BENGALULU, India (ISRO PR) — Growing collision threats of space objects including orbital debris with the operational space assets have become a perennial problem for the safe and sustainable use of outer space. These threats restrict the unhindered access to space and prompt all space actors to take appropriate measures to mitigate them.
Growing Menace of Space Debris
Currently, millions of pieces of space debris together with thousands of operational satellites are orbiting the Earth at different altitudes above the Earth. Space Debris consists of rocket bodies that are used to launch satellites, defunct satellites, materials released during mission operations, fragments from on-orbit breakups of space objects, and fragments from Anti-Satellite (ASAT) tests. These space objects move with an average speed of 27,000 km per hour in Low Earth Orbits, therefore, a collision with even a centimetre sized tiny fragment can be catastrophic to an operational space asset.
These orbiting space debris pose a threat to about 3000 operational satellites presently in orbit, used for critical modern communication, commerce, travel and security systems. Any damage, even minor, to the operational space assets will have cascading impacts on many vital systems including communication, finance, power, transportation, time scheduling and critical defence related aspects. Many state-of-the-art defence technologies such as drones, guided missiles, intelligence data collections, encrypted communications and navigation would be limited or can become inoperable with functionally crippled satellite systems.
Catalogues of space objects, generated and maintained by various space surveillance networks, are limited to larger objects, typically of sizes greater than 10 cm in Low Earth Orbits (LEO, below about 2000 km altitude above the Earth) and greater than 0.3 to 1 m at geosynchronous orbits (GEO, about 36000 km altitude above the Earth). There are even smaller debris with high population density which cannot be tracked due to the limited observational capabilities. The sizes of the objects that can be tracked depends on the observational systems and orbital altitude.
US SPACE COMMAND (USPACECOM) currently tracks using Space surveillance tracking Network and catalogues more than 25,000 space objects including debris and functional spacecraft in Earth’s Orbits. The number of debris that are too small to track are a few millions.
Changing scenario due to large constellation
The outer space, at present, has more defunct objects than functional ones. The proposed large constellations are likely to change the scenario completely.
SpaceX plans to build a large constellation of nearly 12,000 satellites to provide worldwide, high bandwidth internet service. The satellites within the constellation are planned to be deployed in different orbital shells of altitude around 550 km, 540-570 km, 335-345 km, and will be launched in batches. The full constellation is expected to be operational by 2027. In April 2020, SpaceX has also applied for a second generation of Starlink constellation comprising 30000 LEO satellites, the satellites are proposed to be deployed in 8 orbital shells, within an altitude range of 328 to 641 km.
In 2021, a total of 1712 payloads were launched. Among them, nearly 83% are contributed by the proposed constellations. SpaceX is found to be the main contributor, followed by OneWeb, Flock, Lemur, SpaceBee. The number of such satellites are increasing rapidly due to increased applications of Communication and Earth Observation satellites and trend of manufacturing short-lived satellite with commercial components en masse.
The growing usage large constellation of satellites, the growth of private sector investment in space exploration, the on-orbit breakup and collisions (like Iridium-Cosmos collision) and the anti-satellite testing leaving thousands of fragments in the orbits are all contributing to the spurt in the space debris population. It is expected that number of operational satellites will surpass the number of space debris by 2030. Consequently, the total number of space objects of more than 10 cm sized in LEO is expected to be about 60000 by 2030.
The present studies by Inter Agency Space Debris Coordination committee (IADC) and other study groups of space agencies show that the collisional threats will continue to grow in the near future even after spacecraft owner/operators comply with the mitigation guidelines including the ones on Post Mission Disposal put forward by IADC and United Nations.
Long-term growth in space debris leads to two major risks; first, that space debris could potentially create unusable regions of orbit due to proliferation of debris objects and the associated increase in collision risk due to the resultant congestion. Secondly, it may lead to “Kessler Syndrome”, a situation in which a collision in space results in cascading collisions leading to the generation of more and more debris fragments, ultimately prohibiting meaningful use of outer space.
How to minimise collision threats?
