A general is born to be brave and courageous, with a Qishui Yanfei saber across his waist. Hello everyone, this is the editor of Deep Space who can’t hold a knife. The editor of Deep Space spent a long time sorting it out and brought you this article. Without further ado, let’s find out together.
In 2019, SpaceX launched the first large-scale deployment of Starlink satellites into space with 60 satellites in one shot. In the following days, astronomy enthusiasts discovered that these satellites passed over the sky like a train around the world, and even broke into the field of view of astronomical telescopes, causing the photos to be completely covered by satellite tracks.
Artistic illustration of Starlink satellites orbiting above the Earth. Source: SpaceX
The International Astronomical Union and the National Radio Astronomy Observatory have both issued statements expressing concern. As of January 8, 2020, SpaceX has put a total of 120 similar satellites into orbit, and more Starlink satellites will continue to be launched in the future.
Who is coming?
In fact, everyone is quite familiar with SpaceX. This private company has almost single-handedly shaken the global aerospace market, winning numerous orders with its highly cost-effective Falcon rockets. Just a few years ago, with the lessons of the space shuttle still fresh in our minds, people naturally thought that rocket recovery was a dead end. Even if it could be successfully recovered, it would lose most of its capacity and require a lot of investment in maintenance and refurbishment. However, through rapid technological iteration, the Falcon rocket has become increasingly mature, and rocket recycling has become commonplace, and the Falcon can still provide considerable transportation capacity in the recycling state. SpaceX boss Elon Musk has an even more ambitious plan. He wants to go one step further and plans to build a giant rocket that can send humans to Mars. This rocket, which is 9 meters in diameter and weighs more than the Saturn V, uses liquid oxygen methane as fuel. It plans to make fuel from local materials on Mars so that the spacecraft can fly back to Earth again. Of course, what is even more surprising is the "Starship" test machine - it is hand-built in the open air. The stainless steel shell even has obvious concave and convex marks, making it look like a barn. Sure enough, one of the prototypes exploded during pressurization testing.
Figure 1 60 Starlink satellites ready for launch in November 2019. Source: SpaceX
The protagonist this time, the Starlink project, is another “crazy” idea proposed by Musk. Using its own extremely cost-effective launch vehicles, it will launch 42,000 satellites into low-Earth orbit within a few years—yes, this number will be about five times the number of satellites that have been launched by mankind.
According to the plan, "Starlink" is an Internet satellite communication system that plans to achieve complete global coverage through a large number of low-orbit satellites and form a high-throughput, low-latency Internet communication network. The communication services provided by satellite networks are alluring. Once completed, everyone from individual users in remote mountainous areas to corporate users in big cities can enjoy the convenience of communication coverage without blind spots. And satellite networks may also replace ground backbone networks. Optical fibers span thousands of mountains and rivers, but using satellites only requires a few relays.
Corresponding to the overwhelming network deployment plan, Starlink satellites use various means to cut costs. During launch, the plate-shaped satellites are stacked tightly together without the need for a separate separation device. Instead, they are released all at once like playing cards, even if the satellites collide with each other. A Falcon 9 launch vehicle can launch 62 satellites into orbit and can also recover the first stage of the rocket. In future plans, the Falcon Heavy rocket could launch more satellites into space.
Figure 2 About 1,600 satellites in the first phase of the network will orbit the earth at an altitude of 550km. After that, more than 2,700 satellites will enter a higher orbit of 1,100~1,325km to complete the global network. In the second phase, more than 7,000 satellites will enter a lower altitude of 300km. Source: starlink
Each Starlink satellite weighs 227 kilograms and consists of a plate-shaped satellite body and a foldable solar panel. Each satellite is equipped with four phased array antennas for network services. Users on the ground can receive signals from satellites with an elevation angle of 45°. It is expected that satellites will begin to carry laser communication systems for inter-satellite communications in 2020. The satellite uses Hall effect thrusters for orbit change and orbit maintenance, and the fuel is krypton.
In order to avoid collisions with other satellites, the satellite uses data provided by the U.S. Department of Defense to automatically avoid space debris. The on-orbit life of a satellite is 1 to 5 years.
In May 2019, the first batch of 60 satellites launched entered a low-Earth orbit of 550km. The second launch was carried out in November 2019, and the third launch was carried out in January 2020. emission. According to the plan, a batch of Starlink satellites will be launched every less than a month thereafter. In the first phase of the network, about 1,600 satellites will orbit the earth at a distance of 550km. After that, more than 2,700 satellites will enter a higher orbit of 1,100~1,325km to complete the global network. The second phase of the "Starlink" plan will launch more than 7,000 satellites into a lower altitude of 300km to increase the bandwidth of the entire network.
The track surface parameters of the two stages can be seen in Table 1. The first-stage satellites will operate in different orbital planes, while the second-stage low-orbit satellites will individually control individual satellites.
Table 1 These are the orbital parameters of the two stages of Starlink satellite network, from the documents submitted by SpaceX to the FCC for frequency resources. The preliminary coverage plan has been changed to 550km, 72 orbital planes, and 22 satellites in each orbital plane. The first batch of Starlink satellites launched belong to this shell. Source: SpaceX/FCC
According to the U.S. Federal Communications Commission's frequency band approval, SpaceX needs to complete half of the launch work in 2024 and complete the launch of all satellites in 2027. In other words, in just 7 years, SpaceX will launch more than 12,000 satellites.
Just in October 2019, SpaceX submitted a frequency application for a new batch of satellites, planning to add an additional 30,000 satellites, bringing the total number of satellites to 42,000, which is approximately the number of all satellites ever launched by mankind. 5 times. These 30,000 satellites will operate between 328 and 580 kilometers.
