In this paper we perform an assessment of the 2880 Earth impact risk for Asteroid (29075) 1950 DA. To obtain reliable predictions we analyze the contribution of the observational dataset and the ...astrometric treatment, the numerical error in the long-term integration, and the different accelerations acting on the asteroid. The main source of uncertainty is the Yarkovsky effect, which we statistically model starting from 1950 DA’s available physical characterization, astrometry, and dynamical properties. Before the release of 2012 radar data, this modeling suggests that 1950 DA has 99% likelihood of being a retrograde rotator. By using a 7-dimensional Monte Carlo sampling we map 1950 DA’s uncertainty region to the 2880 close approach b-plane and find a 5×10-4 impact probability. With the recently released 2012 radar observations, the direct rotation is definitely ruled out and the impact probability decreases to 2.5×10-4.
We analyze the trajectories of 313 particles seen in the near‐Bennu environment between December 2018 and September 2019. Of these, 65% follow suborbital trajectories, 20% undergo more than one ...orbital revolution around the asteroid, and 15% directly escape on hyperbolic trajectories. The median lifetime of these particles is ∼6 hr. The trajectories are sensitive to Bennu's gravitational field, which allows us to reliably estimate the spherical harmonic coefficients through degree 8 and to resolve nonuniform mass distribution through degree 3. The particles are perturbed by solar radiation pressure, enabling effective area‐to‐mass ratios to be estimated. By assuming that particles are oblate ellipsoids of revolution, and incorporating photometric measurements, we find a median axis ratio of 0.27 and diameters for equivalent‐volume spheres ranging from 0.22–6.1 cm, with median 0.74 cm. Our size distribution agrees well with that predicted for fragmentation due to diurnal thermal cycling. Detailed models of known accelerations do not produce a match to the observed trajectories, so we also estimate empirical accelerations. These accelerations appear to be related to mismodeling of radiation pressure, but we cannot rule out contributions from mass loss. Most ejections take place at local solar times in the afternoon and evening (12:00–24:00), although they occur at any time of day. We independently identify ten ejection events, some of which have previously been reported. We document a case where a particle ricocheted off the surface, revealing a coefficient of restitution 0.57±0.01 and demonstrating that some apparent ejections are not related to surface processes.
Plain Language Summary
The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS‐REx) mission discovered that near‐Earth asteroid (101955) Bennu is periodically ejecting small particles from its surface, placing it in the uncommon class of “active asteroids.” We linked together individual detections of ejected particles and used numerical models of the forces acting on them to ascertain their trajectories and fates. We found that most particles have suborbital trajectories, meaning they fall back to Bennu's surface shortly after being ejected, but some orbit Bennu for days at a time, and some escape directly into space. From the particle trajectories, we are able to estimate their sizes (comparable to pebbles, from a few millimeters to a few centimeters in diameter) and shapes (probably flake like). Their trajectories also make it possible to estimate Bennu's gravity field more precisely than spacecraft measurements and help shed light on the possible causes of the ejections.
Key Points
Most of the 313 particles we study have suborbital trajectories, but some orbit Bennu and others directly escape
The particles appear to have flake‐like shapes and have effective diameters 0.22–6.1 cm with median 0.74 cm
Ejections tend to take place in the local afternoon and evening but can occur anytime
In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped ...accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554-995nm wavelength range, and designated 2008 TC3 (refs 4-6). It subsequently hit the Earth. Because it exploded at 37km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.
The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu: (i) tracking ...and modeling the spacecraft in orbit about the asteroid and (ii) tracking and modeling pebble-sized particles naturally ejected from Bennu's surface into sustained orbits. These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu's equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O'Keefe-Radzievskii-Paddack cycle or arising during Bennu's accretion following the disruption of its parent body.
