Theory:

    Before the President could speak his personal telephone rang. He lifted the receiver and listened quietly for a minute.

    “Colleagues!” he announced, “the first spacecraft has landed.”
 
    It was a red letter day in Tilloo’s life—the day he was waiting for. For his father had at last taken him to the Control Room. From here he could see the alien spacecraft on his TV screen.

    “What a funny looking thing, Daddy! What does it contain?” Tilloo asked.

    His father shook his head, “Can’t say for sure. We are only allowed to observe it from far. But we control all its parts and can exercise our power if it is up to any mischief.” He pointed to an impressive panel with several coloured buttons on it. Tilloo looked at them wistfully.
Explanation:
 
Later the President thought of speaking to his colleagues. Meanwhile, his phone rang, and he was unable to speak to his team. Then, he took up the receiver and listened for a minute silently. It means he was paying attention to the person on the other end of the line. After listening to the phone call, the President announced to his colleagues that the first spacecraft had landed.

According to Tilloo, that day was a red-letter day. The term "red-letter day" refers to a day that was particularly notable or unforgettable. It was a momentous day for him because he had finally got what he wanted. He desired to go to his father's workplace. At last, his father had taken him to the Control Room with him that day. While he entered the Control Room, he saw the alien spacecraft on the big TV screen.

After looking at the alien spacecraft, Tilloo had several doubts in his mind. So he enquired his father about what the spacecraft contained, and also he stated that it looked funny. Later, Tilloo's father nodded his head and said that they were also unsure what was inside the alien spacecraft and that they had received orders from higher officials to observe it from a far distance. Tilloo's father also claimed that they could control all of the components of the alien spacecraft and use their power if they were causing problems. Finally, Tilloo's father pointed to a large panel with a variety of coloured buttons. The control panels were kept for accessing the spacecraft's movements. Otherwise, the spaceship would be controlled using the control panels if it was causing any mischief. Later, Tilloo glanced longingly at them. It meant that he longed to touch the control buttons.
 
button-1785703_1280.png
Coloured buttons
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Red letter day A day that is pleasantly noteworthy or memorable
2
Impressive Evoking admiration through size, quality, or skill; grand, imposing, or awesome
3
Observe Notice or perceive something and register it as being significant
4
MischiefPlayful misbehaviour or troublemaking, especially in children
5
WistfullyWith a feeling of vague or regretful longing
Reference:
National Council of Educational Research and Training (2007). Supplementary. An Alien Hand - Jayant Narlikar (66-73). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.