Theory:

The experts calculated that the bulk of destructive nuclear power available on the Earth would be needed to achieve this mammoth task. A gigantic nuclear explosion suitably placed, suitably directed and suitably timed could do the trick. This could be done by placing the nuclear payload in a spaceship, sending it to intercept the approaching comet and detonating it by remote control. Success or failure, secrecy must be preserved. Finally a time-table was drawn up for the operation which was code-named ‘Project Light Brigade’.
The important dates in it were:
October 10: Despatch the spacecraft with the payload unless by then the comet is already destroyed by natural causes or has changed its path due to unforeseen reasons.
November 15: Rendezvous with the comet and detonation of the payload.
December 15: If the experiment failed this was the day the comet would hit the Earth. If it succeeded, this was the day the comet would pass by at a near but safe distance.
Explanation:

When the scientists decided to deflect Comet Dutta's route, they advised using the bulk of destructive nuclear power available on Earth, since it could help with such a massive undertaking. Such a job was supposed to be accomplished by a huge nuclear explosion that was appropriately positioned, directed, and timed.

It might be done by loading the nuclear payload into a spaceship, sending it out to meet the approaching comet, and then exploding it using a remote control. It was the decision of the worldwide conference participants. It was also decided to keep the plan secret, whether it was a success or a failure, because it would cause a lot of anxiety among people. The project was code-named ‘Project Light Brigade by scientists. The Charge of the Light Brigade was a failed military operation featuring British light cavalry led by Lord Cardigan against Russian forces. It honours heroic soldiers who risked their lives to achieve their goal. The project is named like that as people in the conference were unsure of the operation's result. And, finally, a timeline for the "Project Light Brigade" operation was created.

Experts had already decided that the collision of Comet Dutta with the Earth was unavoidable, and there were few possibilities that it would not hit the Earth or change its course. As a result, experts have opted to hold off the operation until October $$9$$ to see if those few possibilities work out and certain key dates were set for the execution. It was agreed to launch the spacecraft on October $$10$$ with a payload of nuclear power obtained from Earth unless the comet had already been destroyed by natural causes or changed its path due to unexpected circumstances. On November $$15$$, the payload would detonate when the spacecraft approaches the comet. If the charge of the Light Brigade fails, Comet Dutta would most likely strike the Earth on December $$15$$. Fortunately, if the Light Brigade initiative succeeds, Comet Dutta would pass by at a safe distance, even if it gets closer to the Earth.

Meaning of difficult words:

 S.No Words Meaning 1. Mammoth Very large 2. Brigade Group of people who are organised to do something 3. Payload The amount of explosive that a spacecraft or satellite can carry 4. Intercept To stop somebody or somebody that is moving from one place to another 5. Preserve To keep or maintain something safe 6. Approach A method of dealing with something 7. Despatch To send something to some place 8. Brigade A group of people working together to do something 9. Unforeseen An unexpected situation or a problem 10. Rendezvous An arrangement to meet something at a particular place and time 11. Detonation The action of causing an explosive to explode
Reference:
National Council of Educational Research and Training (2008). The Comet - II — JAYANT NARLIKAR (pp. 81-87). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.