Photometry of Pluto-Charon mutual events and Hirayama family asteroids

During 1985 to 1990, nature provided earth bound astronomers with a once-per-century opportunity to observe occultation and transit phenomena between Pluto and its satellite, Charon. Ground based observations of these events are now being used to derive physical parameters for the Pluto-Charon system to a precision that is unlikely to be improved upon until in situ spacecraft observations are obtained. This program supports analysis of photometry observations from McDonald Observatory, a critical location in the International Pluto Campaign network. Knowledge of the diameters, masses, densities, and compositions derived from these observations will augment the understanding of Pluto's origin and its context within the problem of solar system formation

Photometry of Pluto-Charon mutual events and Hirayama family asteroids

Once every 124 years, nature provides earth-bound astronomers with the opportunity to observe occultation and transit phenomena between Pluto and its satellite, Charon. Ground-based observations of these events will allow precise physical parameters for the Pluto-Charon system to be derived which are unlikely to be improved upon until in situ spacecraft observations are obtained. The proposed program will continue to support photometry observations from McDonald Observatory, a critical location in an international Pluto Campaign network. Knowledge of the diameters, masses, densities, and compositions derived from these observations will augment our understanding of Pluto's origin and its context within the problem of solar system formation. A second task will continue to research the evolutionary processes which have occurred in the asteroid belt by measuring the physical properties of specific Hirayama family members. Photoelectric lightcurve observations of Koronis and Themis family members will be used to investigate the individual catastrophic collision events which formed each family. By comparing these properties with results of laboratory and numerical experiments, the outcomes of catastrophic disruptions and collisional evolution may be more precisely determined

Mars Encounters cause fresh surfaces on some near-Earth asteroids

All airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1My) timescales. The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying they have been recently been exposed. Previous work found that Earth encounters were the dominant freshening mechanism and could be responsible for all near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type sample of by a factor of three. We present the orbital distributions of 64 Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces. Our sample reveals two important results: i) the relatively steady fraction of Q-types with increasing semi-major axis and ii) the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection Distances (MOID) that do not have orbit solutions that cross Earth. Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened. The high Earth-MOID asteroids represent 10% of the Q-type population and all are in Amor orbits. While surface refreshing could also be caused by Main Belt collisions or mass shedding from YORP spinup, all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces.Comment: Accepted for publication in Icarus -- 14 pages, 8 figures, 1 table, 2 appendice

Rotationally Resolved Spectroscopy of Asteroid Pairs: No Spectral Variation Suggests Fission is followed by Settling of Dust

We examine the spectral properties of asteroid pairs that were disrupted in the last 2 Myrs to examine whether the site of the fission can be revealed. We studied the possibility that the sub-surface material, perhaps on one hemisphere, has spectral characteristics differing from the original weathered surface, by performing rotationally-resolved spectroscopic observations to look for local variations as the asteroid rotates. We observed 11 asteroids in pairs in the near-IR and visible range. Photometry was also conducted to determine the rotational phases of a spectrum on the asteroid lightcurves. We do not detect any rotational spectral variations within the signal-to-noise, which allow us to constrain the extent of any existing surface heterogeneity. For each observed spectrum of a longitudinal segment of an asteroid, we estimate the maximal size of an un-detected "spot" with a spectral signature different than the average. For 5 asteroids the maximal diameter of such a spot is smaller by a factor of two than the diameter of the secondary member. Therefore, the site of the fission is larger than any area with a unique spectral parameters and the site of the fission does not have a unique spectrum. In the case of an S-complex asteroid, where the site of fission is expected to present non-weathered spectra, a lack of a fission spot can be explained if the rotational-fission process is followed by the spread of dust that re-accumulates on the primary asteroid and covers it homogeneously. This is demonstrated for the young asteroid 6070 that presents an Sq-type spectrum while its inner material, that is presumably revealed on the surface of its secondary member, 54827, has a fresher, Q-type spectrum. The spread of dust observed in the disruption event of asteroid P/2013 R3, might be an example of such a process and an indication that it was indeed formed in a rotational-fission event.Comment: 16 pages, 15 figures, 6 Tables. Accepted for publication in Icaru

