How does Isro save satellites from collision?

India saved its satellites from collision 11 times in 2024, according to Isro’s Indian Space Situational Assessment Report (ISSAR) report.

The Indian Space Research Organisation (Isro) in the report said that it saved six satellites in the low-Earth orbit (LEO) and four in the geostationary orbit (GEO) from collision. The Isro also said that it prevented the Chandrayaan-2 orbiter from a planetary collision.

The Isro said that an orbit maintenance manoeuvre was originally scheduled for Chandrayaan-2 on November 26, 2024, but it was advanced to Nov. 11 to mitigate conjunctions with Nasa’s Lunar Reconnaissance Orbiter (LRO), which were predicted to occur on Nov. 15-16.

The Isro prevents such collisions by performing collision avoidance manoeuvres (CAM). These are planned changes in a spacecraft or a satellite’s trajectory in space to avoid running into some other space object, which can be a body in the space, satellites, or debris from other satellites.

The Isro performs such manoeuvres after conducting Space Situational Awareness (SSA) assessments to see whether an object in the space is going to be in the close vicinity of a spacecraft or a satellite. Such objects can include space debris, natural objects like asteroids and meteoroids, and energy and particle flux.

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Isro’s System for Safe and Sustainable Operations Management (IS4OM) functions as the nodal entity for all space sustainability efforts, including SSA assessments and to improve compliance with internationally-recognised guidelines on the long-term sustainability of outer space activities.

Isro in the report said that it conducts analyses to predict close approaches by other space objects to Indian space assets. In case of any critical close approach, CAMs are carried out to rule out or minimise the collision risk. Last year, the Isro said that the Combatant Space Operations Center of the US Space Command had issued more than 53,000 alerts Isro’s Earth-orbiting satellites. These alerts were analysed using more accurate orbital data from flight dynamics and 11 collision avoidance manoeuvres were conducted — as mentioned above.

The Isro released the following graph to show the CAMs conducted in recent years:

The number of CAMs was less in 2024 compared to the previous year because improved close approach analysis methodology with larger conjunction screening volume and usage of more accurate ephemerides helped to meet collision avoidance requirements by adjusting orbit maintenance manoeuvres on several occasions and avoiding exclusive CAMs, according to Isro.

Last year, the Isro said, all manoeuvre plans, including those of the CAMs, were subjected to close approach risk analysis to rule out any potential close approach with other neighbouring space objects imminently after the manoeuvres. In doing so, the report said that 89 manoeuvre plans were revised to avoid post-manoeuvre close approaches with other space objects for LEO satellites and on two occasions manoeuvre plans were revised such for GEO satellites.