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Raumfahrt - SpaceX to test Starlink “sun visor” to reduce brightness

28.04.2020

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WASHINGTON — SpaceX Chief Executive Elon Musk said April 27 that he hopes to test a new way to reduce the brightness of the company’s Starlink satellites on the next launch for the broadband megaconstellation.

In a briefing to a committee working on the next astrophysics decadal survey, Musk said the experimental “VisorSat,” along with a new approach for orienting Starlink satellites as they raise their orbits, should address concerns raised by astronomers that the Starlink constellation could interfere with their observations.

“Our objectives, generally, are to make the satellites invisible to the naked eye within a week, and to minimize the impact on astronomy, especially so that we do not saturate observatory detectors and inhibit discoveries,” Musk said.

SpaceX first attempted to address the brightness problem with an experimental “DarkSat” included in a batch of Starlink satellites launched in January. The satellite used what the company described as experimental darkening treatments over reflective surfaces, like its antennas, in an effort to reduce the amount of sunlight it reflects and thus make it darker.

While DarkSat has shown some promise, appearing about one magnitude darker than untreated Starlink satellites, the company is moving in a different direction. “We found an option that is even better than that, which is basically to give the satellites shades,” he said.

Musk and others at SpaceX have previously discussed a sunshade that they compared to a patio umbrella that would deploy from a satellite, keeping the antennas in shadow. Musk, at the committee meeting, described a concept called VisorSat that would deploy panels, like sun visors mounted on a car windshield, to block the sun.

“We have a radio-transparent foam that will deploy nearly upon the satellite being released, and it blocks the sun from reaching the antennas,” he said. “They’re sun visors, essentially: they flip out and block the sun and prevent reflections.” He predicted that the visors would have a “massive effect” on the brightness of the satellites.

SpaceX is planning test VisorSat on the company’s next Starlink launch. “It’s a bit of a challenge, but that’s our goal,” he said. He didn’t say how many satellites would be equipped with visors, or when the launch was scheduled. SpaceX has been performing Starlink launches at the rate of at least one a month so far this year, most recently April 22.

A second effort involves the brightness of the satellites as they raise their orbits after launch. Musk said the satellites appear bright because of the orientation of the solar panels, which are aligned differently during orbit raising than once at their operational orbit.

As soon as this week, Musk said SpaceX will try an “orientation roll” to change the alignment of the solar panels relative to the Earth, reducing the amount of sunlight they reflect to the ground. “Early indications are this will have a significant effect on the brightness during orbit raise,” he said. “The satellites will be significantly less visible from the ground.”

The measures SpaceX has taken have come after months of discussions with astronomers, who have been worried about the effects a full constellation of Starlink satellites — about 12,000 according to current plans, with proposals for up to 30,000 more — would have on astronomy. The situation was of particular concern to those operating telescopes with wide fields of view, like the Vera Rubin Observatory under construction in Chile, where Starlink satellites would be visible in a large fraction of images taken each night.

In a separate presentation to the committee earlier in the day, Tony Tyson, chief scientist for the Rubin Observatory, said the concern was that the brightness of unmodified Starlink satellites would cause “nonlinear crosstalk,” or severe image artifacts, in the observatory’s camera. “We would be left with all of these fake trails, fake galaxies, etc., in our data, damaging the science,” he said.

SpaceX has already made progress darkening the satellites, with newer satellites about one magnitude darker than the original “v0.9” satellites launched in May 2019 even without the darkening treatments used on DarkSat. If the satellites can be made about a factor of two darker than DarkSat, Tyson said a technique to correct for the nonlinear crosstalk can work, although it is computer intensive and won’t correct for the original streak left in the images by passing satellites.

The new approaches won’t address the issue of brightness of existing Starlink satellites, but Musk said their lifetime is limited. He estimated the initial generation of satellites will be deorbited in about three to four years to make way for improved satellites. “We’ll just have far greater throughput capability with version two” of the Starlink satellites, he said.

While the focus of the committee presentation and subsequent discussion, which lasted for more than an hour, was on Starlink, there was some talk about the role SpaceX could play in supporting space-based astronomy, which is not affected by Starlink or other megaconstellations.

