[...] large sample of over 25,000 satellites from representative satellite constellations from different companies. With this sample, the number of satellites above the horizon at any given time would be between ~1500 and a few thousand, depending on the latitude. Most of these will appear very close to the horizon, only a few of them passing directly overhead; for instance, about 250 to 300 would have an elevation of more than 30 degrees (Note: i.e. 10-15%) over the horizon (i.e. where the sky is clear from obstructions, and where most of the astronomical observations are performed). [...] When the Sun is 18 degrees below the horizon (i.e. when the night becomes dark), the number of illuminated satellites above the horizon would be around 1000 (with around 160 at elevations higher than 30 degrees). The numbers decrease further towards the middle of the night, when more satellites are in the Earth's shadow [...]In theory, the effects of the new satellites could be mitigated by accurately predicting their orbits and interrupting observations, when necessary, during their passage. Data processing could then be used to further “clean” the resulting images. However, the large number of trails could create significant and complicated overheads to the scheduling and operation of astronomical observations.[...]The IAU, AAS, and National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NSF’s OIR Lab) are planning a workshop of stakeholders in April 2020 to help solve the issue of the brightness of the satellite mega-constellations. SpaceX and OneWeb have expressed an interest in attending.[...]The IAU was peripherally involved in a special session on Challenges to Astronomy from Satellites at the January 2020 AAS conference. The various astronomers and a SpaceX representative presented to an audience of astronomers with the aim of educating both groups on the status of the issues and the efforts for mitigation. Through the efforts of the AAS, constructive conversations with SpaceX have taken place. The IAU has been involved. The exchange has been educational and illuminating for both groups and now conversations have begun with OneWeb.[...]SpaceX may be able to explore adjusting the attitudes of their satellites over the most impacted observatories, especially observatories with wide fields of view for which detectors would be most affected[...]The focus of this Statement has been on the optical wavelengths. This is not to underplay the effect on the radio and submillimetre wavelength ranges, which is still under investigation. The IAU considers the consequences of satellite constellations worrisome. They will have a negative impact on the progress of ground-based astronomy, radio, optical and infrared, and will require diverting human and financial resources from basic research to studying and implementing mitigating measures. A great deal of attention is also being given to the protection of the uncontaminated view of the night sky from dark places, which should be considered a non-renounceable world human heritage.The IAU stresses that technological progress is only made possible by parallel advances in scientific knowledge. Satellites would neither operate nor properly communicate without essential contributions from astronomy and physics. It is in everybody’s interest to preserve and support the progress of fundamental science such as astronomy, celestial mechanics, orbital dynamics and relativity.
IAU update on the professional simulations performed by experts on some of the world's leading astronomical institutions (Vera C. Rubin Observatory, U. Michigan, CAHA, ESO and ESA):<snip>
At the moment it is difficult to predict how many of the illuminated satellites will be visible to the naked eye, because of uncertainties in their actual reflectivity (also since experiments are being carried out by SpaceX to reduce the reflectivity of a Starlink satellite by adopting different coatings). The appearance of the pristine night sky, particularly when observed from dark sites, will nevertheless be altered, because the new satellites could be significantly brighter than existing orbiting man-made objects. The interference with the uncontaminated view of the night sky will be particularly important in the regions of the sky close to the horizon and less evident at high elevation.
Quote from: eeergo on 02/13/2020 09:30 amIAU update on the professional simulations performed by experts on some of the world's leading astronomical institutions (Vera C. Rubin Observatory, U. Michigan, CAHA, ESO and ESA):<snip>Thanks for posting the update here. This sounds backwards:QuoteAt the moment it is difficult to predict how many of the illuminated satellites will be visible to the naked eye, because of uncertainties in their actual reflectivity (also since experiments are being carried out by SpaceX to reduce the reflectivity of a Starlink satellite by adopting different coatings). The appearance of the pristine night sky, particularly when observed from dark sites, will nevertheless be altered, because the new satellites could be significantly brighter than existing orbiting man-made objects. The interference with the uncontaminated view of the night sky will be particularly important in the regions of the sky close to the horizon and less evident at high elevation.Satellites low to the horizon are much further away, and brightness decreases with the square of distance - those at 30 degrees should be ~4x fainter, and those at 10 degrees ~30x fainter, than those at zenith overhead. Also, those near the horizon are more likely to be blocked by local obstructions, as not everyone has a clear view down to 0 degrees elevation. If the satellites overhead are marginally visible in dark areas, those below 30 degrees should be much less visible or not at all, and thus have lower impact, at least to naked-eye viewing, despite the larger numbers
Is Darksat really darker?On a video taken Saturday at astronomical twilight, it's one of the...brightest Starlinks. At final altitude (550 km) they reach mag 2.5!
