Ajit Karn
The Indian Space Research Organisation announced the successful completion of a “particularly challenging” controlled re-entry experiment involving the decommissioned Megha-Tropiques-1 (MT-1) satellite. “The satellite re-entered the Earth’s atmosphere and dissipated over the Pacific Ocean,” the national space agency headquartered in Bengaluru said on Twitter on Tuesday.
ISRO stated that the estimated ultimate impact region is in the deep Pacific Ocean within the anticipated latitude and longitude boundaries.
The low Earth satellite was launched on October 12, 2011 as a joint venture between the Indian Space Research Organization (ISRO) and the French space agency CNES to study tropical weather and climate.
Since August 2022, the perigee of the satellite has been gradually lowered through a series of 20 manoeuvres, consuming approximately 120kg of propellant.
Multiple manoeuvres, including the final de-boost strategy, were designed after taking several constraints into account, such as visibility of the re-entry trace over ground stations, ground impact within the targeted zone, and allowable operating conditions of subsystems, particularly the maximum deliverable thrust and the maximum firing duration constraint on thrusters.
ISRO stated that all manoeuvre plans were analysed to ensure there would be no post-manoeuvre close approaches with other space objects, particularly crewed space stations such as the International Space Station and the Chinese Space Station.
The final two de-boost fires were conducted on March 7 at 11:02 UTC and 12:51 UTC, respectively, using four 11 Newton thrusters for approximately 20 minutes each.
The final perigee was estimated to be less than 80 kilometres, indicating that the satellite would penetrate the denser layers of the Earth’s atmosphere and then disintegrate. The analysis of re-entry aero-thermal flux confirmed that no sizable debris fragments would survive re-entry.
The entire series of events was orchestrated from the Mission Operations Complex at ISTRAC (ISRO Telemetry, Tracking, and Command Network) in Bengaluru.
ISRO announced earlier this week that an uninhabited area in the Pacific Ocean between 5°S and 14°S latitude and 119°W and 100°W longitude has been designated as the re-entry zone for the 1,000-kilogram MT1 satellite.
ISRO had noted in a statement that approximately 125 kilogrammes of unburned propellant remained aboard the spacecraft at the conclusion of its mission, which posed a risk of inadvertent demise.
ISRO estimated that this remaining propellant was sufficient to accomplish a fully controlled atmospheric re-entry and impact the uninhabited area of the Pacific Ocean.
De-orbiting to extremely low altitudes is required for controlled re-entry in order to assure impact within a designated secure zone.
Large satellites and rocket bodies that are likely to sustain aero-thermal fragmentation upon re-entry are typically subjected to controlled re-entry to reduce the risk of ground casualties.
However, these satellites are all designed to endure controlled re-entry at the end of their service lives (EOL).
ISRO stated, “MT-1 was not designed for EOL operations via regulated re-entry, making the entire exercise exceedingly challenging.”
In addition, the on-board limitations of the ageing satellite, in which several systems had lost redundancy and demonstrated degraded performance, and the maintenance of subsystems under harsher environmental conditions at a significantly lower orbital altitude than originally designed added to the operational complexities, the report stated.
The ISRO centres’ mission, operations, flight dynamics, aerodynamics, propulsion, controls, navigation, thermal, and other sub-system design teams collaborated to overcome these obstacles, according to the report. The operations team implemented innovative workarounds as a result of their research, discussions, and exchanges.
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ISRO stated that despite the satellite’s initial three-year mission duration, it continued to provide essential data services until 2021, supporting regional and global climate models.
ISRO reports that UN/IADC (Interagency Space Debris Coordination Committee) space debris mitigation guidelines suggest de-orbiting a LEO (Low Earth Orbit) object at its EOL, preferably through controlled re-entry to a safe impact zone, or by bringing it to an orbit with an orbital lifetime of less than 25 years.
ISRO also recommends “passivation” of on-board energy sources in order to reduce the risk of a post-mission accidental breakup.
In its operational orbit at an inclination of 20 degrees and an altitude of 867 kilometres, MT-1’s orbital longevity would have exceeded 100 years.
ISRO’s statement reads, “As a responsible space agency committed to safe and sustainable operations in outer space, ISRO makes proactive efforts to comply with the UN/IADC space debris mitigation guidelines on post-mission disposal of LEO objects.”
