What’s with Asteroid Ryugu’s Weird Shape?


Recent space missions have served as solar-system paparazzi, stalking a number of near-Earth objects — and the images they’ve sent home give us plenty to ponder. Can we use these observations to determine how one of these photogenic asteroids obtained its shape?

Visits to Asteroids

Ryugu and Bennu

Asteroids Ryugu (top) and Bennu (bottom) have similar spinning-top shapes. Click to enlarge. [JAXA/U. of Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST; GSFC/NASA/U. of Arizona]

Near-Earth objects naturally fall under public scrutiny — it’s in our best interests to learn as much as we can about these neighboring (and potentially hazardous!) bodies. Two spacecraft were launched within the past few years to scope out these objects: NASA’s OSIRIS-REx is currently examining the asteroid 101955 Bennu, and JAXA’s Hayabusa2 is exploring the asteroid 162173 Ryugu.

Both missions plan to eventually return samples of their targets to Earth to help us better understand asteroid composition. In the meantime, the spacecraft have provided stunning images of the asteroids’ surfaces and overall structures, allowing us to learn more about these kilometer-scale inhabitants of our solar system.

Details of a Spinning Top

One striking feature of these asteroids is their shape: both Bennu and Ryugu have so-called “spinning-top” shapes, characterized by a raised equatorial ridge that makes the asteroids look more diamond-like than circular when viewed from the side. Such a shape could conceivably be caused by the flow of materials to the equator, but this requires rapid rotation — quicker than Ryugu’s 7.6-hour or Bennu’s 4.3-hour periods.

Ryugu as seen by Hayabusa2

Ryugu seen from the Hayabusa2 spacecraft from different angles. The western region exhibits a smoother surface and a sharper ridge angle than the rest of the asteroid. [Hirabayashi et al. 2019 via JAXA/U. of Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST]

Hayabusa2’s detailed images of Ryugu reveal other asymmetries of the asteroid: the western region — termed the western bulge — is smoother than the eastern side, and the ridge angle here is sharper: 95° rather than the 105° seen on the rest of the asteroid. Recent research led by Masatoshi Hirabayashi (Auburn University) uses Hayabusa2’s observations to explore the possibility that Ryugu’s deformation was caused by a faster spin rate in its past.

Failed Structure?

Hirabayashi and collaborators use numerical models derived from Hayabusa2’s observations to analyze Ryugu’s current structure, judging how well different regions of the asteroid would hold up at different spin rates. The authors show that the subsurface region of the western bulge is currently structurally intact, whereas other regions are sensitive to structural failure. This suggests that the western bulge’s subsurface is relaxed — likely because it already experienced structural deformation in the past.

Ryugu FMD

“Failure mode diagram” computed for Ryugu. The shaded region indicates combinations for which Ryugu cannot structurally exist because the cohesive strength of its material would be below that necessary to remain structurally intact at a given spin period. [Hirabayashi et al. 2019]

What would it take to cause this deformation and create Ryugu’s current structure? Hirabayashi and collaborators show that a past spin period of around 3 hours — more than twice the asteroid’s current spin rate — could have caused the asteroid’s structure to fail in places. As material shifted, it could have generated the smooth surface of the western region and eventually settled to form its current configuration at a spin period of ~3.5 hours. Ryugu’s spin likely slowed gradually over time after this, eventually reaching its current leisurely 7.6-hour period.

The authors acknowledge that this scenario is not the only possible explanation for Ryugu’s shape, but it’s a model that produces results consistent with Hayabusa2’s detailed images. As we gain more data from Hayabusa2 — and from OSIRIS-REx at Bennu — we can hope to refine our theories further!


“The Western Bulge of 162173 Ryugu Formed as a Result of a Rotationally Driven Deformation Process,” Masatoshi Hirabayashi et al 2019 ApJL 874 L10. doi:10.3847/2041-8213/ab0e8b