Neil Armstrong left a permanent trace in our history. I'm convinced that once most present-day people are gone and forgotten, he will persist in collective memory, just like Gagarin, Columbus, Einstein and Julius Cesar. But his legacy may very well outlive humanity itself. There is no erosion on the Moon. Rain will not wash off that very footprint, wind will not disperse it, it won't be deformed by tectonic movements.
All monuments, plaques or symbols raised in memory of the first man who set his foot on the Moon may not last a tiny fraction of the time that footprint will last. If humanity vanishes this instant, in just a few decades cities will fall. Some structures like pyramids or huge concrete dams will survive much longer, but eventually even those will perish. Plastic may survive longer, but even today we know of organisms that eat plastics. Rushmore monument may last the longest, but in time even that will wither.
Perhaps hundreds of millions years from now, the Earth - with an entirely different set of continents - may be unrecognisable, and that footprint and pile of metal will be all that is left of humanity. The flag is probably white by now from solar radiation, and will eventually crumble into pieces from constant temperature changes (-233'C / -387'F during night, 123'C / 253'F during day). If space tourists from the future don't stomp on it, if a stray meteorite doesn't hit it, the monument Armstrong made himself may last even five billion years, until the Sun changes into a red giant and burns it along with the pale blue dot.
Neil Armstrong (5 VIII 1930 - 25 VIII 2012)
