Why OSIRIS-REx is Courting Asteroid Bennu

KENNEDY SPACE CENTER: The September 8, 2016 launch of the OSIRIS-REx spacecraft begins a two-year journey to catch up to asteroid Bennu. Why was Bennu chosen as the asteroid to study?

Bennu has many characteristics that make it one of five ideal candidates for study. Starting from a dating pool of over 500,000 possible asteroids, the OSIRIS-REx team narrowed their choice to a primitive and carbon-rich asteroid which is expected to have organic compounds and water-bearing minerals like clays. How did they choose Bennu as the asteroid for OSIRIS-REx's first kiss? There were four requirements.

Not too Thin

The sampler head on OSIRIS-Rex will stir up the surface of the asteroid by using compressed Nitrogen gas. The goal is to have small particles on the surface of the asteroid, from dust to pebbles 1 centimeter in size. If the asteroid is spinning too fast, the small particles will have flown off the asteroid. Small asteroids spin too fast. The goal was to find an asteroid with a diameter > 200 meters. This would create a slow enough rotation period that the dust might still be present. Bennu has a diameter of 500 meters (slightly larger than the height of the Empire State Building), and a rotation period of 4.3 hours.

Close Enough

You don't want a dating candidate to be so far away that you end up spending all of your date budget on fuel to get to her house. The target asteroid need to have an optimal orbit that OSIRIS-REx can catch up to the asteroid and later, return the sample. With an orbit that is just 6-degrees different than Earth, Bennu passes by Earth every six years.

Diamonds Are a Spacecraft's Best Friend

Well, if not diamonds, at the very least, a lot of carbon. The "O" in OSIRIS-REx stands for "Origins". The team is trying to find an asteroid that is carbon-rich and might contain the building blocks of life.

A Fatal Attraction

There is a twisted excitement knowing that your potential dating partner might some day destroy you. While Bennu gets close to Earth every six years, it is going to pass between the Moon and the Earth in 2,135. It gets harder to predict this far out, but between 2161 and 2191, there is a one-in-1400 chance that Bennu will strike the Earth and kill all the dinosaurs. The "S" in OSIRIS is Security: The mapping that OSIRIS-REx does to find the sampling location will give us amazing maps of Bennu. It will also teach us about the Yarkofsky Effect - tiny changes in the asteroid's course caused by thermal heating and cooling. Learning more about the Yarofsky Effect will allow scientists to more accurately predict the path of all Near-Earth Asteroids. The detailed mapping of Bennu will allow future generations to plan a mission for 2135 to alter the course of Bennu.

Narrowing the Dating Pool

• There are over 500,000 known asteroids.
• Over 7,000 of those are near-Earth asteroids.
• 192 of those asteroids have orbits optimal for sample return.
• 26 of those have a diameter over 200 meters.
• 5 of those are carbon-rich.

Bennu is the choice.