If you want to ask the world's most well-known private space company, the one that most people know must be SpaceX.
The name of SpaceX is shorthand, and its full name is SpaceeXploration.
This company has a good name, but its reputation in the entire space exploration field and industry is getting worse and worse...
How can this be?
As the private space company with the strongest launch capability in the world, SpaceX has almost single-handedly taken over a large number of government and private companies launch missions in many countries in the world, such as the United States. Even the U.S. government has packaged the resources of the former "state-run" launch pad to SpaceX to operate.
Without the company, space exploration would lose an important provider of transportation. Logically, everyone should respect the value of SpaceX's existence, but in the eyes of some space observers, SpaceX is simply "chaotic".
Among the photos they take from observing space, more and more SpaceX starlink satellites are "grabbing the camera".
Recently, scientists from a number of astronomical observation institutions around the world jointly published a communication paper, lamenting the chaos added to them by the SpaceX starlink satellites. The article pointed out that in recent years, the proportion of observation photos polluted by the Starlink satellite has accounted for nearly 20% of the entire photo library, an increase of up to 35 times.
| Always grab the camera at key moments
Scientists from the U.S. government, the European Observatory, the University of Warsaw, Caltech, UC Berkeley and other institutions recently published such a communication paper in the communication column of the core journal of space exploration, the Journal of Astrophysics: "The Impact of SpaceX Starlink Satellites on Observations of Tswiki Transient Facilities" (click to read the original article at the end of the article)
Note: The Zwicky Transient Facility (ZTF) is a observation facility dedicated to observing transient astronomical events, located at the Palomer Observatory in San Diego, Southern California, operated by Caltech. Transient astronomical events refer to astronomical events of short duration (a few seconds to several years), such as gravitational wave events, gamma-ray bursts, etc.
ZTF Image source: Caltech
The scientists took a large number of astronomical photos taken by ZTF observations from November 2019 to September 2021 and analyzed them to uncover a worrying problem:
In the past two years, more than 5,300 star-chain satellite trajectories have appeared in ZTF observation photos.
The image below, released by Caltech, shows the results of a shot at the Andromeda galaxy with the ZTF telescope last May. It can be seen that a long, bright trajectory line directly crosses the photo, causing significant pollution to the observation subject.
Image source: Caltech
It is worth noting that this figure shows only 1/16 of the ZTF's 47° field of view. Across the entire frame, there is more orbital pollution.
The scientists also found that as SpaceX accelerated its Starlink program, the number of satellites in orbit increased rapidly, and the proportion of pollution caused by these satellites to the ZTF observations also increased significantly.
At the end of 2019, only 0.5% of ZTF's twilight observation photos were contaminated by the starlink satellite trajectory.
By August 2021, the proportion of contaminated photos had increased 35-fold to about 18 percent.
In the chart below, the red line is the number of SpaceX satellites in orbit, and the blue column is the number of ZTF contaminated photos in the same period:
In particular, the authors mentioned that the presence of a large number of star-chain moons in near-ground orbit has caused serious pollution to the twilight observations.
Astronimical twilight refers to the early morning and dusk, when the sun's altitude is between 12-18° below the horizon, and the sunlight scattered by the upper atmosphere into the lower atmosphere is scattered into the lower atmosphere. At this time, the sky is no longer illuminated by the sun, which is convenient for observation of some key celestial bodies, and even some celestial bodies can be seen with the naked eye.
The short twilight period is important for discovering and observing near-Earth objects (NEO). NEO may include comets, meteors, and astronomical objects that pose a potential threat to Earth. In 2020, there was an asteroid with a diameter of more than 1 km and orbiting entirely within the orbit of Venus, which ZTF observed during the twilight period – these observations are critical to protecting the Earth and life on it.
However, the twilight period also happens to be the time when the Satellites of the Starlink are most polluted by astronomical observations.
The authors note that because starlink satellites are low-Earth orbiting satellites, the projected surface density projected by these satellites to the ground is the highest near the horizon and during twilight hours, that is, they have a disproportionate impact on astronomical observations.
The authors also cite models from other scientists to deduce that about 90 percent of observable satellites in low Earth orbit would congregate in the sky near the ground level (within 30°). These satellites significantly increase their brightness due to the refractive reflection of sunlight, interfering with astronomical observations during the twilight period. Their brightness only decreases for a long period of time after the sun sets/before it rises.
The Starlink satellite is currently the most numerous system among all low-Earth orbit satellites, and it has become the first source of pollution for astronomical observations during the twilight hour. As a result, many ground-based astronomy researchers and observation enthusiasts have expressed dissatisfaction with SpaceX.
However, the authors also said that in terms of the ZTF family of data sources alone, its normal scientific observation activities have not been particularly significantly affected by star-chain trajectory pollution.
