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[♪ INTRO]
Every year, thousands of tons of space debris rain down on Earth.
Most of that material is so small it gets totally vaporized
when hitting the atmosphere, but occasionally a chunk will survive its fiery descent,
falling to the surface as a meteorite.
To date, more than 50,000 meteorites have been discovered,
giving astronomers a unique chance
to directly study other places in the solar system.
But to do that, they need to know where those bits came from.
And, while it’s usually possible to figure out roughly where they came from,
like, say, from an asteroid, or from Mars,
pinpointing their exact origin is virtually impossible.
Except, in June 2018, we actually did.
In a remarkable feat of forensic astronomy,
scientists managed to trace one meteorite all the way back to
the exact crater from which it came, more than 200 million kilometers away.
On June 2nd, 2018, astronomers at the Catalina Sky Survey in Arizona
spotted an asteroid headed for Earth.
Which was a really big deal, because it marked only the second time ever
that astronomers spotted an incoming rock before it hit Earth.
At one and a half meters long, and with the mass of an African elephant,
this rock wasn’t going to pose any danger to the planet.
But, it was big enough to likely make it to the surface, and, by detecting it early,
astronomers could predict where on Earth it would fall.
Based on its trajectory, the team predicted it would make landfall
somewhere in a 3,500-square-kilometer area of Botswana.
On one hand, amazing.
On the other, though, that doesn’t exactly pinpoint the location...
But, hey, they were right!
The asteroid burned up and exploded about 30 kilometers
above Botswana’s Central Kalahari Game Reserve.
Almost immediately, meteorite hunters were on the case,
searching for surviving fragments of the explosion.
The whole effort was shockingly like a Hollywood spy film.
Some of the researchers studied gas station and hotel security camera footage
for clues, while others scoured the game reserve
under the protection of park rangers.
By the end of the search, 23 fragments had been found,
each weighing from half a gram up to 20 grams.
The meteorite was named Motopi Pan, after a local watering hole.
Once scientists got these shards back to the lab,
they could begin the tricky forensic task of figuring out where they had come from.
Like more than 99% of meteorites that fall to Earth,
it was clear from its stony composition that
Motopi Pan had come from the asteroid belt.
NASA has been particularly interested in asteroids in the 21st century,
with several current and future missions
studying these building blocks of the solar system.
And, back in 2007, NASA launched the Dawn probe to study the asteroid belt’s
two largest objects: the dwarf planet Ceres, and the asteroid Vesta.
Dawn spent seven years orbiting Ceres and Vesta,
studying their geology and landscape in unprecedented detail.
When planetary scientists compared that data to the meteorites we have on Earth,
they found evidence of a remarkable connection.
More than 2,200 meteorites picked up here on Earth were
a direct match for Vesta’s chemical composition.
And, after its landing, researchers quickly matched
the composition of Motopi Pan to Vesta, too.
But, here’s where the story of this meteorite diverges from the typical path.
To back up this geologic evidence, astronomers reverse engineered the asteroid’s
trajectory based on the Catalina Sky Survey’s sighting before it hit Earth.
They confirmed that, yes, it had in fact come from the direction of Vesta.
But they could go one step further, and figure out the asteroid’s flight time, too.
Prior to being blasted off of Vesta, the minerals that made up Motopi Pan
would have been buried deep underground.
But once they were exposed to space, their chemistry became subtly altered
by interactions with cosmic radiation.
The longer they were exposed, the more the damage would build up.
And because Motopi Pan was collected so quickly,
that evidence wasn’t disturbed by weathering here on Earth.
A research team studied its radiation damage, and calculated that the rock
had been travelling for about twenty-two million years.
Remarkably, knowing how long ago it departed the asteroid belt also helped
scientists to pinpoint more precisely where on Vesta this chunk of rock came from.
The data from Dawn also included detailed maps of Vesta’s terrain,
from which scientists were able to reconstruct the age of craters on its surface.
So they found an ideal candidate for Motopi Pan’s departure point,
in a ten kilometer-long crater called Rubria,
with an age of about twenty-two million years.
This crater sits on a stable hillside near Vesta’s equator,
in an area that’s otherwise unaffected by landslides or other erosion.
That fits with Motopi Pan’s surprising lack of contamination,
suggesting that the rock had sat undisturbed beneath the surface for
billions of years before an impact on Vesta tore it out and sent it on its way.
There are definitely some assumptions involved with linking Motopi Pan directly
to Rubria, but, as we’ve seen, there’s also a remarkable amount of evidence.
Meaning, and I cannot say this enough, astronomers were able to trace the rock
not just to the asteroid it came from, but probably to
the exact spot on the asteroid it came from.
And, with careful observation, they hope to do it again!
If they can use a combination of telescopes, meteorite hunters, and spacecraft
to track more meteorites back to their source, that’ll be a big step towards
decoding the forensic mysteries of the solar system.
Those astronomers needed to know how to understand and analyze their data,
and in today’s world, so do the rest of us.
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