At about 6:30pm last night, a meteor exploded with a bright flash that was widely seen across eastern Australia.
Stunned onlookers from Sydney to Canberra, and beyond, reported seeing the explosion light up the night sky in colourful streaks ranging from blue to green to orange.
In technical terms, the fireball was a “bolide”. Bolides are meteors that are not only brighter than the planet Venus, but which can also seen to explode or break up as they enter the atmosphere. They are rare to see.
So where might this fireball have come from?
First, what it wasn’t
The first point to make is that this fireball was not a piece of space junk. It was moving very fast, likely in excess of 30 kilometres per second.
Space junk, in contrast, enters Earth’s atmosphere at slower speeds of roughly 8km per second. In addition, such junk enters Earth’s atmosphere at a very shallow angle, meaning it can streak from horizon to horizon over the course of a minute or more.
The second point to make is that this fireball was definitely not from the Eta Aquariid meteor shower, which is visible between mid-April and late May.
Meteor showers consist of debris moving through space, crashing into Earth, and coming from a specific direction. Meteors in a shower appear to radiate from a single point in the sky, known as the radiant, which gives the shower its name.
Crucially, if a meteor shower’s radiant is below the horizon, you cannot see meteors from that shower. Earth is in the way, and the debris is hitting the other side of our planet! The radiant for the Eta Aquariids rises during the early hours of the morning across Australia. So a fireball seen in the evening sky cannot be related to the shower.
A fragile fragment
It’s more likely this fireball was an icy fragment of a comet, or a rocky fragment of an asteroid, from the outer reaches of the Solar System.
The speed of the fireball is one indicator of this – the faster an object is moving when it hits Earth’s atmosphere, the more different its orbit around the Sun must be to that of Earth. The high speed of this fireball’s entry suggests it was likely moving on quite an eccentric orbit around the Sun.
Another hint comes from the bright explosion of the fireball. That means it was to some degree a fragile object – icy or rocky. By comparison, a solid lump of iron from the core of a shattered larger asteroid would be strong enough to plough through Earth’s atmosphere without fragmenting, making such a terminal explosion much less likely.
Another important point is that the colours observed as the fireball flew through the atmosphere and exploded are not necessarily strong indicators of exactly what it was made of.
That’s because the vast majority of the observed colour from a fireball is associated with the gases in Earth’s atmosphere. As the fireball passes through the atmosphere, it causes a massive shockwave. This causes the air in front of it to rapidly heat up.
It is this superheated air that provides the vast majority of light from a meteor or fireball, and therefore what creates the striking colours. Many bright fireballs are seen to have a greenish hue, which is often stated as evidence for an iron/nickel composition. In fact, that greenish glow is so commonly observed because it is the result of superheated atmospheric oxygen.
The only way to know for certain what the fireball was made of is to find any pieces of it that reached ground level, and analyse their chemistry in the lab. But given the bright terminal explosion, it’s unlikely any solid material survived the blazing atmospheric entry.
Also, the direction of the observed fireball suggests it entered Earth’s atmosphere above the ocean. So if there are surviving pieces, they are probably buried at sea.
But that does not mean we will never learn the truth of the fireball’s origin. Scientists will gather as much footage of the fireball as they can. This will allow them to triangulate and precisely retrace the fireball’s path, and to accurately determine how fast it was moving and at what altitude it exploded. It will even reveal details on the object’s orbit around the Sun, prior to encountering Earth.
These are all crucial clues in this astronomical detective story.
Unravelling the Solar System’s history
But why might scientists want to know this?
For one thing, it’s interesting to get an idea of the fireball’s provenance. Was it originally from the asteroid belt? A fragment of a comet? Can we link it to a known object?
More importantly, however, this is all part of disentangling the heritage and history of the Solar System’s formation. Every object that enters our atmosphere in this way is a pristine piece of material that can add to our story of the Solar System’s history, and from which we can learn about its current state.
In an ideal world, scientists would be able to get their hands on pieces of this new interloper – fragments of material dating back to the Solar System’s birth. But even by simply gathering information on the object’s orbit prior to encountering Earth, we can learn a great deal.
