In Sixty years ago, The Space Object Population no one would have imagined the present multi-faceted aspects of space activities. As space is a very dynamic environment and is continuously evolving, hence, it is difficult for anyone to envision how space would be some thirty years from now.
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The first human-made satellite Sputnik was launched to study the atmosphere by the Soviet Union on October 4th, 1957. It was spherical, 58 cm in diameter with four external antennas, weighed approximately 84 kg and stayed in space for only 2 months. But this event heralded the beginning of the space age. Since then there have been more than 6000 launches worldwide.
One of the most significant recent trends in space exploration is the increasing participation of the private sector. Private companies, driven by entrepreneurial vision and technological prowess, have made significant strides in space-related ventures. This includes the development and launch of commercial satellites, space tourism initiatives, and even plans for private missions to the Moon and Mars.
Small satellite revolution was marked by the advent of miniaturized satellites, commonly known as CubeSats. CubeSats are small, low-cost satellites that have become increasingly popular due to their flexibility and affordability and they can result in an increase in the number of objects in space.
The deployment of mega-constellations such as SpaceX’s Starlink, OneWeb etc., comprising hundreds or even thousands of small satellites working together, has gained traction in recent years. Moon and Mars have become the points of focus in our recent space exploration endeavours. Multiple countries and private companies have launched spacecraft to these celestial objects and this has widened the reach of human-made objects in space
Space Situational Awareness
In general, situation awareness “is the perception of the elements in the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future” (Endsley, 1987). Space Situational Awareness (SSA) is the situational awareness as applied to the space domain. SSA refers to the comprehensive understanding and knowledge of the space environment, including the presence, location, and behavior of space objects such as satellites, debris, and other celestial bodies and their future evolution.
It comprises the monitoring, tracking, and characterization of these objects to assess their positions and trajectories, thereby assessing the potential threats to operational space assets, human beings, and the Earth’s environment. SSA ensures the safety, security, and sustainability of space operations by mitigating the risks associated with space debris.
Key SSA tasks encompass tracking and monitoring of space objects, producing catalogues on space objects, predicting collision between objects in space, estimating the risks to spacecraft, modelling and tracking the object re-entries, detecting hazards to spacecraft, identifying state ownership and responsibilities, monitoring the behaviour of spacecraft, diagnosing spacecraft failures and malfunctions. These main functionalities are briefly discussed next.
Observation of Space Object Population
Measurements of space objects include the identification and characterization of the space object. Measurements are done with the help of ground-based radars and telescopes. Ground-based radars are well suited to observe space objects in LEO orbits because of all-weather day-night observation capability. Radar power budget and operating wavelength are the limiting factors for the detection of small objects at long ranges. Optical facilities can detect debris when it is sunlit and the background sky is dark.
For objects in geosynchronous orbit, optical observations can be continued during the entire night except during dawn and dusk. Debris tracking capabilities of optical telescopes in the LEO region are limited to one to two hours a day during the twilight due to availability of sunlight for illuminating the object. Higher angular rate of LEO objects results in faster transit of objects which also limits the duration of observation.
Tracking of space objects is very critical for SSA. Global efforts are underway for measuring and cataloguing less than 10cm objects. The US operates the Space Surveillance Network (SSN), a distributed network of about 30 sensors (radars, telescopes) and the Combined Space Command Centre (CSPOC) under USSPACECOM. The ground-based capabilities are complemented by space- based sensors. Russia operates the second-largest network of sensors and maintains an independent catalogue of space objects.
Russian Academy of Sciences in Moscow formed the International Scientific Optical Network (ISON). ISON consists of observatories in about 20 countries and operates more than 30 telescopes for space surveillance. Other space-faring agencies from ESA, China, Japan, France, Germany etc. also operate ground-based observational facilities dedicated to space object monitoring and tracking Worldwide, considerable progress has been made in measurements using electro-optic sensors on board spacecraft.
The advantage of using space-based sensors is to have an unobstructed view of the objects in space without atmospheric distortion and weather, mainly the cloud-cover related constraints. One of the earliest flown sensors is Space Based Visible (SBV) sensors mounted on a gimballed platform on the Mid-Course Space Experiment (MSX) satellite by the US. Human-made objects show up as tiny streaks against the fixed star background by these sensors. Accurate
processing of the measurement data is vital for the precise characterization of the space object. The next section presents an overview of various analysis techniques in SSA
At present, the USSAPCECOM maintains a catalogue which is the primary source of orbital data of space objects. But a more serious problem is the measurement of millimetre size objects. Mission- ending risk is driven by millimetre sized debris in LEO but there is a lack of direct measurement data on such small debris.
Conjunction assessment and collision avoidance against the large (10 cm or greater) tracked objects only address less than one percentage of the space debris mission-ending risk (12).
Objents less than 10 cm are difficult to track individually, hence a statistical estimation is made of the population density based on measurements. These density models are generated over the years with cumulative measurements. The centimetre sized objects are derived from the dedicated radar campaign.
