, i.e., around the time that Mars crosses the orbital plane of C/2013 A1. Ejection velocities larger than currently estimated by a factor >2 would allow impacts for smaller particles ejected as close as 3 AU from the Sun. These particles would reach Mars from 19:13 TDB to 20:40 TDB. [ABSTRACT FROM AUTHOR]

Our previous model (NEOMOD2) for the orbital and absolute magnitude distribution of Near Earth Objects (NEOs) was calibrated on the Catalina Sky Survey observations between 2013 and 2022. Here we extend NEOMOD2 to include visible albedo information from the Wide-Field Infrared Survey Explorer. The debiased albedo distribution of NEOs can be approximated by the sum of two Rayleigh distributions with the scale parameters p V,dark ≃ 0. 03 and p V,bright ≃ 0. 17. We find evidence for smaller NEOs having (on average) higher albedos than larger NEOs; this is likely a consequence of the size-dependent sampling of different main belt sources. These inferences and the absolute magnitude distribution from NEOMOD2 are used to construct the debiased size distribution of NEOs. We estimate 830 ± 60 NEOs with diameters D > 1 km and 20 , 000 ± 2 , 000 NEOs with D > 140 m. The new model, NEOMOD3, is available via the NEOMOD Simulator — an easy-to-operate code that can be used to generate user-defined samples (orbits, sizes and albedos) from the model. • The albedo distribution of NEOs is size and orbit dependent. • There are 830 NEOs larger than 1 km and 20,000 NEOs larger than 140 m. • The average interval between impacts of > 140 m km NEOs is 20,000 yr. [ABSTRACT FROM AUTHOR]

Close approaching (or impacting) asteroids can have slow apparent motion in some directions in the sky as they approach Earth. For some objects that approach Earth from east of opposition, the induced topocentric motion coming from Earth's rotation cancels the natural eastward motion in the sky, making the object appear to be almost stationary, This makes discovery difficult. Near-Earth asteroid 2019 OK passed within about 70 000 km of Earth on 2019 July 25.06. It evaded discovery inbound, and despite its ∼ 100 m size and approaching from the nighttime side of Earth, was first seen only about 24 h before closest approach. The reasons why various surveys did not detect it are analyzed. It is compared to a range of synthetic impactors and the appearance of impactors coming from various directions in the nighttime sky is discussed. 2019 OK appeared to be almost stationary, and as a result was not detected as a moving object. Some impacting objects approaching from east of opposition will likely have similar slow motion to 2019 OK. Detection of slow moving nearby objects is a new, unrecognized challenge for Near-Earth Object surveys. • Asteroids that may impact Earth can have slow skyplane velocity. • Topocentric motion can cancel slow eastward motion of an impacting asteroid, making it appear stationary. • Slow-moving asteroids are challenging for Near-Earth Object surveys to discover. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023±0.025°/dy (41.335±0.005h) prior to the 2000 perihelion passage, 210.448±0.016°/dy (41.055±0.003h) for the interval between the 2000 and 2005 perihelion passages, 211.856±0.030°/dy (40.783±0.006h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625±0.012°/dy (40.827±0.002h) in the interval 2006–2010 following the 2005 perihelion passage. The period decreased by 16.8±0.3min during the 2000 passage and by 13.7±0.2min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024±0.003°/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations. The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4–15) pole and longitude system. The possibility of a 180° error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42. We find that a net torque in the range of 0.3–2.5×107 kgm2 s−2 acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than ∼1° per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H2O out-gassing from the surface is largely responsible for the observed spin-up. [Copyright &y& Elsevier]

