By Tom Metcalfe

Far overhead, out of sight and mind for most people on Earth, two different robotic missions are on the cusp of a revolutionary new era in 21st-century spaceflight.

The idea of sustainability in space is a relatively new concept. Most space technology throughout history has been expendable, and balancing what goes up with what comes down is a recent concern.

“We’ve had a pretty consistent model—‘get it into space, and then we’re finished,’” says Danielle Wood, founder of the Space Enabled research group at the MIT Media Lab. But that model has created tons of trash in the “graveyard orbit” above the geostationary belt, while defunct spacecraft in low-Earth orbit can circle for decades before they burn up when their orbits decay, pumping potentially problematic amounts of vaporized metal into Earth’s sensitive upper atmosphere. “Both are not sustainable,” Wood says; future space missions will have to deal with the problem of cleaning up after they've finished. “Sustainability in space means that we are able to leave to future generations something like the natural state of the Earth, the moon and the solar system.”