Twinkle, Twinkle No More… Will LEO Satellites Block Our View Of The Stars?

Low-Earth Orbit (LEO) communication constellations are rapidly becoming a reality over Africa. Given that the initial launches of the Starlink service were clearly visible in the night sky, this led to valid concerns around the possible impact of LEO constellations on the natural beauty of our night skies. It is truly a distressing thought that successful, large-scale broadband deployments in Africa could rob us of the majesty and wonder of our night skies, whether we’re on a mountaintop in the Drakensberg or on a nocturnal safari in the Okavango Delta.

Introduction

There are currently 36 active global LEO constellation projects, with the top 10 projects jointly planning to launch some 63,418 satellites. These projects will undoubtedly bring major improvements in communications for all regions of Africa, both rural and urban. In an African communications perspective, the most relevant projects are Starlink from SpaceX, Kuiper from Amazon, OneWeb, Telesat’s Lightspeed and the mPower projects from SES, although all 36 global LEO constellations have the potential to impact how we see our night skies.

While amateur stargazers and safari guides naturally worry about reflected light from satellites, the real concern is the effect on astronomers’ observations and our ability to research deep space using terrestrial observatories. For ground-based telescopes, even the smallest degree of reflection can saturate images and compromise research.

To understand the impact that these LEO constellations could have on African night skies as we know and see them, we provide some Q&As on the most pressing points. The information given accurately reflects our current understanding, but the situation may well change as this emerging industry continues to evolve.

Artist’s impression of an in-orbit Starlink satellite in so-called ’shark-fin’ mode, with the solar array pointed away from earth.

1. Why do satellites appear to shine?

The sun’s rays are reflected from the solar arrays of LEO satellites. These structures are typically up to 12m x 3.2m in size, and are designed to reflect the sun’s rays in order to reduce heat, while the body units of satellites often also reflect light.  

The degree to which the sun’s light is reflected very much depends upon the orientation of the satellite relative to the sun, and this is different for orbital raising periods and in-orbit locations. During orbit raising cycles the satellites are placed in ’low drag’ mode with the solar panels orientated horizontally. This presents large surfaces capable of reflecting sunrays. In contrast, satellites that are in orbital slots will be in ’shark-fin’ configuration, with the solar panels pointing away from Earth, thereby minimising reflection.

2. When do satellites reflect the sun’s rays?

LEO satellites are in orbit at between 200km to 500km above Earth, so the reflections occur during twilight hours when the satellites are in full view of the sun, while on Earth it is still dark. The lower the altitude of the satellites above Earth, the brighter the reflections, and the shorter the duration of their visibility from Earth. Conversely, satellites in higher orbits reflect for longer periods of time, but at lower intensity. For example, Starlink satellites that are in orbit at an altitude of 500km will typically reflect for 1 -2 hours per day, depending on your horizon profile at your point of observation.

Now that we understand when and how LEO satellites shine in the sky, we can move onto the more interesting question of what we can we do to mitigate or eliminate this occurrence.  

3. Can satellite orientation be changed to stop reflections?

Yes, the reflections are very much dependent on the orientation of the satellite to the sun and yes, this orientation can be controlled. The most visible ’string of pearls’ effect of the Starlink antennas occurs immediately after they have been released from the second stage of the rocket and while the solar panels are in a low drag mode and horizontal to the earth. This phase lasts for about 2 – 3 weeks until final orbital position is reached, and the satellite changes its configuration and orientation to what is referred to as a ‘shark fin’ or ’knife-edge’. This greatly reduces reflection. Reflections in the low-drag mode during initial deployment can be mitigated by changing the orientation during the twilight hours when the satellite is at its most reflective.

4. Can we change satellite designs to stop the reflection?

Reflections from smooth polished surfaces are much brighter than those from rough, uneven surfaces. However, most current LEO satellite designs focus on making their surfaces highly heat-reflective so as to passively cool the satellite unit without the need for additional cooling. Of course, surfaces that are effective at reflecting heat are also excellent at reflecting light rays.

In Jan 2020 SpaceX launched an experimental DarkSat to test the effectiveness of a special coating designed to reduce the reflectiveness of the antenna. This test showed that it was possible to greatly reduce the visibility of a Starlink satellite and solve the problem for stargazers. Unfortunately, it also made the satellite operate at a higher temperature, which impacted its performance and increased its infrared spectrum radiation. As a result, the experiment DarkSat cannot be considered a viable solution as the additional infrared radiation that was generated negatively impacted the astronomers’ observations, and satellite operation was compromised due to the higher operating temperatures.

5. What about solar screens?

More correctly known as foam visors, these are used in many applications to block light and reduce surface reflections. In the case of LEO satellites, any foam visors would also have to be exceptionally lightweight, as well as being radio transparent so as not to affect the performance of the antenna systems.  

After in-orbit testing of various foam visor designs, SpaceX decided not to continue with this option due to the negative impact that the visors had on infrared inter-satellite communication and the additional drag visors caused.

6. Are there any other possible solutions?

Space-based telescopes give scientists on Earth a view of space that’s unaffected by any form of terrestrial light pollution. The two most famous examples are the Hubble Space Telescope (launched in 1990 and still in operation) and the more modern James Webb Space Telescope or NGST, launched in 2021. The Hubble Space Telescope is at an altitude of 535km above Earth, while the NGST is much further away, in a solar orbit approximately 1.5 million km from our planet.

This means that both these instruments are beyond the orbit of LEO satellites, and therefore immune from impacts caused by reflections from these satellites.

Conclusions

While this discussion is far from over, and this topic is too wide-ranging to cover in this one article, we can in summary note that perceived risks to how African night skies are viewed are being taken very seriously by SpaceX and other LEO satellite constellation operators. The latest satellites to be launched are less visible and represent a significant improvement on the previous generation of LEO satellites.

Also, and most importantly, we should note that the threat to the visibility of astral bodies in African skies is due to reflected sunrays, and is not in any way caused by the principal function of the satellites, i.e., wave radiation transmission. This is good news, because with further engineering and development, a combination of advances in material coatings and satellite orientation control will indeed make LEO constellations invisible from Earth, without impairing their essential functions or making them more expensive.

Article supplied by:

Dr Dawie de Wet (Pr. Eng. M.Sc. Eng.) – Group CEO of Q-KON and Chief Engineer for Twoobii, a southern African supported satellite broadband service. With over 30 years’ experience in designing, engineering, developing and implementing wireless, microwave and satellite communication systems in Africa, Dawie continues to focus on developing Telco solutions that meet the user requirements of emerging markets through developing and deploying world-class technology.