"Give a man a watch and he will know the time; give him two and he will never be sure" - this was my impression when reading the Wikipedia article on 162173 Ryugu.

Mean diameter:

  • 0.865±0.015 km[3]
  • 0.87 km[4]
  • 0.90±0.14 km[5]
  • 0.92±0.12 km[6]
  • 0.980±0.029 km[7]
  • 1.13±0.03 km[8]

And no mass estimates at all.

Now that Hayabusa2 has arrived, do we have solid measurements of Ryugu's size and mass yet?

  • $\begingroup$ Give JAXA some time to collect and process their data. The papers about 25143 Itokawa were published 8 months after the Hayabusa 1 encounter. $\endgroup$
    – Mike G
    Jul 8, 2018 at 13:43
  • $\begingroup$ @MikeG: okay, then the question can just wait for the answer to become available. $\endgroup$
    – SF.
    Jul 8, 2018 at 14:19
  • $\begingroup$ There is a special session on Hayabusa2 at the Division of Planetary Sciences 50th meeting in October. I would expect the first preliminary science results to be announced there but the data analysis takes time. Volume ans Mass in particular take a while as you have to make sure you've mapped all the surface of the asteroid and sampled the variable gravity field $\endgroup$ Jul 27, 2018 at 16:21

1 Answer 1


A science paper from 2019 by K. Kitazato et al. shows a shape model of the surface in Fig. 2, so we now indeed know the exact size and the exact mass, even the surface temperature distribution:

Shape Model of 162173 Ryugu.

The quoted paper refers to S. Watanabe et al., Science, 2019 which has all the details

Ryugu has an oblate body, with an equatorial radius of 502 ± 2 m and polar-to-equatorial axis ratio of 0.872 ± 0.007. The total volume obtained from the SPC-based shape model is $0.377 {\rm km}^3$, with an uncertainty of 1.3%. We conducted a gravity measurement during a spacecraft ballistic descent down to 0.85 km from the asteroid surface and a subsequent ballistic ascent up to 5.4 km. The estimated mass is $4.50\times 10^{11} {\rm kg}$, with an uncertainty of 1.3%, mainly because of uncertainties in the solar radiation pressure on the spacecraft.

The highlighting in the quote is from me.


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