Somewhere beyond the orbit of Mars, under Jupiter's watchful eye, an asteroid rotates silently with a broadsword jammed through its iron ribs. Because this blade was crafted beyond Earth's atmosphere, its metal and that of its resting place are seamlessly conjoined; as such, separating their forms requires extraordinary degrees of both strength and finesse.
It is for the protection of the most foolhardy and ambitious of astronauts that this weapon is so deeply embedded. Newton's third law necessitates that whosoever first draws this sword from its stone will be propelled outward with an equivalent force to that which was required to release it. The resultant launch vector will either stamp their doomed outline into the face of a nearby Jovian moon, or thrust them far beyond the boundary of heliopause, into the greater void.
Out there, in that cave that has no walls, constellations (if such a term even applies) older than myth await them, whose stars collapsed so long ago that their light no longer reaches Earth. Without any matter to call their own, they crave to reclaim their former permanence above all else. Their very mouths have event horizons, and their digestive tracts lead downward into chambers which can never be filled.
Whosoever first draws this sword from its stone will need it more than anyone ever has.
Only the most adept of astronauts need apply.
Given the availability of modern conveniences, there might be a simpler solution.
Certain tricks might even allow a craftier astronaut to take them by surprise.