Aims. From light curve and radar data we know the spin axis of only 43 near-Earth asteroids. In this paper we attempt to constrain the spin axis obliquity distribution of near-Earth asteroids by ...leveraging the Yarkovsky effect and its dependence on an asteroid’s obliquity. Methods. By modeling the physical parameters driving the Yarkovsky effect, we solve an inverse problem where we test different simple parametric obliquity distributions. Each distribution results in a predicted Yarkovsky effect distribution that we compare with a χ2 test to a dataset of 125 Yarkovsky estimates. Results. We find different obliquity distributions that are statistically satisfactory. In particular, among the considered models, the best-fit solution is a quadratic function, which only depends on two parameters, favors extreme obliquities consistent with the expected outcomes from the YORP effect, has a 2:1 ratio between retrograde and direct rotators, which is in agreement with theoretical predictions, and is statistically consistent with the distribution of known spin axes of near-Earth asteroids.
We have identified and quantified semimajor axis drifts in near-Earth asteroids (NBAs) by performing orbital fits to optical and radar astrometry of all numbered NEAs. We focus on a subset of 54 NEAs ...that exhibit some of the most reliable and strongest drift rates. Our selection criteria include a Yarkovsky sensitivity metric that quantifies the detectability of semimajor axis drift in any given data set, a signal-to-noise metric, and orbital coverage requirements. In 42 cases, the observed drifts (~10 super(-3) AU Myr super(-1)) agree well with numerical estimates of Yarkovsky drifts. This agreement suggests that the Yarkovsky effect is the dominant non-gravitational process affecting these orbits, and allows us to derive constraints on asteroid physical properties. In 12 cases, the drifts exceed nominal Yarkovsky predictions, which could be due to inaccuracies in our knowledge of physical properties, faulty astrometry, or modeling errors. If these high rates cannot be ruled out by further observations or improvements in modeling, they would be indicative of the presence of an additional non-gravitational force, such as that resulting from a loss of mass of order a kilogram per second. We define the Yarkovsky efficiency f sub(Y) as the ratio of the change in orbital energy to incident solar radiation energy, and we find that typical Yarkovsky efficiencies are ~10 super(-5).
Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeter‐ to centimeter‐scale particles. The close ...proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft enabled monitoring of particles for a 10‐month period encompassing Bennu's perihelion and aphelion. We found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). We estimate that Bennu ejects ~104 g per orbit. The largest event took place on 6 January 2019 and consisted of ~200 particles. The observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mJ, respectively. Hundreds of particles not associated with the multiparticle ejections were also observed. Photometry of the best‐observed particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. Ground‐based data back to 1999 show no evidence of past activity for Bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. There appears to be a gentle decrease in activity with distance from the Sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor.
Plain Language Summary
We measured the brightness of pebble‐sized particles in the vicinity of near‐Earth asteroid Bennu to better understand their physical characteristics and the events that launched them from Bennu's surface. Our measurements spanned 10 months, encompassing Bennu's closest and farthest distances from the Sun, so that we could assess how the level of ejection activity changes with solar distance. We observed 18 multiparticle ejection events containing anywhere from a few to 200+ particles. Individual particles ranged from millimeters to centimeters in diameter. The energy of the events and a possible decrease in activity with larger distances from the Sun suggest that meteoroid impacts, fracturing of surface boulders due to solar heating, or both may be responsible for ejecting the particles. We estimate that Bennu releases ~10,000 g of material over one orbit or 1.2 years. Although mass loss has been remotely observed for other asteroids, the comparatively low level of particle ejection activity at Bennu was only observable thanks to the close proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft.
