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APRIL 28, 2020
Astronomy Discussion with National Academy of Sciences
SpaceX is launching Starlink to provide high-speed, low-latency broadband connectivity across the globe, including to locations where internet has traditionally been too expensive, unreliable, or entirely unavailable. We also firmly believe in the importance of a natural night sky for all of us to enjoy, which is why we have been working with leading astronomers around the world to better understand the specifics of their observations and engineering changes we can make to reduce satellite brightness. Our goals include:
Making the satellites generally invisible to the naked eye within a week of launch. Were doing this by changing the way the satellites fly to their operational altitude, so that they fly with the satellite knife-edge to the Sun. We are working on implementing this as soon as possible for all satellites since it is a software change.
Minimizing Starlinks impact on astronomy by darkening satellites so they do not saturate observatory detectors. Were accomplishing this by adding a deployable visor to the satellite to block sunlight from hitting the brightest parts of the spacecraft. The first unit is flying on the next launch, and by flight 9 in June all future Starlink satellites will have sun visors. Additionally, information about our satellites orbits are located on space-track.org to facilitate observation scheduling for astronomers. We are interested in feedback on ways to improve the utility and timeliness of this information.
To better explain the details of brightness mitigation efforts, we need to explain more about how the Starlink satellites work.
Starlink Orbits
Starlink has three phases of flight: (1) orbit raise, (2) parking orbit (380 km above Earth), and (3) on-station (550 km above Earth). During orbit raise the satellites use their thrusters to raise altitude over the course of a few weeks. Some of the satellites go directly to station while others pause in the parking orbit to allow the satellites to precess to a different orbital plane. Once satellites are on-station they reconfigure so the antennas face Earth and the solar array goes vertical so that it can track the Sun to maximize power generation. As a result of this maneuver, the satellites become much darker because the solar array visibility from the ground is greatly reduced.
Currently, about half of the over 400 satellites are on-station and the other half are orbit raising or in the parking orbit. Satellites spend a small fraction of their lives orbit raising or parking and spend the vast majority of their lives on-station. Its important to note that at any given time, only about several hundred satellites will be orbit raising or parking. The rest of the satellites will be in the operational orbit on-station.
Starlink Satellite
The Starlink satellite design was driven by the fact that they fly at a very low altitude compared to other communications satellites. We do this to prioritize space traffic safety and to minimize the latency of the signal between the satellite and the users who are getting internet service from it. Because of the low altitude, drag is a major factor in the design. During orbit raise, the satellites must minimize their cross-sectional area relative to the wind, otherwise drag will cause them to fall out of orbit. High drag is a double-edged sword-it means that flying the satellites is tricky, but it also means that any satellites that are experiencing problems will de-orbit quickly and safely burn up in the atmosphere. This reduces the amount of orbital debris or space junk in orbit.
This low-drag and thrusting flight configuration resembles an open book, where the solar array is laid out flat in front of the vehicle. When Starlink satellites are orbit raising, they roll to a limited extent about the velocity vector for power generation, always keeping the cross sectional area minimized while keeping the antennas facing Earth enough to stay in contact with the ground stations.
When the satellites reach their operational orbit of 550 km, drag is still a factor-so any inoperable satellite will quickly decay-but the attitude control system is able to overcome this drag with the solar array raised above the satellite in a vertical orientation that we call shark-fin. This is the orientation in which the satellite spends the majority of its operational life.
Satellite Visibility
Satellites are visible from the ground at sunrise or sunset. This happens because the satellites are illuminated by the Sun but people or telescopes on the ground are in the dark. These conditions only happen for a fraction of Starlinks 90-minute orbit.
This simple diagram highlights why satellites in orbit raise are so much brighter than the satellites that are on-station. During orbit raise, when the solar array is in open book, sunlight can reflect off of both the solar array and the body of the satellite and hit the ground. Once on-station, only certain parts of the chassis can reflect light to the ground.
Physics of Satellite Brightness
The apparent magnitude of an object is a measure of the brightness of a star or object observed from Earth. It is a reverse logarithmic scale, so higher numbers correspond to dimmer objects. A star of magnitude 3 is approximately 2.5 times brighter than a star of magnitude 4. Based on observations that have been taken by us and by members of the astronomical community, current Starlink satellites have an average apparent magnitude of 5.5 when on-station and brighter during orbit raise. Objects up to about magnitude 6.5-7 are visible to the naked eye (naked-eye visibility is closer to 4 in most suburbs), and our goal is for Starlink satellites to be magnitude 7 or better for almost all phases of their mission.
There are two types of reflections off of Starlin
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