Quote from: Rondaz on 02/23/2020 09:25 pmIs Darksat really darker?On a video taken Saturday at astronomical twilight, it's one of the...brightest Starlinks. At final altitude (550 km) they reach mag 2.5!It's not at final altitude, so this observation is premature.
QuoteI don't know to what depth SpaceX has studied the coating but so far there is zero evidence of it having any effect.Some people observed it before it reached operational configuration and noticed that it wasn't darker. You find this to be evidence of... what, exactly? SpaceX said that it probably wouldn't be darker until it reached operational configuration.
I don't know to what depth SpaceX has studied the coating but so far there is zero evidence of it having any effect.
Do you have a link to observations of Starlink 1130?
Quote from: gongora on 02/23/2020 09:40 pmQuote from: Rondaz on 02/23/2020 09:25 pmIs Darksat really darker?On a video taken Saturday at astronomical twilight, it's one of the...brightest Starlinks. At final altitude (550 km) they reach mag 2.5!It's not at final altitude, so this observation is premature.Actually it seems that it did reach the operational altitude now.
The 5 darker satellites in the picture are all at 550+/-1 km. The 4 brighter ones are all at ~555 km. IMO it's pretty obvious that the latter are still phasing and/or precessing to final slots and are not in the low-brightness operational configuration.This is supported by the fact that the 4 brighter ones are still mag ~2.5, which we know is much brighter than the typical to date in operational configuration and inline with the brightness of orbit-raising configuration. Bottom line, this observation tells us nothing we didn't know already. And framing the tweet to imply that SpaceX is either lying or incompetent is, uhhm, highly unprofessional.
Quote from: envy887 on 02/24/2020 12:24 pmThe 5 darker satellites in the picture are all at 550+/-1 km. The 4 brighter ones are all at ~555 km. IMO it's pretty obvious that the latter are still phasing and/or precessing to final slots and are not in the low-brightness operational configuration.This is supported by the fact that the 4 brighter ones are still mag ~2.5, which we know is much brighter than the typical to date in operational configuration and inline with the brightness of orbit-raising configuration. Bottom line, this observation tells us nothing we didn't know already. And framing the tweet to imply that SpaceX is either lying or incompetent is, uhhm, highly unprofessional.Point is: the "DarkSat coating" has done nothing to distinguish it from the others, neither at deployment/thrusting altitudes nor at operational altitude - and some mechanism actually makes it brighter than some of its siblings (it is reasonable to believe not the coating, or it would be a really bad job as far as experimental albedo-reducing measures go). This also means an at best 2-month period of very high-brightness is unavoidable with the current model of hundreds of satellites already in space, or waiting for an imminent launch, and the single feeble attempt at reducing their visibility has no effect on this issue. If this high visibility is mostly due to the solar array, it also means any mitigations will come from attitude tricks, not from "coatings".In a few days we should know for sure whether DarkSat's approach has any effect on the operational attitude - but so far the Starlink visibility issues, acknowledged by SpaceX as such, provide a grim track record on this respect. I fail to see why a "framing" that accurately represents reality would be "highly unprofessional", especially considering SpaceX isn't mentioned or referred to as an entity, launches do continue unaltered by any of these concerns, and exactly 1 albedo-reducing technique has been tried out out of 300 opportunities, with the absence of results described above.
