NASA Astronomy Picture of the Day for September 13, 2026: Comet NEOWISE Rising over the Adriatic Sea (video)
Today's NASA Astronomy Picture of the Day is not a still image but a time-lapse video.
It was filmed before dawn on July 8, 2020, from Italy's Adriatic coast. Three things occupy the frame at three different distances. The sea lies closest. Above it hangs a thin, rippled cloud glowing silver-blue. Behind the cloud, a comet with a long tail slowly rises. Farther still are the stars.
The video combines more than 240 photographs taken over 30 minutes.
The comet is C/2020 F3 (NEOWISE), one of the few comets this century visible to the naked eye and later called the "Great Comet of 2020." But first, consider the cloud. It is the most counterintuitive part of the scene, and the easiest to explain clearly.
The Sun Has Set, but the Cloud Is Still Bright
It is a noctilucent cloud, a name that means "night-shining cloud."
Noctilucent clouds are the highest clouds on Earth. NASA puts them at an altitude of 80 to 85 kilometers. Compare that with familiar heights: Mount Everest rises 8.8 kilometers, while passenger jets cruise at about 11 kilometers. These clouds sit more than nine times higher than Everest and seven times higher than an airliner. They are no longer in the troposphere but in the mesosphere, almost at the edge of space.
Now for the beautiful mechanism.
From the ground, you watch the Sun sink below the horizon. The sky darkens, and ordinary clouds darken with it because they, like you, have entered Earth's shadow. But noctilucent clouds are so high that even after darkness reaches the ground, they remain outside Earth's shadow, lit from below by the Sun that has already set for you.
An official report from Heilongjiang's provincial culture and tourism department puts it clearly: after the lower atmosphere has entered Earth's shadow, ice particles high in the atmosphere can still catch sunlight from below the horizon and scatter it as pale blue or silver-gray light.
NASA even gives a precise geometric window. The best time to see noctilucent clouds is when the Sun lies 6 to 16 degrees below the horizon. Any higher and the sky is too bright; any lower and the clouds themselves enter the shadow.
That silver-blue glow is sunlight that has already said goodbye to you, circling back to shine upward from below.
The Coldest Place in the Sky Freezes in Summer
Here is another counterintuitive fact.
Noctilucent clouds appear only in summer. Why would a cloud made of ice choose the warmest season?
Because temperature changes in Earth's upper atmosphere run opposite to those on the ground. NASA Earth Observatory explains that while the lower atmosphere warms through spring and summer, the upper atmosphere grows colder. In summer, the high-latitude mesosphere becomes the coldest place in Earth's entire atmosphere - NASA's own phrase.
How cold? NASA says noctilucent clouds form only when temperatures fall below minus 130 degrees Celsius.
During the hottest weeks of the year, the coldest place associated with Earth lies 80 kilometers above your head. That is where the ice forms.
Every Ice Crystal Is 3% Burned-Up Meteor
Water vapor needs something on which to freeze, just as a raindrop needs a condensation nucleus. What can provide that nucleus 80 kilometers above Earth?
NASA's AIM satellite supplied my favorite answer in this story: meteor ash.
Meteoroids enter the atmosphere every day and burn up at this height, leaving behind extremely fine metallic dust that scientists call "meteor smoke." NASA explains that specks of meteor smoke act as gathering points where water molecules can assemble themselves into ice crystals.
AIM's SOFIE instrument measured the proportion: about 3% of every ice crystal in a noctilucent cloud is meteoritic material.
So this is how the silver-blue cloud forms. A rock arrives from deep in the solar system and burns to ash 80 kilometers overhead. The ash drifts until summer makes that altitude colder than minus 130 degrees. Water vapor freezes around it. Then sunlight from below the horizon shines upward.
Another NASA source adds that the nuclei may also come from rocket launches and volcanic eruptions.
In 1885, the Truth Was Better Than the Legend
Noctilucent clouds were first recorded in 1885. Two years earlier, in 1883, Krakatoa in Indonesia had erupted catastrophically.
That produced a widely repeated story: volcanic ash reached the upper atmosphere and made the clouds.
But that is not what NASA says.
NASA notes that skywatchers in the Northern Hemisphere first noticed the clouds in 1885, about two years after Krakatoa erupted. Its next sentence contains the important point: ash from the Indonesian volcano produced such spectacular sunsets that watching the evening sky became a worldwide pastime.
Read together, NASA's causal chain is not volcano to cloud. It is this: volcano to spectacular sunsets; spectacular sunsets to a worldwide habit of looking up in the evening; that habit to noticing a kind of cloud that may have been there all along.
The phenomenon was "discovered" in 1885 because a volcano taught humanity to look up together.
That is better than the legend.
They Are Increasing, but NASA Says the Cause Is Unclear
Noctilucent clouds have indeed been changing, and NASA is unequivocal: they have been growing brighter and more frequent and appear to be moving to lower latitudes.
The cause requires more care.
A common explanation holds that human-produced methane reaches the upper atmosphere, oxidizes into water vapor and supplies more water for ice crystals. NASA has described that mechanism: methane entering the upper atmosphere passes through a complex series of reactions and oxidizes into water vapor, which can then help grow noctilucent ice crystals.
