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OSIRIS-REx was designed to retrieve a sample
from asteroid Bennu and return it to Earth.
So, of the more than 1 million known asteroids
in our solar system, why Bennu?
First, as a near-Earth asteroid, Bennu is easy to reach.
It takes about 14 months to orbit the Sun and comes close to Earth
once every six years.
To get there, OSIRIS-REx was launched in 2016 on an Atlas V
rocket with a single booster and a Centaur upper stage.
It also got a big assist from Earth's gravity in 2017,
allowing it to match Bennu's orbital tilt.
The second reason that Bennu was chosen was its size and spin rate.
Bennu measures about half a kilometer in diameter and rotates
on its axis once every 4.3 hours,
which is actually pretty slow compared to many smaller asteroids.
This allowed OSIRIS-REx to map Bennu’s surface up close,
and to match its velocity, briefly touch down, and collect a sample.
Before OSIRIS-REx arrived, scientists thought that Bennu's slow rotation
meant a low risk of flinging away most of its sampleable material.
But shortly after arrival, Bennu was caught on camera ejecting
hundreds of pebbles.
It turns out that Bennu regularly sheds small particles,
but it still had plenty of loose material left on its surface for sample collection.
The third reason for choosing Bennu was its composition.
We don't know what ingredients were present during the formation of life
on Earth, but primitive asteroids like Bennu serve
as time capsules, preserving material from the dawn of the solar system.
Spectrometers on OSIRIS-REx confirmed that
Bennu is rich in carbon-based molecules,
including organics that are the stuff of life.
When the Bennu sample is analyzed, it will help scientists to better
understand the formation of the solar system and our own origins.
So why this particular asteroid?
Proximity to Earth, the right size and spin, and a carbon-rich composition.
That is why we chose Bennu.
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