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[ music ]

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The Milky Way - home to billions of stars,

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rising and setting over billions of worlds, including our own.

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In this vast expanse, how did our Sun, the Earth, and the planets come to be?

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In recent decades, our understanding of the solar system's evolution has greatly improved,

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but deep questions remain.

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To answer those questions, astronomers are preparing to visit someplace very small.

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Asteroid Bennu. A lump of rock and organic material,

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the early building blocks of the solar system, of Earth, of us.

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Bennu is a time capsule, and its journey takes us way, way back...four and a half billion years.

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The raw ingredients of Bennu, and our solar system, originated in a stellar nursery: 

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a vast cloud of hydrogen, helium, and dust.

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Our own Sun doesn't yet exist.

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Nearby are hot stars like this one, quickly burning up its fuel...

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and destroying itself in a colossal explosion called a supernova.

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The explosion destabilizes our cloud, causing it to collapse.

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In the geologic blink of an eye, a hundred thousand years,

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gravity and angular momentum flatten the cloud into a swirling disc.

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In the center, where molecules crash together tightest,

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a proto-star revs up to incredible pressures and temperatures.

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Deep within the disc, clumps of dust not much larger than a grain of wheat

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are flash heated into droplets of molten rock, called chondrules.

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The source of this heat remains a mystery.

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Chondrules are destined to become the building blocks of the solar system.

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Coaxed by gravity and turbulence, the chondrules clump.

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They grow into the first asteroids, into mountains, into planets.

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The asteroids are rubble piles of rock, metal, ice and organics.

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This large asteroid is the parent body of Bennu, a proto-planet whose size we can only guess.

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Closer to the proto-star, a planet begins to form.

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And then...dawn in the solar system.

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The proto-star undergoes fusion and ignites, revealing our Sun.

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But the solar system is far from finished.

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Jupiter most likely forms near its outer edge,

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but just 500 million years after the Sun ignites, some believe that it slowly moves inward.

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Its massive gravity ripples the asteroid belt, disrupting countless asteroids and comets,

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flinging them toward the Sun.

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They rain down on the inner planets, hammering and re-melting large portions of their crust.

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Did these impacts also deliver organics and water, key ingredients for life?

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Back in the asteroid belt, Bennu's parent body is lucky,

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it survives this period of heavy bombardment.

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The solar system cools and calms. Jupiter and its many moons assume the orbits that we see today.

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Billions of years of quiet follow...[ impact ] more or less.

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Then a billion years ago, one theory suggests a collision shatters the proto-planet.

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[ loud explosion ]

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Some of the debris loosely coalesces into a new, smaller body: Bennu.

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But Bennu will not stay in place. Dull, non-reflective, it slowly migrates toward the Sun.

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Solar heating turns its warm side into a low-intensity thruster.

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Through millions of years, Bennu's orbit gradually tightens,

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until it interacts with Saturn's gravity, altering its trajectory and hurling it into the inner solar system.

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Close encounters with Earth and Venus follow.

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Their gravitational tugs may have repeatedly stretched and reformed Bennu...

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turning it inside out and pulling off loose material.

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As a result, it has no satellites of its own...until now.

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Today, NASA is sending a spacecraft called OSIRIS-REx to explore Bennu and retrieve a sample.

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Why? Bennu has survived its long journey

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and settled into a near-Earth orbit, bringing its secrets within our reach.

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Now it is ready to teach us more about the solar system's history,

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its formation, its evolution, and our own place among the stars.

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[ music fades ]

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[ satellite beeping ]