What does "in" mean in the following sentence? "NASA lanched the satellite. But several days in, something went wrong." Does it mean after several days?
Several days in
Expressions like this can be interpreted as equivalent to "after" ("several days after") in this case.
Typically, in would be used in an expression like this when it's relative to another given time period
4 hours into the 12 hour flight, I became sick
We were going on a 12 day vacation. Three days in, ....
It is another example of idiom born of lazy language usage. Using "in" in such context shortens the idea and depends on the implication of the previous part of the conversation. Using in for into the journey, into the process, into the treatment, etc. relieves the writer of repeating what may already have been said and understood,