Why didn't the nuke detonate after reaching the Stark Tower?

Question

In the end of Avengers (2012) movie, S.H.I.E.L.D. fired a nuke towards Manhattan to take out the portal and Chitauri army. Also, it looked like the nuke was precisely targeted to Stark Tower on which portal device was stationed.

Although, Iron Man was able to intercept it and change its trajectory, the nuke still reached its destination i.e. Stark Tower (Iron Man took the nuke into the portal which was directly above Stark Tower). Why didn't the nuke detonate upon reaching its destination?

Note: A nuke trigger is generally GPS location, timer, switch etc, but it's never impact because there's no incentive of impact (impact can't trigger nuclear reaction and it can even damage circuitry).

Answer 1

It was on a timer. Per the film script.

TONY: I got a nuke coming in, it's gonna blow in less than a minute. And I know just where to put it.

The missile detonated at precisely the right moment, but Tony took it off course, accelerating at a much faster pace than the missile was supposed to travel, hence why it reached its destination with time to spare to take it a mile into the sky.

[With a mighty effort, he wrenches the rocket off its course. Steering it from behind, he accelerates quickly, flying straight up into sky, towards the portal.]