How NASA manages the ISS to avoid collisions with space junk
Located in orbit about 400 kilometers above us, the International Space Station is one of the most complex and expensive engineering projects in the history of mankind. The station weighs about 400 metric tons, and its dimensions slightly exceed the size of the field for American football. It took dozens and dozens of launches from Russia and the United States (including 37 shuttle flights), as well as more than 155 cosmonauts and astronauts in outer space — twice as many as the total number of space missions at that time.
Comparison of the size of the ISS and American football field
The construction and launch of the ISS cost mankind approximately $ 150 billion and 13 years of work. And at this very moment, somewhere out there, high above our heads, six people are working in space. Of course, the station is not just hanging there without moving. By itself, the ISS orbit tends to decrease due to atmospheric resistance - the speed of this decrease is about 2 kilometers per year - therefore, the station needs to be pushed from time to time to help it stay at the required height. Moreover, this whole huge structure is quite mobile - it can be moved, rotated and tilted in three dimensions in order to avoid possible collisions with space debris. So how do you manage to move 400 tons of a rather fragile ISS if, for example, an asteroid moves onto it?
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MKS control room
In order to figure out how to move the station in orbit, I rode to the Johnson Space Center and met with Josh Parris. Parris is one of the people sitting at the console in the ISS flight control room. In fact, his position is called TOPO - Trajectory Operations Officer. As it was from the first days of manned flights, people occupying such positions have a lot of experience and an extensive set of knowledge - Parris and his colleagues spent years studying to earn the right to sit at the helm of the ISS.
Josh parris
“We are responsible for where the station is at the moment and wishing to dock with the ships, where they will be after a certain time, and also for ensuring that they do not collide with anything,” explains Parris. At the height of the ISS orbit (400 km) there are not many satellites, but there is plenty of space debris. Over the past couple of years, hundreds of radar reports have been received of possible collisions between the station and the wreckage. Only in 2013 there were 67 such potentially dangerous situations.
“What are these fragments?” - I asked, - “Is it because of the Chinese experiments on the destruction of satellites?”
“Pretty much yes,” says Parris, “In addition, a large amount of debris remains after the collision of the satellites Cosmos and Iridium . Notice that all these fragments are what have already descended to the height of our orbit. In higher orbits, too, is full of all kinds of garbage that can come down to us. ”
“And who is tracking all this? Do you have a big, computer-generated map with beautiful graphics, like a movie show? ”
“The guys from USSTRATCOM [The Strategic Command of the US Armed Forces , - comment.] At Vandenberg Air Base are doing this,” explains Parris. “They maintain a catalog of all known pieces of space debris, and about three times a day they compare the ISS trajectory with data from the catalog. So what exactly they notify us about dangerous rapprochements "
Dangerous rapprochement? It sounds scary. I asked Parris to tell in more detail, and he explained that around the ISS there is a “protective perimeter” in the shape of a pizza box four kilometers in height (2 kilometers up and down from the station), and 25 kilometers in length and width. If one of the debris falls into this perimeter, USSTRATCOM notifies NASA.
The main display showing the position and trajectory of the ISS, and the main ground stations that serve it
Operators monitor everything that goes inside this “pizza box” and calculate the probability of a collision for each of the tracked objects. Each object receives a hazard class based on the probability of a collision. Objects with a chance of collision from 1/10000 to 1/100000 receive the "yellow" level. The flight rules prescribe that the station be moved if such objects appear, unless this maneuver affects the current programs - for example, because of the maneuver, the opportunity to launch the Union in time is lost. A red hazard level is declared for any object, the probability of collision with which ranges from 1 (100%) to 1/10000. For such cases, the rules are stricter - the station must necessarily be moved, unless the movement is more dangerous than the piece of garbage itself (for example, if the ISS has damaged equipment, and the maneuver of moving the station will damage it even more).
ISS, for its size and fragility, is quite mobile. It is equipped with four gyrodynes (Control Momentum Gyros - CMG), which allow it to change its orientation in space. For the gyrodynes, the person whose position is called the ADCO (Attitude Determination and Control Officer) is responsible and who helps TOPO to decide on the best way to avoid collisions with garbage.
