One of Earth's minimoons hit Australia
Updated: Aug 21, 2020
On August 22, 2016 over South Australia, the Earth likely lost one of its moons. Yeah, you heard me.
The Desert Fireball Network is a system of observatories covering over 1/3 of Australia. These observatories monitor the entire night sky for fireballs. Fireballs are very bright meteors that occur when larger (typically asteroidal) objects impact the atmosphere.
In August of 2016, a rock hit the atmosphere above South Australia. Instead of coming from interplanetary space, this rock was slow enough that it was actually previously captured by the Earth (yanno before it crashed into it...). These objects are really cool and interesting. Affectionately referred to as 'minimoons', these bodies are temporarily captured by Earth. This happens when an object (usually with an orbit similar to the Earth) approaches the Earth slowly enough with a favorable geometry.
Minimoons have become very popular over the previous couple of years as they are prime targets for asteroid redirect or capture missions. They are the easiest objects to get to in the solar system. Missions sent to them can test technogies important for planetary defense and asteroid mining.
Only 2 minimoons have ever been detected by telescopes: 2020 CD3 and 2006 RH120 . Additionally, only 2 fireballs ever have been precisely measured and determined to originate from a minimoon. A fireball over the Czech Republic in 2014 and the fireball over Australia in 2016 were likely produced by minimoons. For both of these fireballs, the meteoroids (the rocks) had close encounters with the Moon while orbiting the Earth. This makes predicting their orbits over time impossible, the trajectory is just too chaotic.
Although..... we can probabilistically characterize the history of the Australian minimoon. Below is a plot of the possible capture locations for the rock. The meteoroid was likely captured by the Earth through either the first or second Lagrange points (the red X's). This is consistent with models of the minimoon population. Whereas, there is also a non-negligible chance that it was wrangled in by a close encounter with the Moon (yellow ring of points).
Below you can see the orbital simulation of the event observed by the DFN. The animation is in the Earth-centered inertial (ECI) frame, i.e., the Earth is in the middle and not rotating with the Earth's surface.
If you would like to read more about this fireball, check out the paper in the Astronomical Journal!
The research from this study has been featured in:
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