Clearing out near space is much more difficult than meets the eye
The problem of space pollution is of concern to the entire aerospace community. Such hypothetical developments in low-Earth orbit, such as Kessler's syndrome, which predicts the formation of space debris out of control, has stirred even the popular media. It is clear that there is a need for fundamental research in order to understand what danger even a small fragment is fraught with, and to calculate how much we are willing to pay to clean up outer space.
Today, politicians, scientists, technicians and the general public are deeply aware of the proliferation of space debris. Thanks to the fundamental work of J-K. Liouville and Nicholas Johnson, published in 2006, we understand that the debris rate is likely to continue to rise in the future, even if all launches are stopped. The reason for this sustained growth is the collisions that are expected to occur between satellites and rocket stages already in orbit. This is of great concern to many satellite operators, who are forced to take appropriate measures to protect their assets.
Some experts believe that these incidents will be just the beginning of a series of collisions that will make it nearly impossible to access low Earth orbit. This phenomenon, which was first described in detail by NASA consultant Donald Kessler, is commonly referred to as Kessler's syndrome. But the reality is likely to be very different from similar predictions or events shown in the feature film "Gravity". Indeed, the results presented to the Inter-Agency Space Debris Coordination Committee (IADC) at the sixth European conference on the subject indicated an expected increase in debris of only 30 percent over 200 years with continuous launches.
Collisions will still occur, but the reality will be far from the catastrophic scenario that some fear. The growth in the amount of space debris can be reduced to quite a modest level. The IADC proposal is to widely disseminate and strictly adhere to the space debris mitigation guidelines, especially with regard to the neutralization of energy sources, which should be fully developed by the end of the flight, and disposal after the end of the flight. Nevertheless, from the point of view of the IADC, the expected increase in the amount of waste, despite the ongoing efforts, still requires the introduction of additional measures to combat the existing risk factors.
No progress?
Significant interest in the reclamation of the space environment was noted nine years after the publication of the work of Liouville and Johnson. In particular, steps have been taken around the world to develop methods for removing objects from low Earth orbit. The European Space Agency, for example, recently announced its intention to secure government support for the launch of a European spacecraft in the next decade. The agency has conducted numerous studies to determine rational and reliable ways to achieve the goal. A key element of the planning was computer models of the debris space, which showed that debris growth could be prevented by removing a specific spacecraft or rocket stage. In computer simulations, these objects are identified as the most prone to collisions, so after they are removed from orbit, the number of collisions should sharply decrease, which will prevent the appearance of new debris as a result of the scattering of debris.
Almost ten years have passed since the publication of the work of Liouville and Johnson, and it is surprising that at the international or national level there are no methodological principles that clearly define measures to eliminate the consequences of pollution of the near-Earth space. There seems to be some apathy about developing a debris disposal procedure, despite calls for action. But is it really so?
In fact, the situation is not as simple as it seems. Regarding the space debris removal procedure, there are some fundamental questions that still need to be answered. Of particular concern are issues related to ownership, accountability and transparency. For example, many of the technologies offered for debris removal can also be used to remove or disable an active spacecraft. Consequently, one can expect accusations that these technologies are weapons. There are also questions regarding the cost of a consistent garbage disposal program. Some technicians have estimated it at tens of trillions of dollars.
However, perhaps the most important reason for the lack of adequate methodological principles lies in the fact that we do not yet know how to carry out reclamation, by which in practice we mean the purification of outer space. But this does not mean that we do not know what technologies we need.
Algorithms for one-time use have already been practically developed. The real problem arises from a seemingly simple task: to determine the "correct" debris to remove from orbit. And until we can solve this problem, it seems that we will not be able to reclaim space.
Playing wreckage
To realize the problematic nature of solving such a seemingly simple task as identifying garbage to be removed, we use the analogy of a game with a deck of 52 ordinary playing cards. In this analogy, each map represents an object in outer space that we might want to remove to prevent a collision. After the cards have been dealt, we place each card individually face down on the table. Our goal now is to try to identify the aces and remove them from the table, since these cards represent satellites or other large objects of space debris that may become participants in the collision at some point in the future. We can remove as many cards from the table as we want, but whenever we remove one card, we have to pay $ 10. In addition, as we move away, we have no right to look at the map (if a satellite is removed from orbit, we cannot say with certainty what exactly it could become a participant in the collision). Finally, we have to pay $ 100 for every ace that remains on the table, which represents the potential losses resulting from collisions involving our satellites (in reality, the cost of replacing a satellite can range from $ 100,000 to $ 2 billion).
