Around 6.45pm on Wednesday, September 2 a bright green meteor lit up the Canberra sky. While meteors this spectacular over a populated area are rare, meteors themselves are far more common than you might think.
Every day around 100 tonnes of extraterrestrial material rains down on the Earth from space. This is an incredible boon for scientists interested in studying the Solar System: free samples of space rocks literally rain down from the sky every day! But there's a catch.
If all you have is a rock in your hand and all you know is that it came from the sky, it's impossible to say for certain where it came from. And the Solar System is a very large place, filled with enormously diverse worlds. To find out a meteorite's origins, we need to watch it fall to see its orbit.
But when or where most meteorites will fall is impossible to predict, so the simplest way to observe them is to watch the sky all night, every night, all over the continent. Thankfully this doesn't mean there are legions of lonely, bleary-eyed, caffeine-fuelled PhDs scanning the outback night. We have cameras and computers to do that.
These camera systems, developed by Curtin University as part of their Global Fireball Observatory, are able to automatically identify streaks of light and notify us that they saw something. If more than one camera catches the same fireball, we can calculate its trajectory and orbit, and hone in on where it might have landed. This is the same way that having two eyes can help you catch a ball using depth perception.
Unfortunately, only one camera caught this recent fireball so we don't have a good trajectory to pin it to. However, based on past statistics it's most likely to be what's known as a "chondrite", an ancient group of meteorites that predate the planets themselves, and make up over 85 per cent of all falls.
These chondrites formed from the same cloud of gas and dust the sun formed from, and by clumping together the chondrites eventually formed the planets we know today, including the Earth. Today this rubble of planet formation is concentrated in the Asteroid Belt, between Mars and Jupiter.
The Asteroid Belt is a chaotic place, where mountain-sized worlds are made and unmade, steadily clumping together and occasionally blasting apart in collisions. A key reason for this commotion is the gravitational pull of the gas giants Jupiter and Saturn, which can gradually alter the orbits of asteroids, stirring the Belt over tens of thousands of years. Most of the meteorites that impact the Earth were sent here from the inner edge of the Asteroid Belt by the steady effect of Saturn, stretching their orbits from circles into ovals until they crossed our path.
However, the chondrites are extremely diverse and roamed much farther than just the Asteroid Belt in the days before the planets herded them there. Some meteorites in our collections may have even formed beyond Saturn, almost a quarter of the way to Pluto.
If we can understand the migration history of the populations of the Asteroid Belt we can begin to build a map of the Solar System at its birth and understand the cosmic ingredients that went into building the Earth.
- Geoff Bonning is PhD Candidate in cosmochemistry at the Research School of Earth Sciences, at the Australian National University.