The population of near-Earth asteroids (NEAs) shows a large variety of objects in terms of physical and dynamical properties. They are subject to planetary encounters and to strong solar wind and radiation effects. Their study is also motivated by pr
actical reasons regarding space exploration and long-term probability of impact with the Earth. We aim to spectrally characterize a significant sample of NEAs with sizes in the range of $sim$0.25 - 5.5 km (categorized as large), and search for connections between their spectral types and the orbital parameters. Optical spectra of NEAs were obtained using the Isaac Newton Telescope (INT) equipped with the IDS spectrograph. These observations are analyzed using taxonomic classification and by comparison with laboratory spectra of meteorites. A total number of 76 NEAs were observed. We classified 44 of them as Q/S-complex, 16 as B/C-complex, eight as V-types, and another eight belong to the remaining taxonomic classes. Our sample contains 27 asteroids categorized as potentially hazardous and 31 possible targets for space missions including (459872) 2014 EK24, (436724) 2011 UW158, and (67367) 2000 LY27. The spectral data corresponding to (276049) 2002 CE26 and (385186) 1994 AW1 shows the 0.7 $mu$m feature which indicates the presence of hydrated minerals on their surface. We report that Q-types have the lowest perihelia (a median value and absolute deviation of $0.797pm0.244$ AU) and are systematically larger than the S-type asteroids observed in our sample. We explain these observational evidences by thermal fatigue fragmentation as the main process for the rejuvenation of NEA surfaces. In general terms, the taxonomic distribution of our sample is similar to the previous studies and matches the broad groups of the inner main belt asteroids. Nevertheless, we found a wide diversity of spectra compared to the standard taxonomic types.
We study the visible and near-infrared (NIR) spectral properties of different ACO populations and compare them to the independently determined properties of comets. We select our ACOs sample based on published dynamical criteria and present our own
observational results obtained using the 10.4m Gran Telescopio Canarias (GTC), the 4.2m William Herschel Telescope (WHT), the 3.56m Telescopio Nazionale Galileo (TNG), and the 2.5m Isaac Newton Telescope (INT), all located at the El Roque de los Muchachos Observatory (La Palma, Spain), and the 3.0m NASA Infrared Telescope Facility (IRTF), located at the Mauna Kea Observatory, in Hawaii. We include in the analysis the spectra of ACOs obtained from the literature. We derive the spectral class and the visible and NIR spectral slopes. We also study the presence of hydrated minerals by studying the 0.7 $mu$m band and the UV-drop below 0.5 $mu$m associated with phyllosilicates. We present new observations of 17 ACOs, 11 of them observed in the visible, 2 in the NIR and 4 in the visible and NIR. We also discuss the spectra of 12 ACOs obtained from the literature. All but two ACOs have a primitive-like class spectrum (X or D-type). Almost 100% of the ACOs in long-period cometary orbits (Damocloids) are D-types. Those in Jupiter family comet orbits (JFC-ACOs) are $sim$ 60% D-types and $sim$ 40% X-types. The mean spectral slope $S$ of JFC-ACOs is 9.7 $pm$ 4.6 %/1000 AA and for the Damocloids this is 12.2 $pm$ 2.0 %/1000 AA . No evidence of hydration on the surface of ACOs is found from their visible spectra. The slope and spectral class distribution of ACOs is similar to that of comets. The spectral classification, the spectral slope distribution of ACOs, and the lack of spectral features indicative of the presence of hydrated minerals on their surface, strongly suggest that ACOs are likely dormant or extinct comets.
In the framework of a 30-night spectroscopic survey of small near-Earth asteroids (NEAs) we present new results regarding the identification of olivine-rich objects. The following NEAs were classified as A-type using visible spectra obtained with 3.6
m NTT telescope: (293726) 2007 RQ17, (444584) 2006 UK, 2012 NP, 2014 YS34, 2015 HB117, 2015 LH, 2015 TB179, 2015 TW144. We determined a relative abundance of $5.4% $ (8 out of 147 observed targets) A-types at hundred meter size range of NEAs population. The ratio is at least five times larger compared with the previously known A-types, which represent less than $sim1%$ of NEAs taxonomically classified. By taking into account that part of our targets may not be confirmed as olivine-rich asteroids by their near-infrared spectra, or they can have a nebular origin, our result provides an upper-limit estimation of mantle fragments at size ranges bellow 300m. Our findings are compared with the battered-to-bits scenario, claiming that at small sizes the olivine-rich objects should be more abundant when compared with basaltic and iron ones.
The Asteroid Redirect Mission (ARM) under development by NASA is being planned to collect a multi-meter boulder from a near-Earth asteroid (NEA), and to bring it to the cis-lunar space in the mid-2020s for future study and exploitation by a crewed mi
ssion. The MarcoPolo-M5 project is being proposed in 2016 for the M5 mission opportunity by ESA, to bring back to Earth a sample from a very primitive D-type NEA. We aim to further characterize the physical properties of two optimal targets for sample return space missions, the low-DeltaV NEAs (341843) 2008 EV5 and (52381) 1993 HA. 2008 EV5 is the baseline target of ARM, but only one spectrum of this object exists in the literature. 1993 HA is a very favourable target for a space mission based on its dynamical properties: here we intend to assess if it is a suitable target for MarcoPolo-M5. We obtained visible spectroscopy of 2008 EV5 with the FORS2 instrument at ESO-VLT, at different rotational phases. We also obtained visible and near-infrared spectroscopy of 1993 HA, using the EFOSC2 and SOfI instruments at ESO-NTT. Visible photometry of 1993 HA was carried out within the IMPACTON project at the Observatorio Astronomico do Sertao de Itaparica (Brazil). Our new observations are in agreement with the C-type classification of 2008 EV5. We obtained five visible spectra which do not show any variability within the limits of noise, suggesting a homogeneous surface. We obtained the first ever spectroscopic dataset for 1993 HA, finding a featureless, red-sloped behaviour typical of D-types. We found that the synodic rotation period of 1993 HA is 4.107+-0.002 h. The derived lightcurve also suggests an elongated shape (axis ratio a/b>=1.71). At this stage 1993 HA does indeed seem to be the most favourable target for MarcoPolo-M5, though future observations are necessary to study it further.
The ESO/MPG WFI and the INT WFC wide field archives comprising 330,000 images were mined to search for serendipitous encounters of known Near Earth Asteroids (NEAs) and Potentially Hazardous Asteroids (PHAs). A total of 152 asteroids (44 PHAs and 108
other NEAs) were identified using the PRECOVERY software, their astrometry being measured on 761 images and sent to the Minor Planet Centre. Both recoveries and precoveries were reported, including prolonged orbital arcs for 18 precovered objects and 10 recoveries. We analyze all new opposition data by comparing the orbits fitted before and after including our contributions. We conclude the paper presenting Mega-Precovery, a new online service focused on data mining of many instrument archives simultaneously for one or a few given asteroids. A total of 28 instrument archives have been made available for mining using this tool, adding together about 2.5 million images forming the Mega-Archive.