Deep Impact successfully met its primary mission objective on 4 July 2005 when the smart impactor guided itself into the path of Comet 9P/Tempel 1. The mother flyby spacecraft then observed and recorded the breathtaking collision and subsequent plume development. Ground-based navigators targeted the prerelease trajectory using optical navigation image planning and data analysis, trajectory correction maneuver design and evaluation, and orbit determination using both radiometric and optical data. In-flight improvements to the Tempel 1 ephemeris were also a critical part of the overall navigation design and operations success. The achieved navigation accuracy established a new standard for comet encounters, and this difficult task taught several important lessons. This definitive Work provides a mission overview, summarizes the navigation requirements, compares the achieved navigation performance with a baseline design that reflects in-flight updates, and identifies operational procedures that may benefit future comet-bound navigators. [ABSTRACT FROM AUTHOR]

Since its discovery in 1867, periodic comet 9P/Tempel 1 has been observed at 10 returns to perihelion, including all its returns since 1967. The observations for the seven apparitions beginning in 1967 have been fit with an orbit that includes only radial and transverse nongravitational accelerations that model the rocket-like thrusting introduced by the outgassing of the cometary nucleus. The successful nongravitational acceleration model did not assume any change in the comet’s ability to outgas from one apparition to the next and the outgassing was assumed to reach a maximum at perihelion. The success of this model over the 1967–2003 interval suggests that the comet’s spin axis is currently stable. Rough calculations suggest that the collision of the impactor released by the Deep Impact spacecraft will not provide a noticeable perturbation on the comet’s orbit nor will any new vent that is opened as a result of the impact provide a noticeable change in the comet’s nongravitational acceleration history. The observing geometries prior to, and during, the impact will allow extensive Earth based observations to complement the in situ observations from the impactor and flyby spacecraft. [ABSTRACT FROM AUTHOR]

Astrometric uncertainty is a crucial component of the asteroid orbit determination process. However, in the absence of rigorous uncertainty information, only very crude weighting schemes are available to the orbit computer. This inevitably leads to a correspondingly crude characterization of the orbital uncertainty and in many cases to less accurate orbits. In this paper we describe a method for carefully assessing the statistical performance of the various observatories that have produced asteroid astrometry, with the ultimate goal of using this statistical characterization to improve asteroid orbit determination. We also include a detailed description of our previously unpublished automatic outlier rejection algorithm used in the orbit determination, which is an important component of the fitting process.To start this study we have determined the best fitting orbits for the first 17,349 numbered asteroids and computed the corresponding O&z.sbnd;C astrometric residuals for all observations used in the orbit fitting. We group the residuals into roughly homogeneous bins and compute the root mean square (RMS) error and bias in declination and right ascension for each bin. Results are tabulated for each of the 77 bins containing more than 3000 observations; this comprises roughly two thirds of the data, but only 2% of the bins. There are several interesting results, including substantial bias from several observatories, and some distinct non-Gaussian characteristics that are difficult to explain. Several limitations of our approach and possible future improvements are discussed.The correlation of errors among observations taken closely together in time is in many cases an important issue that has rarely been considered. We have computed the mean correlation between observations with varying time separations and obtained empirical functions that model the results. This has been done for several observatories with very large data sets (the remainder has been lumped into a single mixed batch). These functions could be used for estimating correlation coefficients among different observations of the same observatory, supplying a new observation weighting scheme based upon an empirically tested observational error model. [Copyright &y& Elsevier]

