Why can't I use ' after' in the following sentence which imples 40 years should pass before we estalish a colony on mars?

The famous British scientist, Stephen Hawking, belives that there could a colony on Mars in/within/( After?) 40 years. NASA scientists are at present working on models of a space colony, permanent living quarters orbiting the Earth.

Unlike some other languages which you may be familiar with, after is usually not used to refer to the future as seen from the present moment.

He'll come in an hour.
Your son will learn to walk in a week.

It is sometimes used to refer to a point in time in the past.

He began to read a book but after an hour he became drowsy and fell asleep.

After is used in conjunction with a past tense verb

He went to school after he ate lunch

A colony was built on Mars after 40 years of exploration of the planet . . . . the action has been completed.

The sentence here is more in the future tense and the action is not definite . . . there could be. If the action has not been completed, you can't use after, you must use in or within.
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