The encountered sub-millimetre debris population are inferred from the analysis of retrieved surfaces and in-situ impact sensors. The population is also characterized based on the ground-based simulations of hypervelocity collisions with satellite and rocket bodies, and ground- based simulations of explosive fragmentations.
These data generated from experiments and campaigns are used for validation and improvement of the debris flux models. The episodic variations of the dynamic small debris population are captured through short-term and long-term models. A short-term model is an engineering model that enables the mission to assess the operational risks to be faced. NASA’s Orbital Debris Environment Model (ORDEM)  and ESA’s Meteoroid and Space Debris Terrestrial Environment Reference (MASTER) are examples of the short-term debris environment model.
The long-term models are evolutionary models which are used to get better insight into future evolution, and thereby devise effective remediation measures. NASA’S LEO-to-GEO Environment Debris Model (LEGEND), and JAXA-Kyushu University’s LEO Near Earth Orbital Debris Environment Evolution Model (NEODEEM) are examples of long-term debris environment models.
Analysis and Risk Mitigation
Assessment of Close Approach Risk due to Catalogued Objects A conjunction is a close approach between two orbiting objects. Conjunction assessment is an essential prerequisite for collision avoidance. It is required to monitor the orbital evolution of space objects and predict their path to identify potentially risky conjunctions.
This activity is carried out three to seven days before the conjunction with some uncertainties. If the probability of collision is high, a Collision Avoidance Manoeuvre (CAM) is performed by the operational spacecraft. Most of the spacecraft operators use this mitigation strategy to protect their space assets against any collision risk
There are several mechanisms and algorithms in place to assess the probability of collision and carry out the CAM. To perform a reliable assessment, the accuracy of the measured data is important. To improve the positional accuracy of the target advanced orbit determination algorithms are used. These algorithms fuse data from a combination of sensor types and different viewing geometries to reduce positional inaccuracy.
Advanced orbit determination algorithms can substantially improve orbit determination accuracy and prediction. These methods also improve the responsiveness to detect, process and characterize manoeuvres in near real-time which essentially allows the orbit solutions to be updated as soon as post manoeuvre data is received.
Protection against Uncatalogued Objects
A collision is called catastrophic when an object completely disintegrates due to the collision. When a 1 mm debris hits an operational spacecraft, the impact may not be catastrophic but the impact can perforate the fuel tanks, and cause problems to batteries or other critical components which may lead to a premature termination of the mission life. In such a case we need to properly protect the asset from such small debris.
We need to employ techniques like active and passive protection. Active protection uses sensors to provide a warning of impact and then protect its critical components or flag a signal to move the spacecraft to avoid the potential impact. A passive protection schemes like shielding of a spacecraft or its critical components can help safeguard a space asset against small uncatalogued objects.
Space Debris Remediation
Long-term effect of the Space debris population can be remediated using techniques like Active Debris Removal (ADR). Active debris removal refers to the intentional removal of space debris from Earth’s orbit using various technological approaches. The goal of ADR is to reduce the population of space debris, lower the risk of collisions, and safeguard operational satellites and future space missions.
Unlike passive debris mitigation measures, which focus on preventing the creation of new debris, ADR actively targets and removes existing debris objects. Debris capture and deorbiting docking and rendezvous using a controllable spacecraft, drag enhancement by deploying drag sails or inflatable structures on the debris, harpoon and tether systems to physically capture and secure debris are some of the techniques which are still in a nascent stage of development RemoveDEBRIS mission of ESA and Active Debris Removal Vehicle (ARDV) of NASA are some of the missions taken up by the international space community for active debris removal.
Space weather forecasting sensors are used to generate forecasts that may adversely affect space operations. Specialized telescopes that detect visible light, ultraviolet light, gamma rays and X-rays are used for monitoring the space weather phenomenon. Space weather events, such as solar flares and CMEs, lead to changes in atmospheric density, which in turn affect the accuracy of object tracking and predictions.
Such events, particularly CMEs, can introduce uncertainties in object trajectories which require rapid adjustments and heightened vigilance during collision avoidance efforts. Integrating space weather monitoring into the SSA framework enhances overall situational awareness by providing real-time data on solar activity, geomagnetic storms, and other space weather phenomena.
NEO Impact Threat Detection
Astronomers use ground-based telescopes equipped with advanced imaging and spectroscopic techniques to observe and track asteroids and meteoroids. Space-based telescopes, such as NASA’s Near-Earth Object Observations (NEOO) program and the European Space Agency’s Gaia mission, provide a complementary perspective to ground-based observatories.
These space-based surveys offer advantages such as an unhindered view of the cosmos and the ability to detect fainter and smaller objects. Radar systems are powerful tools for tracking asteroids and meteoroids. Tracking and monitoring these objects is crucial for understanding their behaviour, identifying potential hazards, and developing mitigation strategies.
Collaboration among space agencies, research institutions, and International organizations is crucial in advancing our SSA capabilities. Sharing data, knowledge, and expertise fosters a comprehensive understanding of the space environment and enables effective response strategies. Continued research and technological advancements in space weather modelling, prediction, and SSA systems will further improve our ability to anticipate and mitigate the impacts of space weather and space objects on space operations.