Bright-green fireballs that streaked across the skies over New Zealand last month may finally have an explanation.
On July 7, 2022, a bright-green meteor was spotted crashing into Cook Strait between New Zealand's North Island and South Island. The meteor, likely about 3.3 feet (1 meter) in diameter, struck with explosive power equivalent to 2,000 tons (1,800 metric tons) of TNT (opens in new tab) and led to a massive sonic boom. Two weeks later, another rare green fireball was photographed over Canterbury, on New Zealand's South Island.
Fireballs are unusually bright meteors that can exceed one meter in size. Only around four are reported each year over any one region, so why are so many fireballs lighting up the skies over this island nation?
"We have seen a tremendous increase in the number of reports from New Zealand," said Robert Lunsford, a fireball report coordinator at the American Meteor Society, who told Live Science that fireball reports from New Zealand have now surpassed those from Australia, which is nearly 30 times larger. These fireballs' intense green color makes it more likely that people will report the sightings, he added.
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It's also recently become easier than ever to report a fireball. "The general public in New Zealand is now becoming aware that they can actually report these events through Fireballs Aotearoa (opens in new tab)," said Lunsford. Fireballs Aotearoa is a collaboration between astronomers and citizen scientists aimed at finding meteorites that have just fallen to Earth. The nonprofit has now joined the International Meteor Organization, which collects meteor observations from all around the world and makes it easy for members of the public to report a sighting online (opens in new tab).
So what gives these meteors their unforgettable green hue? Whether meteor wakes — which can last a few seconds — are green depends on the size, height and chemistry of the meteors.
"The source of green in a long-lasting meteor wake behind the meteor is restricted to those above about 62 miles (100 km)", said Jack Baggaley, a professor emeritus in physics and astronomy at the University of Canterbury in New Zealand, who recently wrote an article (opens in new tab) on the topic.
As much smaller meteors moving as fast as 45 miles (70 km) per second strike Earth's atmosphere, solar particles ionize oxygen in the upper atmosphere. The same process creates green auroras.
Green can also appear in slower meteors if they contain meteors, Baggaley told Live Science.
"Fireballs — as were the case observed here — occur lower down, less than 37 miles (60 km) up, and the green color in them is produced by large bodies composed of metallic material such as nickel, iron and magnesium," Baggaley said.
Were these fireballs possibly related to the Perseid meteor shower, which occurs annually from mid-July to late August?
The Perseids' swift velocity can cause ionized oxygen atoms to glow with a greenish hue as meteors pass through the atmosphere, according to Lunsford, but he doesn't think the New Zealand fireballs are connected to the Perseids.
This year, the Perseids last from July 17 until Aug. 24 and peak from Aug. 12 to Aug. 13, so that timeline doesn't fit well with the early-July sightings. Nor does celestial geography: Perseus, the radiant point of the Perseid meteor shower, is seen only in the Northern Hemispere's night sky.
"Perseid meteors are generally not visible from New Zealand due to the southerly location of the islands," Lunsford said, "though there is a chance on the night of maximum activity to see a couple of these meteors shoot upwards from below the northern horizon from the northernmost portion of North Island."
Unfortunately, this year's Perseids peak is likely to be less impressive than in past years. That's because the peak coincides with August's full moon, which will bleach the night sky and make most meteors hard to see.
Originally published on Live Science.