updates
Starlink brightness and the astronomy talks
On April 28, 2020, SpaceX described sun visors, a knife-edge climb attitude, and a goal of about magnitude 7 so satellites would not stand out to the eye or saturate detectors.
Topics: space-civilization
This page is an original Muskularity summary. It does not republish SpaceX’s full post. Figures are the company’s statements on the cited date unless another primary link is named.
Summary
SpaceX said it was working with astronomers to make Starlink less bright while it built a low-latency broadband constellation. Two goals are stated. First, make the satellites generally invisible to the naked eye within a week of launch, by flying knife-edge to the Sun during the climb, a software change. Second, keep them from saturating observatory detectors, using a deployable visor that shades the brightest surfaces. The first visor was to fly on the next launch, and from flight 9 in June every later satellite would have one. Orbits were to be published on space-track.org.
The orbit story has three phases: raising, a parking orbit at 380 kilometers, and station at 550 kilometers. Of more than 400 satellites then up, about half were on station. On station the array goes vertical, which SpaceX called shark-fin, because drag at 550 kilometers still matters. Brightness: the company said a magnitude 3 star is about 2.5 times brighter than magnitude 4, that naked-eye limit is roughly 6.5 to 7 in a dark sky and closer to 4 in many suburbs, and that operational Starlinks then averaged about magnitude 5.5, brighter while raising. The goal was magnitude 7 or fainter for almost the whole mission. An earlier dark coating, DarkSat, was said to have cut brightness about 55 percent in differential measurements. The visor stows for launch and opens after separation. SpaceX also argued that a typical astronomy camera sees less than a degree of sky, so a satellite crossing a given exposure is unlikely. That is the company’s statistical claim.
The post cites astronomer discussions, including an American Astronomical Society note and a National Science Foundation workshop paper, as the conversation it was joining.
Key facts
| Fact | Label |
|---|---|
| Parking orbit 380 km; operational orbit 550 km; more than 400 satellites, about half on station | Company status in April 2020 |
| Average on-station magnitude about 5.5; goal about magnitude 7 | Company brightness figures |
| DarkSat coating described as about 55 percent dimmer | Company measurement claim |
| Visors on every satellite after flight 9 in June 2020 | Company deployment plan |
Why it matters
Brightness is the astronomy half of the Starlink controversy, and this is the company’s 2020 mitigation plan with numbers. A goal of magnitude 7 is not a measurement that every later satellite met. The debris rules come later, in the 2022 sustainability post.
On this site
Sources
- SpaceX update, April 28, 2020 — Grade A, company post
- AAS note linked from the SpaceX post — Grade A, professional society
- Space-Track orbit catalog referenced by SpaceX — Grade A, catalog
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