Catalina Sky Survey (CSS) is a major survey of Near-Earth Objects (NEOs). In a recent work, we used CSS observations from 2005–2012 to develop a new population model of NEOs (NEOMOD). CSS's G96 telescope was upgraded in 2016 and detected over 10,000 unique NEOs since then. Here we characterize the NEO detection efficiency of G96 and use G96's NEO detections from 2013–2022 to update NEOMOD. This resolves previous model inconsistencies related to the population of large NEOs. We estimate there are 936 ± 29 NEOs with absolute magnitude H < 17. 75 (diameter D > 1 km for the reference albedo p V = 0. 14) and semimajor axis a < 4. 2 au. The slope of the NEO size distribution for H = 25 –28 is found to be relatively shallow (cumulative index ≃ 2. 6) and the number of H < 28 NEOs (D > 9 m for p V = 0. 14) is determined to be (1. 20 ± 0. 04) × 1 0 7 , about 3 times lower than in Harris & Chodas (2021). Small NEOs have a different orbital distribution and higher impact probabilities than large NEOs. We estimate 0. 034 ± 0. 002 impacts of H < 28 NEOs on the Earth per year, which is near the low end of the impact flux range inferred from atmospheric bolide observations. Relative to a model where all NEOs are delivered directly from the main belt, the population of small NEOs detected by G96 shows an excess of low-eccentricity orbits with a ≃ 1 –1.6 au that appears to increase with H (≃ 30 % excess for H = 28). We suggest that the population of very small NEOs is boosted by tidal disruption of large NEOs during close encounters to the terrestrial planets. When the effect of tidal disruption is (approximately) accounted for in the model, we estimate 0. 06 ± 0. 01 impacts of H < 28 NEOs on the Earth per year, which is more in line with the bolide data. The impact probability of a H < 22 (D > 140 m for p V = 0. 14) object on the Earth in this millennium is estimated to be ≃ 4. 5 %. • Catalina Sky Survey observations (2013–2022) are used to update our NEO model. • There are 936 ± 29 NEOs with absolute magnitude H ¡17.75. • The population of very small NEOs is boosted by tidal disruption of large NEOs. • We estimate 0. 06 ± 0. 01 impacts of H ¡28 NEOs on the Earth per year. • The impact probability of a H ¡22 NEO on the Earth in this millennium is 4.5%. [ABSTRACT FROM AUTHOR]

When optical navigation images acquired by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission revealed the periodic ejection of particles from asteroid (101955) Bennu, it became a mission priority to quickly identify and track these objects for both spacecraft safety and scientific purposes. The large number of particles and the mission criticality rendered time‐intensive manual inspection impractical. We present autonomous techniques for particle detection and tracking that were developed in response to the Bennu phenomenon but that have the capacity for general application to particles in motion about a celestial body. In an example OSIRIS‐REx data set, our autonomous techniques identified 93.6% of real particle tracks and nearly doubled the number of tracks detected versus manual inspection alone. Key Points: We describe autonomous techniques for the identification and tracking of particles in motion about a celestial bodyWe demonstrate these techniques using images from the OSIRIS‐REx mission to the active asteroid (101955) BennuIn the OSIRIS‐REx dataset, our autonomous algorithms detected 93.6% of real particle tracks, including 244 tracks not identified by manual inspection [ABSTRACT FROM AUTHOR]

Highlights • We study the mutual orbit period change of asteroid Didymos by the DART impact. • Shape deformation causes additional change in the mutual orbit period of Didymos. • Further analyses of this scenario allow for better evaluating the DART impact. Abstract The Asteroid Impact & Deflection Assessment (AIDA) targets binary near-Earth asteroid (65803) Didymos. As part of this mission, the NASA-led Double Asteroid Redirection Test (DART) will make a kinetic impactor collide with the smaller secondary of Didymos to test kinetic impact asteroid deflection technology, while the ESA-led Hera mission will evaluate the efficiency of the deflection by conducting detailed on-site observations. Research has shown that the larger primary of Didymos is spinning close to its critical spin, and the DART-impact-driven ejecta would give kinetic energy to the primary. It has been hypothesized that such an energy input might cause structural deformation of the primary, affecting the mutual orbit period, a critical parameter for assessing the kinetic impact deflection by the DART impactor. A key issue in the previous work was that the secondary was assumed to be spherical, which may not be realistic. Here, we use a second-order inertia-integral mutual dynamics model to analyze the effects of the shapes of the primary and the secondary on the mutual orbit period change of the system. We first compare the second-order model with three mutual dynamics models, including a high-order inertia-integral model that takes into account the detailed shapes of Didymos. The comparison tests show that the second-order model may have an error of ∼ 10 % for computing the mutual orbit period change, compared to the high-order model. We next use the second-order model to analyze how the original shape and shape deformation change the mutual orbit period. The results show that when the secondary is elongated, the mutual orbit period becomes short. Also, shape deformation of the secondary further changes the mutual orbit period. A better understanding of this mechanism allows for detailed assessment of DART's kinetic impact deflection capability for Didymos. [ABSTRACT FROM AUTHOR]

