Is time travel possible and if so why have no future time astronauts left their mark for us here in the 20th century ?

You could potentially slow down time for yourself by travelling at close to the speed of light, or perhaps by some stasis technology we haven't invented yet, but we're a long way off moving forward more than a few seconds compared to everyone else. They did tests with jet planes and atomic clocks, but you know, that only shows it's possible for time to be relative, and you can't ever travel backwards, according to Einstein.

How do you know they haven't?

No.

The fact that there are grandfathers alive and well today proves that time travel is not possible.

Cheers!

Time travel is not possible.

So far, it’s been found that time can only move forward...

Oddly, it does seem to be possible. Maybe we just become extinct before we work out how to do it.

How do you know they havent?

It's not possible, except for the trivial case of travel into the future at the rate of 1 hour per hour, that we all do.