Advancing technology in near-infrared instrumentation and dedicated planetary telescope facilities have enabled nearly two decades of reconnoitering the spectral properties for near-Earth objects (NEOs). We report measured spectral properties for more than 1000 NEOs, representing>5% of the currently discovered population. Thermal flux detected below 2.5 μm allows us to make albedo estimates for nearly 50 objects, including two comets. Additional spectral data are reported for more than 350 Mars-crossing asteroids. Most of these measurements were achieved through a collaboration between researchers at the Massachusetts Institute of Technology and the University of Hawaii, with full cooperation of the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. We call this project the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS; myth-neos). While MITHNEOS has continuously released all spectral data for immediate use by the scientific community, our objectives for this paper are to: (1) detail the methods and limits of the survey data, (2) formally present a compilation of results including their taxonomic classification within a single internally consistent framework, (3) perform a preliminary analysis on the overall population characteristics with a concentration toward deducing key physical processes and identifying their source region for escaping the main belt. Augmenting our newly published measurements are the previously published results from the broad NEO community, including many results graciously shared by colleagues prior to formal publication. With this collective data set, we find the near-Earth population matches the diversity of the main-belt, with all main-belt taxonomic classes represented in our sample. Potentially hazardous asteroids (PHAs) as well as the subset of mission accessible asteroids (ΔV≤7 km/s) both appear to be a representative mix of the overall NEO population, consistent with strong dynamical mixing for the population that interacts most closely with Earth. Mars crossers, however, are less diverse and appear to more closely match the inner belt population from where they have more recently diffused. The fractional distributions of major taxonomic classes (60% S, 20% C, 20% other) appear remarkably constant over two orders of magnitude in size (10 km to 100 m), which is eight orders of magnitude in mass, though we note unaccounted bias effects enter into our statistics below about 500 m. Given the range of surface ages, including possible refreshment by planetary encounters, we are able to identify a very specific space weathering vector tracing the transition from Q-to Sq-to S-types that follows the natural dispersion for asteroid spectra mapped into principal component space. We also are able to interpret a shock darkening vector that may account for some objects having featureless spectra. Space weathering effects for C-types are complex; these results are described separately by Lantz, Binzel, DeMeo. (2018, Icarus 302, 10-17). Independent correlation of dynamical model...

Telescopic measurements of asteroids' colours rarely match laboratory reflectance spectra of meteorites owing to a 'space weathering' process that rapidly reddens asteroid surfaces in less than 10(6) years. 'Unweathered' asteroids (those having spectra matching the most commonly falling ordinary chondrite meteorites), however, are seen among small bodies the orbits of which cross inside Mars and the Earth. Various explanations have been proposed for the origin of these fresh surface colours, ranging from collisions to planetary encounters. Less reddened asteroids seem to cross most deeply into the terrestrial planet region, strengthening the evidence for the planetary-encounter theory, but encounter details within 10(6) years remain to be shown. Here we report that asteroids displaying unweathered spectra (so-called 'Q-types') have experienced orbital intersections closer than the Earth-Moon distance within the past 5 x 10(5) years. These Q-type asteroids are not currently found among asteroids showing no evidence of recent close planetary encounters. Our results substantiate previous work: tidal stress, strong enough to disturb and expose unweathered surface grains, is the most likely dominant short-term asteroid resurfacing process. Although the seismology details are yet to be worked out, the identification of rapid physical processes that can produce both fresh and weathered asteroid surfaces resolves the decades-long puzzle of the difference in colour of asteroids and meteorites.

A comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much 'redder', with the spectral difference explained by 'space weathering', though the actual processes and timescales involved have remained controversial. A recent study of young asteroid families concluded that they suffered only minimal space weathering. Here we report additional observations of those families, revealing that space weathering must be a very rapid process-the final colour of a silicate-rich asteroid is acquired shortly after its 'birth' (within 10(6) years of undergoing a catastrophic collision). This rapid timescale favours solar wind implantation as the main mechanism of space weathering, as laboratory experiments have shown that it is the most rapid of several competing processes. We further demonstrate the necessity to take account of composition when evaluating weathering effectiveness, as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance. The rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface-particle-size properties, rather than to different relative ages. Apparently fresh surfaces most frequently seen among small near-Earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters.

Meteorites have long been considered as reflections of the compositional diversity of main belt asteroids and consequently they have been used to decipher their origin, formation, and evolution. However, while some meteorites are known to sample the surfaces of metallic, rocky and hydrated asteroids (about one-third of the mass of the belt), the low-density icy asteroids (C-, P-, and D-types), representing the rest of the main belt, appear to be unsampled in our meteorite collections. Here we provide conclusive evidence that the surface compositions of these icy bodies are compatible with those of the most common extraterrestrial materials (by mass), namely anhydrous interplanetary dust particles (IDPs). Given that these particles are quite different from known meteorites, it follows that the composition of the asteroid belt consists largely of more friable material not well represented by the cohesive meteorites in our collections. In the light of our current understanding of the early dynamical evolution of the solar system, meteorites likely sample bodies formed in the inner region of the solar system (0.5-4 AU) whereas chondritic porous IDPs sample bodies that formed in the outer region (>5 AU).

Although petrologic, chemical, and isotopic studies of ordinary chondrites and meteorites in general have largely helped establish a chronology of the earliest events of planetesimal formation and their evolution, there are several questions that cannot be resolved via laboratory measurements and/or experiments alone. Here, we propose the rationale for several new constraints on the formation and evolution of ordinary chondrite parent bodies (and, by extension, most planetesimals) from newly available spectral measurements and mineralogical analysis of main-belt S-type asteroids (83 objects) and unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter, we suggest that spectral data may be used to distinguish whether an ordinary chondrite was formed near the surface or in the interior of its parent body. If these constraints are correct, the suggested implications include that: (1) large groups of compositionally similar asteroids are a natural outcome of planetesimal formation and, consequently, meteorites within a given class can originate from multiple parent bodies; (2) the surfaces of large (up to ∼200 km) S-type main-belt asteroids mostly expose the interiors of the primordial bodies, a likely consequence of impacts by small asteroids (D < 10 km) in the early solar system; (3) the duration of accretion of the H chondrite parent bodies was likely short (instantaneous or in less than ∼10 5 yr, but certainly not as long as 1 Myr); (4) LL-like bodies formed closer to the Sun than H-like bodies, a possible consequence of the radial mixing and size sorting of chondrules in the protoplanetary disk prior to accretion.

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Abstract. The new Rosetta mission baseline to the comet 67P/Churyumov-Gerasimenko includes two asteroid fly-bys. To help in target selection we studied all the candidates of all the possible scenarios. Observations have been carried out at ESO-NTT (La Silla, Chile), TNG (Canaries), and NASA-IRTF (Hawaii) telescopes, in order to determine the taxonomy of all the candidates. The asteroid targets were chosen after the spacecraft interplanetary orbit insertion manoeuvre, when the available total amount of ∆V was known. On the basis of our analysis and the available of ∆V, we recommended to the ESA Science Working Group the asteroids 21 Lutetia and 2867 Steins as targets for the Rosetta mission. The nature of Lutetia is still controversial. Lutetia's spectral properties may be consistent with a composition similar to carbonaceous chondrite meteorites. The spectral properties of Steins suggest a more extensive thermal history. Steins may have a composition similar to relatively rare enstatite chondrite/achondrite meteorites.