Why Can We See Stars Through Telescopes but Not Satellites Like the Hubble or ISS?

Many people wonder why it's possible to see stars through telescopes but not from space-based observatories such as the Hubble Space Telescope or the International Space Station (ISS). The answer lies in the conditions of light, orbit, and observation techniques. Let's explore the reasons behind this fascination.

Stars in Perspective: The Role of Light

Nature emphasizes the contrast between light and darkness, with stars appearing at night when the sky is free from the glare of the sun. However, this doesn't mean stars aren't visible from space on a regular basis. The key factor is the availability of light. As Scott Kelly, an astronaut who spent a year on the ISS, explains, the stars are easily seen during the night but are too dim to be observed during the day due to the overwhelming brightness of sunlight.
Here’s a simplified analogy: Just as the night sky is filled with stars, you might not notice them if you’re standing in a brightly lit room. Similarly, during the day, the stars are overwhelmed by the intense light of the sun.

Daytime on the ISS: An Ongoing Sunlight Dazzle

The ISS orbits the Earth in about 90 minutes, and it spends half of each orbit in sunlight and the other half (called night) in darkness. However, the day on the ISS mirrors what we experience on Earth. The sunlight makes the stars too faint to be seen from inside the ISS during the day. This is why, although the sky outside the ISS is much darker, astronauts do not see stars when the Sun is shining.

One common misconception stems from the idea that any time the sky is black, it’s prime star-watching time from the ISS. However, as asteroids have shown in moon landing conspiracy theories, the absence of stars in images taken by the Apollo missions does not negate their authenticity. These missions did not occur at night on the moon; the images were taken when the moon was not illuminated by the sun, not when the crew was in the dark.
Moreover, the Cupola module of the ISS, which is the best viewing point, always points towards Earth. Even during nighttime, all that can be seen are city lights, making the sky look clear of stars.

Astronauts' Experience and Observations

During Extra Vehicular Activities (EVA), astronauts often focus on city lights and atmospheric phenomena like lightning storms and auroras, rather than the stars. This is because the harsh sunlight during the daytime makes it difficult to see and photograph faint stars. Even at night, the challenge in observing stars for distant viewing is significant because astronaut activities are scheduled around ensuring safety and efficiency. As Sun Wang, an astronaut specializing in EVA, explained, visibility of stars is often compromised due to the challenges of performing tasks in the dark while maintaining visibility and safety.

Light-Dimming Conditions: Stars in Clear Skies

When the ISS is in the "night" portion of its orbit, where the Sun is blocked by the Earth, the stars become much more visible. NASA has documented instances where astronauts have captured stunning images of star-filled skies from the ISS, showing a night sky teeming with stars. The key factor here is the lack of sunlight, which allows the stars to be visible and photographed.

Cameras on Earth and on the ISS, like human eyes, have mechanisms to adapt to light levels. During periods of bright light, the pupils of the eyes constrict, and the camera iris closes to protect the sensitive retinas and film from overexposure. These adaptive mechanisms mean that even a single bright light, such as a streetlight, can significantly impact the visibility of fainter stars. In space, the only real interference to star visibility is sunlight, which is almost always present except during the dark parts of the ISS's orbit.

Conclusion: Stars in the Cosmos

In summary, the inability to see stars from the ISS during daylight is a consequence of the intensity of sunlight and the challenges of maintaining proper visibility for spacewalks. However, thanks to careful alignment, inflatable modules, and critical observation windows, astronauts are indeed able to see and photograph the stars during their orbits in the dark. This phenomenon highlights the delicate balance between light and darkness in the vast cosmos.

Understanding these conditions enhances our appreciation of the vastness of the universe and the remarkable achievements of space exploration. Whether through telescopes on Earth or observatories like the Hubble, the beauty of the starry night sky continues to inspire and amaze us.