“I’m very excited about the future of space-based telescopes that could be very large,” he said. He mentioned Starship, the company’s next-generation large reusable launch system, which will begin regular flights “I think within a couple of years,” he promised. “It allows for space telescopes to be transported to orbit at probably an order of magnitude lower cost than in the past.”

“I’m pretty interested in trying to figure out how to help launch and possibly build a big observatory in space,” he said, offering to meet with astronomers to discuss mission concepts. “Like a planet imager or something like that.”

Quelle: SN

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Update: 30.04.2020

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SpaceX to debut satellite-dimming sunshade on Starlink launch next month

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SpaceX plans to debut a new sunshade structure on its future Starlink satellites. Credit: SpaceX

A new sunshade, or visor, designed to reduce the brightness of SpaceX’s Starlink broadband Internet satellites will debut on the company’s next launch, a measure intended to alleviate astronomers’ concerns about impacts on observations through ground-based telescopes, SpaceX founder Elon Musk said Monday.

Beginning with the next launch of Starlink satellites — scheduled as soon as May 7 from Cape Canaveral — SpaceX will try out a new light-blocking panel to make the spacecraft less visible to skywatchers and astronomers.

“We have a radio-transparent foam that will deploy nearly upon the satellite being released (from the rocket),” Musk said Monday in a virtual meeting of the National Academies’ Decadal Survey on Astronomy and Astrophysics 2020 panel, a committee charged with setting the top priorities for U.S. astronomy for the next decade.

Musk said the new material will block sunlight from reaching the satellites’ antennas, comparing it to a sun visor in a car’s windshield. Reflected sunlight at dawn and dusk is what makes the satellites visible from the ground.

The sunshade is transparent to radio signals. SpaceX has nicknamed the new Starlink satellite design “VisorSat.”

On seven launches since last May, SpaceX has deployed 420 Starlink satellites to begin building out the Starlink network. The satellites are designed to provide global broadband Internet service, and SpaceX eventually plans to launch thousands of the quarter-ton, flat-panel data relay stations to orbits below an altitude of about 354 miles (570 kilometers).

“Using our low orbital altitude and flat satellite geometry to our advantage, we designed an RF-transparent deployable visor for the satellite that blocks the light from reaching most of the satellite body and all of the diffuse parts of the main body,” SpaceX wrote in an update posted on the company’s website this week. “This visor lays flat on the chassis during launch and deploys during satellite separation from Falcon 9. The visor prevents light from reflecting off of the diffuse antennas by blocking the light from reaching the antennas altogether.”

SpaceX says that every future Starlink satellite will be fitted with the sun visor beginning with a subsequent launch in June.

Five Falcon 9 rocket launches so far this year have carried 300 Starlink satellites into orbit — 60 at a time — most recently on April 22. The next Falcon 9 launch for the Starlink network is set for May 7 at approximately 7:30 a.m. EDT (1130 GMT).

The sunshade is the latest change SpaceX has introduced on Starlink satellites in response to complaints from astronomers about the network’s impacts on observations from ground-based telescopes.

This 333-second exposure taken last year by the Dark Energy Camera on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory shows 19 streaks attributed to Starlink satellites passing through the camera’s field-of-view shortly after their launch Nov. 11 from Cape Canaveral. Credit: NSF’s National Optical-Infrared Astronomy Research Laboratory/NSF/AURA/CTIO/DELVE

The first 60 Starlink satellites launched last May were much brighter than SpaceX or astronomers anticipated, prompting a series of discussions between the aerospace company and the astronomy community.

SpaceX launched a satellite with a darker coating in January, and astronomers noticed an improvement. SpaceX said last month that preliminary data indicated a “notable reduction” in the brightness of that satellite, which has been dubbed “DarkSat.”

“The darkening that they did on DarkSat is about a factor of two-and-a-half fainter — so about 1 magnitude in astronomical units — and it’s still visible to the unaided eye under excellent conditions,” said Pat Seitzer, an astronomer and orbital debris expert at the University of Michigan.. “That is you’re a person with great vision sitting on top of a mountaintop faraway from the city lights.”

But the darker coating has drawbacks, SpaceX said. Black surfaces in space get hot, so the company is moving forward with the sun visor solution instead.

