Nova

If spacecraft OSIRIS-REx can grab a piece of an asteroid and bring it back to Earth, scientists could gain great insight into our planet's origins, and even how to defend against rogue asteroids. But NASA only gets three shots at collecting a sample.

As global temperatures rise, scientists are exploring geoengineering solutions, from planting trees to sucking carbon out of the air to physically blocking out sunlight. But would it work? And what are the risks of engineering Earth's climate?

The Colosseum is a monument to Roman imperial power and cruelty. Its graceful lines and harmonious proportions concealed a highly efficient design and advanced construction methods that made hundreds of arches out of 100,000 tons of stone. In its elliptical arena, tens of thousands of gladiators, slaves, prisoners and wild animals met their deaths. Ancient texts report lions and elephants emerging from beneath the floor, as if by magic, to ravage gladiators and people condemned to death.

More than two thousand years ago, the thriving city of Petra rose up in the bone-dry desert of what is now Jordan. An oasis of culture and abundance, the city was built by wealthy merchants whose camel caravans transported incense and spices across hundreds of miles from the Arabian Gulf. They carved spectacular temple-tombs into its soaring cliffs, raised a monumental Great Temple at its heart, and devised an ingenious system that channeled water to vineyards, bathhouses, fountains, and pools.