Binary near-Earth asteroid (65803) Didymos is the target of the proposed NASA Double Asteroid Redirection Test (DART), part of the Asteroid Impact & Deflection Assessment (AIDA) mission concept. In this mission, the DART spacecraft is planned to impact the secondary body of Didymos, perturbing mutual dynamics of the system. The primary body is currently rotating at a spin period close to the spin barrier of asteroids, and materials ejected from the secondary due to the DART impact are likely to reach the primary. These conditions may cause the primary to reshape, due to landslides or internal deformation, changing the permanent gravity field. Here, we propose that if shape deformation of the primary occurs, the mutual orbit of the system would be perturbed due to a change in the gravity field. We use a numerical simulation technique based on the full two-body problem to investigate the shape effect on the mutual dynamics in Didymos after the DART impact. The results show that under constant volume, shape deformation induces strong perturbation in the mutual motion. We find that the deformation process always causes the orbital period of the system to become shorter. If surface layers with a thickness greater than ∼0.4m on the poles of the primary move down to the equatorial region due to the DART impact, a change in the orbital period of the system and in the spin period of the primary will be detected by ground-based measurement. [ABSTRACT FROM AUTHOR]

The impact of asteroid 2008 TC 3 was an unprecedented event—the first ever predicted impact of a near-Earth object. When it was first detected about 20 h before impact, 2008 TC 3 was still farther away than the Moon. Once it was recognized as an impactor and announced as such, 2008 TC 3 began to receive considerable attention from astronomical observers. Using the unprecedented dataset of nearly 900 astrometric observations and the latest observation debiasing and weighting techniques, we estimate the precise trajectory of 2008 TC 3 and its impact ground track. At the entry point into the atmosphere, the 3- σ formal uncertainty in predicted position is an ellipse only 1.4 km × 0.15 km in size. The locations of the many meteorites recovered from the desert floor mark the asteroid’s actual ground track and provide a unique opportunity to validate trajectory models. We find that the second-order zonal harmonics of the Earth gravity field moves the ground track by more than 1 km and the location along the ground track by more than 2 km, while non-zonal and higher order harmonics change the impact prediction by less than 20 m. The contribution of atmospheric drag to the trajectory of 2008 TC 3 is similar to the numerical integration error level, a few meters, down to an altitude of 50 km. Integrating forward to lower altitudes and ignoring the break-up of 2008 TC 3 , atmospheric drag causes an along-track deviation that can be as large as a few kilometers at sea level. [ABSTRACT FROM AUTHOR]

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. • Apophis' tumbling spin state will change significantly due to gravitational torques. • If a contact binary, Apophis' components could shift at closest approach. • The Yarkovsky drift rate will change, affecting future encounter predictions. • Flyby outcome sensitivity to inertias will provide insights on Apophis' mass distribution. [ABSTRACT FROM AUTHOR]

