This OSIRISREx tweet says:

It's official! I'm in orbit around #asteroid Bennu -- now the smallest body ever orbited by a spacecraft. My snug path around the asteroid also sets a new record for the closest orbit of a planetary body by any spacecraft.

How close is OSIRIS REx orbiting Bennu? What is it's periBennu? And how does it compare to Dawn's periCeres? And if JAXA's Hayabusa 2 can be considered "in orbit" around Ryugu, to it's periRyugu?

How far inside Bennu's Hill sphere will this be?

(for more on that uncertainty, see Nature of Hayabusa-2's “Home Position” relative to Ryugu? Is it Ruygu's L1?

There are some numbers in AsteroidMission.org's NASA’s OSIRIS-REx Spacecraft Enters Close Orbit Around Bennu, Breaking Record and a nice animation in the video, but nothing really clear about the actual size of the orbit once the maneuvers are complete.

  • $\begingroup$ What part of the sentence "the spacecraft will circle Bennu about a mile (1.75 kilometers) from its center" is "not really clear"? $\endgroup$
    – Hobbes
    Jan 2 '19 at 14:01
  • $\begingroup$ @Hobbes I don't understand what you mean; "about a mile" is not the same as a periapsis, and "circle Bennu" doesn't mean the eccentricity is zero. If you watch the video you will see the orbit looks elliptical, although that could be an artifact like these: space.stackexchange.com/a/26588/12102 I'm looking for a quantitative answer about Osiris' orbit, plus a comparison to what's happening with the other two spacecraft: "circles at about a mile" isn't really addressing my question. $\endgroup$
    – uhoh
    Jan 2 '19 at 14:22
  • $\begingroup$ Is this orbit stable? Youd think a nonspheroid would make it nonuniform to say the least. $\endgroup$ Jan 5 '19 at 13:48

Bennu's hill sphere is 29.5 km roughly. The orbit is roughly circular, about 1.75 km above the center, or about 1.5 km above the surface of the asteroid. It is well inside of the Hill Sphere. This doesn't even compare to Dawn, which orbited about 35 km above the surface at closest, and that was with a very eccentric orbit. Hayabusa 2 isn't really orbiting Ryugu, but is rather remaining stationary above it about 6 km above the surface.

The specific orbit, as outlined in this article, goes from 1.4 km to 2 km from the surface. Thus it will be only a bit over 1 km at the closest approach.

  • $\begingroup$ I'm not sure it would be considered "closest approach", but sometime in 2020 the spacecraft will come within about 3 meters of the surface of Bennu so the TAGSAM can get its sample. The TADSAM's (Touch-And-Go Sample Arm Mechanism) arm is 3.35 meters at maximum extent. $\endgroup$
    – Vince 49
    Mar 2 '19 at 0:43
  • $\begingroup$ That won't be a true orbit, however. $\endgroup$
    – PearsonArtPhoto
    Mar 2 '19 at 12:51

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