On August 22, 2016, a fireball shot through the sky over South Australia. A low, bright meteor. It was one of hundreds of space rocks that plunge through Earth’s atmosphere each year — and are large enough to survive its fiery descent.
The Earth is mostly water, so most meteors splash toward the middle of the ocean without fanfare. The one we notice also land on dry ground are rare and, to astronomers, precious. They are fragments of the mysterious environment in that vast dark space between the planets in our solar system… or even in the dark ranges beyond the solar system.
The Desert Fireball Network, a loose organization of Australian scientists based around Curtin University in Perth, set out to find the August 2016 meteorite, which we call a meteor after it lands. They noticed that the rock had orbited the Earth for a short while before tumbling into the desert, and they even gave it a clever nickname: Minimoon.
They finally found it two years later! Celebrations were in order. Astronomers could add the 1.2-ounce rock, about the size of an AA battery, to their small but growing collection of recovered meteorites, each a piece of the interplanetary puzzle.
But the party didn’t take place. DFN’s closer inspection of the meteorite found in the orange Australian desert led to a shocking conclusion. It was definitely a rock from outer space. But it was the wrong rock from space.
The DFN meteorite recovered from the sand, roughly within the predicted impact zone of the 2016 fireball, was not the same meteorite that caused the fireball. “A deceptive meteorite,” Martin Towner, a Curtin University researcher and DFN operations manager, described the recovered rock to The Daily Beast.
Incredibly, the Australians went in search of a rare space rock and found a not related rare space rock. The probability of that is difficult to estimate, but the Australian team has tried. Spoiler: they are low.
Now the Minimoon’s misfortune stands as a warning. As astronomers scour the planet for meteorites, they must be careful to trace the origin of each rock. If they connect a meteorite to the wrong fireball—evidence of a rock’s journey through the atmosphere—they run the risk of drawing the wrong conclusions about the area of space where a particular meteor originated. They can ruin entire fields of science.
“This is an important example of demonstrating that fireball-meteorite pairs must be carefully verified,” Towner and other members of the DFN team wrote in a new study.
When calculating the fireball’s trajectory, the DFN team had reduced the impact zone to an area of approximately 170 hectares. It took a few years to organize an expedition due to the extreme remoteness of the impact zone: a two-day journey by car from Perth.
“Finding on the clay pans was good,” Towner told The Daily Beast, using a term for a clay-rich depression, “but in the dunes was a bit hit and miss, with some loose sand that could bury things and thick bushes under trees .”
“If they connect a meteorite to the wrong fireball—evidence of a rock’s journey through the atmosphere—they run the risk of drawing the wrong conclusions about the area of space where a particular meteor originated. They can ruin entire fields of science.”
The team of four crawled across the potential impact zone, 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.
In case there were more fragments of the same meteor, the team searched for two more days – and found nothing. They rushed back to their labs to analyze the rock. An obvious test was to assess how slippery the meteorite was. The smoother a space rock is, the longer on Earth it undergoes a slow, steady process of polishing by windblown dirt or sand.
The DFN team rated their newly recovered meteorite as only “mildly” weathered. “It might have been tempting to attribute the mild degree of weathering to the two years it spent on the ground,” the team wrote in its study (which appeared online on July 12 and has not yet been peer-reviewed). In other words, the smoothness was consistent with a recently landed rock.
So the scientists had reason to believe they had found Minimoon. “It sat on the sand, it was about the right size, in about the right place, and it looked pretty fresh, and it’s not like you often find meteorites when searching,” Towner said. “So we were quite happy then!”
But the next test shattered their joy. The DFN team chopped off 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 took on different charges, changed their weight and made it possible to differentiate them.
It’s 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 contained. Certain subatomic particles called radionuclides – from cobalt and manganese atoms, among others – originate from space and do not survive long on Earth.
If a meteorite still has these nuclides, it is ‘fresh’. That is, landed in the last thousand years or so. If it not have the nuclides, it had an impact on the earth Lake than a thousand years ago.
The rock had no telltale nuclides. The DFN team estimated that it hit the desert at least 1,900 years ago. In other words, it wasn’t a Minimoon. It was a whole different meteorite that just landed in the same area as Minimoon probably, based on the latter’s trajectory.
The scientists searched previous studies and concluded that erroneous fireball-meteorite pairs — in which scientists see a meteor fall, search for it on the ground, and find the wrong space rock — are likely rare. As in, it occurs in no more than one in 50 meteorite surveys that also involve strong fireballs.
These mistaken identities, however rare, are a major problem. We are getting better at detecting and analyzing fireballs. The US military even tracks them using various sensors and periodically releases the data. The most recent releasethis spring contained data on about a thousand fireballs dating back to 1988.
The data—the speed, duration, brightness and color of a fireball—offer hints about a meteor’s internal structure. The faster a meteor is, the further away from Earth it may have originated. Color, brightness and duration can indicate the mineral composition and size of a meteor.
Finding a meteorite on the ground gives scientists a chance to confirm and supplement any conclusions they might draw from observing a fireball. Perhaps a particularly fast fireball appears to be coming from very far away — perhaps even traveling to Earth from outside the solar system. Scientists would like to know what minerals make up such a strange, far-traveling rock. The implications for planetary formation are profound.
“Scientists would like to know what minerals make up such a strange, far-traveling rock. The implications for planetary formation are profound.”
But these extensive analyzes of fireball-meteorite pairs only work if space rock hunters combine the right fireballs and meteorites. When fireballs and meteorites don’t match, they can draw the wrong conclusions.
Because fireballs are rare and finding an intact meteorite even is rarercomplacency can occur. Scientists witness a fireball, look for the meteorite, find one in or near the projected impact zone, and simply assume the two things are related.
As the Australians discovered, that’s not a safe assumption. There are just enough meteorites littering the Earth that scientists sometimes go looking for one space rock and accidentally find another.
Together, the Minimoon fireball and the unrelated South Australian meteorite make up “a cautionary tale,” Towner said. “Just because it looks good and is in the right place isn’t enough – you need to go through the full chain of analysis in the lab whenever possible to confirm it’s the right one.”
If you don’t, you could end up doing bad science.
There is a consequence in this unlikely story. Minimoon should still be there, somewhere in the Australian desert. “If it landed, it would still hang out,” Towner said. “Although a bit of time has now passed and the fall area does have sand dunes and plants that can move or grow in the wind, so chances are it has been buried and lost by now.”