We use the recently determined rotation state, shape, size and thermophysical model of Apophis to predict the strength of the Yarkovsky effect in its orbit. Apophis does not rotate about the shortest principal axis of the inertia tensor, rather its rotational angular momentum vector wobbles at an average angle of ≃37° from the body axis. Therefore, we pay special attention to the modeling of the Yarkovsky effect for a body in such a tumbling state, a feature that has not been described in detail so far. Our results confirm that the Yarkovsky effect is not significantly weakened by the tumbling state. The previously stated rule that the Yarkovsky effect for tumbling kilometer-size asteroids is well represented by a simple model assuming rotation about the shortest body axis in the direction of the rotational angular momentum and with rotation period close to the precession period is confirmed. Taking into account uncertainties of the model parameters, as well as the expected density distribution for Apophis’ spectral class, we predict the secular change in the semimajor axis is ( - 12.8 ± 3.6 ) × 10 - 4 au/Myr (formal 1 σ uncertainty). The currently available astrometric data for Apophis do not allow an unambiguous direct detection of the Yarkovsky effect. However, the fitted secular change in semimajor axis of ( - 23 ± 13 ) × 10 - 4 au/Myr is compatible with the model prediction. We revise the Apophis’ impact probability information in the second half of this century by extending the orbital uncertainty derived from the current astrometric data and by taking into account the uncertainty in the dynamical model due to the thermal recoil accelerations. This is done by mapping the combined uncertainty to the close encounter in 2029 and by determining the statistical weight of the known keyholes leading to resonant impact orbits. Whereas collision with the Earth before 2060 is ruled out, impacts are still possible from 2060 with probabilities up to a few parts in a million. More definitive analysis will be available after the Apophis apparition in 2020–2021. [ABSTRACT FROM AUTHOR]

Asteroid (410777) 2009 FD could hit Earth between 2185 and 2196. The long term propagation to the possible impacts and the intervening planetary encounters make 2009 FD one of the most challenging asteroids in terms of hazard assessment. To compute accurate impact probabilities we model the Yarkovsky effect by using the available physical characterization of 2009 FD and general properties of the near Earth asteroid population. We perform the hazard assessment with two independent methods: the first method is a generalization of the standard impact monitoring algorithms in use by NEODyS and Sentry, while the second one is based on a Monte Carlo approach. Both methods generate orbital samples in a seven-dimensional space that includes orbital elements and the parameter characterizing the Yarkovsky effect. The highest impact probability is 2.7 × 10-3 for an impact during the 2185 Earth encounter. Impacts after 2185 corresponding to resonant returns are possible, the most relevant being in 2190 with a probability of 3 × 10-4. Both numerical methods can be used in the future to handle similar cases. The structure of resonant returns and the list of the possible keyholes on the target plane of the scattering encounter in 2185 can be predicted by an analytic theory. [ABSTRACT FROM AUTHOR]

- On October 7, 2008, asteroid 2008 TC3 impacted Earth and fragmented at 37 km altitude above the Nubian Desert in northern Sudan. The area surrounding the asteroid's approach path was searched, resulting in the first recovery of meteorites from an asteroid observed in space. This was also the first recovery of remains from a fragile 'cometary' PE = IIIa/b type fireball. In subsequent searches, over 600 mostly small 0.2-379 g meteorites (named 'Almahata Sitta') with a total mass 10.7 kg were recovered from a 30 × 7 km area. Meteorites fell along the track at 1.3 kg km−1, nearly independent of mass between 1 and 400 g, with a total fallen mass of 39 ± 6 kg. The strewn field was shifted nearly 1.8 km south from the calculated approach path. The influence of winds on the distribution of the meteorites, and on the motion of the dust train, is investigated. The majority of meteorites are ureilites with densities around 2.8 g cm−3, some of an anomalous (porous, high in carbon) polymict ureilite variety with densities as low as 1.5 g cm−3. In addition, an estimated 20-30% (in mass) of recovered meteorites were ordinary, enstatite, and carbonaceous chondrites. Their fresh look and matching distribution of fragments in the strewn field imply that they were part of 2008 TC3. For that reason, they are all referred to as 'Almahata Sitta.' No ureilite meteorites were found that still held foreign clasts, suggesting that the asteroid's clasts were only loosely bound. [ABSTRACT FROM AUTHOR]

