The asteroid is steadily disintegrating. By the time it reaches the Earth, it'll be probably be no larger than an ordinary meteor.
7 seconds sound clip from Superman (1948) - The Reducer Ray (Chapter 3).
You can hear this line at 00:12:11.621 in the Blu-ray version of the episode.
Quote context
[...]
- If he interferes, I'll find a way to destroy him.
- Hold page one for possible re-plate. City desk...
- What does it all add up to, Morgan?
- Ordinary? I doubt that. Its behavior's been very strange.
- What is its present location?
- Approximately 30000 miles away.
- As near as we can calculate, it should strike our atmosphere in about 10 hours.
- Oh. That would be about 8 tonight.
- You better inform the press, but avoid anything sensational. We don't want any panic.
- Well, I was wondering if you'd heard anything about my raise.
- Yes, I have. I meant to tell you the board turned down your request for a raise.
- I can't get along on what they pay me! I need money! I...
[...]