“This avoids thermal issues due to black paint, and is expected to be darker than DarkSat since it will block all light from reaching the white diffuse antennas,” SpaceX said.

The Starlink satellites are brightest soon after a launch, when they are flying at an altitude of around 200 miles (300 kilometers). Once deployed from the Falcon 9 rocket, the spacecraft unfurl their solar array wings to generate electricity, then activate krypton ion thrusters to begin climbing into their higher operational orbit some 341 miles (550 kilometers) above Earth.

SpaceX commands the satellites to fly in a special attitude, or orientation, during the orbit-raising maneuvers. This minimizes drag on the satellites.

“This low-drag and thrusting flight configuration resembles an open book, where the solar array is laid out flat in front of the vehicle,” SpaceX said. “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.”

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Credit: SpaceX

After the satellites reach their operational orbit, SpaceX flies the spacecraft in a different configuration called the “shark-fin” attitude. In this orientation, the solar panel points away from the Earth.

When the satellites are in the “open book” configuration at lower altitudes, sunlight reflects off the craft’s solar arrays, making them more visible from the ground.

The sun visor is designed to reduce brightness while the Starlink satellites are on station at their operational altitude. SpaceX is debuting a different technique to address brightness concerns during the early weeks of each satellite’s life, when the spacecraft are flying closer to Earth.

“We’re taking several steps to reduce brightness in orbit-raising, which has definitely startled a lot of people around the world,” Musk said Monday. “I’ve gotten quite a lot of feedback on that front. So we’re changing the angles. That should be something that happens even this week — the satellites being commanded to a different angle that is less bright.”

“We’re currently testing rolling the satellite so the vector of the sun is in-plane with the satellite body, i.e. so the satellite is knife-edge to the sun,” SpaceX said. “This would reduce the light reflected onto Earth by reducing the surface area that receives light.”

The new roll maneuver can be implemented when the satellites are climbing to higher altitude, and when they pause in an intermediate orbit to align with their operational planes within the Starlink network.

SpaceX said the new “orientation roll” maneuver will diminish the amount of power generated by each satellite’s solar panel, and reduce contact time between the spacecraft and ground controllers. The change also points star tracker cameras at the Earth and the sun, reducing the satellite’s attitude knowledge, according to SpaceX.

“There will be a small percentage of instances when the satellites cannot roll all the way to true knife edge to the sun due to one of the aforementioned constraints,” SpaceX said. “This could result in the occasional set of Starlink satellites in the orbit raise of flight that are temporarily visible for one part of an orbit.”

Musk described the changes as “quite simple.”

“I think, with the benefit of hindsight, the changes seem quite simple, which is to make sure that the orientation of the satellites is not such that we’re reflecting the sun,” he said. “And to either darken the specular surfaces or the white surfaces, or shade them, is really quite simple actually. It’s a little bit silly in hindsight. It’s not that hard.

“Our objectives, generally, are to make the satellites invisible to the naked eye within a week, and to minimize the impact on astronomy, especially so that we do not saturate observatory detectors and inhibit discoveries,” Musk said.

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Credit: SpaceX

The satellites that launched without the brightness mitigations will likely be retired and will re-enter the atmosphere and burn up within three or four years, Musk said. They will be replaced with improved satellites.

SpaceX has regulatory approval from the Federal Communications Commission to eventually field a fleet of up to 12,000 small Starlink broadband stations.

Officials say 24 launches are needed to provide global broadband service through the Starlink service. But the company could provide an interim level of service over parts of the Earth — such as Canada and northern parts of the United States — later this year, once SpaceX has launched around 720 satellites on 12 Falcon 9 flights.

Musk tweeted last week that SpaceX aims to begin private “beta testing” of the Starlink service within about three months, followed by a public trial period in six months.

SpaceX has modified the architecture of the Starlink network several times. Most recently, SpaceX submitted an application to the Federal Communication Commission on Friday proposing to operate more satellites in lower orbits than the FCC previously authorized.

