Autor Wątek: [AS]SpaceX Readies First Batch of Starlink Satellites for Wednesday Night Launch  (Przeczytany 1148 razy)

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SpaceX launches fourth batch of Starlink satellites, tweaks satellite design
by Caleb Henry — January 29, 2020 [SN]

SpaceX has launched 242 Starlink satellites, counting two demo spacecraft, but not all will be used for broadband service. Credit: SpaceX webcast

WASHINGTON — SpaceX completed its second Starlink launch of the month Jan. 29, conforming to a target cadence the company set last year to launch two dedicated Starlink missions monthly throughout 2020.

SpaceX’s Falcon 9 rocket lifted off at 9:07 a.m. Eastern from Cape Canaveral Air Force Station, Florida, with another 60 Starlink internet satellites. The rocket’s upper stage deployed the satellites into a 302-kilometer low Earth orbit about an hour after liftoff.

SpaceX said it will test its satellites around that low altitude, where it expects any failures would burn up in Earth’s atmosphere after a few months. After completing checkouts, SpaceX plans to raise the satellites to a 550-kilometer operational orbit.

The launch — SpaceX’s fourth for Starlink not counting two demonstration satellites launched in 2018 — carried an upgraded set of satellites designed for better spectral efficiency and throughput. Poor weather delayed the mission by about a week.

Falcon 9’s first-stage booster landed on the droneship “Of Course I Still Love You” in the Atlantic Ocean, completing its third trip to space. SpaceX previously used the booster to launch the company’s Crew Dragon capsule on a March 2019 demonstration mission for NASA, and to launch three Canadian radar satellites last June.

SpaceX successfully caught a payload fairing half with “Ms. Tree,” a boat equipped with a large net. Jessica Anderson, a SpaceX manufacturing engineer co-narrating the launch, said the second fairing half missed its recovery boat, “Ms. Chief,” but appeared to have a soft water landing.

“We will be pulling that fairing half out of the water and hopefully reusing it again in the future,” she said.

Changes to Starlink

SpaceX has now launched 242 Starlink broadband satellites, though not every satellite will be part of the constellation when it starts service, a milestone anticipated later this year in Canada and the United States.

Some 10 Starlink satellites have not raised their orbits, according to observations by Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics who tracks satellite movements.

SpaceX spokesman James Gleeson, when asked about the 10 satellites, said SpaceX is “performing a controlled de-orbit of several first iteration Starlink satellites,” using onboard propulsion. 

“While these satellites are operable and capable of providing service, the second iteration of Starlink satellites that SpaceX has started to deploy provide better spectrum efficiency, more capacity and optimized service to the end user,” he said.

SpaceX’s Starlink satellites launched Jan. 29 each weigh approximately 260 kilograms, an increase of 33 kilograms from the 60 satellites launched in May 2019.

SpaceX specified that the newest Starlink satellites have four phased array antennas. Previous satellites were described as having “multiple” phased array antennas.

SpaceX has been modifying Starlink’s design since early on in the program. The first 60 satellites were described as 95% demisable upon atmospheric reentry, meaning some components risked reaching the Earth’s surface. By the second dedicated launch in November, Starlink’s design featured fully demisable parts.

SpaceX is also experimenting with ways to lessen Starlink’s impact on astronomy. Earlier this month the company launched a satellite nicknamed “DarkSat” that features a darkening coating to make it less visible to stargazers and ground-based observatories.

SpaceX CEO Elon Musk said in May 2019 that later versions of Starlink would include inter-satellite links. He said then that the company would like to keep Starlink satellites in orbit for four to five years before deorbiting and replacing them with newer, more capable models.


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Understanding the impact of satellite constellations on astronomy
by Staff Writers  Garching, Germany (SPX) Feb 13, 2020 [SD]

Around 19 Starlink satellites were imaged shortly after launch in November 2019 by DECam on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO) by astronomers Clara Martinez-Vazquez and Cliff Johnson. The gaps in the satellite tracks are due to the gaps between the DECam CCD chips.

In June 2019, the International Astronomical Union expressed concern about the negative impact that the planned mega-constellations of communication satellites may have on astronomical observations and on the pristine appearance of the night sky when observed from a dark region. We here present a summary of the current understanding of the impact of these satellite constellations.

Following the statement of June 2019, IAU's Commission B7 Protection of Existing and Potential Observatory Sites and the Executive Committee Working Group Dark and Quiet Sky Protection were asked by the IAU Executive Committee to assess the situation and to start discussions with the companies that are responsible for launching and operating the mega-constellations in order to study measures to mitigate their interference.

Commission B7 has requested the input of astronomers from different organisations (Vera C. Rubin Observatory, U. Michigan, CAHA, ESO and ESA) skilled in modeling the frequency, location and brightness of satellite mega-constellations. Some of those results are presented below. The results of the simulations, given the large number of parameters involved and the associated assumptions and uncertainties, are to be considered preliminary.

While there is large uncertainty about the future number of satellites, some simulations were conducted on the basis of a 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 over the horizon (i.e. where the sky is clear from obstructions, and where most of the astronomical observations are performed). The vast majority of these will be too faint to be visible to the naked eye.

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 (e.g., no reflected sunlight).

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.

The prominent trains of satellites ("strings of pearls"), often seen in images and videos, are significant immediately after launch and during the orbit-raising phase when they are considerably brighter than they are at their operational altitude and orientation. The global effect depends on how long the satellites are in this phase and on the frequency of launches.

Apart from their naked-eye visibility, it is estimated that the trails of the constellation satellites will be bright enough to saturate modern detectors on large telescopes. Wide-field scientific astronomical observations will therefore be severely affected. For instance, in the case of modern fast wide-field surveys, like the ones to be carried out by the Rubin Observatory (formerly known as LSST), it is estimated that up to 30% of the 30-second images during twilight hours will be affected.

Instruments with a smaller field of view would be less affected. 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.

A summary of the findings and of the actions that have so far been undertaken is presented in a specific IAU Theme.

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. This is one of the main messages communicated on the dedicated IAU-UNESCO web site on astronomical heritage.

In order to mitigate the impacts of satellite constellations that may interfere with professional and amateur astronomical observations, the IAU, in close collaboration with the American Astronomical Society (AAS), will continue to initiate discussions with space agencies and private companies that are planning to launch and operate currently planned and future satellite constellations.

The IAU notes that currently there are no internationally agreed rules or guidelines on the brightness of orbiting manmade objects. While until now this was not considered a priority topic, it is now becoming increasingly relevant.

Therefore the IAU will regularly present its findings at the meetings of the UN Committee for Peaceful Uses of Outer Space (COPUOS), bringing the attention of the world Government representatives to the threats posed by any new space initiative on astronomy and science in general. In addition, the specific theme of the mega-satellites will be included in the Programme of the IAU/UNOOSA/IAC Conference Dark and Quiet Skies for Science and Society, which will be held in Santa Cruz de La Palma, Canary Islands, Spain, on 5-8 October 2020.

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.

A summary of the findings and of the actions that have so far been undertaken is presented in a specific IAU Theme here