Theory:

     “Let me assure you, Sir John, that it is correct. I stake my reputation on it,” James could not contain himself any more.
 
     “Do you realise what will happen if Comet Dutta collides with the Earth, as you predict it will?”
 
     “The effects will be catastrophic! That is why I have taken extra care to verify my calculations. Barring rare circumstances, the collision is inevitable.” James was confident. But Sir John picked out the one qualifying phrase: “What are those rare circumstances?”
 
     “Well, it might collide with some asteroid before reaching here. Or it might just split up when near the Sun, or it might evaporate...”
Explanation:
 
James assured John that the information contained in the paper was accurate. After all, he was confident in his real work, which he completed with great care, accuracy, and commitment. James provided a guarantee of his findings, putting his own reputation on the line, and he knew that if his claim was proven to be false, his reputation would be ruined overnight. James had taken efforts to disclose the information as he believed in his ability. And, so James could neither tolerate criticism on his work nor could he permit it be left without published. Sir. John then asked him whether he realised what would happen if Comet Dutta crashed with the Earth, as James had predicted.
 
James arguing with John.jpg
James arguing with John
 
James answered to him, saying it would result in unimaginable devastation as it would completely destroy the Earth's ecosystem and all living things on it. This is what motivated James to put in extra effort in order to arrive at the confirmed calculation.
 
The collision of Comet Dutta was unavoidable, and it would most likely happen unless there are any circumstances that prevent it. After hearing it, John enquired about the peculiar circumstances in which the Earth-Comet Dutta collision would not occur, as James had predicted.
 
James explained to John that, as per his prediction, Comet Dutta could collide with an asteroid, break apart, or evaporate if it gets close to the sun. But these are the least chances that could happen and there are more chances of a collision between Earth and Comet Dutta. If a collision occurs, it may inflict a great deal of damage.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Asteroid
A mass of rock that revolves around in space
2.
Stake
Taking risk on something, hoping to bring good result
3.
Reputation
The opinion of people towards someone or something
4.
Collide
Two objects hitting or crashing each other
5.
Catastrophic
Something which brings in lot of damage and destruction to people' lives
Reference:
National Council of Educational Research and Training (2008). The Comet I - Jayant Narlikar (pp. 73-80). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.