Key Points
Asteroid (101955) Bennu is active from perihelion through aphelion with a possible decrease in activity further from the Sun
Bennu's activity is less than that detected by telescope for other active asteroids and is only observable up close
The particles' shallow phase functions resemble those of similarly sized individual rocks rather than those of ensemble asteroid surfaces
Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss. The activity manifests itself in the form of ejection events emitting up to hundreds of millimeter‐ to ...centimeter‐scale particles. The Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer spacecraft monitored particle activity for a 10‐month period that included Bennu's perihelion and aphelion. Novel and classical methods were utilized to detect the particles and characterize their orbital and physical properties. Roughly 30% of the observed particle mass escaped to heliocentric orbit. A majority of particles fell back onto the surface of Bennu after ejection, with the longest‐lived particle surviving for 6 days on a temporary orbit. Particle ejection events appear to preferentially take place in the afternoon and evening and from low latitudes, although they can occur at any time or latitude. The reaccumulation of material is biased toward low latitudes resulting in the possible in‐fill of craters and growth of Bennu's equatorial bulge. Of the potential mechanisms behind this activity that were investigated in focused studies, meteoroid impacts, thermal fracturing, and ricochet—but not water ice sublimation—were found to be consistent with observations. While phyllosilicate dehydration was not investigated with a focused study, it remains a possible mechanism. These mechanisms are not unique to Bennu, suggesting that many near‐Earth asteroids may exhibit activity that has gone undetected thus far. Spacecraft missions with wide‐field imagers are encouraged to further characterize this phenomenon.
Plain Language Summary
Near‐Earth asteroid Bennu is the target of the Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer asteroid sample return mission. Navigational imagery acquired by the spacecraft showed that pebble‐sized particles of rock were being ejected repeatedly from the surface of Bennu. Modified and new techniques were deployed to study these particles and the ejection events that launched them. Some particles escape into space, others temporarily orbit Bennu, and most fall back onto Bennu's surface after being launched. Ejections most often occur in the local afternoon and evening. The most well‐characterized potential mechanisms causing the ejection events are cracking of surface rocks due to thermal stress and impacts by meteoroids. Ricochet, when a particle reimpacts the surface and bounces off again, can also occur. Bennu thus belongs to a class of solar system objects known as “active asteroids”, meaning those that shed mass from their surface, a characteristic once thought to be reserved for comets. The discovery of Bennu's activity, which is modest enough that it could only have been detected by a nearby spacecraft (not by telescopes), suggests that there may be more active asteroids than previously believed.
Key Points
Novel and classical methods were applied to detect and characterize particles ejected from near‐Earth asteroid Bennu
Of the mechanisms investigated, meteoroid impacts, thermal fracturing, and ricochet are consistent with observations
Some ejected particles escape or temporarily orbit and most reimpact, with implications for Bennu's geophysics and surface properties
•Successfully computed ephemeris of Comet C/2013 A1 for the Oct 2014 Mars flyby.•Used high quality astrometry and applied strict selection criteria to observations.•Detected significant out-of-plane ...nongravitational perturbations.•Used the Rotating Jet Model for nongravitational perturbations acting on C/2013 A1.•Estimated C/2013 A1 pole as (RA, DEC)=(63°, 14°).
The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportunity for imaging a long-period comet and resolving its nucleus and rotation state. Because of the small encounter distance and the high relative velocity, the goal of successfully observing C/2013 A1 from the Mars orbiting spacecraft posed strict accuracy requirements on the comet’s ephemeris. These requirements were hard to meet, as comets are known for being highly unpredictable: astrometric observations can be significantly biased and nongravitational perturbations affect comet trajectories. Therefore, even prior to the encounter, we remeasured a couple of hundred astrometric images obtained with ground-based and Earth-orbiting telescopes. We also observed the comet with the Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE) camera on 2014 October 7. In particular, these HiRISE observations were decisive in securing the trajectory and revealed that out-of-plane nongravitational perturbations were larger than previously assumed. Though the resulting ephemeris predictions for the Mars encounter allowed observations of the comet from the Mars orbiting spacecraft, post-encounter observations show a discrepancy with the pre-encounter trajectory. We reconcile this discrepancy by employing the Rotating Jet Model, which is a higher fidelity model for cometary nongravitational perturbations and provides an estimate of C/2013 A1’s spin pole (RA,DEC)=(63°,14°).
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.
PDF
