The photograph above shows a bright green meteor streaking across the night sky over the town of Mettupalayam, at the foot of the Western Ghats in southern India. It was taken by the Indian natural history photographer Prasenjeet Yadav, and the most remarkable part of the story is that he was asleep when it happened. His camera had been left running through the night on a time-lapse sequence, and he only found the meteor in the frames the next day.
The image circulated widely online in early 2017 and has been shared many times since, often with exaggerated captions about a meteor "hitting the Earth". Here is what the picture really shows, how it was made, and why meteors sometimes glow green.
The photographer and the project
Yadav grew up on his father's farm in Maharashtra, where he became fascinated by wildlife. He trained in molecular biology and later worked in molecular ecology at the National Centre for Biological Sciences in Bengaluru before turning to photography full time. A grant from National Geographic funded a project documenting the "sky islands" of the Western Ghats: isolated mountaintops separated by deep valleys, each with plants and animals found nowhere else.
The grant was meant to cover ten months, but Yadav has said he spent around two and a half years on the work. Alongside the wildlife and the scientists studying it, he wanted to show how fast towns at the base of the hills were growing. That is how the meteor picture came about.
How the image was made
On a hillside overlooking Mettupalayam, Yadav set up a Nikon D600 to shoot a time-lapse of the town's lights, a way of illustrating urbanisation in the region. The camera was programmed to take a long run of 15-second exposures, one after another, until the early hours of the morning. He was travelling with fellow photographer Anand Varma at the time, and both went to sleep while the camera worked.
When Yadav reviewed the sequence the following day, one frame stood out: a thick, glowing green streak cutting across the stars above the town. He first suspected a glitch. After he sent it to several astronomers, they confirmed it was a meteor. Speaking to reporters, he summed up the lesson in a way any photographer will recognise, calling it a case of "being there in the right place at the right time."
Long exposures help explain why the meteor looks so dramatic. A 15-second frame collects all the light the meteor gave off during its brief flight, so a streak that lasted perhaps a second or two is recorded as a single solid band. The same long exposure brightens the stars and turns the town lights into a warm glow below.
Meteor, meteorite or fireball?
Many headlines claimed the photo showed a meteor "hitting" the Earth. That is not what it shows. The terms are often mixed up, so they are worth separating:
- Meteoroid: a small rocky or metallic body travelling through space, from the size of a grain of sand up to about a metre across.
- Meteor: the streak of light produced when a meteoroid plunges into the atmosphere and heats the air around it. This is what Yadav photographed.
- Meteorite: any fragment that survives the fall and reaches the ground. Most meteors burn up completely high in the atmosphere, so there is no evidence that this one produced a meteorite.
- Fireball: an especially bright meteor, brighter than the planet Venus. A fireball that ends in a flash or visibly breaks apart is often called a bolide.
Meteors usually light up at altitudes of roughly 80 to 120 km and fade well before they get close to the ground. Even though the streak appears to be just above the town in the picture, it was almost certainly tens of kilometres up and possibly far away along the horizon.
Why some meteors glow green
Meteoroids enter the atmosphere at speeds of about 11 to 72 km per second. At those speeds the air in front of them is compressed and heated so intensely that atoms in the air and in the vaporising rock become excited and give off light. Each chemical element glows at particular wavelengths, rather like the colours in a fireworks display.
According to the American Meteor Society, most of that glowing cloud is made of atmospheric atoms, with only a small share coming from the meteoroid itself. Sodium produces yellow-orange light, magnesium blue-white, and nickel contributes green. Glowing oxygen in the upper atmosphere also emits a strong green line, the same colour seen in many auroras. The combination of speed, altitude and composition decides the final hue, which is why some fireballs flash green while others look white, yellow or red.
The colour you remember can also be misleading. The human eye is not good at judging the colour of brief bright flashes, which is one reason a photograph like this is valuable: the camera recorded the green directly.
How rare is a shot like this?
Meteors themselves are not rare. Wikipedia's summary of the scientific estimates puts the material entering Earth's atmosphere at around 25 million meteoroids, micrometeoroids and bits of space debris a day, adding up to roughly 15,000 tonnes a year. The vast majority are tiny grains that produce faint shooting stars or nothing visible at all.
Bright fireballs are much less common at any one spot. The American Meteor Society notes that several thousand fireball-class meteors enter the atmosphere every day, but most occur over oceans or empty land, or in daylight when nobody can see them. An experienced observer might expect to see a really bright one only after hundreds of hours watching the sky. Capturing one in a well-composed frame, with a town and mountains beneath it, takes luck as much as skill, which is exactly what made this image so popular.
Tips for catching your own meteor
- Use a time-lapse. Set a camera on a sturdy tripod with a wide-angle lens and shoot continuous exposures of 10 to 30 seconds. Yadav's result shows the value of simply letting the camera run.
- Go dark and go during a shower. Annual showers such as the Perseids in August and the Geminids in December produce far more meteors than an average night. Get away from city glow if you can, while keeping an interesting foreground.
- Check every frame. A meteor may appear in just one image out of hundreds.
- Report bright fireballs. Groups such as the American Meteor Society collect sightings, and several reports from different places can help scientists work out a meteor's path, and sometimes where fragments may have landed.
Yadav's photograph is a reminder that some of the best images of the night sky come from patient preparation followed by a stroke of luck. He set out to document a growing town and ended up with one of the most widely shared meteor pictures taken in India.
Sources: Inquirer Technology: Emerald green meteor photographed over India's 'Sky Islands'; NPR (2019): A farmer's son aims his camera at the world's changing environment; American Meteor Society: Fireball FAQ; Wikipedia: Meteoroid; photo: Prasenjeet Yadav.
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