Workplace ADCO. Note the baseball bat - to adjust the orientation
In addition to this, the station has several sets of accelerators, allowing it to move and turn. The Star service module is equipped with accelerators; there are also accelerators on ships docked to the station (for example, on Progress). For a typical station avoidance maneuver, a delta V value of about 1 m / s is usually required.
One of the important parameters that have to be monitored is the station mass, since the exact amount of thrust required to reach the desired delta V depends on the displaced mass. Basically, the mass of the station changes when ships dock or unlock to it. The delta V value achieved during the maneuver is too small to somehow affect the daily work of the crew - they know when the maneuver is made, but the movement itself is controlled from Earth, and the crew does not have to do anything special.
"That is, they do not sit there with joysticks, steering station?"
“No,” laughs Parris, “Everything is completely controlled from Earth.”
So, when a piece of debris is assigned a yellow or red code, it is time to act. 28.5 hours before the closest approach to the station, the operators begin the process of planning an avoidance maneuver. But why exactly 28.5 hours? It looks like a rather strange number. “The turnaround will be carried out by the Russian segment of the station,” explains Parris. “Considering the time difference between Houston and the Russian MCC, 28.5 hours is quite enough for the specialists in Russia to compile the so-called cyclogram — the exact sequence of actions necessary to complete the maneuver, which is transmitted to the station. Well, and besides, such a stock leaves us more time to track garbage. ”
TOPO, together with experts in ballistics from Russia, determines the parameters sufficient for evasion, including the delta V value. Then they contact USSTRATCOM to make sure that the planned maneuver will dodge the dangerous wreck, but at the same time will not lead them onto the trajectory of some other object. When the maneuver itself is finally carried out, the station’s computers are responsible for it, and the specialists on Earth only follow the process. So it is actually impossible to just grab the steering wheel and “steer” the ISS - all its movements are carried out after careful planning, and the start of any engines always takes place only automatically, in accordance with the program.
If it seems to you that the process described above is excessively complicated, then remember that human lives are at stake. “If we run into one of the monitored objects, this could lead to the loss of the station,” says Parris. "The ISS is moving at such a high speed [about 7.6 km / s, approx. Per.] That any collision will be a disaster." The lower limit of the size of the tracked space debris is 10 centimeters. And yes - “loss of the station” is exactly what you thought. A large enough piece of garbage can not only damage the equipment of the station, but also lead to the death of the crew.
Fortunately, Parris assesses our data on debris greater than 10 cm in the orbit of the station as exhaustive. “In addition, the station has shields [sorry, not force fields, comment approx.] To protect against micrometeorites, and other similar things. Unfortunately, there is a small gap between the strength of shields and the minimum size of debris that we can track. However, we do our best to prevent any abnormal situations. ”
Trying not to look too gloomy, I wondered what would happen in the worst case - if NASA finds out that the ISS is facing an unforeseen collision. Most often, USSTRACOM employees give NASA a three-day warning of possible collisions, but sometimes they may miss something. “Usually we can find a way to fly around something we know about in advance,” explains Parris. But if he does not have the 28.5 hours we need in order to create an evasion maneuver together with our colleagues from Russia, they can carry out the so-called PDAM - Predetermined Debris Avoidance Maneuver. This is a pre-recorded maneuver, passing from a delta V of about 0.5 m / s, which can be performed very quickly. Finally, even if it cannot be carried out - for example, if it would lead the ISS to the trajectory of some other piece of garbage - then the station’s crew sits in the docked Soyuz ship and waits for the collision, in full readiness for evacuation, if necessary . From the moment when the first person appeared at the station, this procedure had to be carried out three times.
The configuration of the station in May 2013. Showing docked ships Soyuz and Progress
In fact, the PDAM maneuver was developed after the 2011 incident. Then NASA received a warning about a possible collision too late, and the station crew had to take refuge in the Union. It ended with the fact that the debris missed from the ISS at a distance of about 725 meters. NASA hopes that with the development of PDAM, such situations will not recur.
