On August 22, 2016, a fireball streaked the sky over South Australia. A low, bright meteor. It was one of hundreds of space rocks that tumble through Earth’s atmosphere each year — large enough to survive the fiery descent.
Earth is mostly water, so most meteors splash down in the middle of the ocean without fanfare. We notice that too Land on dry ground are rare and precious to astronomers. They are fragments of the mysterious surroundings in that vast dark space between the planets in our solar system…or even in the murky expanses beyond the solar system.
That Desert Fireball Network, a loose organization of Australian scientists based at Curtin University in Perth, searched for the August 2016 meteorite, which we call a meteor after it landed. Noting that the rock had orbited the earth briefly before plunging toward the desert, they even gave it a clever nickname: Minimoon.
Two years later they finally found it! It was time to celebrate. Astronomers could add the 1.2-ounce stone, about the size of an AA battery, to their small but growing collection of recovered meteorites, each of which is a piece of the interplanetary puzzle.
But the party didn’t last. The DFN’s closer examination of the meteorite found in the orange Australian desert led to a shocking result. It was definitely a rock from outer space. But it was her not correct rocks from space.
Meteorite DFN, recovered from the sand roughly within the predicted impact zone of the 2016 fireball, was not the same meteorite that caused the fireball. “A renegade meteorite,” is how Martin Towner, a research associate at Curtin University and operations manager at DFN, described the recovered rock to The Daily Beast.
Incredibly, Australians went in search of a rare space rock and found an unrelated one Rare space rock. The odds of that are hard to calculate, but the Australian team tried. Spoilers: They’re low.
Now the Minimoon mishap stands as a warning. As astronomers scan the planet for meteorites, they must be careful to trace the origins of each rock. By associating a meteorite with the wrong fireball—evidence of a rock’s journey through the atmosphere—they risk drawing incorrect conclusions about the region of space from which a particular meteor originated. They could corrupt entire fields of science.
“This is a key example that fireball-meteorite pairings should be carefully reviewed,” wrote Towner and other members of the DFN team a new study.
When calculating the trajectory of the fireball, the DFN team narrowed the impact zone to an area of about 170 hectares. Due to the extreme remoteness of the impact area, it took a few years to organize an expedition: a two-day road trip from Perth.
“Searching on the clay pans was good,” Towner told The Daily Beast, using a term for a clayey dip, “but it was a bit of a back and forth on the dunes, with some loose sand that could bury things, and seal.” bushes under trees.”
“By associating a meteorite with the wrong fireball—evidence of a rock’s journey through the atmosphere—they risk drawing incorrect conclusions about the region of space from which a particular meteor originated. They could corrupt entire fields of science.”
The four-man team crawled over the potential impact zone, searching for the telltale signs of an alien rock. Round shape. Dark in colour. 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 it to be.
In case there were other fragments of the same meteor, the team searched for another two days – and found nothing. They rushed back to their labs to analyze the rock. An obvious test was to assess how smooth the meteorite was. The smoother a space rock, the longer it will undergo a slow, steady polishing process on Earth by windswept dirt or sand.
The DFN team rated their newly recovered meteorite as only “slightly” 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 their study (which appeared online July 12 and has not yet been peer-reviewed). In other words, the smoothness matched a recently landed stone.
So the scientists had reason to believe that they had found Minimoon. “It was in 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 you’re looking,” Towner said. “So we were pretty happy back then!”
But the next test thwarted her joy. The DFN team chiseled out, crushed and burned a quarter gram of the meteorite. Using a technique called accelerator mass spectrometry, they bombarded the resulting gas with electrons. Different elements took on different charges, changing their weight and making it possible to distinguish them.
It’s all very technical, but the result was that after careful analysis, the DFN team was able to estimate how many rapidly decaying radioactive particles the rock still contained. Certain subatomic particles called radionuclides — made up of cobalt and manganese atoms, among others — come out of space and don’t survive long on Earth.
If a meteorite still contains these nuclides, it is “fresh”. That is, to have landed in the last thousand years or so. If it doesn’t have the nuclides, it has affected the earth more than a thousand years ago.
The rock had no telltale nuclides. The DFN team estimates that it hit the desert at least 1,900 years ago. In other words, it wasn’t Minimoon. It was quite different Meteorite that randomly landed in the same area as Minimoon based on its trajectory.
The scientists reviewed previous research and concluded that false fireball-meteorite pairings — in which scientists see a meteor fall, search for it on the ground, and find the wrong space rock — are likely rare. Strong fireballs do not occur in more than one in 50 meteorite investigations.
These mistaken identities, rare as they are, are a big deal. We’re getting better and better at spotting and analyzing fireballs. The US military even tracks them with various sensors and publishes the data regularly. The latest versionthis spring, contained data on around a thousand fireballs dating back to 1988.
The data – the speed, duration, brightness and color of a fireball – provide clues to the inner structure of a meteor. The faster a meteor is, the farther from Earth it can have originated. Color, brightness, and duration can indicate a meteor’s mineral composition and size.
Finding a meteorite on the ground gives scientists the opportunity to confirm and supplement any conclusions they might draw from observing a fireball. Perhaps a particularly fast fireball appears to have come from far away — perhaps even traveled to Earth from outside the solar system. Scientists would want to know what minerals such a strange, far-traveling rock is made of. The implications for planet formation are profound.
“Scientists would want to know what minerals such a strange, far-traveling rock is made of. The implications for planet formation are profound.”
But these comprehensive analyzes of fireball-meteorite pairings only work if space rockhunters pair the right fireballs and meteorites. When fireballs and meteorites don’t match, they may draw the wrong conclusions.
Since fireballs are rare, and finding an intact meteorite is even rarer, complacency can set in. Scientists witness a fireball, look for the meteorite, find one in or near the projected impact zone, and just assume the two things are related.
As the Australians found, that’s not a safe assumption. There are just so many meteorites on Earth that scientists sometimes look for one space rock and accidentally find another.
Together, the Minimoon fireball and the unrelated South Australia meteorite are “a cautionary tale,” Towner said. “Just because it looks good and it’s in the right place isn’t enough – you have to go through the entire chain of analysis in the lab wherever possible to confirm it’s right.”
If you don’t, you could end up doing bad science.
There is a sequel to this unlikely story. Minimoon should still be out there somewhere in the Australian desert. “If it had landed, it would still be hanging around,” Towner said. “Although a bit of time has passed now and the fall area has sand dunes and plants that can move or grow in the wind, there’s a chance it may have been buried and lost by now.”