The "new" starry sky
We can often see artificial satellites streaking across the sky in the early morning and evening. These satellites often appear in deep-sky photos taken by astronomers, and removing the tracks of these satellites is an almost inevitable step in the post-processing process. Starlink satellites are no exception. The brightness of a single satellite is actually not outstanding, but the problem is-there are too many of them.
Figure 3: Images of the NGC 5353 and NGC 5354 galaxy groups taken with a telescope at the Lowell Observatory in Arizona, USA, on the evening of May 25, 2019. Running diagonally across the image are the traces left by Starlink satellites as they pass through the telescope's field of view. Source: Victoria Girgis/Lowell Observatory
In the first few days after the launch of the Starlink satellite, 60 satellites were operating in similar orbits. Therefore, the phenomenon of a large number of satellites passing through the sky in a row can be seen on the ground. To the naked eye, the brightness of these satellites is 3rd magnitude or darker. The brightness of different satellites is different, and the brightness of the satellite is related to its attitude. Since these satellites have similar orbits, they will pass through the same area of ??the sky one after another. When it appears in the telescope's field of view, the consequences are devastating, as shown in Figure 3.
Figure 4: Satellite flash captured by an enthusiast with his own SLR. Source: What’s more, some enthusiasts have even photographed these satellites “flashing”. In a video uploaded to the Internet, this enthusiast used his SLR to capture a long series of satellites flashing in sequence, with a brightness exceeding that of Vega.
The brightest star in Figure 5 is Vega. Source: This suddenly caused a stir in the astronomy circle. If the sky is full of such satellites in the future, astronomers and astronomy enthusiasts will not be able to observe them at all. The satellite trajectories will all cover up distant celestial bodies. The IAU issued a statement saying that such a giant satellite network would have a serious impact on astronomical observations. In response to the doubts of astronomers, SpaceX initially stated that "the satellite is very small and will not have any impact." However, as the opposition grew louder, it changed its tune and stated that "it will take measures to reduce the brightness of subsequent satellites." The November 2019 launch included a blacked-out satellite as an attempt to reduce the satellite's brightness. But blackening can adversely affect the performance of satellites, which absorb large amounts of heat from sunlight.
In addition, Starlink satellites operate in the Ku and Ka frequency bands, and radio astronomy observations in these bands may also be affected. Since the working band of satellites contains an important water vapor spectrum line, a large number of satellite communications may also interfere with meteorological satellite observations of water vapor, resulting in reduced accuracy of weather forecasts.
Traces of interlopers
So, with what density will these satellites appear in our night sky, and how will they affect our astronomical observations? The author made a simple calculation: taking 12,000 satellites as the research object, and based on the orbit distribution in public data, calculated the number of satellites seen in the sky by observers on the ground.
The results show that in mid-latitudes, there are more than 100 satellites located in the sky above an elevation angle of 20° at any time. At around 45 degrees north latitude, the number reaches more than 160.
Figure 6 Taking 12,000 satellites as the research object, the author calculated the distribution of the number of satellites seen in the sky by observers at different latitudes on the ground based on the orbit distribution in public data. Blue, red, and yellow are respectively Represents initial coverage, final coverage and second phase LEO satellites.
In Figure 6, blue represents the first batch of 550km orbit satellites launched, red represents satellites with orbits above 1,000km, and yellow represents the second phase of low-orbit satellites. It can be seen that satellites with lower orbital inclinations are not visible at high latitudes. At the same time, the density of satellites above latitudes 35° to 55° is the highest, with more than 120 satellites at any time, and a maximum of more than 160 satellites.
The number of satellites in the sky remains basically stable throughout the day, but the number of satellites illuminated by the sun is related to the position of the sun. For example, around the spring and autumn equinox at 40° north latitude, within one hour after sunset, all 150 satellites overhead are reflecting sunlight. As the height of the sun gradually decreases, the number of luminous satellites gradually decreases, and the number of luminous satellites at the end of the astronomical twilight gradually decreases. Only about half remains. There are basically no luminous satellites in the sky for about 4 hours before and after midnight, which means that it can be considered "safe" to conduct astronomical observations during this period.
Figure 7. In the spring and autumn seasons near 40 degrees north latitude, the time changes of all Starlink satellites and luminous Starlink satellites in the sky at the same position.
For north latitude 40°, there are no luminous satellites in the sky for a longer period of time during the winter solstice, while there are luminous satellites in the sky all night during the summer solstice. For low latitudes near the equator, the situation is slightly better than at mid-latitudes, and the annual changes are not obvious. For the Arctic and Antarctic regions, the influence of satellites also exists. For example, at Kunlun Station in Antarctica, when the local polar night begins, dozens of satellites are visible all night long, and there are also quite a few satellites visible during the local winter solstice.
Epilogue
When the Starlink satellites dispersed in orbit, the "pollution" of the sky was obvious. Our skies will not be completely taken away, but optical astronomical observations will be pushed into a corner and the available observation time will be greatly shortened. A single exposure of many telescopes may take up to 15 minutes, and during this period there is a high chance that a satellite will break into the photo, causing the entire photo to fail. It is true that these traces can be removed algorithmically, but these steps inevitably leave invisible flaws in the picture, making the data no longer reliable.
Figure 8 A schematic diagram of the distribution of Starlink satellites drawn by an artist. Source: SpaceX
With the launch of Starlink satellites, the transit of these "satellite trains" has also become a popular astronomical phenomenon, comparable to the gradually disappearing "iridium flash" phenomenon. However, the Iridium system only has dozens of satellites, and the frequency of Iridium flashes observed at the same location is only about once a day. If the frequency of Iridium star flashes were increased hundreds of times, I'm afraid people would view it very differently.
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