Journey to Bennu


The OSIRIS-REx spacecraft was launched just over a year ago – on September 8, 2016, and right now it’s headed back toward Earth. It’s not stopping, though. On Friday, September 22nd, just before noon CDT, OSIRIS-Rex will zip by Earth, passing about 10,000 miles over Antarctica. This maneuver, called a “gravity assist,” will allow the spacecraft to ‘borrow’ some of Earth’s orbital energy and convert it into momentum. The maneuver will slingshot OSIRIS-REx back out into deep space on an intercept course to its target, an asteroid known as Bennu.

The acronym that makes up the spacecraft’s name, OSIRIS-REx, stands for Origins, Spectral Interpretation, Resource Identification and Security, Regolith Explorer. While that’s a mouthful, it covers the mission’s ambitious goals: to send a spacecraft to an asteroid, gather a sample, and return it to Earth.

Bennu is relatively small as asteroids go. It’s only about 500 meters in diameter – about 5 football fields – so why go there? Why not to a larger asteroid a gather a sample from that one?

Bennu was selected for several reasons. First, it is thought to be a very primitive, carbon-rich asteroid, a preserved sample of the very early solar system and possibly older than Earth itself. The sample from Bennu will be studied to see if it contains early organic molecules and amino acids, the building blocks of life here on Earth. Second, Bennu’s orbit classifies it as a Near Earth Asteroid or NEA. Once every six years the asteroid comes within 200,000 miles of Earth – about the distance from Earth to the moon – which makes it close enough that we can not only get a spacecraft there, we can return that spacecraft to Earth. The characteristics of this orbit also means that Bennu actually crosses Earth’s orbit from time to time, which classifies it as a Potentially Hazardous Asteroid. While there are no known impact risks from Bennu until well into the 22nd Century, the mission will nonetheless study Bennu’s orbit to analyze for any risks in the far future, in the event that future scientist need to develop any impact mitigation missions.

If all goes according to plan, OSIRIS-REx will arrive at Bennu sometime in November 2018. Bennu is far too small for a landing, so the spacecraft will fly in formation with the asteroid, spending about a year performing a detailed survey. Once the survey is complete, mission controllers will select a site and command OSIRIS-REx to gather the sample. Once on board, the sample will be stored, sealed, and placed under a pure nitrogen purge for return to Earth in September 2023.

Check out the mission’s website at for more information.

Until next time… Clear Skies!
Leonard Ferguson

My name is Leonard Ferguson, and I will be your guide to all things magical in our Fort Bend skies. I am an active amateur astronomer, have lived in Richmond TX for over 25 years and am a longtime volunteer at the Houston Museum of Natural Science's George Observatory. The night sky is something we all share, and it's a fascinating and wondrous place. I hope you will join me in this journey!

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