Spacecraft have successfully landed on the Moon, Venus, and Mars, and have penetrated the atmosphere of Jupiter. On 2001 February 12, the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft landed on the surface of the asteroid (433) Eros after a year of observations in orbit about the asteroid. NEAR Shoemaker was not designed to land on an asteroid, complicating the design of operations needed to accomplish this feat. However, the NEAR Shoemaker team wanted to attempt a landing after the year of orbital operations that consumed most of the remaining spacecraft fuel, operations funding, and planned Deep Space Network tracking. This would be a fitting end to the mission, and it would be possible to obtain images at much greater resolution during the descent than could be obtained from orbit. The operations were more successful than the NEAR Shoemaker team had hoped, obtaining 70 high-resolution images during the descent and two weeks of gamma-ray spectrometer data from the surface after the successful soft landing. [Copyright &y& Elsevier]

Triangulated observations of fireballs allow us to determine orbits and fall positions for meteorites. The great majority of basaltic meteorites are derived from the asteroid 4 Vesta. We report on a recent fall that has orbital properties and an oxygen isotope composition that suggest a distinct parent body. Although its orbit was almost entirely contained within Earth's orbit, modeling indicates that it originated from the innermost main belt. Because the meteorite parent body would likely be classified as a V-type asteroid, V-type precursors for basaltic meteorites unrelated to Vesta may reside in the inner main belt. This starting location is in agreement with predictions of a planetesimal evolution model that postulates the formation of differentiated asteroids in the terrestrial planet region, with surviving fragments concentrated in the innermost main belt. [ABSTRACT FROM AUTHOR]

Astrometric positions of moving objects in the Solar System have been measured using a variety of star catalogs in the past. Previous work has shown that systematic errors in star catalogs can affect the accuracy of astrometric observations. That, in turn, can influence the resulting orbit fits for minor planets. In order to quantify these systematic errors, we compare the positions and proper motion of stellar sources in the most utilized star catalogs to the second release of the Gaia star catalog. The accuracy of Gaia astrometry allows us to unambiguously identify local biases and derive a scheme that can be used to correct past astrometric observations of solar system objects. Here, we provide substantially improved debiasing tables for 26 astrometric catalogs that were extensively used in minor planet astrometry. Revised corrections near the galactic center eliminate artifacts that could be traced back to reference catalogs used in previous debiasing schemes. Median differences in stellar positions between catalogs now tend to be on the order of several tens of milliarcseconds (mas) but can be as large as 175 mas. Median stellar proper motion corrections scatter around 0.3 mas/yr and range from 1to 4 mas/yr for star catalogs with and without proper motion, respectively. The tables presented in this work contain a posteriori corrections meant to improve orbit fits based on optical observations that were measured against astrometric catalogs other than Gaia. However, astrometrists are strongly encouraged to make use of the most recent Gaia astrometric catalog when submitting new observations. Since previous debiasing schemes already reduced systematics in past observations to a large extent, corrections beyond the current work may not be needed in the foreseeable future. • The Gaia astrometric catalog published with the second Gaia data release is the new standard for minor planet astrometry. • We have used Gaia DR 2 as a reference to calculate debiasing tables for 26 other astrometric catalogs. • The quality of catalog corrections is significantly improved compared to previous work. • The new debiasing scheme delivers incremental improvement of the quality of near-Earth asteroid orbits. [ABSTRACT FROM AUTHOR]

The B-plane is a fundamental tool to analyze planetary encounters of small bodies and spacecraft flybys. In this paper, we review the B-plane formulation with a full derivation of its coordinates and their partial derivatives, which allow the mapping of orbital uncertainties onto the B-plane. We find that this mapping can be sensitive to variations in the inbound asymptote, especially for low-velocity encounters, and to non-Keplerian dynamics for distant encounters. Under linearity assumptions, we show how to derive close approach boundaries and impact probabilities from the orbital uncertainty mapped onto the B-plane. [ABSTRACT FROM AUTHOR]