It measures speed comet interacts with Earth (10)
I believe the answer is:
tachometer
'it measures speed comet interacts with earth' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for tachometer that I've seen before include "rev counter" , "Instrument measuring speed of rotation" , "It measures revolutions" , "Teacher Tom (anag.)" .)