Theory:

     On December 15 Comet Dutta came closest to the earth — at a distance of 80,000 kilometres. Millions saw it and admired it. Only a handful knew how close they had come to total annihilation.
 
     When the comet had gone far away and was seen no more, Duttada felt it safe to make the following comment to his wife: “Now that the comet came and went, are you satisfied that no major disaster took place that can be attributed to it?”
 
     “I agree that there has been no major disaster; but there could have been some. Do you know how they were averted?” Indrani Debi said with quiet confidence.
Explanation:
 
On December 15th, Comet Dutta came within \(80,000\) kilometres of the Earth. It grabbed the attention of millions of people all around the world. Comet Dutta was admired by them. But, only those connected to the Project Light Brigade realised how close it (Comet Dutta) had come to destroying the planet. However, as predicted by experts, it did not strike the Earth since its path was diverted owing to the successful operation of the Light brigade's spacecraft. After a while, Comet Dutta had faded away and was no longer visible.
 
After a lengthy period of mental turmoil, Duttada felt at ease. He inquired whether his wife was happy that Comet Dutta had caused no harm to anyone on Earth. She agreed with his statement and told him that Comet Dutta could have destroyed the Earth, but it had not happened. Hearing her remark, Duttada was taken aback. He was quite certain that he hadn't told anyone about the Project Light Brigade. She went on to ask Duttada, confidently, whether he knew why Comet Dutta had vanished without causing harm to the Earth.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Annihilation
The state of being completely destroyed
2.
Avert
To prevent something bad or dangerous from happening
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.