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Theory:

The outer ice is melting
Causing vapor from the force,
And leaves a trail behind it
As it travels on its course.
Explanation:
  
In the third stanza, the speaker further explains how a tail of a comet is formed.
 
A comet is basically a flying ice mountain of compressed gases and dust. While travelling through space, and as it comes closer to the sun, the comet’s surface (which is made up of ice) starts melting. This causes the gases within the comet to vaporise. This also gives the otherwise rocky structure a fuzzy head (known as 'coma'). On the other hand, the radiation from the sun and the solar wind pushes out the gas and dust particles out of the comet to form a tail-like structure known as the comet's tail. As the comet travels, the tail follows.
Comet_Wild2.jpg
A comet without its head and tail*
  
Important!
Click here to learn more about the different parts of a comet .

Back to the poem, the speaker states that 'the comet leaves a trail behind it'. Here, the word "trail" can mean two things.
 
Firstly, one can substitute the word "trail" with the word "tail". The substances that form the tail are seen as an extension of the comet itself. It goes where the comet goes.
 
Secondly, the speaker might suggest that the comet leaves behind the particles as it travels. As we recall, the tail is said to be about \(150\) million km in length. Since the tail is very long, the speaker is probably trying to tell us that the length of the comet's tail makes it look like a spillage rather than an extension of the comet. In this perspective, the tail is detached from the comet, and so, when the comet moves, it leaves behind the particles, but interestingly enough, the trail doesn't follow the comet.
 
Halley's_Comet,_1910.jpg
A comet leaves behind a trail**
 
Meanings of difficult words:
 
S.No
Words
Meanings
1
TrailA mark or a series of signs or objects left behind by the passage of someone or something
2
CourseThe route or direction followed by a ship, aircraft, road, or river
3
SpillageThe act or process of spilling
4
RadiationThe process of emitting radiant energy in the form of waves or particles
Reference:
State Council of Educational Research and Training (2019). Term-3 English Standard-9. The Comet- Norman Littleford (pp. 95). Published by the Tamil Nadu Textbook and Educational Services Corporation.