Using a hyphen when describing measures (e.g., a 300-meter asteroid)

Question

Here's some examples. Please tell me which ones are right, and which ones are wrong, and why.

There's also extended versions.

Etcetera. Which ones are right/wrong, and why?

Perhaps surprisingly, an answer on an apparently-unrelated question actually answers this. We are going to need that canonical answer on a more obviously-related question. — Andrew Leach, Jun 19 '15 at 07:12

score 1 · Answer 1 · answered Jun 19 '15 at 06:55

"A 350-meter asteroid impacted the Moon." is the right form.

"A 350-meter-wide asteroid impacted the Moon." is the right extended form.

Here you are combining two or more words to form a compound adjective in front of a noun and in this case, you need put a hyphen between these words.

We can also say ->

"A 350 meters wide asteroid impacted the Moon."

Here the number is not used as compound adjective preceding a noun and hence there is no need to hyphenate.

"A 350 meters wide asteroid" is incorrect. – phoog Jun 19 '15 at 07:41 — phoog, Jun 19 '15 at 07:41

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