When the space shuttle Atlantis returned to Earth this morning, it brought home some unusual cargo — seed pods grown in space. They’re the harvest of an 8-week-long commercial gardening experiment on board the International Space Station (ISS). Astronauts on the ISS have been tending a batch of fast-growing Arabidopsis plants (better known as “mustard weed”) to discover whether plants can complete their entire seed-to-seed life cycle in a weightless environment. If normal, healthy seeds were, the experiment will be a good sign that future astronauts can grow multiple generations of plants in space. Such self-perpetuating gardens will be a practical necessity for humans as they explore and colonize the solar system. Hardy space plants could provide fresh food, oxygen, and even clean water for explorers living for long stretches aboard orbiting outposts or on the Moon and Mars.