Learn French: Sans doute le voyage spatial serait-il trop long. - Probably the space journey would be too long.

This is called complex inversion, which is triggered when you start a sentence with certain adverbs like Sans doute. Because the true subject is a noun (le voyage spatial), you cannot simply swap it with the verb. Instead, the noun stays in its normal position at the front, and you add the matching dummy pronoun (il) right after the verb with a hyphen.

Doesn't sans doute mean 'without a doubt'? Why is the translation 'probably'?

Although it literally translates to 'without doubt', its meaning has weakened over the centuries. In modern French, sans doute almost always means 'probably' or 'very likely'. If you want to express absolute certainty, you must use sans aucun doute.

Is it absolutely mandatory to use complex inversion after sans doute?

It is required in formal and written French. However, in casual spoken French, you can avoid this complex structure by adding que right after the adverb. For example, you might hear: Sans doute que le voyage spatial serait trop long.

Why does spatial come after voyage here?

In French, classifying adjectives that define a specific category or technical type (like spatial, médical, or historique) always follow the noun. Unlike some short descriptive adjectives, they can never be placed before the noun.