Monday marked the 46th anniversary of humanity’s greatest achievement. But it was images last week from a much more distant land that reminded us how exciting space exploration can be.The appropriately named New Horizons unmanned probe sent back startling pictures of Pluto, once designated the solar system’s outermost planet before being downgraded to dwarf planet status in 2006 — the same year the New Horizons mission launched from Cape Canaveral.