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Space & Planetary Science

OSIRIS-REx Arrives at Bennu

By Keith Cowing
NASA Watch
December 4, 2018
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OSIRIS-REx Arrives at Bennu

OSIRIS-REx Spacecraft Arrives at Asteroid Bennu
“The spacecraft executed a maneuver that transitioned it from flying toward Bennu to operating around the asteroid. Now, at about 11.8 miles (19 kilometers) from Bennu’s Sun-facing surface, OSIRIS-REx will begin a preliminary survey of the asteroid. The spacecraft will commence flyovers of Bennu’s north pole, equatorial region, and south pole, getting as close as nearly 4 miles (7 kilometers) above Bennu during each flyover.”

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2 responses to “OSIRIS-REx Arrives at Bennu”

  1. Michael Spencer says:
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    All of JPL’s previous magic aside, this particular mission is simply stunning.

    For instance: the gravity at Bennu is minuscule (it is 500 meters across, for Pete’s sake!). I imagine that because gravity is so weak any adjustments Bennu must be even more exact and precise, lest the bird fly into space. I’ve not seen figures that relate Benn’s velocity relative to the sun, nor that of the Bennu, nor a discussion of what is involved getting Bennu into orbit.

    I’ve not also seen Bennu’s motion characterized. Is spin the same as for most other solar system objects? What are the other considerations gaining orbit while also changing from an equatorial approach to a polar orbit?

    Rocket science indeed.

    • fcrary says:
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      If I did it right, the surface escape velocity from Bennu is about a third of a mile per hour. You can technically talk about orbits (within a couple miles, outside of which solar tides start to make whole idea of an orbit meaningless.) But that’s not exactly a useful approach. In practice, they’re flying in formation with the asteroid, and will eventually dock with it. That’s much more like a spacecraft approaching a space station (also with virtually no gravity), than a spacecraft orbiting a planet. A friend working on the Rosetta project told me they had similar issues with “orbiting” 67P/Churyumov-Gerasimenko.

      The rotation rate is a bit fast (4.2 hour period) and basically upside down (within 10 deg. of pure retrograde.) I think that’s not common, but asteroids have a lot of variety. And for something that small, it wouldn’t take much of an impact to mess with it’s rotation.