On October 13, 2026, thousands of people across the US East Coast woke to a glowing jellyfish drifting across the pre-dawn sky. It was not a UFO, a missile, or a strange weather event — it was the exhaust plume of a SpaceX Falcon 9 launching the CRS-35 cargo mission to the International Space Station, lit by the rising sun. The phenomenon, sometimes called a space jellyfish, is one of the most striking sights in modern spaceflight. Here is the science behind why it happens and why it glows.

The first reason is timing. The jellyfish appears only when a rocket launches near sunrise or sunset. At that moment the ground is still in darkness, but the rocket has climbed high enough to catch direct sunlight. The exhaust plume, tens of miles above the surface, becomes brightly illuminated while everything below remains dark — a contrast that makes the glowing cloud look enormous and otherworldly against the twilight sky. Launches in full daylight are too bright for the effect to stand out, and deep-night launches leave the plume unlit.

The second reason is what the plume is made of. Rocket exhaust is mostly water vapor and carbon dioxide, and at high altitudes the frigid mesosphere — roughly thirty to sixty miles up — freezes that water vapor into tiny ice crystals. These crystals scatter sunlight, which is why the cloud glows white, silver, and blue, sometimes shifting toward orange where the light passes through more atmosphere. Technically, what you are watching is a man-made noctilucent cloud, the same family of glowing high-altitude clouds that sometimes appear naturally in summer twilight.

The third reason is shape. In the near-vacuum of the upper atmosphere, there is almost no air pressure to contain the exhaust, so it expands dramatically into a huge rounded bulb — the jellyfish's body. Behind it, the rocket's continuing climb leaves thin, trailing filaments of exhaust and ice that become the tentacles. Upper-atmosphere winds then stretch and shear these trails, which is why the crisp jellyfish gradually softens into a wispy streak over several minutes. Enthusiasts who study these displays can compare them with the slow-motion footage of a Starship launch to see plume physics in fine detail.

So why does the jellyfish not appear at every launch? All three ingredients have to line up: the sun must sit just below the horizon for ground observers, the sky must be clear, and the trajectory has to carry the plume into the sunlit zone. Launches heading northeast from Florida, like CRS-35, put the show on display for the most populated stretch of coastline. A launch at noon, or one flying away from populated areas, produces no jellyfish at all — even though the physics is identical.

The good news for skywatchers is that the ingredients line up regularly. With Falcon 9 flying frequently and twilight windows recurring, the eastern US sees several of these displays a year, and photographers now treat them as a seasonal tradition. If you want to catch the next one, track dawn and dusk launches and be outside with a clear eastern view — and read about the Starship single-engine test fire to follow how SpaceX keeps pushing flight test boundaries.