Over the past few days, a pair of spectacular fireballs have graced Australia's skies.
The first, in the early hours of Monday, May 20, flashed across the Northern Territory, and was seen from both Tennant Creek and Alice Springs, over 500km apart.
The second came two days later, streaking over South Australia and Victoria.
Such fireballs are not rare, and serve as another reminder that Earth is sitting in a celestial shooting gallery. In addition to their spectacles, they hold the key to understanding the solar system's formation and history.
Crash, bang, boom!
On any clear night, if you gaze skyward long enough, you will see meteors.
Specks of debris vaporise harmlessly in the atmosphere, 80-1
The more the object, the more spectacular the flash. Where your typical meteor is caused by an object of the size of a grain of dust, fireballs such as those seen this week are caused by much larger bodies – the size of a grapefruit, and melon or even a car
Such impacts are rarer than their tiny siblings because there are many more small objects in the solar system than larger bodies
Moving to still larger objects, you get really spectacular but rare events like the incredible Chelyabinsk fireball in February 2013.
That was probably the biggest impact on Earth for 100 years, and caused a lot of damage and injuries. It is the result of the explosion of an object of 10,000 tonnes in mass, around 20 meters in diameter.
On the longer timescales, the greatest impacts are truly enormous. Some 66 million years ago, and a comet or asteroid around 10km in diameter plowed into what is now the Yucatan Peninsula, Mexico. The result? A crater some 200 kilometers across, and a mass extinction that included the dinosaurs.
Even that is not the biggest impact Earth has experienced. Back in our planet's youth, it was a victim of a truly cataclysmic event, when it collided with an object of the size of Mars.
When the dust and debris cleared, our once solitary planet was accompanied by the moon.
Impacts that could threaten life on Earth are, thankfully, very rare. While scientists are actively seeking to make sure that no extinction-level impacts are coming in the near future, it's really not something we should lose too much sleep about
Smaller impacts, like those seen earlier this week, come far more Often, the footage of another fireball was reported earlier this month over Illinois in the United States.
In other words, it's not that unusual to have two bright fireballs in the space of a couple of days over a country as vast as Australia.
These bright fireballs can be an incredible boon to our understanding of the formation and evolution of the solar system. When an object is large enough, it is possible for fragments (or the whole thing) to penetrate the atmosphere intact, delivering a new meteorite to our planet's surface
Meteorites are incredibly valuable to scientists. They are celestial time capsules-relatively pristine fragments of asteroids and comets that formed when the solar system was young
Most meteorites we find have lain on Earth for long periods of time before their discovery. These are termed "finds" and while still valuable, they are often degraded and weathered, chemically altered by our wet, warm environment.
By contrast, "falls" (meteorites whose fall has been observed and that are recovered within hours or days of the event) are far more precious. When we study their composition, we can be sure that we are studying something ancient and pristine, rather than worrying that we are seeing the effect of Earth's influence