If TOPO does not calculate the station’s position relative to any space debris, it is busy with hundreds of other things. “If we are not tracking any debris - now, it seems, just such a lull - we have a number of tasks for each day of the week that we must perform. I have already said that we track the exact position of the station, so that we work with different MCC, especially with the Goddard flight dynamics center. ” Parris explained that TOPO is supplying people in Goddard with information about the exact location of the station, so that various NASA tracking systems can precisely position their antennas.
Parris at his workplace
In general, with the exception of planning evasive maneuvers, the work of TOPO consists mainly in passive monitoring. At his workplace, Parris and his colleagues work on three monitors. “On the first monitor, our ground-based GPS data. On the left, where you see three columns, we compare the data on the ground with the station telemetry data. ” This is necessary to compare where, in the opinion of the ISS, it is located with where it is located in the opinion of ground tracking stations. The average monitor also tracks the spatial position of the ISS. “The graphs show the difference between different data sources. Usually they all look like one straight line, ”explains Parris.
On the right side of the middle monitor, behind the comparison graphs, there is a window that shows the state of the station equipment that TOPO needs. Finally, on the right monitor is the software that NASA uses to calculate the ephemeris . “Above you see a list of station motion vectors, and below — a catalog of all the ephemeris we have for the ISS.” Parris also stressed that if TOPO were in the process of working on an evasion maneuver, there would be a completely different set of applications on the screens. The computers themselves are ordinary PCs running Linux (there are also computers on the Windows 7 in the flight control room, but the main control monitors work on Linux). This is a consequence of those old days when each console was unique, and its reconfiguration took hours, if not days.
The ISS control room is located on the second floor of the Johnson Space Center building 30, and occupies one of two rooms that were once used as the Apollo mission control room (the second room, on the third floor, was restored to about the same condition as in Apollo's mission time, and you can visit her as part of a trip to the Johnson Space Center). When I left, Parris had already taken up his job, studying the parameters of the station, flying through space, and carefully watching for possible collisions. This is a job that never ends, and which requires constant vigilance.
Comparison of the size of the ISS and American football field
The construction and launch of the ISS cost mankind approximately $ 150 billion and 13 years of work. And at this very moment, somewhere out there, high above our heads, six people are working in space. Of course, the station is not just hanging there without moving. By itself, the ISS orbit tends to decrease due to atmospheric resistance - the speed of this decrease is about 2 kilometers per year - therefore, the station needs to be pushed from time to time to help it stay at the required height. Moreover, this whole huge structure is quite mobile - it can be moved, rotated and tilted in three dimensions in order to avoid possible collisions with space debris. So how do you manage to move 400 tons of a rather fragile ISS if, for example, an asteroid moves onto it?
')
Ground control to Major Tom
MKS control room
In order to figure out how to move the station in orbit, I rode to the Johnson Space Center and met with Josh Parris. Parris is one of the people sitting at the console in the ISS flight control room. In fact, his position is called TOPO - Trajectory Operations Officer. As it was from the first days of manned flights, people occupying such positions have a lot of experience and an extensive set of knowledge - Parris and his colleagues spent years studying to earn the right to sit at the helm of the ISS.
Josh parris
“We are responsible for where the station is at the moment and wishing to dock with the ships, where they will be after a certain time, and also for ensuring that they do not collide with anything,” explains Parris. At the height of the ISS orbit (400 km) there are not many satellites, but there is plenty of space debris. Over the past couple of years, hundreds of radar reports have been received of possible collisions between the station and the wreckage. Only in 2013 there were 67 such potentially dangerous situations.
“What are these fragments?” - I asked, - “Is it because of the Chinese experiments on the destruction of satellites?”