SpaceX said the underside coating wouldn't do anything until it's in the operational attitude because brightness during orbit-raising is primarily from the uncoated solar array). It's obvious from the picture and orbital data that 1130 is not yet in the operational attitude, so saying "the coating hasn't done anything" is completely pointless at the moment. It's basically baskseat "are we there yet?" whining.If anyone wants to complain about brightness during orbit-raising, then fine - do that. But that's another topic, and conflating the two is misleading at best.
Quote from: edzieba on 01/31/2020 04:06 pmQuoteI don't know to what depth SpaceX has studied the coating but so far there is zero evidence of it having any effect.Some people observed it before it reached operational configuration and noticed that it wasn't darker. You find this to be evidence of... what, exactly? SpaceX said that it probably wouldn't be darker until it reached operational configuration. Quote from: edzieba on 01/31/2020 04:06 pmDo you have a link to observations of Starlink 1130? "DarkSat" aka Starlink 1130 now at operational altitude at 550 km - and brighter than some other comparable (already bright: mag 2.5 (!) ) Starlinks. So one less unknown: operational altitude doesn't significantly ameliorate visibility concerns.
Quote from: envy887 on 02/24/2020 01:58 pmSpaceX said the underside coating wouldn't do anything until it's in the operational attitude because brightness during orbit-raising is primarily from the uncoated solar array). It's obvious from the picture and orbital data that 1130 is not yet in the operational attitude, so saying "the coating hasn't done anything" is completely pointless at the moment. It's basically baskseat "are we there yet?" whining.If anyone wants to complain about brightness during orbit-raising, then fine - do that. But that's another topic, and conflating the two is misleading at best.One would expect *some* effect to be noticeable even in the non-optimized attitude - remember SpaceX stated they aren't planning on implementing any special attitude profile with DarkSat. Most of the brightness might be coming from the array during commissioning, but the difference in *steady* brightness between the commissioned satellites and those still under commissioning is not that large. In fact, very roughly eyeballing it you could say there is as much variability between 1084 and 1130 as between 1121 and 1073 in the second image.In any case, the point here is that this rules out that coating works on the most reflective parts of the satellite during ascent/commissioning (which was doubtful for some until now), effectively meaning there are no visibility abatement measures for that longish period yet. It also further confirms the array necessitates from attitude changes to become less visible and could not somehow be "coated". Finally, it also puts to rest theories about major reductions in magnitude brought about by the operational altitude. I'd say that's some good applicable data, while we wait for the final verdict once the nominal attitude is reached.Probably further discussion should be moved to the relevant astronomy-vs-megaconstellations thread.
So one less unknown: operational altitude doesn't significantly ameliorate visibility concerns. Only remaining factor is attitude. Of course, what an experimental satellite might be able to achieve wrt attitude might not be viable for an operational one, so future results will only be "best case" (less concern about impacting ground connectivity, no inter-satellite connections present, much less with fine tolerances as with the case of optical comms...)
Did the darkening of the #Starlink DarkSat work after all? The report https://www.reddit.com/r/Starlink/comments/f9704t/measurement_of_the_brightness_of_darksat_in_its/ from yesterday - *after* its attitude went operational - has it 1 to 1.5 mag. fainter than the others, at only 5.7 mag. More observations across the sky needed! h/t @sinus_iridium
According to the chief space situational awareness expert of the AAS *now* is the time to do the crucial photometry of that #Starlink 'DarkSat': "Starlinks were at altitude but not at operational orientation yet when those images were taken - should be there now. Try now."
So, if confirmed, a significant decrease in brightness and probably enough to knock most of the Starlinks (with that coating) below naked-eye visibility when at 550km. It wouldn't make much difference to the effect on astronomical observations.The surface treatment (presumably to some part of the Earth-facing base of the Starlink) does not change the Starlink brightness when in the low-drag configuration.The reduction in magnitude I saw suggests that only a portion of the earth-facing base of Darksat has been coated in anything vaguely black. Changing the coating of the Ku and Ka-band antenna panels on that face is probably a bigger deal and will take longer to re-qualify. If the whole of that panel could be brought down to a few percent reflectivity another 2 optical magnitudes might be knocked off.
