Space & Science

Bright green meteors seem to be raining down on New Zealand, but why?

New Zealand may appear to be under a meteor bombardment right now. After a huge meteor exploded over the sea near Wellington on July 7, creating a sonic boom that could be heard deep in the South Island, a smaller fireball was captured two weeks later over Canterbury.

Aotearoa Fireballs, a collaboration between astronomers and citizen scientists that aims to recover freshly fallen meteorites, has received many questions about these events. One of the most frequent concerns the bright green color and if it is the same green produced by the auroras.

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BrightGreenMeteors 5A bright green meteor. (Crude prices)

Green fireballs have been reported and filmed regularly in New Zealand. Bright meteors often signal the arrival of a piece of asteroidwhich can measure between a few centimeters and a meter in diameter when it crashes into the atmosphere.

Some of these asteroids contain nickel and iron and they hit the atmosphere at speeds of up to 60 km (37 miles) per second. This very quickly releases an enormous amount of heat and the vaporized iron and nickel emit a green light.

But is it the same as the bright green of an aurora? For the most recent meteor, the answer is mostly no, but it’s actually not that simple.

BrightGreenMeteors 5An aurora australis observed from the International Space Station. (Wikimedia Commons, CC BY-ND)

The colors of a meteor trail

The green glow of the aurora is caused by oxygen ions in the upper atmosphere, created by collisions between atmospheric oxygen molecules and particles ejected by the sun.

These oxygen ions recombine with electrons to produce oxygen atoms, but the electrons can persist in an excited state for several seconds. In a so-called “forbidden” energy transition because they do not obey the usual quantum rules, they then radiate auroral green light at a wavelength of 557 nm.

A meteor can also shine through this route, but only if it is extremely fast. Very fast meteors heat up in the thin atmosphere above 100 km where the aurora forms.

If you want to see a green auroral wake from a meteor, watch out for the Perseids meteor showerwhich has now begun and will peak on August 13 in the southern hemisphere.

Also arriving at around 60 km per second, the Perseids are extremely fast chunks of the Comet Swift–Tuttle. Some Perseids trail a beautiful, glowing, distinctly green trail behind them, especially at the beginning of their path.

Once the Canterbury meteor hit on July 22, wayward upper atmosphere winds twisted the contrail slightly bright, resulting in a faint yellow glow towards the end (as seen in the GIF below, also recorded by Greg Price for an earlier meteor).

This is because the sodium atoms are continuously excited in a catalytic reaction involving ozone.

BrightGreenMeteors 5(Crude prices)

Are we bombarded by meteors?

Yes and no. The arrival of large, booming green meteors and falling meteorites are not uncommon in New Zealand, but recovering the rock is rare. Fireballs Aotearoa strives to improve the recovery rate.

In an average year, maybe four meteorites hit New Zealand. We encourage citizen scientists to build their own meteor camera systems so they can capture these events.

By comparing the meteor against the starry background and by triangulating the images captured by several cameras, we can determine the position of the meteor in the atmosphere to within a few tens of meters.

BrightGreenMeteors 5The July 22 meteor seen by a specialized meteor camera. (Campbell Duncan/NASA/CAMS NZ)

Not only does this help us find the rock, but it tells us what the meteoroid’s orbit was before impact, which in turn tells us what part of the solar system it came from. It’s a pretty efficient way to sample the solar system without ever having to launch a space mission.

BrightGreenMeteors 5(Fireballs Aotearoa and International Meteor Association)

Above: Witness reports and high-resolution meteor cameras help calculate a meteor’s path. This map shows the approximate path of the July 22 meteor at the top of the red shape in the center.

Aotearoa fireballs are quickly populating Otago with meteor cameras and there are half a dozen more in other parts of the South Island. The North Island is not yet well covered, and we wish more people (on either island) would 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, maybe we can pick up the meteorite and do some good science with it.

Many thanks for the contribution of Jim Rowe of the UK Fireball Alliance and Greg Price who photographed the July 22 meteor and the lingering train.The conversation

Jack Baggaleyprofessor emeritus of physics and astronomy, University of Canterbury.

This article is republished from The conversation under Creative Commons license. Read it original article.

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