US meteorite adds to origins mystery - BBC News
Scientists are getting closer to tracing the sources of meteorites that fall to Earth.
Artwork: Scientists are trying to work out the likely paths meteorites took as they fell toward Earth

In January 2018, a falling meteorite created a bright fireball that arced over the outskirts of Detroit, Michigan, followed by loud sonic booms.

The visitor not only dropped a slew of meteorites over the snow-covered ground, it also provided information about its extra-terrestrial source.

Although tens of thousands of meteorites have been recovered by humans, scientists have only been able to trace the orbits of a small number. Most of these have been calculated in the last decade.

Scientists can use information about how the meteorite burned through Earth's atmosphere to calculate how the rocky object moved through space before it transformed into a fireball.

Researchers cannot trace the specific path of an object back through time - there are too many variables that could have affected its motion. But they can determine the most likely paths. Studying the likely orbits of similar asteroids can help to reveal their parent body, the larger asteroid they once were part of.

Video of the fireball over Michigan:

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"This is a great way to do what amounts to a low-cost asteroid sample return mission," says Dr Peter Brown, who studies asteroids at Canada's University of Western Ontario. "In this case, the sample comes to us. We don't have to go to the sample."

Dr Brown and his colleagues gathered information from fireball surveys as well as videos posted on social media to reconstruct a potential orbit for the Hamburg meteorite, named after the small Detroit suburb it buzzed.

The team then worked with several of the amateur photographers to calibrate their observations. "We spent a lot of time scouring YouTube and Twitter," he says.

The researchers found that the Hamburg meteorite was a fairly typical fireball. It likely entered the atmosphere with a mass ranging from 60kg to 220kg and a diameter between 0.3m and 0.5m.

Travelling at about 16 km/s, it produced two major flares at 24.1km and 21.7km above the ground. The total energy produced by the fireball equalled somewhere between two and seven tonnes of TNT.

While some researchers took to the ground to hunt for dark meteorites in the Michigan snow, Dr Brown and his colleagues took to the internet to find reports of the fall. Because the region was densely populated, Dr Brown said there were a lot of video recordings that captured the fall.

Out of the wealth of camera phone and security footage, they tracked down almost 30 unique videos that were sharp enough to reveal their location. Of these, only a handful was good enough for the team members to perform detailed calibration.

How do you calibrate a casual fireball video? First you need to have a positional reference that helps to pinpoint where the video was taken from. Ideally, the same camera would be placed in the exact spot where the meteorite fall was originally viewed - though often a similar camera was used instead.

Measurements from those videos revealed the angle that the incoming meteorite was travelling on.

The Chelyabinsk meteor in 2013 was also filmed from multiple locations

"A lot of the legwork was just talking to people," Dr Brown says.

In addition to the casual imagery, the researchers looked at images from fireball surveys, where the calibration had already been performed.

While the official data was easier to work with, Dr Brown says that smart phones and dashboard cameras often tend to have higher resolution, providing better precision data if they can be calibrated. The growing prevalence of these kinds of cameras "has almost revolutionised this area," he says.

While humans have collected meteorites for thousands of years, it wasn't until 1959 that the first meteorite orbit was recovered. Cameras operated by the Ondrejov Observatory in the Czech Republic recorded the fall of the Pribram meteorite, allowing the researchers to trace its orbit back to the asteroid belt.

For the first time, astronomers were confident that meteors came from asteroids. "That orbit really sort of sealed it," Dr Brown says.

Fireball networks came online through the 1960s, 70s, and 80s, and by 2000, four meteorite orbits were known. Three of those were H-chondrites, the iron-rich class of meteorites that most commonly falls, and the group that Hamburg belongs to.

Since 2000, those meteorites with orbits that can be calculated have increased. Another 10 were spotted by 2010. The last few years have produced a handful of traceable meteorites annually, Dr Brown says.

H-chondrites, like this example that fell in Kansas in 1929, are the most common type of meteorite falls

Today, there are about 30 meteorites whose orbits have been calculated. While the spread of cameras dedicated to tracking fireballs has played an important role, Dr Brown says that casual recordings have also advanced the field.

The Hamburg fall "was very well recorded, and that's what makes it so interesting", Dr Brown says. After the more powerful 2013 Chelyabinsk fireball, "there's no other fall that had so many video records".

But casual video recordings have their downfall. Because they are so much more difficult to calibrate than official surveys, they take more time. That can move them down the priority list for swamped scientists.

Dr Brown knows of researchers working on nearly 10 more meteorite orbits, but he estimates that others exist. "There are data for probably another 20 that people just haven't tried to do because it's so much work," he says. "It's a difficult process."

Although H-chondrites make up the bulk of the meteorites that survive the plunge through Earth's atmosphere, their origin remains a mystery. In 1998, astronomers proposed the large main-belt asteroid (6) Hebe as the primary parent body because it resembled H-chondrites.

Hebe's orbit sits in a location where Jupiter's gravitational forces can stir up material, allowing it to escape from the asteroid belt. Near-Earth asteroids similar to Hebe have also been spotted, suggesting that something - probably the giant planet Jupiter - slung material from the asteroid belt.

However, other main-belt asteroids similar to H-chondrites have been identified in recent years, muddying the picture.

The asteroid (6) Hebe has been proposed as one source of H-chondrites

Of the 30 or so meteorites with known orbits, nearly half are H-chondrites. So far, however, those objects don't seem to be coming from the outer asteroid belt - the side facing Jupiter - where Hebe orbits. Instead, they appear to start their journey from the middle and inner belt, closer to the Sun. And the new discovery isn't helping.

"Hamburg, unfortunately, adds more questions about the orbit of H-chondrites than it answers," Dr Brown says.

Narrowing things down will take more meteorite samples. Dr Brown estimates that doubling the existing known orbits for H-chondrites will allow researchers to make more solid associations with a parent body.

That assumes the iron-rich asteroids come from a single source; it's possible they come from two or more locations in the asteroid belt.

"It's a very complicated story," Dr Brown says. "We need to get more of these if we're going to answer these questions more fully."