To minimise the collision threats posed by space debris, IADC brought out a set of Space Debris Mitigation Guidelines in 2002. Later in 2007, the Scientific and Technical Subcommittee (STSC) of the United Nations’ Committee on the Peaceful Uses of Outer Space (UNCOPUOS) adopted a consensus set of space debris mitigation guidelines based on these IADC guidelines which was endorsed by the General Assembly of the United Nations in late 2007. These guidelines describe the practices that have to be adopted for limiting the generation of space debris in the environment. In order to limit the collision risk to spacecraft and also to contain proliferation of space debris environment, the guidelines are to be considered for the mission planning, design, manufacture and operational (launch, mission and disposal) phases of spacecraft and launch vehicle orbital stages.
The guidelines typically cover on the topics such as limiting debris released during normal operations, minimising the potential for break-ups during operational phases, limiting the probability of accidental collision in orbit, avoiding intentional destruction and other harmful activities in space, minimising potential for post-mission break-ups resulting from stored energy, limit the long-term presence of spacecraft and launch vehicle orbital stages in the Low Earth Orbit (LEO) region after the end of their mission and limit the long-term interference of spacecraft and launch vehicle orbital stages with the Geosynchronous Orbit (GEO) region after the end of their mission.
As a responsible space faring nation, India follows all the above guidelines in its space activities and complies with them to the maximum extent possible and practicable under the constraints of national priorities, payload availability and mission operations.
Observational facilities plays a key role
Radar and optical telescopes are the main ground based facilities for tracking space objects including space debris. Accurate orbital information is a pre-requisite for mitigating any collision threats to an operational space asset from other resident space objects. Presently orbital data available in public domain lack the requisite accuracy leading to ambiguity in the decision making process pertinent to collision avoidance manoeuvres.
Space faring nations and private SSA entities have their own observational facilities to get more accurate orbital data and hence to avoid collision threats more confidently. India has also progressed in establishing indigenous space object observational facilities through Multi Object Tracking Radar at SHAR and few optical telescopes. In order to track closely and frequently the operational spacecraft and the threat objects for estimation of their positions accurately, it is essential to have a network of observational facilities with wide geographical distribution.
Space Situational Awareness (SSA)
SSA deals with the comprehensive knowledge of the space environment, assessment of any threats to space activities and the implementation of necessary mitigation measures to safeguard the space assets. SSA plays a crucial role in ensuring safe and sustainable space activities complying with domestic and international guidelines, standards and other norms.
Satellites orbiting the Earth being susceptible to collision from other space objects, it is essential to analyse the close approach of all space objects to detect collision threats well in advance. Whenever a critical collision threat is identified for a spacecraft, an evasive manoeuvre known as Collision Avoidance manoeuvre (CAM) needs to be performed to mitigate the collision risk. The essentially global nature of space activities makes data sharing and collaboration very important for SSA, especially in the context of collision avoidance. Because, in case a close approach event involves two operational assets, exchange of relevant information through coordination and collaboration between the concerned agencies helps to improve the accuracy of close approach analysis.
Space Situational assessment for 2021
Global Scenario: Space object population is found to be growing which is indicative of the ease of accessibility to space and varied applications of space technology in day-to-day life.
Space object to Launch ratio is highest in 2021. In other words, more space objects are placed in orbit per launch. In 2020, for 522 objects were placed in space with 102 launches comparing to 1860 objects in 135 launches in 2021. In 2021, more than 60% of objects added to space are from large LEO satellite constellations, mostly from SpaceX Starlink constellation.
In 2021, Spacetrack has reported 4 on-orbit break-up events generating 150 fragment objects and 2 collision events producing 942 objects, including that of Russian ASAT test. 515 space objects re-entered the Earth’s atmosphere as a result of natural decay. Clearly, the space object population has increased by many folds in comparison to the reduction through natural decay.
As on 1st January, 2022, India has 21 operational satellites in Low Earth Orbit (LEO) and 28 operational satellites in Geostationary Orbit. In 2021, ISRO carried out launch of one PSLV-DL variant (PSLV-C51) mission and one GSLV-MkII variant (GSLV-F10) mission. GSLV-F10 could not accomplish the mission successfully. In 2021, India placed 5 satellites and 1 PSLV rocket body (PS4 stage) in Low Earth Orbits.