Unexpected D-type Interlopers in the Inner Main Belt

Very red featureless asteroids (spectroscopic D-types) are expected to have formed in the outer solar system far from the sun. They comprise the majority of asteroids in the Jupiter Trojan population, and are also commonly found in the outer main belt and among Hildas. The first evidence for D-types in the inner and middle parts of the main belt was seen in the Sloan Digital Sky Survey (SDSS). Here we report follow-up observations of SDSS D-type candidates in the near-infrared. Based on follow up observations of 13 SDSS D-type candidates, we find a ~20% positive confirmation rate. Known inner belt D-types range in diameter from roughly 7 to 30 kilometers. Based on these detections we estimate there are ~100 inner belt D-types with diameters between 2.5 and 20km. The lower and upper limits for total mass of inner belt D-types is 2x$10^{16}$ kg to 2x$10^{17}$ kg which represents 0.01% to 0.1% of the mass of the inner belt. The inner belt D-types have albedos at or above the upper end typical for D-types which raises the question as to whether these inner belt bodies represent only a subset of D-types, they have been altered by external factors such as weathering processes, or if they are compositionally distinct from other D-types. All D-types and candidates have diameters less than 30km, yet there is no obvious parent body in the inner belt. Dynamical models have yet to show how D-types originating from the outer solar system could penetrate into the inner reaches of the Main Belt under current scenarios of planet formation and subsequent Yarkovsky drift.Comment: 16 pages, 3 figures, 4 tables -- accepted for publication in Icaru

Observations of "Fresh" and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-Fission Mechanism

The rotational-fission of a rubble-pile asteroid can result in an "asteroid pair", two un-bound asteroids sharing similar orbits. This mechanism might exposes material that previously had never have been exposed to the weathering conditions of space. Therefore, the surfaces of asteroid pairs offer the opportunity to observe non-weathered fresh spectra. We report near-IR spectroscopic observations of 31 asteroids in pairs. We analyze their spectral slopes, 1 {\mu}m absorption band, taxonomy, and estimate the time elapsed since their separation. Analyzing the 19 S-complex objects in our sample, we find two fresh Q-type asteroids that are the first of their kind to be observed in the main-belt over the full visible and near-IR range. This solidly demonstrates that Q-type objects are not limited to the NEA population. The pairs in our sample present a range of fresh and weathered surfaces with no clear evidence for a correlation with the ages of the pairs. However, our sample includes old pairs (1 to 2 My) that present low spectral slopes. This illustrates a timescale of at least ~2 My before an object develops high spectral slope that is typical for S-type asteroids. We discuss mechanisms that explain the existence of weathered pairs with young dynamical ages and find that the "secondary fission" model (Jacobson & Scheeres 2011) is the most robust with our observations since: 1) the secondary members in our sample present fresh parameters that tend to be fresher than their weathered primaries; 2) most of the fresh pairs in our sample have low size ratios between the secondary and the primary; 3) 33% of the primaries in our sample are fresh, similar to the prediction set by this model; 4) known satellites orbit two of the pairs in our sample with low size ratio and fresh surface; 5) there is no correlation between the weathering state and the primary shape as predicted by other models.Comment: 19 pages, 17 figures, 4 tables. Accepted to Icaru

The Compositional Structure of the Asteroid Belt

The past decade has brought major improvements in large-scale asteroid discovery and characterization with over half a million known asteroids and over 100,000 with some measurement of physical characterization. This explosion of data has allowed us to create a new global picture of the Main Asteroid Belt. Put in context with meteorite measurements and dynamical models, a new and more complete picture of Solar System evolution has emerged. The question has changed from "What was the original compositional gradient of the Asteroid Belt?" to "What was the original compositional gradient of small bodies across the entire Solar System?" No longer is the leading theory that two belts of planetesimals are primordial, but instead those belts were formed and sculpted through evolutionary processes after Solar System formation. This article reviews the advancements on the fronts of asteroid compositional characterization, meteorite measurements, and dynamical theories in the context of the heliocentric distribution of asteroid compositions seen in the Main Belt today. This chapter also reviews the major outstanding questions relating to asteroid compositions and distributions and summarizes the progress and current state of understanding of these questions to form the big picture of the formation and evolution of asteroids in the Main Belt. Finally, we briefly review the relevance of asteroids and their compositions in their greater context within our Solar System and beyond.Comment: Accepted chapter in Asteroids IV in the Space Science Series to be published Fall 201

Ultra-low delta-v objects and the human exploration of asteroids

Missions to near-Earth objects (NEOs) are key destinations in NASA's new ‘Flexible Path’ approach. NEOs are also of interest for science, for the hazards they pose, and for their resources. We emphasize the importance of ultra-low delta-v from LEO to NEO rendezvous as a target selection criterion, as this choice can greatly increase the payload to the NEO. Few such ultra-low delta-v NEOs are currently known; only 65 of the 6699 known NEOs (March 2010) have delta-v <4.5 km/s, 2/3 of typical LEO-NEO delta-v. Even these are small and hard to recover. Other criteria – short transit times, long launch windows, a robust abort capability, and a safe environment for proximity operations – will further limit the list of accessible objects. Potentially there is at least an order of magnitude more ultra-low delta-v NEOs, but finding them all on a short enough timescale (before 2025) requires a dedicated survey in the optical or mid-IR, optimally from a Venus-like orbit because of the short synodic period for NEOs in that orbit, plus long arc determination of their orbits.United States. National Aeronautics and Space Administration (Contract NAS8-39073