But given that SpaceX has received approval from U.S. communications regulator, the FCC, to launch more satellites, if the total number of satellites on the Starlink network increases to 12,000 (7 times the current number) in the future according to the company's plan, the consequences will be unimaginable.
The authors estimate that when the number of satellites in the Starlink system reaches 10,000, all of ZTF's astronomical observations will be contaminated by the satellite's trajectory — note that all the photos, that is, the pollution rate will be 100%.
Not to mention, spaceX has also submitted a new plan to the U.S. government on top of its current plan of 12,000, and will launch an additional 30,000 satellites. At that time, all observations based on the surface of space will inevitably be polluted.
To exaggerate, the existence of numerous starlink moons will pose a greater obstacle to human discovery and prevention of near-Earth object threats.
The trajectory of the Starlink satellite appears in infrared photographs of space observation Image source: NASA Observation Laboratory
| SpaceX is not without effort to correct it
In May 2019, SpaceX launched its first starlink satellites. At that time, the astronomical community had already expressed concern about the pollution caused by the large number of low-Earth orbit satellites from astronomical observations.
However, given the greater power of the FCC, the federal communications regulator to which Starlink is attached, and spaceX's high voice in the space industry, these concerns have hardly hindered Starlink's advancement. In addition, the FCC has also approved SpaceX's plan to increase the number of Starlink satellites several times.
(If you've seen don't Look Up on Netflix recently, don't Look Up, remember how dismissive the opinions of astronomy experts are among the people in power and tech billionaires.) Now it seems that this plot is not a fictional irony, but a depiction of reality... ）
Stills from Don't Look Up Image credit: Netflix
As of last November, the number of SatelliteLink satellites that had been launched had reached 1,791: 124 of them had fallen into the atmosphere and been decommissioned, the number of satellites in orbit had reached 1,667, and the number of satellites in normal operation (capable of providing Internet services to the ground) was 1,468.
The numbers are still growing rapidly, because spaceX can launch satellites to the sky not one by one, but can be launched in batches. The number of satellites per batch is typically between 48 and 53 satellites, while the SpaceX Falcon 9 rocket has a maximum payload of 60. SpaceX's first two launches this year alone will increase the number of starlink satellites in orbit to nearly 2,000.
According to the plan that SpaceX has disclosed and submitted to the regulatory authorities, the total number of satellites in the entire Starlink program is 42,000.
Launch load pattern stacked starlink satellite Image source: SpaceX
As the absolute leader in today's private space industry, SpaceX's continued operation and growth inevitably involves dealing with the entire astronomy research community.
In fact, the company has also made some efforts to minimize the visibility of starlink satellites in the sky and reduce the impact of their presence on ground-based astronomical observations.
In April 2020, SpaceX published a public article on its website saying that it had held talks with experts from the National Academy of Sciences of the United States on the situation of satellite interference with astronomical observations. The company said it will make technical modifications to future launches of starlink satellites, using a dark coating and installing a set of expandable masks in the brightest parts of the satellite, thereby reducing the albedo of satellites in astronomical observations.
Starting in November 2019, SpaceX began to use version V1.0 starlink satellites (although the version number is 1, which is not the first version of SpaceX satellites to launch into the sky). One of the satellites of this generation, called DarkSat, uses a special dark coating on the surface of the shell to reduce the albedo.
Unfortunately, this coating was later discovered to cause the satellite to absorb excess visible radiation and reflect excess infrared rays, causing serious overheating problems. As a result, the DarkSat scheme was quickly deprecated
A few V1.0 series satellites, and subsequent V1.5 series satellites, added a deployable unfoldable and retractable dark mask (see figure below) to replace the previously failed coating scheme called VisorSat. In principle, during the period when the satellite has the greatest impact on astronomical observations, these masks can be unfurled and retracted after the period.
The above-mentioned communication paper pointed out that this masking scheme is more obvious for reducing the visibility of satellites during critical periods, but perhaps due to mechanical failures, some of the recently launched Starlink satellites have not been equipped with masks, or have been installed but not unfolded.
SpaceX claims on its website that it is working with some well-known astronomical societies and research institutions to further explore the impact of satellites on astronomical observations and deploy solutions in a timely manner.
However, from the observations provided in the January newsletter paper, SpaceX's efforts have had little practical effect — rather, spaceX continues to increase the number of satellites in orbit while the problem has not been solved. Just last Thursday, Feb. 4, SpaceX just launched the second batch of 49 Starlink satellites of the year.
"Given the gradual growth in the number of satellites in orbit, the rate of growth in the number of observational photographs affected by their trajectories is also alarming," the paper reads, "We will continue to observe the northern hemisphere sky and expect to record thousands of additional satellite trajectories." ”