ISRO plans controlled re-entry of decommissioned satellites into Earth's orbit

The experiment has been undertaken as this satellite had sufficient leftover fuel, which presented a unique opportunity to test the relevant methodologies and understand the associated operational nuances of post mission disposal by direct re-entry into the Earth’s atmosphere.

The orbital lifetime of MT1  would have been more than 100 years in its 20 degree inclined operational orbit of 867 km altitude. About 125 kg on-board fuel remained unutilised at its end-of-mission that could pose risks for accidental break-up. This leftover fuel was estimated to be sufficient to achieve a fully-controlled atmospheric re-entry to impact an uninhabited location in the Pacific Ocean.

Inter-Agency Space Debris Coordination Committee space debris mitigation guidelines recommend deorbiting a LEO (low Earth orbit) object at its end-of-life (EOL), preferably through controlled re-entry to a safe impact zone or by bringing it to an orbit where the orbital lifetime is less than 25 years, according to ISRO. It is also recommended to carry out passivation of on-board energy sources to minimise the risk of any post-mission accidental break-up.

Controlled re-entries involve deorbiting to very low altitudes to ensure impact occurs within a targeted safe zone. Furthermore, the on-board constraints of the aged satellite, where several systems had lost redundancy and showed degraded performance, and maintaining subsystems under harsher environmental conditions at much lower than originally designed orbital altitude added to the operational complexities.