Fireball over SA could have been a “Minimoon”

Rocks explode in Earth’s atmosphere fairly regularly, and at first glance a bolide spotted over South Australia in 2016 seemed like just any other. However, researchers using an outback network of cameras called the Desert Fireball Network have revealed this was no ordinary rock.

Instead, they have suggested the 2016 bolide was in fact a minimoon. Minimoons are what their name suggests – rocks captured by Earth’s gravity that end up orbiting our planet.

“It is an extremely rare event that only the largest fireball networks in the world would have a chance of observing,” says Curtin University’s Patrick Shober, who led the research.

The findings have been published in The Astronomical Journal.

Unfortunately for the astronomers there won’t be a rock to retrieve and examine closely – bolides are rocks that explode during entry into our atmosphere.

Minimoons in our neighbourhood

While Earth famously has a moon – The Moon – astronomers think there could be an additional metre-wide minimoon orbiting Earth at any time.

It wouldn’t be unique, astronomers have spotted temporary moons around other planets before. Jupiter in particular has a habit of capturing comets as temporary moons, sometimes for decades at a time. Some of these moons impact the planet, others are flung off to continue their journey.

A 2012 simulation which crunched the data on 10 million virtual asteroids found that only 18,000 would be likely to enter an Earth orbit. However, while astronomers aren’t certain how many asteroids are near Earth, a recent estimate was around 21,500. That means that while it’s expected that some might get trapped into Earth’s gravity, it’s still a rare event.

Scientists have spotted just two Earth minimoons before. In 2006 a tiny asteroid named 2006 RH120 orbited Earth for nearly a year before escaping Earth’s gravity and resuming its travels through the solar system.

Then, in 2014 a fireball camera network in the Czech Republic spotted the first bolide identified as a minimoon. That rock was estimated to be roughly 5 kg and diameter of just 15 cm.

The Desert Fireball Network comprises 50 automatic cameras which constantly take long-exposure photographs of the sky. Covering over 2.5 million square kilometres of outback, the system can use the images to calculate the trajectories of fireballs as they burn up. This information can then be used to understand the journey of the meteor.

When the meteor survives its plunge – which wasn’t the case for this minimoon, the network can also calculate the where the rock hit the ground, allowing researchers to retrieve meteorites after impact.

Slow velocity and trajectory suggests Earth orbit

The first thing that tipped the researchers off to the origin of the new detection was the slow velocity of the fireball, named DN160822_03. Travelling at just 11.0 km/s the characteristics suggested an object that had been orbiting Earth, however the trajectory did not match with satellite debris. Assessing all the factors, the team found a 95% likelihood it was a captured minimoon.

The team’s calculations also show that there is an increased capture probability during Earth’s aphelion and perihelion – ie when Earth is closest or furthest from the Sun.

“We also discover that the probability of capture occurring as a result of a close lunar encounter varies according to the lunar month for this event,” they write.

It’s not known how long DN160822_03 had been orbiting Earth before beginning its final plunge, or if there is another one out there. As camera trackers like Desert Fireball Network are expanded, and new ones established, it could be that we discover more minimoons in their final journeys.

However, the gold scenario would be to identify a minimoon still in orbit. With the push to visit and possibly retrieve asteroids gaining momentum, being able to make a small jump into Earth’s orbit would make that task much more straightforward.

“Minimoons are really awesome because they are the most accessible object to get to from Earth in the solar system,” Shober told Discover Magazine.

To make that early detection possible, the Large Synoptic Survey Telescope (LSST) is currently being built in Chile.

“The LSST is the dream instrument for discovering tiny, fast-moving asteroids, and we expect it will regularly discover temporarily captured objects within the next five years,” says Robert Jedicke, from the University of Hawaii, who was not involved in the current research.

This article was first published on Australia’s Science Channel, the original news platform of The Royal Institution of Australia.