Meanwhile OSIRISREx Is Now Orbiting A Tiny Asteroid

1. It’s wasn’t clear, at least to me, in what sense this motion could be termed an ‘orbit’. In fact, the PI (Dante Lauretta) was quoted 8.23.2016 in Spaceflightnow.com:

   Its very much like a docking,” Lauretta said. “We’re under our own thrust authority the whole time. The gravity field is, more or less, insignificant at that point.

   So, it’s not an ‘orbit’. Then I stared wondering: what are the minimum parameters under which one could call the relative motions of two bodies an ‘orbit’?

   Sounds like. trick physics question, I know; junior high physics, probably 🙂

   1. No, it’s not junior high physics. People like Isaac Newton have worried about it (and, in his case, reached some mistaken, theological conclusions…) In a strictly technical sense, you might say that an orbit has to be a closed path, so you’d periodically return to exactly the same place. But that never happens. There are always some perturbations which prevent returning to _exactly_ the same place.

      In practice, a good working definition is that the gravity between the two bodies is much greater than any other force affecting them. (And even that isn’t quite right, since there could be two, much stronger forces at work, as long as they balanced and cancelled each other out.)

      The most common criteria is the Hill distance, which would be about 200 body radii or 5 km for 101955 Bennu. Inside that, the Sun’s gravity and the centrifugal force for asteroid/spacecraft velocity around the Sun are nearly balanced and the difference is smaller than the asteroid’s gravity. But we’re talking about something like 10 micro-g’s of gravity.

      At that point, I wouldn’t be shocked if there were large perturbations from photon pressure. Once you say all other forces must be under a micro-g, it’s a good, two-week homework problem for an aerospace engineering student: List off and estimate all the other forces that might be that big.

Comments are closed.