Big bangs and sky stones — SA’s meteorite hunters on exactly what that flash in the sky was

Roger Gibson, professor of structural geology and metamorphic petrology at Wits. (Photo: LinkedIn)

By Estelle Ellis

1 Sep 2024

The fireball that streaked across the heavens was heard from Gqeberha to the Karoo on Sunday, 25 August.

On Sunday, 25 August, people on the Garden Route, the Sunshine Coast, in Nelson Mandela Bay and even as far as the Karoo heard a big bang. Many looked up to the sky and saw a bright flash streaking through the winter air. Some reached for their phones to take pictures, others to post that the aliens are coming.

But Dr Leo Vonopartis, from the geoscience department at the University of the Witwatersrand, said it was just a meteorite.

“When the solar system was forming around 4.57 billion years ago, matter orbiting around the sun started to coalesce and form a ‘protoplanetary disk’ of gas and dust-sized particles,” he explained.

“The particles in the disk started to agglomerate and form bigger bodies that orbit the sun. These bodies grew in size and collected more material, ultimately forming the planets that we see today. Less-volatile and denser material in the disk formed the rocky inner planets, and the more volatile gases in the disk formed the gas giants farther from the sun.”

Not all of the disk formed planets, though, and this material is still orbiting the solar system, mainly in the asteroid belt between Mars and Jupiter. Collisions between asteroids or passing gravitational fields occasionally deflect this rocky material into an Earth-crossing orbit.

The ensuing collisions occur at incredible speeds – about 20km/s, or 72,000km/h. This natural phenomenon is seen in the short-lived “shooting stars”. The light from these meteors is caused by burning friction between the rock and Earth’s atmospheric molecules.

Bigger pieces burn for longer and the brightest ones are called fireballs or bolides that, as seen on Sunday, are bright enough to be visible in daylight. In short, Earth is under daily bombardment by space rocks, but most are dust- to pea-sized; the large ones like the one on Sunday are relatively rare.

Meteorite What Eastern Cape residents saw streaking through the winter air. (Screengrab: TikTok)

The passage of a rock mass at such high speed through Earth’s atmosphere is a very violent event. Compare it with a jet aircraft that exceeds the speed of sound (about 1,235km/h), which causes a sonic boom that is strong enough to blast windows out of houses (which is why there are no-go zones for such jets over cities).

“The rock mass in the fireball is slowed down as it drops deeper into the atmosphere, but it is still travelling at many times the speed of sound. Because it is still so high up – usually above 30km altitude, or about three times the height that aeroplanes fly – the sound waves can travel very far. They are usually most intense and involve multiple, loud, sharp bangs close to the point of final disintegration, called the airburst, and merge into a duller, rolling thunder further away. Sounds appear to have been heard hundreds of kilometres from the site of this bolide,” said Professor Roger Gibson, Vonopartis’s colleague.

Gibson said there had been some reports from witnesses of pieces of stone so far, but they had only managed to get one stone.

“Sifting through the reports is a work in progress, but we are ever hopeful. Metallic meteorites are quite distinctive and can be large, weighing many kilograms to tens of kilos, but very rare.

Meteorite Dr Leo Vonopartis of the Wits School of Geoscience is asking people to report fragments they find. (Photo: LinkedIn)

“The majority of meteorites are made of silicate minerals, with varying amounts of metal, and usually form smaller stones of a few tens of grams to a few kilograms.

Read more: Meteors, supermoons, a comet and more: your guide to the southern sky in 2024

“Among these meteorites, the principal feature is to find a fusion crust caused by melting of the outer edge of the stone in the fireball that contrasts with a light-coloured, unmelted interior,” Gibson added.

“This contrast is commonly visible as the brittle crust is chipped, or the stones break open, when they hit the ground. The pieces that survived are probably pebble-sized and could be hard to find,” he said.

Vonopartis said that, based on the size of the fireball seen, the meteorite described was the size of a car.

“So there wasn’t a lot to start with. The outer parts were then stripped away by frictional melting and vaporisation – this is the source of the bright ‘tail’ of the fireball – and the stresses of trying to punch through the atmosphere at such high speed eventually cause it to shatter into small pieces that scatter at high altitude and fall to Earth over a large area.

“Only one stone has been found so far, but once we know the direction in which the fireball was moving and its exact location, and allowing for atmospheric wind directions at the time, we can build a model of where pieces should have fallen,” he said.

“The biggest surviving parts of the meteorite have the most momentum and are commonly found at the end of the trajectory. We are still trying to define these parameters and the size of the strewn field.

Meteorite Roger Gibson, professor of structural geology and metamorphic petrology at Wits. (Photo: wits.ac.za)

“This is why we are asking anyone who thinks they have found a fragment to photograph it carefully, measure the precise GPS location and, only if it is not possible to leave it in place, to carefully collect it. The location and size will help us refine the strewn field and help us predict where other pieces might lie,” Vonopartis added.

He said that he, Gibson, Dr Carla Dodd – from Nelson Mandela University – or Dr Deon van Niekerk from Rhodes University can be contacted for guidance. Van Niekerk has obtained a permit from the South African Heritage Society to collect the stones produced by the explosion.

They have asked people not to pick up the stones produced by the explosion, but this is not because of health reasons.

“These samples carry vital and delicate scientific information that is sensitive to the conditions here on Earth and to moisture or oils on people’s skin. Touching the samples may contaminate the sample or degrade the minerals in the meteorite, and some of the fragments may be brittle and could break if not handled with care.

“The analytical techniques used to study these rocks are very sensitive and future analysis of these samples, measuring aspects that we are unable to detect now, may be compromised by people touching the sample now, or storing it poorly.

“If people suspect that they have a meteorite, they should handle the sample with care, use gloves if possible, wrap the specimen in aluminium foil, and place it in an airtight plastic bag with the air pushed out,” Vonopartis advised. DM

This story first appeared in our weekly Daily Maverick 168 newspaper, which is available countrywide for R35.

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