Today marks the 112th anniversary of the Tunguska explosion, the worst meteor event in recent history. On 30 June 1908, witnesses saw an asteroid enter the atmosphere and explode, creating an airburst that flattened trees and killed hundreds of reindeer in Siberia. Due to the remoteness of the location, it’s believed there were no human casualties. In 2014, in remembrance of this catastrophe, a group of scientists established Asteroid Day. This campaign aims to raise awareness of the existential threat of asteroids and promote increased asteroid tracking capabilities and systems. Their website reports that although there are over 1 million asteroids that have the potential to impact Earth, just 1% are known to science.
Most asteroids in our Solar System are located within the asteroid belt, in orbit between Mars and Jupiter. Asteroids are stony or metallic space rocks that formed at the same time as the Solar System, 4.6 billion years ago. Occasionally asteroids are knocked out of their orbit and are sent hurtling through space – sometimes towards Earth.
Though their impacts can be devastating, asteroid strikes are rare. Relatively more common are meteoroid events, every year an estimated 6000 meteoroids fall to Earth. Meteoroids have similar composition to asteroids but are smaller, ranging from the size of a pebble to hundreds of metres across, they are formed from the debris of asteroids. Most meteoroid impacts are not witnessed, but occasionally impressive displays are caught on camera, like the mesmerizing footage captured on vehicle dash cams in Chelyabinsk, Russia in 2013. Most meteorite falls are harmless, although there is one curious incident in 1954 when unlucky Ann Hodges was struck in her Alabama home.
Scientists study meteorites, the fragments of asteroids and meteoroids that have landed on Earth, to learn more about the conditions of the early Solar System. Meteorites are found everywhere on Earth – but the majority are collected from snow fields and deserts where they are most easily seen. There are many examples of meteorites collected in Australia, and the Museum is lucky to have some in its vast collection.
In honour of Asteroid Day, we will look at select space rocks that landed in our Australian backyard, as part of the Powerhouse collection.
The Henbury Meteorite fragments in Figure 1 and Figure 2 are pieces of a large iron meteoroid that struck Australia approximately 4700 years ago. The impact caused a series of 12 small craters, located within the Henbury Meteorites Conservation Reserve, 125 km southwest of Alice Springs. These fragments in the Museum collection represent a small proportion of the approximate 500 kg of meteorite fragments recovered in the vicinity of the craters. The meteor would have weighed several tonnes before it broke apart upon impact.
Of the three classifications of meteorites (iron, stony, stony-iron), iron meteorites like these Henbury fragments are the rarest. They originate from the core of asteroids, and consist mostly of iron-nickel metal with trace amounts of other elements.
Figure 3 and Figure 4, are both examples of stony meteorites. Most meteorites discovered are remants of smaller meteoroids and are not associated with large crater systems like the Henbury Crater or Australia’s famous Wolfe Creek Crater. While most meteor falls go unsighted, some are lucky to witness meteors streaking through the sky, like the meteorite in Figure 4 that was observed on 5 February 1924. Some recovered meteorites may have fallen to Earth millions of years ago, while others may have fallen much more recently.
Stony meteorites, like Figure 3 and Figure 4 consist mostly of silicate materials. Although to a novice, they may appear like other Earth rocks, some stony meteorites are examples of the oldest material in the Solar System, and have changed little over billions of years. They are different from planetary rocks that have been subject to billions of years of geological activity, having melted and reformed multiple times. These meteorites can be thought of as a fossil record, holding valuable clues about planetary evolution.
Although they can be formidable, most space rocks that enter our atmosphere are harmless. But for scientists, they are a gateway to understanding the cosmos and help unlock some of the mysteries of our Solar System.
If you are lucky to live somewhere with dark skies, you have a much greater chance of spotting a meteor. There are some great initiatives and projects that you can get involved with, like the citizen science project Fireballs In the Sky, an app that helps log meteor sightings and recover meteorites throughout Australia.
To learn more about Asteroid Day, visit their website for a program of panel discussions and interviews with experts, as they reveal the latest research on this fascinating topic.