Well, how can we solve this problem? On the reverse side, all the cards are the same, so there is no way to tell where the aces are, and the only way to make sure we have removed all the aces is to clear all the cards off the table. In our example, this will cost a maximum of $ 520. In outer space, we face the same problem: we do not know exactly which objects may be involved in collisions, but it is too expensive to remove all of them, so we have to choose. Let's assume we have made a choice: to remove one card worth $ 10, what is the likelihood that we have removed an ace? Well, the probability that the card is an ace is four divisible by 52, in other words, roughly 0, 08, or 8 percent. Thus, the probability that the card is not an ace is 92 percent. This is the probability that we wasted our $ 10.
What happens if this time we take a second card (which will cost us another $ 10)? The likelihood that the second card is an ace depends on whether the first card was an ace. If this were the case, then the probability that the second card is also an ace is three divided by 51 (because now there are only three aces in the deck, which has decreased by one card). If the first card is not an ace, then the probability that the second card is an ace: four divide by 51 (because there are still four aces in the decreasing deck).
We can use this method to determine the likelihood that we have removed both aces by simply multiplying the probabilities to find the answer: 4/52 times 3/51, which gives us a probability of 0.0045 or 0.45 percent for $ 20 per two cards removed. Not very encouraging.
However, we can also determine the probability of removing at least one of the aces. After drawing two cards, there is a 15 percent chance that we have successfully removed at least one of the aces. This sounds more promising, but the odds are not very good now either.
It turns out that in order to increase the chances of drawing at least one of the aces, we need to remove more than nine cards (worth $ 90) or more than 22 cards (worth $ 220) if we want to be 90 percent sure that we have removed one of the aces. Even if we succeed, three aces are still on the table, so in total we still have to pay $ 520, which coincidentally is the same amount that we had to pay if we had chosen the option with the removal. all cards.
The games are over
Returning from our analogy back to the real space environment, the situation seems more alarming. Currently, approximately 20,000 objects are tracked in orbit using the US network of space observing stations, with about six percent of them being objects weighing more than one ton, which could hypothetically participate in a collision and which we might want to remove. … In the card analogy, our problem is that the back of all cards is the same and the probability that one is an ace of spades is the same as the probability that the other is also an ace. There is no way to identify the cards you want and remove them from the table. In reality, our chances of avoiding a collision are much higher than in a card game, because in orbit we can see the likelihood of some objects being involved in collisions and we can focus our attention on them. For example, objects that are in densely populated orbits such as heliosynchronous at altitudes between 600 and 900 kilometers are most likely to be involved in collisions due to congestion in this zone. If we focus our attention on similar objects (and others on similarly congested orbits) and take into account the predictions of the possibility of their collision, it turns out that we must remove about 50 objects in order to reduce the expected number of catastrophic collisions by only one unit, which follows from the research results undertaken by members of the IADC space agency.
And it turns out that even if several objects can be removed by a single cleaner spacecraft (and five targets appear to be a versatile alternative), multiple flights - often challenging and ambitious - have to be undertaken just to prevent one collision.
Why are we not able to more accurately predict the likelihood of collisions and remove only those objects that we know for sure will be dangerous? There are many parameters that can affect the trajectory of a satellite, including the orientation of the satellite, whether it is erratic movement or space weather (which can affect the drag experienced by satellites). Even small errors in the initial values can lead to large discrepancies in the results of calculating the position of the satellite in comparison with reality, and after a relatively short period. In fact, we use the same technique as forecasters: we use models to generate the likelihood of specific outcomes, but not the fact that these results will ever be obtained.
Thus, we have technologies that can be used occasionally to remove space debris. This is the position taken by the European Space Agency with their planned mission e. Deorbit, but there are still problems that need to be solved in order to identify the objects most suitable for removal. These challenges must be addressed before the necessary guidelines and methodological principles can be provided to those interested in preparing a long-term space debris removal program that is essential for effective environmental remediation.
Methodological principles in terms of specific sites, their numbers, requirements and constraints are essential to increase the likelihood that efforts to remediate the environment will be effective and worthwhile. To develop such methodological principles, we must reconsider our unreasonable expectations of a favorable outcome.
