Space launches, specifically morning space launches, are causing spectacular shining clouds to appear in new places, according to recent research published in the journal Advancing Earth and Space Science.
Some 80 kilometres above the ground float the highest clouds in the Earth’s atmosphere. Called noctilucent, mesospheric, or polar mesospheric clouds, these aggregations of crystalized water vapour are high up enough to reflect sunlight, even after the Sun has set or even before it has risen.
Because of their position in the upper atmosphere, when they’re present at the right time, noctilucent (i.e. “night-shining”) clouds shimmer with an other-worldly glow. They can make the sky at dusk or dawn look something like the surface of the ocean on a bright day — ripples of silvery light between patches of darkness.
“You see them for about 30 minutes to an hour and a half after after sunset, or before sunrise,” said Cora Randall, an atmospheric scientist at the University of Colorado, Boulder, and one of the researchers on the new study, in video call with Gizmodo. “And that’s because they’re very tenuous, they’re very thin clouds. You can only see them when the sunlight scatters off of them and it’s dark where you’re standing.”
And emissions from rocket launches, which pump cloud fuel (i.e. water vapour) directly into the mesosphere, are likely making these unique formations a more common sight. Launches happening at the Kennedy Space Centre in Cape Canaveral, Florida, could be correlated with the formation of noctilucent clouds thousands of miles away and days later.
Usually, noctilucent clouds are invisible because they form most commonly over the poles and occur during each hemisphere’s respective summer; the season for noctilucents is from mid-May to August in the North and from mid-November to February in the South. The poles never get dark during their summer times, so noctilucent clouds tend to get washed out by the ever-present sunlight.
But, in the past few decades, more noctilucent clouds have been observed at so-called “mid-latitudes,” away from the poles — as far south as California and Colorado. “We had noticed that they were becoming brighter and more frequent, basically over the last 50 years or so,” said Randall. Scientists hypothesised that climate change or solar cycles were to blame.
For the new study, Randall and her co-researchers amassed data on the clouds from between 2007 and 2021, collected by NASA’s Aeronomy of Ice in the Mesosphere (AIM) satellite. They focused their attention on northern latitudes between 56 and 60 degrees (a narrow band bordered by Bergen, Norway, to the north and Edinburgh, Scotland, to the south), as that mid-latitude data was most reliable. What they found was the clouds didn’t seem to be following any apparent trend.
Instead, they noted that mid-latitude noctilucent clouds are super variable, appearing a lot at some times but not at others. Surprisingly, that year-to-year variability tracks strongly with the frequency of morning rocket launches, based on data from NASA’s Aeronomy of Ice in the Mesosphere (AIM) satellite.
The researchers also observed that, in the morning, atmospheric winds quickly carried particles in the mesosphere poleward, which aligned with the observed rocket/noctilucent cloud relationship. The theory goes that water vapour from rocket launch emissions gets pulled northward by the winds into areas of the upper atmosphere that’s cold enough for the shining clouds to form.
Previous case studies have linked noctilucent clouds to Space Shuttle launch emissions. However, this is the first research to demonstrate that even much smaller rockets seem to be having an effect. ”The shuttle of course, is enormous compared to some of these other transport vehicles,” pointed out Michael Stevens, an astrophysicist at the Naval Research Laboratory and a co-author of the recent paper. “So the fact that the all the smaller vehicles are contributing, not only contributing, but actually sort of driving the [year-to-year cloud] variability, that was kind of a surprise. We didn’t expect that,” he said.
Though it’s probably not just rockets. Other factors are likely still influencing the appearance of noctilucent clouds. Climate change, Randall specified, is almost certainly still playing a role, even if the data set she and her colleagues analysed didn’t capture a clear signal. “There’s no question in my mind that anthropogenic activities are affecting the clouds. It’s really more a matter of to what extent quantitatively,” she explained.
Interestingly, a relationship between volcanic eruptions and mesospheric clouds has been thought to exist for more than a century. The first known recorded instance of a noctilucent cloud sighting was in 1885, two years following the eruption of Krakatoa.
But having a clearer picture of the influence of rocket launches could offer a clearer picture of all the ways that humans are changing our planet. Aside from being beautiful to look at, noctilucent clouds don’t have much of a known impact here on Earth. Instead, their importance is as a potential indicator of human-caused change.
The mesosphere, and the clouds that appear within it, are very sensitive to small shifts. If we better understand the impact that, for instance, rockets are having, we can use that knowledge to accurately assess the effects of larger atmospheric alterations like massive greenhouse gas emissions, said both Randall and Stevens.
Researching noctilucent clouds is also, at its core, a basic expression of human curiosity. “The fact that they’ve become more and more prevalent in the late 20th century and into the 21st century has piqued the interest of a lot of scientists. We would like to know why this is happening,” said Stevens. For decades, its been a mystery. Now, it’s slightly less of one. If you look to the sky at dusk or dawn and see a sea of shining sky, know a rocket might have made the clouds.