S. R. Chesley

The Pan-STARRS Moving Object Processing System

ABSTRACT.We describe the Pan-STARRS Moving Object Processing System (MOPS), a modern software package that produces automatic asteroid discoveries and identifications from catalogs of transient detections from next-generation astronomical survey telescopes. MOPS achieves >99.5%>99.5% efficiency in producing orbits from a synthetic but realistic population of asteroids whose measurements were simulated for a Pan-STARRS4-class telescope. Additionally, using a nonphysical grid population, we demonstrate that MOPS can detect populations of currently unknown objects such as interstellar asteroids. MOPS has been adapted successfully to the prototype Pan-STARRS1 telescope despite differences in expected false detection rates, fill-factor loss, and relatively sparse observing cadence compared to a hypothetical Pan-STARRS4 telescope and survey. MOPS remains highly efficient at detecting objects but drops to 80% efficiency at producing orbits. This loss is primarily due to configurable MOPS processing limits that a...

ExploreNEOs. V. Average Albedo by Taxonomic Complex in the Near-Earth Asteroid Population

Examining the albedo distribution of the near-Earth object (NEO) population allows for a better understanding of the relationship between absolute (H) magnitude and size, which impacts calculations of the size frequency distribution and impact hazards. Examining NEO albedos also sheds light on the differences between the NEO and Main Belt populations. We combine albedo results from the ExploreNEOs Warm Spitzer Exploration Science program with taxonomic classifications from the literature, publicly available data sets, and new observations from our concurrent spectral survey to derive the average albedos for C-, D-, Q-, S-, V-, and X-complex NEOs. Using a sample size of 118 NEOs, we calculate average albedos of 0.29+0.05 –0.04, 0.26+0.04 –0.03, and 0.42+0.13 –0.11 for the Q-, S-, and V-complexes, respectively. The averages for the C- and D-complexes are 0.13+0.06 –0.05 and 0.02+0.02 –0.01, but these averages are based on a small number of objects (five and two, respectively) and will improve with additional observations. We use albedos to assign X-complex asteroids to one of the E-, M-, or P-types. Our results demonstrate that the average albedos for the C-, S-, V-, and X-complexes are higher for NEOs than the corresponding averages observed in the Main Belt.

ExploreNEOs. I. Description and First Results from the Warm Spitzer Near-Earth Object Survey

We have begun the ExploreNEOs project in which we observe some 700 Near-Earth Objects (NEOs) at 3.6 and 4.5 μm with the Spitzer Space Telescope in its Warm Spitzer mode. From these measurements and catalog optical photometry we derive albedos and diameters of the observed targets. The overall goal of our ExploreNEOs program is to study the history of near-Earth space by deriving the physical properties of a large number of NEOs. In this paper, we describe both the scientific and technical construction of our ExploreNEOs program. We present our observational, photometric, and thermal modeling techniques. We present results from the first 101 targets observed in this program. We find that the distribution of albedos in this first sample is quite broad, probably indicating a wide range of compositions within the NEO population. Many objects smaller than 1 km have high albedos (0.35), but few objects larger than 1 km have high albedos. This result is consistent with the idea that these larger objects are collisionally older, and therefore possess surfaces that are more space weathered and therefore darker, or are not subject to other surface rejuvenating events as frequently as smaller NEOs.

The internal structure of asteroid (25143) Itokawa as revealed by detection of YORP spin-up

Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa’s spin rate of dω/dt = (3.54 ± 0.38) × 10-8 rad day-2, equivalent to a decrease of its rotation period of ~45 ms year-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ~21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m-3 and 2850 ± 500 kg m-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.

Yarkovsky-driven impact risk analysis for asteroid (99942) Apophis

We assess the risk of an Earth impact for asteroid (99942) Apophis by means of a statistical analysis accounting for the uncertainty of both the orbital solution and the Yarkovsky effect. We select those observations with either rigorous uncertainty information provided by the observer or a high established accuracy. For the Yarkovsky effect we perform a Monte Carlo simulation that fully accounts for the uncertainty in the physical characterization, especially for the unknown spin orientation. By mapping the uncertainty information onto the 2029 b-plane and identifying the keyholes corresponding to subsequent impacts we assess the impact risk for future encounters. In particular, we find an impact probability greater than 10 −6 for an impact in 2068. We analyze the stability of the impact probability with respect to the assumptions on Apophis’ physical characterization and consider the possible effect of the early 2013 radar apparition.

DETECTION OF SEMIMAJOR AXIS DRIFTS IN 54 NEAR-EARTH ASTEROIDS: NEW MEASUREMENTS OF THE YARKOVSKY EFFECT

We have identified and quantified semimajor axis drifts in near-Earth asteroids (NEAs) 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 detectabilityofsemimajoraxisdriftinanygivendataset,asignal-to-noisemetric,andorbitalcoveragerequirements. In 42 cases, the observed drifts (∼10 −3 AU Myr −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 fY as the ratio of the change in orbital energy to incident solar radiation energy, and we find that typical Yarkovsky efficiencies are ∼10 −5 .

The Pan-STARRS Synthetic Solar System Model: A Tool for Testing and Efficiency Determination of the Moving Object Processing System

We present here the Pan-STARRS Moving Object Processing System (MOPS) Synthetic Solar System Model (S3M), the first-ever attempt at building a comprehensive flux-limited model of the major small-body populations in the solar system. The goal of the S3M is to provide a valuable tool in the design and testing of the MOPS software, and will be used in the monitoring of the upcoming Pan-STARRS 1 all-sky survey, which started science operations during late spring of 2010. The model is composed of synthetic populations of near-Earth objects (NEOs with a subpopulation of Earth impactors), the main-belt asteroids (MBAs), Jovian Trojans, Centaurs, trans-Neptunian objects (classical, resonant, and scattered trans-Neptunian objects [TNOs]), Jupiter-family comets (JFCs), long-period comets (LPCs), and interstellar comets. The model reasonably reproduces the true populations to a minimum of V = 24.5, corresponding to approximately the expected limiting magnitude for Pan-STARRSs ability to detect moving objects. The NEO synthetic population has been extended to H < 25 (corresponding to objects of about 50 m in diameter), allowing for close flybys of the Earth to be modeled.

OSIRIS-REx: Sample Return from Asteroid (101955) Bennu

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

Orbital Identification for Asteroid 152563 (1992 BF) through the Yarkovsky Effect

Often a newly discovered near-Earth asteroid is linked to old observations of a formerly lost object. This orbital identification is done using a standard dynamical model that accounts for gravitational perturbations from planets and relativistic effects. Here we report the first case where such an identification requires consideration of the Yarkovsky effect, a tiny non-gravitational perturbation due to the recoil of thermal radiation from the body. Moreover, this implies that the Yarkovsky force is revealed in the orbital motion of the body, asteroid 152563 (1992 BF), only the second case so far. Orbital fits indicate a drift in the orbital semi-major axis of –(10.7 ± 0.7) × 10–4 AU Myr–1, which we ascribe to Yarkovsky forces. This yields a correlated constraint of physical parameters such as the obliquity, rotation rate, surface thermal inertial, and bulk density. The magnitude and direction of drift point to an obliquity in excess of 120°. Observations taken during 2011 and subsequent close encounters with the Earth might help establish rotation parameters and thereby constrain thermal inertia of 1992 BF, thus making the Yarkovsky strength a measure of this asteroids bulk density.

Assessment of the 2880 impact threat from Asteroid (29075) 1950 DA

Abstract 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 .