Understanding Airglow: A Q&A with NASA's Space Station View

Welcome to an illuminating exploration of Earth's atmospheric glow, captured from a unique vantage point. On April 13, 2026, NASA astronaut Chris Williams photographed the Milky Way rising above our planet's natural luminescence while aboard a SpaceX Dragon capsule docked to the International Space Station. This phenomenon, known as airglow, offers a stunning display similar to auroras but driven by different mechanisms. Below, we dive into the details with a series of questions and answers that unpack the science behind this celestial sight.

What did NASA astronaut Chris Williams capture from the ISS on April 13, 2026?

On that date, Chris Williams took a photograph showing the Milky Way galaxy rising above Earth's atmospheric glow. He was inside a SpaceX Dragon spacecraft docked to the International Space Station. The image highlights a natural phenomenon called airglow, which appears as a soft, ethereal band of light hugging the planet's limb. This photo serves as a vivid reminder of the beauty visible from orbit and the ongoing scientific observation of our atmosphere.

What is airglow and how does it occur?

Airglow is a faint emission of light from the Earth's upper atmosphere. It happens when atoms and molecules there become excited by sunlight during the day and later release that excess energy by emitting photons (particles of light) at night. Alternatively, airglow can result when atoms or molecules that have been ionized by sunlight collide with and capture a free electron, also releasing a photon. The process is a form of chemiluminescence, where chemical reactions produce light without high temperatures. This glow is constantly present, though usually too dim for human eyes to see from the ground; from space, however, it becomes clearly visible as a colorful band.

How is airglow different from auroras?

While both airglow and auroras produce light in Earth's atmosphere, their energy sources differ. Auroras are driven by high-energy particles from the solar wind that collide with atoms and molecules, causing them to emit light. In contrast, airglow is energized by ordinary, day-to-day solar radiation—sunlight itself—without the need for solar wind particles. Additionally, auroras are typically concentrated near the polar regions, while airglow occurs globally in the upper atmosphere (around 80–300 km altitude). Both phenomena involve excited atoms releasing photons, but the root cause distinguishes them.

Where was Chris Williams when he took the photo?

Chris Williams was aboard a SpaceX Dragon capsule that was docked to the International Space Station (ISS). The ISS orbits Earth at an altitude of about 400 kilometers (250 miles), providing an unparalleled view of both the planet's surface and its atmospheric phenomena. From this vantage point, the Milky Way appears as a bright, sweeping band across the sky, and the airglow becomes a visible crescent along Earth's horizon.

Why does Earth's atmosphere glow at night?

Earth's atmosphere glows at night primarily due to airglow. During the day, sunlight excites atoms and molecules in the upper atmosphere, giving them extra energy. As night falls, these excited particles release that energy by emitting light—a process called relaxation. The emitted photons produce a faint, steady glow. Additionally, recombination of ions with free electrons also contributes. Unlike sunlight reflected off clouds or the moon, airglow originates from the atmosphere itself and is always present, though its intensity varies with solar activity and altitude.

What causes the emission of photons in airglow?

Photons in airglow are emitted through two main processes. First, atoms or molecules in the upper atmosphere absorb energy from sunlight, becoming excited. When they return to a lower energy state, they release that energy as a photon—a particle of light. Second, if sunlight ionizes an atom or molecule (strips away an electron), that ion can later collide with a free electron. When the ion captures the electron, it releases the excess energy as a photon. In both cases, the emission occurs as the particle “relaxes” to a more stable configuration. The specific wavelengths (colors) depend on the type of atom or molecule involved, such as oxygen (green or red) or sodium (yellow).

How is airglow similar to auroras?

Both airglow and auroras involve the emission of light from atoms and molecules in Earth's upper atmosphere. In each phenomenon, excited particles release photons as they return to a lower energy state. The resulting colors—often green, red, or yellow—can appear similar, and both can be seen from space as glowing bands near the horizon. However, the key similarity lies in the fundamental process of photon emission due to energy relaxation. Despite different energy sources, the basic physics of light production is the same.