India placed 65 rocket bodies in orbit till date from the first launch, of which 42 are still in orbit around the Earth and 23 have re-entered and burnt up in the Earth’s atmosphere. The break-up event of the 4th stage of PSLV-C3 in 2001 generated 386 debris of which 76 are still in orbit.
Safeguarding Indian space assets from debris in 2021
ISRO regularly carries out close approach monitoring for all space objects with Indian Space assets. ISRO monitored 4382 events with close approach distance less than 1 km to Indian LEO space assets and 3148 close approach events less than 5 Km with GEO objects during the year 2021. The maximum number of threats objects were from fragments of Fengyun 1C (Chinese ASAT test in 2007) and Cosmos – Iridium Collision. 84 close approaches less than 1 km were observed between Starlink satellites and Indian Space Assets.
Combined Space Operations Centre of USPACECOM issued 543 conjunction alerts regarding the close approaches to the Indian operational assets. The number of alerts is expected to increase significantly in the coming years with the increase in the number of space objects especially due to proliferation of the large satellite constellations.
Collision avoidance manoeuvre needs to be carried out to avoid any critical close approach which might lead to collision. The number of such manoeuvres have increased significantly in recent years. CAM is carried out by thrusting the satellite with the fuel stored in the spacecraft. Penalties incurred due to CAM include reduction in the lifetime of the spacecraft due to fuel expenditure and disruption of payload operations. The graph above depicts the number of such manoeuvres carried out by ISRO over the last few years. In 2021, ISRO carried out 19 CAMS, out of which 14 were executed for LEO and 5 for GEO spacecraft. It can be seen that number of CAMS are going up year to year and the increasing trend is anticipated to be continued in future with the unabated growth of the space object population.
In 2021, Chandrayaan-2 Orbiter (CH2O) and Lunar Reconnaissance Orbiter (LRO) of NASA were predicted to come critically close to each other in lunar orbit on 20 October, 2021. NASA and ISRO agreed that the situation merited a collision avoidance manoeuvre which was executed by Chandrayaan-2 on 18th October, 2021, well before the conjunction.
Spacecraft Decommissioning in 2021
Post Mission Disposal (PMD) was carried out for Cartosat-2, initially operating at a 630 km altitude circular orbit, through a series of apogee manoeuvres to lower its perigee altitude to 380 km from. Hence, the post mission orbital lifetime of Cartosat-2, otherwise expected to be more than 30 years, has been now reduced to less than 5 years in full compliance with the IADC recommended “25-year rule” on the post mission lifetime of LEO objects. The satellite is expected to be decommissioned in 2022.
Prior to its decommissioning on 24 Jan 2022, INSAT-4B was raised to a super-synchronous graveyard orbit followed by passivation of propulsion and electrical systems in complete compliance with the UN and IADC recommended post mission disposal guidelines for GEO objects.
With the ever-increasing trend in the object population in outer space, it is imperative for all responsible space faring entities to limit the creation of space debris by conforming to space debris mitigation guidelines. Monitoring of space objects is an essential component for continual assessment of the space situation and devising appropriate mitigation strategies for space asset protection. Towards this goal, space object observation capabilities need to be enhanced through the establishment of necessary infrastructure.
With the announced space reforms with private entities participation in space activities, India’s responsibility and the contributions in the long term sustainability of outer space need to be stressed. Indian efforts in SSA is coordinated centrally at SSA Control Centre in Bengaluru and managed by the Directorate of Space Situational Awareness and Management in ISRO HQ. ISRO has taken up the establishment of Space Surveillance and Tracking network with RADARS and Optical Telescopes under the project Network for Space Objects Tracking and Analysis (NETRA). To analyse and assess the potential threats to operational spacecraft and to attain self-reliance in safeguarding the valuable space assets, it is essential to augment and expand the network of observational facilities.
In the context of the global implication of space-based activities, collaboration amongst international space agencies and entities also plays a key role in effective monitoring and mitigation of the space debris environment.