Near-Earth Asteroids (NEAs) are a population of asteroids in a steady state, constantly being replenished with asteroids from the main belt. NEAs have orbits that come close to or cross the Earth's orbit and therefore some could have impacting trajectories and pose a threat. Small NEAs (diameter < 300 m) pose a greater threat compared to large NEAs because they are more abundant and can cause significant damage on impact. The characteristics of small NEAs can give an indication of the most likely properties of potential future impactors. Even though in recent years the number of discovery and characterisation programmes of NEAs have increased, the characterisation of the small NEA population still lags behind because they can only be observed with 1-m class telescopes when they pass close to the Earth and become bright enough. Presented here in this MSc thesis are 20 NEAs that were successfully observed and characterised with the South African Astronomical Observatory (SAAO) 40-inch telescope and the Sutherland HighSpeed Optical Camera. Out of the 20 NEAs, 14 had diameters < 300 m (H > 21). Characterisation involved assigning taxonomic probabilities to each NEA based on spectra from the Bus-DeMeo classification scheme and thereby inferring its most probable composition, as well as using a Lomb-Scargle periodogram to extract the rotation period from multi-band photometry. The taxonomic probabilities were determined with the colours g0−r 0 and r0−i 0 , in combination with a machine learning (ML) algorithm trained on synthetic colours from observed spectra obtained from literature. The taxonomies considered were the S-, C-, and X-complexes, and the D-, Q-, and V-types. In this thesis, the taxonomic probabilities are reported for all of the targets. A distinct taxonomic class was assigned to 15 NEAs that had a probability >50% in a specific taxonomy. New taxonomic classes are reported for 11 of the targets. A notable result of this study is the confirmation of the prediction that the most common meteorite, ordinary chondrites, are due to S-complex and Q-type asteroids. The fraction of meteorite falls due to ordinary chondrites are similar to the combined fraction of Scomplex and Q-type asteroids in this study (∼80%). This confirmation was only possible by including the Q-type asteroids in the classification and being able to differentiate between the C-complex and Q-type asteroids with two colours and a ML approach. A rotation period was extracted for nine NEAs that were observed for long enough to resolve a light curve period. The remaining targets had only partial or flat light curves and no period could be resolved from the periodogram. Reported here are also three small NEAs with H > 22 magnitude which were found to have rotation periods smaller than the 2.2 hour spin barrier and could be rigid pieces of rock instead of rubble piles.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/33725 |
| Date | 06 August 2021 |
| Creators | Janse van Rensburg, Petronella |
| Contributors | Erasmus, Nicolas, Bershady, Matthew A |
| Publisher | Faculty of Science, Department of Astronomy |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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