Fireballs Aotearoa, a collaboration between astronomers and citizen scientists that aims to recover recently fallen meteorites, has received a lot of questions about these events. One of the most common is light green, and whether it is the same green as the aurora borealis.
Green fireballs have been reported and photographed in New Zealand regularly. Bright meteors often indicate the arrival of a piece of the asteroid, which can be anywhere from a few centimeters to a meter in diameter when it hits the atmosphere.
Some of these asteroids contain nickel and iron and strike the atmosphere at speeds of up to 60 kilometers (37 miles) per second. This releases a huge amount of heat very quickly, and the vaporized iron and nickel radiate a green light.
But is this the same as the bright green of the aurora borealis? For the last meteorite, the answer is basically no, but it’s actually not that simple.
Meteor Trail Colors
The green glow of the aurora is caused by oxygen ions in the upper atmosphere, caused by collisions between oxygen molecules in the atmosphere and particles emitted by the sun.
These oxygen ions combine with electrons to produce oxygen atoms, but the electrons can remain in the excited state for several seconds. In an energy transition known as “forbidden” because it does not obey usual quantum rules, then auroral green light radiates at the wavelength of 557 nm.
A meteor can also shine this way, but only if it is extremely fast. Very fast meteors heat up in the thin atmosphere over 100 km high where the aurora is formed.
If you’d like to see green aurora borealis from a meteor, watch out for the Perseid meteor shower, which is starting now and will peak on August 13 in the Southern Hemisphere.
The Perseids also reach about 60 km per second, which is very fast slivers of Comet Swift-Tuttle. Some Perseids follow behind them a distinctive beautiful, glowing, green awakening, especially at the beginning of their path.
As soon as the meteor hit Canterbury on July 22, choppy upper-atmosphere winds gently twisted the glowing path, creating a faint yellow glow near the end (shown in the GIF below, also recorded by Greg Price for a previous inhale).
This occurs because sodium atoms are constantly excited in a catalytic reaction involving ozone.
Do meteors bombard us?
Yes and no. Big thriving green meteorite arrivals and meteorite fall aren’t rare in New Zealand, but rock recovery is rare. Fireballs Aotearoa improves recovery rate.
In an average year, perhaps four meteorites struck New Zealand. We encourage citizen scientists to build their own meteor camera systems so they can capture these events.
By comparing the meteor to the starry background and trigonometric images taken by several cameras, we can determine the position of the meteor in the atmosphere to within tens of meters.
This not only helps us find the rock, but also tells us about the pre-collision orbit of the meteorites, which in turn tells us what part of the solar system it came from. This is a fairly effective way to sample the solar system without having to launch a space mission.
Above: Witness reports and high-resolution meteor cameras help calculate the meteor’s path. This map shows the approximate path of the July 22 meteor at the top of the red figure in the center.
Aotearoa fireballs are rapidly filling Otago with meteor cams and there are half a dozen more in other parts of the South Island. The North Island isn’t well covered yet, and we’re keen to get more people (on either island) to build or buy a meteor camera and keep it pointed skyward.
Then the next time a bright meteor explodes with a boom over New Zealand, we might be able to catch the meteor and do some good science with it.
Many thanks for the input from Jim Rowe of the Fireball Alliance UK, and Greg Price who photographed the July 22 meteor and the continuous train.
Jack Bagley, Emeritus Professor of Physics and Astronomy, University of Canterbury.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.