Rather than launching more than 2,800 of the Starlink satellites to higher orbits between 690 miles (1,110 kilometers) and 823 miles (1,325 kilometers) in altitude, SpaceX will instead deploy the spacecraft closer to Earth. The change allows the network to provide consumers with better Internet service, and also reduces the number of Starlink satellites that might be visible in the sky at any one time.

“We think, for a lot of reasons, that 550 (kilometers) and below is the right approach for a LEO (Low Earth Orbit) broadband situation,” Musk said. “Your data rate is going to be four times better than say at 1,100 kilometers. That’s a close approximation. It’s also better for astronomy.”

The lower orbit also ensures atmospheric drag will cause dead satellites to re-enter the atmosphere more quickly, SpaceX said.

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This diagram illustrates when the Starlink satellites are most visible, shortly after sunset and shortly before sunrise. Credit: SpaceX

Musk said 20,000 to 30,000 Starlink satellites may be needed to provide the level of Internet service envisioned by SpaceX, which aims to provide connectivity to the “3 or 4 percent of the least-served portion of the Earth.”

“So it’s not a huge percentage, but it’s for those that have the least service,” Musk said Monday. “You do want to have probably on the order of 20,000 to 30,000 satellites, something like that. But not 200,000, I don’t think.”

SpaceX has filed documentation with the International Telecommunication Union for up to 30,000 additional Starlink satellites beyond the 12,000 spacecraft already authorized by the FCC.

It’s the large number of Internet satellites planned by SpaceX and other companies, such as Amazon, that worries astronomers.

“SpaceX is committed to the progress of science, both in the United States and elsewhere,” Musk said. “So we’ll do our best to ensure that we’re not going to interfere with any facilities anywhere in the world.”

One of the ground-based facilities most at risk of interference from the Starlink satellites is the U.S. government-funded Vera Rubin Observatory, formerly known as the Large Synoptic Survey Telescope. The observatory under construction in Chile will capture deep, wide-field images of the entire southern sky, allowing astronomers to learn more about dark energy and dark matter, and detect potentially hazardous asteroids with orbits near Earth, among other objectives.

Steve Kahn, director of the Vera Rubin Observatory, said last week that SpaceX is responsive to astronomers’ concerns.

“They’ve been quite cooperative in working with us,” Kahn told Spaceflight Now in an interview.

Scientists will measure the brightness of the new VisorSat spacecraft after launch to gauge the effectiveness of the sunshade. If it works as advertised, the sun visor could limit the impacts of the Starlink satellites on the Vera Rubin Observatory.

Musk pitches astronomers on Starship’s ability to launch giant space telescopes

Toward the end of Monday’s virtual meeting, Musk expressed interest in building and launching a new space-based observatory using SpaceX’s next-generation Starship launch vehicle.

“I’d be pretty interested in trying to figure out how to help launch and possibly build a big satellite, a big observatory in space,” Musk said. “Maybe … we can get together and talk about what would be a really exciting space observatory, like a planet imager or something like that.”

The decadal survey panel was chartered to prioritize which space-based observatories NASA should pursue after the launch of the James Webb Space Telescope and the Wide Field Infrared Survey Telescope.

“I’m really interested in the advancement of science, and to understand what the heck is going on in this universe,” Musk said.

SpaceX says from the Starlink Internet business will help fund development of the Starship. When coupled with a new first stage booster SpaceX calls the Super Heavy, the Starship will be able to loft more than 100 metric tons, or 220,000 pounds, of cargo to low Earth orbit.

“Starship has the capability to transport satellites, payloads, crew, and cargo to a variety of orbits and Earth, lunar, or Martian landing sites,” SpaceX wrote in a Starship user’s guide released last month.

The Starship’s payload envelope is also significantly larger than any other existing rocket. Its diameter will measure around 30 feet, or 9 meters, allowing the Starship to launch big telescopes without requiring the mirrors be folded to fit inside a payload fairing.

“The launch situation has changed quite dramatically from where it was in 2010,” Musk said. “It will be very much changed, I think, in even five years. I think Starship … will be flying quite soon. I think you’ll see regular flights within a couple of years, and that’s a very big rocket.

“It allows for space telescopes to be at least transported to orbit at probably an order of magnitude lower cost than in the past,” Musk said. “So that’s pretty important to factor into future plans.”

Quelle: SN

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