Astronomy as an esoteric field rarely weaves a thread neat enough to connect with humanity’s daily trudges. Yet, the study of Near-Earth Asteroids is both scary and fascinating since the rotation as well as the composition of the asteroids are valuable tracers in determining the history of the formation of the solar system, also the associated risk should an asteroid collide with us while we are going about our joyful Earthly business.
Annually the world observes Asteroid Day on the 30th of June because of the 1908 Siberian, Tunguska event which was the largest asteroid impact in recent history whose direct damage has been recorded by modern technology through photographs. The blast flattened about 80 million trees. While there are many theories around what actually happened, an asteroid impact is most plausible. Large asteroids have entered the Earth’s atmosphere resulting in varying incidents, like tsunamis or huge craters. Closer to home, the barely visible Vredefort crater in the Free State was 300 kilometres in diameter. The impact happened 2 billion years ago, what is still visible today can only been seen in satellite images because of erosion. Knowing about these events and their impact on Earth makes studying Near-Earth Asteroids (NEAs) not only important to astronomers but also to ordinary citizens as their impact could be dreadful to humanity.
NEAs are a stable population continuously replenished with asteroids from the main asteroid belt. The asteroid belt is located between the inner and outer planets within our solar system. The formation of some asteroids is attributed to collisions in space. Noteworthy is their role in the formation and shaping of our planet. Most asteroids are left-over material from the primordial solar system that never gravitationally clumped together to form into planets. Their orbits sometimes come close or cross the orbit of our planet, hence some could have harmful trajectories that pose a threat. Compared to the larger asteroids, the smaller ones are a more likely threat since they are more abundant, and on impact they can cause significant damage.
In recent years, the number of discovery and classification programs of Near-Earth Asteroids has grown, whereas the characterisation of small NEAs has not caught up. When small NEAs pass close to Earth, they can become very bright, meaning astronomers do not need large telescopes for their observation. Instead, they can use more abundant and undersubscribed smaller telescopes to study them.
NEAs studies are classification services designed to understand how these objects could impact humanity. In her MSc project, Petro Janse van Rensburg presented results of 20 NEAs that were successfully observed and characterised with the South African Astronomical Observatory (SAAO) 40-inch telescope and the Sutherland High Speed Optical Camera. From the 20 NEAs, 14 were less than 300 metres in diameter. The characterisation involved using a Lomb-Scargle periodogram to extract the rotation period from multi-band photometry, as well as allocating taxonomic probabilities to each NEA using an asteroid classification scheme and the observed colours of the asteroid. This would give a suggestion of the most likely composition.
Petro considers her contribution to the larger body of work to be compositional. She further explained that as a team, “We found that the fraction of stony (S-type) NEAs in the sample was ~54%. This result agreed with other studies. The sample was really small, but the results could be added to the results of previous studies so that we can get an idea of the taxonomic distribution of small NEAs.” She further explained that in her study and some others mentioned in her thesis, generally about 50% of the NEAs are stony-type compared to less than 20% for carbon-type. Therefore, it would be beneficial for mitigation strategies to be developed to manage stony asteroids if they become a threat instead of carbon-based asteroids.
She concluded by saying, “The results also suggest that the combined fraction of stony and a similar type of asteroid (Q-type) agree with the fraction of meteorites that are ordinary chondrites (∼80%). Ordinary chondrites are stony and the most common meteorite. What this result agrees with is the prediction that the most common meteorites originate from S- and Q-type asteroids. Q-type and S-type asteroids are made of the same composition, but Q-type asteroids have different colours because the surface properties have been modified by effects like space weathering and possibly close planetary encounters that shift material on the surface.
Among the many skills she has acquired during her master’s project, academic writing and coding with Python are transferable expertise that will help Petro immensely with her next project and in future. Her PhD project has observing possibilities of nearby starburst and Luminous Infrared Galaxies with SAAO 1-metre telescope in Sutherland. This will certainly be a minor challenge for her since she has already obtained foundational experience during her Near-Earth Asteroids observations.