Explained: ISRO starts countdown for launch of 'PSLV-C55', but why the one-minute delay?

The Indian Space Agency (ISRO) commenced the countdown for the Saturday launch of its PSLV rocket carrying two Singaporean satellites. The 25 hours 30 minutes countdown commenced at 12:50 on Friday,and the vehicle liftoff will happen at 2:20 pm (Indian time) on Saturday. This will be the 57^thflight of India's PSLV rocket, at present the oldest and most-flown rocket in the country's fleet.

The primary passenger satellites is TeLEOS-2, a 741kg satellite developed under a partnership between DSTA (representing the Government of Singapore) and ST Engineering. Once deployed and operational, it will be used to support the all-weather, day and night, satellite imagery requirements of various agencies within the Government of Singapore. TeLEOS-2 carries a Synthetic Aperture Radar (SAR) payload.

The secondary passenger is LUMELITE-4, a 16-kg satellite developed by Singaporean academia. LUMELITE-4 is developed for the technological demonstration of the High-Performance Space-borne VHF Data Exchange System (VDES). It aims to augment Singapore’s e-navigation maritime safety and benefit the global shipping community.

In addition to the two primary satellites, there are seven non-separating payloads that are on-board the PSLV rocket. These experiments would be housed within the fourthand final stage of the rocket.According to ISRO,PSLV-C55 mission will carry out in-orbit scientific experiments by using the spent PS4 stage as an orbital platform. This is the third time that PS4 will be used after satellite separations as a platform for experiments.

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Earlier this week, ISRO anounced that the PSLV vehicle in its core-alone configuration (without strap on boosters) would lift-off from the first launch pad in Satish Dhawan Space Centre at Sriharikota, at 2:19pm. The two satellites will be placed in a circular obrit, 586kms above the earth's surface. However, the launch is now learnt to have been delayed by a minute. What's in a minute you may ask. In orbital rocketry, a minute could mean the difference between catastrophe and celebration.

How are rocket launch timings determined?

There is an entire branch of space science known as 'Orbital Mechanics' that is used to determine what is known as a 'Launch window', a period of time within which a certain launch must happen. Launch windows depend on a plethora of factors - place of launch, type of orbit intended, final destination, whether the satellite is headed for another planet, number of satellites being carried, the time of the year, position of the relevant heavenly bodies and also minimal risk of space debris.

We know thatLow Earth orbit is getting increasingly populated with constellations of communication satellites from various firms, and also pre-existing satellites, space debris and much more. Under such circumstances, space agencies look at the safest time of flight, when there is nil risk of collission or proximate approaches with space debris. Even, millimetre-sized debris can pose a major threat to space assets, as the debris in Low Earth orbit circles the earth at speeds close to 27,000kmph. A collission at such speeds could have unimaginable impacts.

How ISRO monitors space debris, safeguards space assets?

The Indian Space agency has a facility aimed at performing what in simple terms can be explained as a holistic 'Traffic Management in Space' and to ensure the sustainable use of space. Known as the'ISRO System for Safe & Sustainable Operations Management' (IS4OM), the new facility based in Bengaluru will perform a host of functions - safeguarding India's space assets, protecting them from space debris, predicting the orbital movement of debris, predicting likely collisions and coordinating with global agencies for all of the above.

"This is not about (space)traffic management alone, it is about keeping orbits available to everyone, operating in space without causing problems to one another and ensuring sustainable use" explained ISRO Chairman, DrS. Somanath had earlier told WION.

Elaborating on the different functions of 'IS40M', DrSomanath said, "Firstly it is about using radars and optical observation to watch and tag objects in space, then we will have toconsider the effect of earth's gravitational pull, impact of sun and moon etc. andpredict the movement of the said object/debris. Based on this, we will have to look for possibilities of collisions. This entire process has to be carried out on a daily basis. In case an object poses a threat to our satellite, we must accordingly move our satellite out of harm's way and also inform foreign agencies(whose satellites are in the vicinity) about our collision avoidance maneuver."

In addition to this, the objects that are likely to fall back onto earth), asteroids that are on collision course with earth will also have to be tracked. These are complex calculations and are error-prone, owing to the large number of factors involved, thus necessitating various corrections.DrSomanath added that these mathematical models used to calculate and project the path of debris and their impact etc. are indigenously developed and even if a country wanted, such models are not available for purchase in the International market.

On India's capabilities, he had explained that India has radars that aremeant to observe satellites and the Indian space agency is also installing optical tracking facilities. However, the Indian space agencydoes not have all facilities required to track objects of 10cm size and for this it relies on data provided by foreign agencies.

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