Space is the final frontier of humanity. Space Debris and exploration of space enable us to unravel the mysteries of the world that lie beyond the Earth and seek out answers to some of the most fundamental quests about our universe, Including deeper insights into the evolution and extinction of species due to extraterrestrial factors to have better preparedness for survival. On the other hand, decades of space-based activities have provided immense socio-economic benefits to improve the quality of life. Apart from vital applications.
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In Earth observation and monitoring, meteorology and disaster warning, communication; navigation, astronomy, and scientific studies, space has catalyzed numerous groundbreaking discoveries, cutting-edge technological development, and innovations along with spinoffs, especially in miniaturized electronics and medical sciences, and opened up novel economic opportunities like space tourism. Furthermore, space-based services play crucial roles in national security. With a wide array of military applications like precision navigation, network-centric warfare, advanced surveillance techniques, anti-satellite weaponry etc., space is now considered the fourth dimension of modern warfare, in addition to ground, air, and sea.
Space assets operating in outer space are vulnerable to various environmental hazards- natural objects like asteroids, comets, and meteoroids along with energy and particle flux, and other Resident Space Objects (RSO), consisting of operational satellites, rocket bodies, and space debris. Therefore. updated knowledge and continual awareness of the space environment is imperative to protect valuable space assets during their on-orbit operation.
As our dependence on space-based applications steadily grows, we also become increasingly aware that near-earth space is a finite, shared resource that must be utilized responsibly to ensure it remains accessible for utilization and exploration for future generations. Hence understanding the current status and future evolution of the space environment plays an important role in ensuring that space activities are carried out in a safe, sustainable, and responsible manner.
This chapter introduces the foundational concepts of SSA and its importance for the safety and sustainability of outer space activities. It also familiarises the reader with Space Traffic Management (STM) and Space Domain Awareness (SDA), which are becoming increasingly vital with the rapid diversification of the scope of space activities.
Understanding the Space Environment:
An understanding of the space environment is essential is critical for safe and sustainable operations In outer space. The following section provides an overview of our space environment.
The term space debris is defined as non-functional, human-made objects in Earth’s orbit that no longer serve any useful purpose. These are also called orbital debris and are distinguished from naturally occurring objects like meteoroids. These objects can vary in size, from tiny paint flecks to large defunct satellites and spent rocket stages. Space debris is not spread uniformly through space. but they are concentrated near the regions that are heavily used by satellites
Low Earth Orbit (LEO) region extends up to an altitude of 2000 km. This is the most densely populated region in space. The Geosynchronous (GEO) region is a toroid-shaped zone at the +/-200 km altitude band around the Geostationary altitude of 35786 km (Le from 35586 to 35986 km altitude) and +/-15 deg latitude about the equator. These two regions are called protected regions because they are highly utilized and any space debris creation will have a potentially severe impact on the space environment Figure 1-1 shows pictorially orbital regimes around Earth.
Here is a table of the world’s tallest trees organized by their habitat:
The US. Space Command (USSAPCECOM) tracks space debris and has cataloged objects sized typically 10 cm or more in LEO and 0.3 m or more in GEO. These measurements are made through ground-based (generally radars and optical telescopes) and space-based sensors. The non-cataloged objects less than 10 cm in size are measured through the detection of samples by ground-based sensors, examination of returned spacecraft, and through active measurements in orbit. The knowledge of the smaller debris is based on the extrapolation of the incomplete data.
Sources and Sinks of Space Debris:
Space debris is created in space by the accumulation of defunct satellites, spent rocket stages, and various fragments, which now pose a significant threat to both current and future space missions.
Defunct Satellites: Satellites have limited lifespans and eventually become inoperable or reach the end of their missions. However, they remain in orbit, posing a risk of collision and further debris creation.
Mission operations related: Materials released during spacecraft mission operations such as cameras, lens caps, solid fuel, items like gloves, toolboxes, solid motor exhausts, and payload adaptors. Etr, contributes to the debris related to mission operations.
Rocket Stager: Launching satellites into space involves multi-stage rockets. After the final stages burn out, they are often discarded and left to orbit as space debris.
Fragmentation/Break-ups and Collisions: Break-up events may be high-energy separation from the parent body or a low-energy separation for which the fragmentation root cause is mostly not known.
Intentional activities like the anti-satellite tests and self-destruct missions such as the Cosmos self-destruct at the End of life, Anti-Satellite Test (ASAT) on Fengyun 1C and Russian Cosmos spacecraft also resulted in the creation of a large number of fragments. Accidental collision of satellites such as the infamous collision between the Cosmos 2251 and Iridium 33 also created a large number of space debris
Micro-debris, or small particles less than a centimeter in size, are generated when larger objects break apart due to mechanical stresses, impact with other objects, or natural causes like micrometeoroid impacts. Breakup of smaller substances from larger surfaces due to reasons like thermal flexing, atomic erosion, and small particulate impact also produces such small-sized space debris.
The orbits of space debris evolve due to various perturbations and may naturally decay over time to eventually re-enter the atmosphere. This natural cleansing mechanism is mostly applicable to Low Earth orbits. The debris can also be removed from the space through active means by direct retrieval or relocation. Figure 1-2 shows the sources of space debris and the mechanisms of removal from outer space.
Growth of Catalogued Population:
According to the latest figures provided by ESA’s Space Debris Office, as of 06 June 2023, there are about 36500 space debris objects greater than 10 cm, 1000000 space debris objects greater than 1 cm to 10 cm and 130 million space debris objects greater than 1 mm to 1 cm.
Total number of cataloged space objects as a function of time. The four curves represent the population breakdown. The curves show that fragmentations have dominated the population. Three major chunks have been created by the ASAT tests on Fengyun IC satellite by China
in 2007 which has created more than 3000 debris. The accidental collision of Russia’s defunct Cosmos 2551 satellite generated more than 2000 pieces of debris, and the recent ASAT test on Cosmos 1408 by the Russian Federation in 2001 resulted in around 2000 debris. There are about 3000 retired spacecraft and around 7100 operational spacecraft.
The mass of objects in space also shows a continuously increasing trend and there is no sign of slowing down. The mass of spacecraft is the major contributor and the total mass has exceeded 9000 metric tonnes In LEO (up to 2000 km altitude region) orbit alone, the total mass has exceeded more than 4000 metric tonnes. It is to be noted that the history of mass increase is different from the distribution of mass
The distribution of objects at a given orbital regime is represented by spatial density, which is defined as the number of objects per unit volume. The spatial density of objects as a function of altitude in LEO orbits. The proliferation of CubeSats and the deployment of large constellations were primarily responsible for the increase below 600 km. It is to be noted that collision rates will vary not only with the spatial density but also with the inclination-dependent relative velocity.