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The NYTimes' article Hayabusa2 Prepares to Drop Rovers on Asteroid Ryugu says

Sept. 19 Rock Hoppers

Hayabusa2 is preparing to deploy two small rovers this week, each about 7 inches wide. Ryugu’s gravity is so weak that the Minerva rovers will be able to slowly hop and float across the surface using internal rotors.

and includes the images below of the designated landing site and two alternates, as well as a "close-up".

Is it known what criteria were used to select landing sites?

With a mass of about 5E+11 kg and radius of about 450 meters (density about 30% higher than water) the escape velocity is only about 7 cm/sec, and even if you dropped the ~1 kg landers from far away the specific kinetic energy upon "impact" is only about 0.07 Joules, so "crash landings" are not really a huge issue.

So what criteria did they use? Particular scientific interest based on spectroscopy or imaging? Suspected parallelism of the surface normal to local gravity? Something else?

Question: What criteria was used to select potential landing sites for Hayabusa-2's rovers on Ryugu?

below: "The Hayabusa2 team selected a planned landing site (in purple) and two backup options (in orange) near the asteroid’s equator." From here

enter image description here

below: "During an experiment to measure the asteroid’s gravity, the spacecraft took these images of the surface from less than a mile away." From here

enter image description here

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The JAXA document Candidates for landing sites for the Hayabusa2 mission explains in detail how landing sites were selected for Hayabusa2, MASCOT, and MINERVA-II1. (MINERVA-II was a two-part project consisting of MINERVA-II1 and MINERVA-II2; this document probably omits MINERVA-II2 because it was an optional payload and wasn't scheduled for release until the following year.) Note that the landing sites in the photo in the question were potential landing sites for Hayabusa2, not for the MINERVA-II rovers. You can see all the final landing site candidates for Hayabusa2, MASCOT, and MINERVA-II1 on page 9 of the linked PDF (labeled as page 17).

There is significantly more detail about how sites were selected for Hayabusa2 than for MINERVA-II1, so I would encourage you to read the entire document, but page 35 of the PDF (labeled as page 70) lists the following criteria for MINERVA-II1 landing sites:

  • Landing site does not overlap with spacecraft touchdown candidates.
  • Landing site does not overlap with MASCOT landing site candidates.
  • The altitude of the spacecraft after separation must not be lower than 30m.
  • Ensure communication with ground station.
  • Not high temperature region, and fewer parts in shadow

("Spacecraft" here refers to Hayabusa2.)

It goes on to say that:

  • Due to the equatorial ridge, separation near the equator results in widely spaced landing points to the north and south.
  • Separating in the southern hemisphere may result in a spacecraft altitude below 30m.

(Asteroid Ryugu has a pronounced ridge at its equator.)

Therefore, the decision was to:

Separate in northern hemisphere, more than 100m north of the equator.

I'm not sure why releasing the rovers over the southern hemisphere would require Hayabusa2 to fly lower than for a release over the northern hemisphere. Kent Yoshikawa from JAXA gave an interactive presentation at the International Astronautical Congress in 2019 titled "Hayabusa2 Operational Design and Evaluation of MINERVAII-1A/B Rovers Deployment" (abstract, video) that looks like it touched on those issues, but he doesn't go into much detail in that video; I guess you would have had to be there to ask questions.

The MINERVA-II1 rovers were much smaller and had fewer scientific instruments than Hayabusa2 and MASCOT, so science objectives did not appear to factor into the choice of MINERVA-II1 landing sites (at least, not directly; Hayabusa2 and MASCOT were given priority in site selection and they did consider science objectives).

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  • $\begingroup$ Thank you for the very thorough answer, wow!! $\endgroup$ – uhoh Sep 11 at 15:52

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