Transcripts of Bennu’s Journey

[ music ] The Milky Way - home to billions of stars, rising and setting over billions of worlds, including our own. In this vast expanse, how did our Sun, the Earth, and the planets come to be? 

In recent decades, our understanding of the solar system's evolution has greatly improved, but deep questions remain. To answer those questions, astronomers are preparing to visit someplace very small. Asteroid Bennu. A lump of rock and organic material, the early building blocks of the solar system, of Earth, of us. Bennu is a time capsule, and its journey takes us way, way back...four and a half billion years. 

The raw ingredients of Bennu, and our solar system, originated in a stellar nursery: a vast cloud of hydrogen, helium, and dust. Our own Sun doesn't yet exist. Nearby are hot stars like this one, quickly burning up its fuel... and destroying itself in a colossal explosion called a supernova. The explosion destabilizes our cloud, causing it to collapse. 

In the geologic blink of an eye, a hundred thousand years, gravity and angular momentum flatten the cloud into a swirling disc. In the center, where molecules crash together tightest, a proto-star revs up to incredible pressures and temperatures. Deep within the disc, clumps of dust not much larger than a grain of wheat are flash heated into droplets of molten rock, called chondrules. The source of this heat remains a mystery. 

Chondrules are destined to become the building blocks of the solar system. Coaxed by gravity and turbulence, the chondrules clump. They grow into the first asteroids, into mountains, into planets. The asteroids are rubble piles of rock, metal, ice and organics. This large asteroid is the parent body of Bennu, a proto-planet whose size we can only guess. 

Closer to the proto-star, a planet begins to form. And then...dawn in the solar system. The proto-star undergoes fusion and ignites, revealing our Sun. But the solar system is far from finished. 

Jupiter most likely forms near its outer edge, but just 500 million years after the Sun ignites, some believe that it slowly moves inward. Its massive gravity ripples the asteroid belt, disrupting countless asteroids and comets, flinging them toward the Sun. They rain down on the inner planets, hammering and re-melting large portions of their crust. Did these impacts also deliver organics and water, key ingredients for life? 

Back in the asteroid belt, Bennu's parent body is lucky, it survives this period of heavy bombardment. The solar system cools and calms. Jupiter and its many moons assume the orbits that we see today. Billions of years of quiet follow...[ impact ] more or less. 

Then a billion years ago, one theory suggests a collision shatters the proto-planet. [ loud explosion ] Some of the debris loosely coalesces into a new, smaller body: Bennu. But Bennu will not stay in place. Dull, non-reflective, it slowly migrates toward the Sun. Solar heating turns its warm side into a low-intensity thruster. 

Through millions of years, Bennu's orbit gradually tightens, until it interacts with Saturn's gravity, altering its trajectory and hurling it into the inner solar system. Close encounters with Earth and Venus follow. Their gravitational tugs may have repeatedly stretched and reformed Bennu... turning it inside out and pulling off loose material. 

As a result, it has no satellites of its own...until now. Today, NASA is sending a spacecraft called OSIRIS-REx to explore Bennu and retrieve a sample. Why? Bennu has survived its long journey and settled into a near-Earth orbit, bringing its secrets within our reach. Now it is ready to teach us more about the solar system's history, its formation, its evolution, and our own place among the stars. [ music fades ] [ satellite beeping ]