But NASA immediately adds a line that is rarely repeated: so far it is unclear which factor, increasing water vapor or cooling, is driving the changes in polar mesospheric clouds. Both probably contribute, it says, but that remains an active research question.
The accurate formulation is therefore: NASA clearly describes brighter and more frequent noctilucent clouds extending to lower latitudes as an observed fact. Whether climate change is responsible, and through precisely which pathway, remains unclear even to NASA.
It is worth learning to separate observation from explanation, even when they occupy the same sentence. That is the second habit this story hopes to leave behind.
In the Summer of 2020, Two Rare Events Met
Now return to the video.
Noctilucent clouds are normally visible only between 50 and 65 degrees north or south. Paolo Girotti filmed at about 44 degrees north in Italy, a place where these clouds should not ordinarily appear.
But 2020 was exceptional, and NASA recorded the fact.
NASA Earth Observatory wrote that year that the clouds had been seen as far south as Joshua Tree, California, near 34 degrees north. The same report offered a reason: on May 24, 2020, the mesosphere reached its lowest temperature in the 14-year record.
It was an unusually remarkable noctilucent-cloud season. NASA's standard is practical: when the clouds extend into midlatitudes, where people live and can notice them in everyday life, the season is noteworthy.
At the same time, Comet NEOWISE was passing through its brightest weeks.
The video is precious not because its photographer was merely lucky, but because he encountered two rare events at once. One of the century's few naked-eye comets overlapped with low-latitude noctilucent clouds produced during the coldest mesosphere in 14 years. Both occupied the same sky in early July 2020.
The Comet and the Telescope That Found It
Comet C/2020 F3 was discovered on March 27, 2020. The discoverer was not a mountaintop observatory but an infrared space telescope, NASA's NEOWISE survey spacecraft. The Jet Propulsion Laboratory says the mission found the icy visitor through its two infrared channels. At first, it appeared in the data as a string of fuzzy red dots.
The next events came quickly. On July 3, it passed perihelion inside Mercury's orbit, about 43 million kilometers from the Sun. Five days later, on July 8, Girotti made this video. On July 23, the comet came closest to Earth at about 103 million kilometers, roughly 268 times the distance from Earth to the Moon.
NASA has published two slightly different estimates for the nucleus. JPL says "about 5 kilometers"; the Hubble team says it may be no more than 4.8 kilometers across. APOD's "about five kilometers" is consistent with both.
The fact that the comet did not break apart was itself unusual. Hubble's account notes that other comets often fragment under thermal and gravitational stress during such close encounters, while observations showed NEOWISE's solid nucleus apparently remained intact.
Notice that NASA says only that it survived, not why. A common online explanation is that its nucleus was large enough to endure the encounter, but that claim was not found on NASA's pages. The evidence lets us go only this far: it survived, and NASA does not say why.
As for its return, orbital calculations give different inbound and outbound periods: about 4,400 years on the way in and about 6,700 years on the way out. The difference comes from the small amount of energy added by planetary gravity as the comet passed through the inner solar system. (These two figures come from JPL's orbital solution; no NASA webpage was found that states the periods directly.)
In short, no one who saw it will see it again.
What follows is quieter still: the telescope that found it is gone too.
The NEOWISE spacecraft began as WISE, launched on December 14, 2009. It entered hibernation in February 2011 and was revived in 2013 to survey near-Earth objects. NASA's official page records that its mission ended on July 31, 2024, and that it reentered the atmosphere and burned up on November 1, 2024.
A telescope worked for more than 14 years, found a comet that will not return for 6,700 years, and then burned away.
Why Today: Another Comet-Finding Telescope
September 13 does have a connection to comets, though not a proud one.
On September 13, 1985, an American satellite named P78-1, or SOLWIND, was destroyed in orbit by an anti-satellite missile launched from an F-15 fighter. It broke apart at an altitude of 525 kilometers and left 285 cataloged pieces of space debris.
The satellite deserves to be remembered because its coronagraph accomplished something unprecedented: it enabled humanity's first discovery of a comet from space.
The images were taken on August 30, 1979, but no one recognized the object at the time. Howard, Koomen and Michels found it while reanalyzing the data in September 1981. Contemporary accounts say they were initially alarmed by an enormous bright streak, thinking the camera had failed. It had not. The streak was a comet. SOLWIND later found another nine or ten sungrazing comets.
That gives today's date a peculiar meaning: 41 years ago today, humanity deliberately shot down the first telescope to discover a comet from space.
Today's APOD shows the work of another space telescope that found a comet. It too is gone, burned up in the atmosphere 22 months ago.
Both comet-finding machines are gone. Their comets are still flying.
(One harmless detail: APOD has published this video three times. The earlier appearances were July 13, 2020, and July 26, 2022. The 2022 version began "two years ago this month." Today's version changed that to "over four years ago," although July 2020 to September 2026 is actually six years and two months. One of the world's longest-running astronomy websites is made by people too.)
The Photographer Catches Muons
I had planned to describe Paolo Girotti only as an Italian astrophotographer. But the official magazine of Italy's National Institute for Astrophysics, INAF, interviewed him, and the fuller account matters, especially for children.
