On August 22, 2016, a fireball streaked across the sky over South Australia. A low, bright meteor. It was one of hundreds of space rocks that plunge into Earth’s atmosphere each year and are large enough to survive the fiery descent.
Earth is mostly water, so most meteors crash into the middle of the ocean without much fanfare. Those we also notice dry land is rare and, for astronomers, precious. They are fragments of the mysterious environment in this vast dark space between the planets of our solar system…or even in the dark reaches beyond the solar system.
The Desert Fireball Network, an informal organization of Australian scientists based at Curtin University in Perth, set out to find the August 2016 meteorite, what we call a meteor after it landed. Finding that the rock had briefly circled the Earth before tumbling towards the desert, they even gave it a clever nickname: Minimoon.
They finally found him two years later! Celebrations were in order. Astronomers could add the 1.2-ounce, AA battery-sized rock to their small but growing collection of recovered meteorites, each a piece of the interplanetary puzzle.
But the party did not last. Further inspection by DFN of the meteorite found in the Orange Australian Desert has led to a shocking conclusion. It was a rock from outer space, that’s for sure. But it was the Wrong space rock.
The DFN meteorite recovered from the sand, roughly inside the predicted 2016 fireball impact area, was not the same meteorite that caused the fireball. “A rogue meteorite,” is how Martin Towner, a Curtin University researcher and director of operations for DFN, described the recovered rock to The Daily Beast.
Incredibly, Australians went in search of a rare space rock and found an unrelated stone rare space rock. The odds of that happening are hard to calculate, but the Australian team tried. Spoiler: They’re weak.
Now the Minimoon misadventure comes as a warning. As astronomers scour the planet for meteorites, they must be careful to trace the origins of each rock. If they connect a meteorite to the wrong fireball – evidence of a rock’s journey through the atmosphere – they risk drawing the wrong conclusions about the region of space a given meteor originated from. They could corrupt entire fields of science.
“This is a key example to show that fireball-meteorite pairings need to be carefully checked,” Towner and other DFN team members wrote in a new study.
By calculating the trajectory of the fireball, the DFN team had reduced the impact area to an area of approximately 170 acres. It took a few years to organize an expedition due to the extreme remoteness of the impact zone: a two-day road trip from Perth.
“The claypan research was good,” Towner told The Daily Beast, using a term for a clay-rich depression, “but on the dunes it was a bit hit and miss, with loose sand that could bury things. and dense bushes under the trees.”
“If they connect a meteorite to the wrong fireball – evidence of a rock’s journey through the atmosphere – they risk drawing the wrong conclusions about the region of space a given meteor originated from. They could corrupt entire fields of science.”
The four-person team crawled over the area of potential impact, looking for telltale signs of an alien rock. Round in shape. Dark in color. Dense and therefore heavy. When they finally found a meteorite, after six days of searching, it was only a hundred yards from where they expected to find one.
Just in case there were more fragments of the same meteor, the team searched for another two days and found nothing. They rushed to their labs to analyze the rock. An obvious test was to judge the softness of the meteorite. The smoother a space rock is, the longer it has spent on Earth undergoing a slow, steady process of polishing by windswept dirt or sand.
The DFN team felt that their newly recovered meteorite was only “slightly” weathered. “It might have been tempting to attribute the slight degree of weathering to the two years spent in the field,” the team wrote in their study (which was published online July 12 and has not yet been assessed. by peers). In other words, the smoothness corresponded to a recently landed rock.
Scientists therefore had reason to believe that they had found Minimoon. “It was sitting on the sand, it was about the right size, about the right place and it looked pretty cool, and it’s not like you often find meteorites when searching” , said Towner. “So we were pretty happy back then!”
But the next test shattered their joy. The DFN team chiseled out a quarter gram piece of the meteorite, crushed it and burned it. Using a technique called accelerator mass spectrometry, they bombarded the resulting gas with electrons. Different elements pick up varying loads, changing their weight and allowing them to be differentiated.
This is all very technical, but the result, after careful analysis, was that the DFN team was able to estimate how many rapidly decaying radioactive particles the rock still had. Some subatomic particles called radionuclides — from cobalt and manganese atoms, among others — come from space and don’t survive long on Earth.
If a meteorite still contains these nuclides, it is “fresh”. That is, having landed within the last thousand years or so. If it doesn’t have the nuclides, it impacted Earth more than a thousand years ago.
The rock did not contain any telltale nuclides. The DFN team estimated that it hit the desert at least 1,900 years ago. In other words, it wasn’t Minimoon. It was quite different meteorite that has just landed in the same area as Minimoon probably, based on the trajectory of the latter.
The scientists sifted through past surveys and concluded that erroneous fireball-meteorite pairings — where scientists watch a meteor fall, pick it up on the ground, and find the wrong space rock — are likely rare. As in, occurring in no more than a survey of 50 meteorites also involving strong fireballs.
These mistaken identities, while rare, are a big problem. We are becoming better and better able to detect and analyze fireballs. The US military even tracks them using various sensors and periodically releases the data. The most recent versionthis spring included data on about a thousand fireballs dating back to 1988.
The data – a fireball’s speed, duration, brightness and color – offers clues to a meteor’s internal structure. The faster a meteor, the faster it can come from Earth. The color, brightness, and duration can indicate the mineral composition and size of a meteor.
Finding a meteorite on the ground gives scientists the opportunity to confirm and add to any conclusions they might draw from observing a fireball. Perhaps a particularly fast fireball appears to come from far away, perhaps even traveling to Earth from beyond the solar system. Scientists would like to know what minerals make up such a strange, far-travelling rock. The implications for planetary formation are profound.
“Scientists would like to know what minerals make up such a strange, far-travelling rock. The implications for planetary formation are profound.”
But these comprehensive analyzes of fireball-meteorite pairings only work if space rock hunters match the right fireballs and meteorites. Fireballs and meteors don’t match, and they could draw all the wrong conclusions.
Because fireballs are rare, and finding an intact meteorite is even rarer, complacency can set in. Scientists witness a fireball, go in search of the meteorite, find one in or near the projected impact area, and just assume the two things are connected.
As the Australians have discovered, this is not a safe assumption. There are just enough meteorites littering the Earth that sometimes scientists go in search of one space rock and accidentally find another.
Together, the Minimoon fireball and the unrelated South Australian meteorite are “a cautionary tale,” Towner said. “It’s not good enough just because it looks okay and is in the right place – you have to go through the whole chain of lab analysis where possible to confirm it’s the right one. Well.”
If you don’t, you could end up doing bad science.
There is a corollary to this unlikely story. Minimoon should still be out there, somewhere in the Australian desert. “If he landed he would still be hanging around,” Towner said. “Although a little time has passed now and the fall area has sand dunes and plants that can move in the wind or grow, so it is possible that they have been buried and lost now .”