“Pretty much yes,” says Parris, “In addition, a large amount of debris remains after the collision of the satellites Cosmos and Iridium . Notice that all these fragments are what have already descended to the height of our orbit. In higher orbits, too, is full of all kinds of garbage that can come down to us. ”
“And who is tracking all this? Do you have a big, computer-generated map with beautiful graphics, like a movie show? ”
“The guys from USSTRATCOM [The Strategic Command of the US Armed Forces , - comment.] At Vandenberg Air Base are doing this,” explains Parris. “They maintain a catalog of all known pieces of space debris, and about three times a day they compare the ISS trajectory with data from the catalog. So what exactly they notify us about dangerous rapprochements "
Dangerous rapprochement? It sounds scary. I asked Parris to tell in more detail, and he explained that around the ISS there is a “protective perimeter” in the shape of a pizza box four kilometers in height (2 kilometers up and down from the station), and 25 kilometers in length and width. If one of the debris falls into this perimeter, USSTRATCOM notifies NASA.
The main display showing the position and trajectory of the ISS, and the main ground stations that serve it
Operators monitor everything that goes inside this “pizza box” and calculate the probability of a collision for each of the tracked objects. Each object receives a hazard class based on the probability of a collision. Objects with a chance of collision from 1/10000 to 1/100000 receive the "yellow" level. The flight rules prescribe that the station be moved if such objects appear, unless this maneuver affects the current programs - for example, because of the maneuver, the opportunity to launch the Union in time is lost. A red hazard level is declared for any object, the probability of collision with which ranges from 1 (100%) to 1/10000. For such cases, the rules are stricter - the station must necessarily be moved, unless the movement is more dangerous than the piece of garbage itself (for example, if the ISS has damaged equipment, and the maneuver of moving the station will damage it even more).
Accelerators and gyrodynes
ISS, for its size and fragility, is quite mobile. It is equipped with four gyrodynes (Control Momentum Gyros - CMG), which allow it to change its orientation in space. For the gyrodynes, the person whose position is called the ADCO (Attitude Determination and Control Officer) is responsible and who helps TOPO to decide on the best way to avoid collisions with garbage.
Workplace ADCO. Note the baseball bat - to adjust the orientation
In addition to this, the station has several sets of accelerators, allowing it to move and turn. The Star service module is equipped with accelerators; there are also accelerators on ships docked to the station (for example, on Progress). For a typical station avoidance maneuver, a delta V value of about 1 m / s is usually required.
One of the important parameters that have to be monitored is the station mass, since the exact amount of thrust required to reach the desired delta V depends on the displaced mass. Basically, the mass of the station changes when ships dock or unlock to it. The delta V value achieved during the maneuver is too small to somehow affect the daily work of the crew - they know when the maneuver is made, but the movement itself is controlled from Earth, and the crew does not have to do anything special.
"That is, they do not sit there with joysticks, steering station?"
“No,” laughs Parris, “Everything is completely controlled from Earth.”
To battle!
So, when a piece of debris is assigned a yellow or red code, it is time to act. 28.5 hours before the closest approach to the station, the operators begin the process of planning an avoidance maneuver. But why exactly 28.5 hours? It looks like a rather strange number. “The turnaround will be carried out by the Russian segment of the station,” explains Parris. “Considering the time difference between Houston and the Russian MCC, 28.5 hours is quite enough for the specialists in Russia to compile the so-called cyclogram — the exact sequence of actions necessary to complete the maneuver, which is transmitted to the station. Well, and besides, such a stock leaves us more time to track garbage. ”
TOPO, together with experts in ballistics from Russia, determines the parameters sufficient for evasion, including the delta V value. Then they contact USSTRATCOM to make sure that the planned maneuver will dodge the dangerous wreck, but at the same time will not lead them onto the trajectory of some other object. When the maneuver itself is finally carried out, the station’s computers are responsible for it, and the specialists on Earth only follow the process. So it is actually impossible to just grab the steering wheel and “steer” the ISS - all its movements are carried out after careful planning, and the start of any engines always takes place only automatically, in accordance with the program.