Girotti was born in Cattolica in 1994 and grew up in Riccione. His profession is not photography but physics. He was a particle-physics doctoral student at the University of Pisa, working on the Muon g-2 experiment at Fermilab. By day, he measured the muon's anomalous magnetic moment; by night, he went to the seaside to photograph the sky.
He filmed from a meadow called "Roof of the World" in Mount San Bartolo Park, in the Province of Pesaro and Urbino. At an elevation of about 200 meters, it faces the Adriatic, with the Croatian coast visible in good weather.
The equipment list is worth reading in full: a Nikon D3300 body, an entry-level APS-C DSLR; an 18-105mm kit zoom set to 75mm; a tripod; and an interval timer. The settings were f/5.3, ISO 3200 and a five-second exposure for each of roughly 240 frames.
An entry-level camera and a kit lens produced a scene NASA has now selected three times.
The Chinese Thread: Nine Comets and One Night in Mohe
First, the comets.
The first comet discovered by a Chinese observatory appeared on New Year's Day 1965 at Purple Mountain Observatory in Nanjing. Observers used a 40-centimeter refracting telescope to find a new comet. Ten days later, on January 11, they found another. The official website of Nanjing's Yuhuatai Martyrs Memorial Park records the scene: a cheer suddenly rose from the observatory on January 1, and the team agreed to name the new comet Zijinshan, or Purple Mountain. The youngest team member was Yang Jiexing.
The two objects are now numbered 62P/Tsuchinshan 1 and 60P/Tsuchinshan 2. The numerical order is the reverse of their Chinese sequence, an easy detail to mix up. In 2023, Purple Mountain Observatory used data from its Yao'an station to discover a stable spiral structure in the coma of Tsuchinshan 2. Its activity depends not only on heliocentric distance and composition but also on the size of the nucleus.
Sixty-one years later, the observatory still searches for comets, though the method has changed. On January 9, 2023, the Near-Earth Object Telescope at Purple Mountain Observatory's Xuyi station first detected a moving object. More than a month later, on February 22, ATLAS in South Africa reported cometary characteristics. After confirmation, the Minor Planet Center formally announced it on March 1 as Tsuchinshan-ATLAS.
Purple Mountain saw it first and ATLAS confirmed it later. That is why "Tsuchinshan" comes first in the name.
This was the comet visible in the autumn of 2024. Xinhua reported that it reached perihelion on September 28, 2024, and came closest to Earth on October 12. For the following week or two, people across China saw it with the naked eye low in the southwest after sunset.
In January 2025, the same telescope at Xuyi discovered C/2025 A3 (Tsuchinshan). The Chinese Academy of Sciences records it as the ninth comet discovered by Purple Mountain Observatory, the seventh to bear the Tsuchinshan name, and the observatory's first comet on a centaur-like orbit.
From the 40-centimeter refractor installed in 1964 to a survey telescope that automatically marks moving objects for human review today: 60 years on the same mountain.
Now, the clouds.
This happened only three months ago. A report published by Heilongjiang's provincial culture and tourism department on June 18, 2026, says that on the night of June 15, residents and visitors in Mohe, in the Greater Khingan Range, saw noctilucent clouds. It listed three necessary conditions - low temperature, water vapor and dust - and explained that Mohe lies near 53 degrees north. Around the summer solstice, short intervals between evening and morning twilight create an excellent window for the clouds to appear.
At 53 degrees north, Mohe sits just inside the usual 50-degree boundary.
Mohe is a Chinese location with an officially recorded noctilucent-cloud sighting. Northern Xinjiang, around Altay, may also be suitable by latitude, but no confirmed observation was found, so Mohe is the only place named here.
Finally
The video contains three things, and each asks you to notice time.
The comet will not return for 6,700 years; no one who saw it will see it again. The cloud is lit by sunlight that has already set, and 3% of every ice crystal is ash left by a meteor that burned away. The person pressing the shutter is a young scientist who measures muons in a laboratory by day and carries an entry-level DSLR to the coast at night.
Image: NASA Astronomy Picture of the Day (APOD) | Image credit and copyright: Paolo Girotti | Daily page (This is a video entry, so no image is embedded in the article.)
Sources: NASA's APOD page for the day and the APOD website at science.nasa.gov/apod; NASA JPL, "Comet NEOWISE Sizzles as It Slides by the Sun"; NASA's Hubble and NEOWISE mission pages; NASA Earth Observatory reporting on noctilucent clouds; NASA AIM mission material and "Meteor Smoke Makes Strange Clouds"; NASA Night Sky Network; NASA Space Cloud Watch citizen-science project; MEDIA INAF's interview with Paolo Girotti; U.S. Naval Research Laboratory records on P78-1/SOLWIND and The Planetary Society, "The Pivotal Discovery"; the Chinese Academy of Sciences; Guangming Online; Xinhua; China News Service; the official website of Nanjing's Yuhuatai Martyrs Memorial Park; the Heilongjiang provincial culture and tourism department; and the British Astronomical Association, Comet Prospects for 2026.