Space Traffic Boosts Noctilucent Clouds

July 29, 2022: Never before have so many rockets been launched. In 2021, the space agencies of Earth broke the all-time record for global rocket launches with 133, and in 2022 it looks like the record will be broken again with more than 150. China and SpaceX are big contributors to this increase.

High above Earth, something else is increasing: Noctilucent clouds (NLCs). It’s no coincidence. A new paper just published in the AGU journal for Earth and Space Science confirms that “space traffic has a strong influence on the interannual variability of these bright mesospheric clouds.”

Above: Noctilucent clouds over Anchorage AK on July 26, 2022. Credit: Todd Salat.

Noctilucent clouds are a natural phenomenon. During summer months, wisps of water vapor rise up to the mesosphere, 83 km high, and crystalize around specks of disintegrated meteoroids. Sky watchers at northern latitudes can easily see the clouds, which are filled with fine ripples and shine at night with an electric blue color.

Rocket launches are boosting NLCs. A team of researchers led by Michael Stevens of the Naval Research Lab in Washington, DC, looked at data from NASA’s AIM spacecraft, which was launched in 2007 to study noctilucent clouds. They found a strong correlation between the number of rockets launched each July and the abundance of clouds in the mesosphere. 

Above: July rocket launches (black) vs. the abundance of noctilucent clouds (red)

The link is simple: Rockets produce plumes of water vapor. Winds carry these plumes toward the polar mesosphere where they become raw material for NLCs. Rockets launched in the “morning” between 11 pm and 10 am local time are most effective. During those times, diurnal wind patterns push the plumes toward the noctilucent zone.

Researchers have long known that rockets can produce NLCs. Seminal studies led by Stevens in the early 2000s linked specific space shuttle launches to outbursts of the clouds over both of Earth’s poles (refs: #1, #2). The shuttle program ended in 2011, but the “rocket effect” has continued and probably increased (researchers are still investigating the trends at mid-latitudes during the satellite era).

This is good news for sky watchers who love seeing “frosted meteor smoke” light up the night. Rockets make NLCs both brighter and more widespread. Researchers, on the other hand, may have mixed feelings. NLCs can be a sensitive indicator of changes to Earth’s climate system–e.g., revealing long-range teleconnections and the abundance of  greenhouse gases in the upper atmosphere. Rocket launches could swamp these delicate signals.

Either way, the launch schedule continues. Be alert for noctilucent clouds!

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