Whether astronauts can see stars in space is one of the most common queries. The answer is a nuanced explanation involving atmospheric physics, human vision, and the technical limitations of photography. While stars are always present, the conditions of the astronaut’s environment often dictate their visibility.
The Simple Answer: They See Stars Clearly
Astronauts can indeed see stars, and they see them with a clarity and brilliance impossible to experience from Earth. This enhanced view is due to the complete absence of an atmosphere to interfere with starlight. On Earth, the atmosphere scatters light, causing stars to twinkle and dim.
Outside of Earth’s atmosphere, starlight travels directly to the eye without being scattered or refracted. This results in the stars appearing as sharp, unwavering pinpoints of light against a deep, inky black sky.
The lack of atmospheric scattering also means the sky is fundamentally dark, even when the Sun is shining. On Earth, the atmosphere scatters sunlight, creating the bright blue sky that washes out fainter stars during the day. In orbit, the stars are always present, but their visibility is conditional on competing light sources.
The Brightness Problem in Orbit
Although stars are clearer from space, they are often not visible when the spacecraft is on the Sun-facing side of its orbit. The intense, unfiltered sunlight overwhelms the eye’s ability to perceive dimmer objects. The human eye automatically adjusts its sensitivity to the brightest light source, which is often the Sun-illuminated Earth or the spacecraft structure itself.
The International Space Station (ISS) orbits Earth approximately every 90 minutes, cycling rapidly between day and night. During the orbital “day,” direct sunlight is extremely powerful and causes the astronaut’s eyes to constrict, making the faint light of distant stars impossible to detect. Even when the Sun is blocked, light reflected off the Earth, known as Earthshine, keeps the eyes adjusted to a lower sensitivity.
Stars only become truly visible to the unaided eye during the orbital “night,” when the ISS is shadowed by the Earth. At this time, the ambient light drops dramatically, allowing the astronauts’ eyes to adapt to the dark. This dark-adapted vision is then sensitive enough to perceive the vast field of brilliant stars.
Why Cameras Exclude the Stars
The common misconception that stars are invisible from space stems primarily from the vast majority of iconic photographs taken from orbit, which show a starless black background. This visual paradox is a limitation of camera technology compared to the dynamic range of the human eye. Cameras operate on the principle of exposure, balancing aperture, ISO, and shutter speed.
To capture a bright object like the Earth, a spacecraft, or an astronaut on a spacewalk, the camera must be set to a low exposure with a fast shutter speed. If the shutter were left open for a longer duration, the bright object would become completely overexposed, appearing as a featureless white blob. This quick exposure is necessary to capture detail on the brightly lit primary subject.
Conversely, stars are extremely faint light sources that require a long exposure to register on a camera sensor. Successful astrophotography from Earth typically uses exposures ranging from 15 to 30 seconds or more to gather enough photons from the stars. When a camera in space is set to a fast shutter speed to properly expose a brilliantly lit Earth, the exposure time is far too short to collect any light from the stars, which consequently do not appear in the resulting photograph.
The human eye possesses a much wider dynamic range and can adjust its sensitivity almost instantly, allowing it to see both the bright Earth and the faint stars. A camera, however, must be set to a single, fixed exposure for the entire frame, forcing the photographer to choose between capturing the bright foreground object or the dim background stars. Photographs that do show stars from the ISS are taken specifically using long-exposure settings when the Earth is blocked or dark.