So. With that in mind, here is a rough (and only partial) inventory of the stuff mankind has left on the moon:

It's easy to be a little bit appalled by this—by the waste we have left on our pristine planetary neighbor, by our treatment of the moon as yet another landfill (lunefill?). After all, literally tons of garbage!

But the trash, in this case, is strategic—the cost of returning to Earth being high, it is the price we pay for discovery. The items we've left on the moon are also members of an enormous ecosystem of space debris: At the moment, more than 21,000 pieces of space junk—abandoned satellites, spent rocket stages, fragments of disintegrated spacecraft—are orbiting the planet, shrouding Earth in a man-made mantle.

But jettisoned spacecraft aside, many of the objects that reside on the moon are there because humans have selected them, specifically, to be relics of the long history of human exploration. Swirled somewhere in the gray powder of Tranquility Base, there is an olive branch, wrought of gold and human industry. That item is technically space junk; junk, however, it is not. Space has its own way of merging loss and eternity. The location of NASA's most recently crashed lunar probes was selected specifically so as to be far away from Tranquility Base and other "lunar heritage sites." And the site of the vehicles' wreckage, the agency announced this week, will be named after Sally Ride.

This seems fitting. Not only did Ride dedicate her final years to the GRAIL mission; the lunar landscape is also a frontier that represents, to us frail humans, both proximity and eternity. Whatever remains there—whatever we choose to leave there—takes on, by default, a special kind of perpetuity. One other earthly object that resides on that landscape is an urn containing the ashes of Eugene Shoemaker, the famed planetary geologist who dreamed, during his life, of going to the moon. The container, composed of polycarbonate, is 1.75 inches long and 0.7 inches in diameter, and carried in a vacuum-sealed, aluminum sleeve. Around it is wrapped a piece of brass foil inscribed with this passage from Shakespeare's "Romeo and Juliet":

And, when he shall die,

Take him and cut him out in little stars,

And he will make the face of heaven so fine

That all the world will be in love with night,

And pay no worship to the garish sun.

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