gemination orthography

Question

In this sentence

ミサイルは高【たか】さ100kmより低【ひく】い所【ところ】を東【ひがし】に750km飛【と】びました
The missile flew 750 kilometers east at a height lower than 100 kilometers.

appearing here https://www3.nhk.or.jp/news/easy/k10013261541000/k10013261541000.html the recording sounds like there is gemination between the end of ひゃく and the beginning of キロメートル.

What is the orthographically correct way of writing "100km"?

ひゃっキロメートル

or possibly

ひゃッキロメートル

Answer 1

I agree with @Simon in the comments: since the gemination comes from the く in ひゃく, ひゃっ should be the natural choice when writing ひゃっキロ in kana and I would not be too surprised to see

[100]{ひゃっ} [km]{キロメートル}

as furigana in a textbook. (That said, in all textbooks for beginners I could find, there actually seem to be no furigana at all — try to search the web for images with something like 単位 教科書.)

Note that numbers are usually not written in kana, so one should not expect any reliable corpus data to confirm this — e.g. the Balanced Corpus of Contemporary Written Japanese has 0 hits for both ひゃっキロ and ひゃッキロ.

However, @a20 has found a paper titled "Morphology of numerical expressions in contemporary Japanese", where the pronunciation of numbers plus counters in written in kana and you can find the following:

Also interesting is the following:

which says that for 6 and 100 (which end in く), gemination essentially always occurs, but for 1 and 8 (which end in ち), there is some variation.

Answer 2

In response to the comment above...

since the gemination comes from the く in ひゃく, ひゃっ is more sensible than ひゃッ

I can add the following sentence found here

https://www.asahi.com/articles/ASN7F2QDMN76UPQJ009.html?iref=pc_extlink

長期休【ちょうきやす】みが来【く】るとホッとしたし、終【お】わりに近【ちか】づくと「また学校【がっこう】が始【はじ】まる」とつらかった

I was relieved when the long vacation came, and when it was nearing the end, it was hard to think that school would start again.

in which we see the following vocabulary item with two different gemination orthographies, according to Jim Breen's wwwjdic:

ホッとした from : ほっと(P); ホッと(P); ホっと (adv,vs) (1) (on-mim) with a feeling of relief; with a sigh of relief; (adv) (2) (on-mim) deeply (sigh)

In light of this example, it consequently appears hard to justify the statement that "gemination comes from" a word, or that any particular orthography is "more sensible." Perhaps responses should be based on objectifiable information, such as frequency of a word's or a spelling's occurrence or at least percentages of their acceptability as determined by polls.