Worst case scenario
If it seems to you that the process described above is excessively complicated, then remember that human lives are at stake. “If we run into one of the monitored objects, this could lead to the loss of the station,” says Parris. "The ISS is moving at such a high speed [about 7.6 km / s, approx. Per.] That any collision will be a disaster." The lower limit of the size of the tracked space debris is 10 centimeters. And yes - “loss of the station” is exactly what you thought. A large enough piece of garbage can not only damage the equipment of the station, but also lead to the death of the crew.
Fortunately, Parris assesses our data on debris greater than 10 cm in the orbit of the station as exhaustive. “In addition, the station has shields [sorry, not force fields, comment approx.] To protect against micrometeorites, and other similar things. Unfortunately, there is a small gap between the strength of shields and the minimum size of debris that we can track. However, we do our best to prevent any abnormal situations. ”
Trying not to look too gloomy, I wondered what would happen in the worst case - if NASA finds out that the ISS is facing an unforeseen collision. Most often, USSTRACOM employees give NASA a three-day warning of possible collisions, but sometimes they may miss something. “Usually we can find a way to fly around something we know about in advance,” explains Parris. But if he does not have the 28.5 hours we need in order to create an evasion maneuver together with our colleagues from Russia, they can carry out the so-called PDAM - Predetermined Debris Avoidance Maneuver. This is a pre-recorded maneuver, passing from a delta V of about 0.5 m / s, which can be performed very quickly. Finally, even if it cannot be carried out - for example, if it would lead the ISS to the trajectory of some other piece of garbage - then the station’s crew sits in the docked Soyuz ship and waits for the collision, in full readiness for evacuation, if necessary . From the moment when the first person appeared at the station, this procedure had to be carried out three times.
The configuration of the station in May 2013. Showing docked ships Soyuz and Progress
In fact, the PDAM maneuver was developed after the 2011 incident. Then NASA received a warning about a possible collision too late, and the station crew had to take refuge in the Union. It ended with the fact that the debris missed from the ISS at a distance of about 725 meters. NASA hopes that with the development of PDAM, such situations will not recur.
Day after day
If TOPO does not calculate the station’s position relative to any space debris, it is busy with hundreds of other things. “If we are not tracking any debris - now, it seems, just such a lull - we have a number of tasks for each day of the week that we must perform. I have already said that we track the exact position of the station, so that we work with different MCC, especially with the Goddard flight dynamics center. ” Parris explained that TOPO is supplying people in Goddard with information about the exact location of the station, so that various NASA tracking systems can precisely position their antennas.
Parris at his workplace
In general, with the exception of planning evasive maneuvers, the work of TOPO consists mainly in passive monitoring. At his workplace, Parris and his colleagues work on three monitors. “On the first monitor, our ground-based GPS data. On the left, where you see three columns, we compare the data on the ground with the station telemetry data. ” This is necessary to compare where, in the opinion of the ISS, it is located with where it is located in the opinion of ground tracking stations. The average monitor also tracks the spatial position of the ISS. “The graphs show the difference between different data sources. Usually they all look like one straight line, ”explains Parris.
On the right side of the middle monitor, behind the comparison graphs, there is a window that shows the state of the station equipment that TOPO needs. Finally, on the right monitor is the software that NASA uses to calculate the ephemeris . “Above you see a list of station motion vectors, and below — a catalog of all the ephemeris we have for the ISS.” Parris also stressed that if TOPO were in the process of working on an evasion maneuver, there would be a completely different set of applications on the screens. The computers themselves are ordinary PCs running Linux (there are also computers on the Windows 7 in the flight control room, but the main control monitors work on Linux). This is a consequence of those old days when each console was unique, and its reconfiguration took hours, if not days.
What is the result?
The ISS control room is located on the second floor of the Johnson Space Center building 30, and occupies one of two rooms that were once used as the Apollo mission control room (the second room, on the third floor, was restored to about the same condition as in Apollo's mission time, and you can visit her as part of a trip to the Johnson Space Center). When I left, Parris had already taken up his job, studying the parameters of the station, flying through space, and carefully watching for possible collisions. This is a job that never ends, and which requires constant vigilance.
Source: https://habr.com/ru/post/188286/
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