By Yiwen Zhu

Original article:

The proliferation of space objects is a rapidly increasing source of artificial night sky brightness

M. Kocifaj, F. Kundracik, J. C. Barentine, S. Bara

If you look up into the sky at night, what do you see? The Milky Way or a few stars? “If you are among the 80 percent of Americans living near urban areas, the answer is probably: not much” (Johnson 2019).

But why can’t we see the Milky Way from the city? It is said that light pollution has blocked our view of the night sky. “The streetlights, security lights, and lit-up buildings we use to illuminate urban areas don’t just shine on our sidewalks and streets; they also shine up into the sky.” (Johnson 2019). While we appreciate the convenience brought by the bright lights at night, we also should not ignore the hidden threats. As Chris Elvidge, a physical scientist said, “as light hits molecules and particles in the atmosphere, it gets scattered. It’s light coming from the sky down toward us that blocks our ability to see the stars” (Dennis 2016).

Beyond streetlights and other sources of light pollution in the city, however, shiny satellites and other space junk orbiting Earth may also pose a threat to our viewing of the universe, found by a new study. The study, conducted by M. Kocifaj, F. Kundracik, J. C. Barentine, and S. Bara, examines the potential contribution of space objects to night sky brightness. They concluded that increased night sky brightness caused by sunlight reflected and scattered by large space objects influences the view of a naked eye or with low-resolution equipment. Also, artificial objects orbiting the Earth increases 10 percent of the brightness of a typical natural night sky.

Behind the striking results, what motivates the researchers to explore the new light pollution source is even more concerning and urgent. The ongoing deployment of satellites and space junk (parts of launch vehicles, unwanted satellites, and small particles) will not only hinder the observation from the ground but also increase the night sky's brightness. Even worse, as they predicted, in the next decade, the launches of fleets of new, large satellites are expected to significantly increase this light pollution source. Moreover, the researchers aim to fill the research gap, a problem that other researchers haven’t solved or realized. While other research focuses on the impact of a single space object and how it affects astronomer’s view, they explore the overall impact of space objects and how they make the night sky brighter.

To examine the issue, the researchers build equations based on theoretical assumptions and compute to what extent space objects will make the night sky brighter. The work explains two main factors that researchers pay attention to how space objects scatter light and reflect light. Since the shapes and directions of space objects are so different from each other, researchers find a more reliable way to estimate how space objects scatter light. They use the cross-section, the view when you cut through the middle of it, of the object to estimate, instead of how the object looks like. Similarly, the ability to reflect light, which means throw back light from a surface, is also very different in space objects. They have different shapes, directions, and are made up of different materials. These factors all influence how they reflect light and make it hard to measure. Therefore, researchers use the average value to estimate.

Kocifaj’s team finds that space junk and satellites, as the new source of light pollution, can increase the brightness of the night sky by 10 percent of the brightness of a typical natural night sky. Even worse, this value exceeds “the IAU’s limiting light pollution ‘red line’ for astronomical observatory sites” (M. Kocifaj 2021). They also highlight that the brightness of the night sky is so high that the human eye or less professional telescopes can’t identify stars. This will also increase the information loss in astronomy.

This is a global problem. As more and more people have never seen a true night sky, we don’t even realize what we are missing and the threats we are facing. More large space objects increasingly reflect and scatter sunlight, which increases the brightness of the night sky and blocks the view of astronomical observations. As the researchers suggest, we should be aware and solve the even worse light pollution problem caused by more space objects launched in the future.

References

Dennis, Brady. 2016. "Thanks to light pollution, the Milky Way is now hidden from a third of humanity ." The Washington Post. 6 10. Accessed 7 22, 2021. https://www.washingtonpost.com/news/energyenvironment/wp/2016/06/10/light-pollution-keeps-much-of-humanityfrom-seeing-the-night-sky/.

Johnson, Kelsey. 2019. "Is the Evening Sky Doomed?" The New York Times. 8 17. Accessed 7 22, 2021.
https://www.nytimes.com/2019/08/17/opinion/sunday/lightpollution.html.

M. Kocifaj, F. Kundracik, J. C. Barentine, S. Bara,. 2021. "The proliferation of space objects is a rapidly increasing source of artificial night sky brightness." Monthly Notices of the Royal Astronomical Society, 3 29: 40 44.