Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Highly porous nature of a primitive asteroid revealed by thermal imaging


Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Thermal images of Ryugu taken at 5 km altitude during the Mid-Altitude Observation Campaign.
Fig. 2: Comparison of a temperature plot on the three-dimensional shape model with calculated images for thermal inertia of 50–1,000 tiu.
Fig. 3: Maximum temperature distribution during one rotation and diurnal temperature profiles on Ryugu.
Fig. 4: Cold spots discovered in the close-up thermal images.

Similar content being viewed by others

Code availability

The display tool for the TIR thermal images is AiGIS, developed by author Naru Hirata of the University of Aizu, which is available at The basic code of the asteroid thermal model that supports the calculation of thermal images using several thermal inertia values was constructed by ref. 17, and is applicable to any asteroid shape model even of more than one million nodes and capable of calculating the self-heating effect between nodes facing each other and the shadowing effect of insolation by geological features. This code is accessible from the corresponding author upon reasonable request.

Data availability

The source data of the TIR global thermal infrared images used for Fig. 1a–d are prepared from for Fig. 1a, from for Fig. 1b, from for Fig. 1c, and from for Fig. 1d. The source data of the TIR close-up thermal images used for Fig. 4a, c are prepared from, and, respectively. A TIR radiance image plotted on the Ryugu shape model (SHAPE_SFM_200k_v20180804) in Fig. 2 is from hyb2_tir_20180801_181632_l3b.txt. The maximum temperature plot in Fig. 3a and the plots of Fig. 3b–g are obtained from the datasets of hyb2_tir_20180801_150432~215056_l3b.txt. All the data related to this manuscript, including the raw thermal images (L1), the temperature-converted thermal images (L2), and the temperature plots on the shape model (L3) of TIR and the corresponding ancillary data for the SPICE tool are archived in the DARTS database (, and will be archived in the PDS4 within one year after the end of its nominal mission, according to the Hayabusa2 science data policy.


  1. Rivkin, A. S. et al. in Asteroids IV (eds Michel, P. et al.) 65–87 (Univ. Arizona Press, 2015).

  2. Tholen, D. J. in Asteroids II (eds Binzel, R. P. et al.) 1139–1150 (Univ. Arizona Press, 1989).

  3. DeMeo, F. E., Alexander, C. M. O., Walsh, K. J., Chapman, C. R. & Binzel, R. P. in Asteroids IV (eds Michel, P. at al.) 13–41 (Univ. Arizona Press, 2015).

  4. Okada, T. et al. Thermal infrared imaging experiments of C-type asteroid 162173 Ryugu on Hayabusa2. Space Sci. Rev. 208, 255–286 (2017).

    Article  ADS  Google Scholar 

  5. Tsuda, Y. et al. Flight status of robotic asteroid sample return mission Hayabusa2. Acta Astronaut. 127, 702–709 (2016).

    Article  ADS  Google Scholar 

  6. Flynn, G. J., Consolmagno, G. J., Brown, P. & Macke, R. J. Physical properties of the stone meteorites: implications for the properties of their parent bodies. Chem. Erde 78, 269–298 (2018).

    Article  CAS  Google Scholar 

  7. Rozitis, B. & Green, S. F. Directional characteristics of thermal-infrared beaming from atmosphereless planetary surfaces—a new thermophysical model. Mon. Not. R. Astron. Soc. 415, 2042–2062 (2011).

    Article  ADS  Google Scholar 

  8. Shimaki, Y. et al. Surface roughness and thermal inertia of asteroid Ryugu inferred from TIR on Hayabusa2. Lunar Planet. Sci. Conf. 50, 1724 (2019).

    ADS  Google Scholar 

  9. Britt, D. T., Yeomans, D., Housen, K. & Consolmagno, G. in Asteroids III (eds Bottke, W. F. Jr et al.) 485–500 (Univ. Arizona Press, 2002).

  10. Arakawa, S., Tanaka, H., Kataoka, A. & Nakamoto, T. Thermal conductivity of porous aggregates. Astron. Astrophys. 608, L7 (2017).

    Article  ADS  Google Scholar 

  11. Müller, T. G. et al. Hayabusa-2 mission target asteroid 162173 Ryugu (1999 JU3): searching for the object’s spin-axis orientation. Astron. Astrophys. 599, A103 (2017).

    Article  Google Scholar 

  12. Watanabe, S. et al. Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu—a spinning top-shaped rubble pile. Science 464, 268–272 (2019).

    ADS  Google Scholar 

  13. Sugita, S. et al. The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes. Science 464, eaaw0422 (2019).

    Article  Google Scholar 

  14. Kitazato, K. et al. Surface composition of asteroid 162173 Ryugu as observed by the Hayabusa2 NIRS3 instrument. Science 464, 272–275 (2019).

    ADS  Google Scholar 

  15. Fujiwara, A. et al. The rubble-pile asteroid Itokawa as observed by Hayabusa. Science 312, 1330–1334 (2006).

    Article  CAS  ADS  Google Scholar 

  16. Housen, K. R. & Holsapple, K. A. Ejecta from impact craters. Icarus 211, 856–875 (2011).

    Article  ADS  Google Scholar 

  17. Takita, J., Senshu, H. & Tanaka, S. Feasibility and accuracy of thermophysical estimation of asteroid 162173 Ryugu (1999 JU3) from the Hayabusa2 Thermal Infrared Imager. Space Sci. Rev. 208, 287–315 (2017).

    Article  ADS  Google Scholar 

  18. Biele, J. et al. Effects of dust layers on thermal emission from airless bodies. Prog. Earth Planet. Sci. 6, 48 (2019).

    Article  ADS  Google Scholar 

  19. Groussin, O. et al. The temperature, thermal inertia, roughness and color of the nuclei of comets 103P/Hartley 2 and 9P/Tempel 1. Icarus 222, 580–594 (2013).

    Article  ADS  Google Scholar 

  20. Yoshimitsu, T., Kubota, T. & Tomiki, A. MINERVA-II rovers developed for Hayabusa-2 mission. In 11th Low Cost Planetary Missions Conf. (2015);

  21. Ho, T.-M. et al. MASCOT—the Mobile Asteroid Surface Scout onboard the Hayabusa2 mission. Space Sci. Rev. 208, 339–374 (2017).

    Article  ADS  Google Scholar 

  22. Jaumann, R. et al. In-situ investigation of asteroid (162173) Ryugu by the Mobile Asteroid Surface Scout (MASCOT) camera (MASCam). Science 465, 817–820 (2019).

    Article  ADS  Google Scholar 

  23. Grott, M. et al. Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu. Nat. Astron. 3, 971–976 (2019).

    Article  ADS  Google Scholar 

  24. Opeil, C. P., Consolmagno, G. J., Safarik, D. J. & Britt, D. T. Stony meteorite thermal properties and their relationship with meteorite chemical, and physical states. Meteorit. Planet. Sci. 47, 319–329 (2012).

    Article  CAS  ADS  Google Scholar 

  25. Okada, T. Thermal inertia of surface materials of solar system small bodies and its dependence on porosity. Lunar Planet. Sci. Conf. 47, 1457 (2016).

    ADS  Google Scholar 

  26. Delbo, M., Mueller, M., Emery, J., Rozitis, B. & Capria, M. T. in Asteroids IV (eds Michel, P. et al.) 107–128 (Univ. Arizona Press, 2015).

  27. DellaGiustina, D. N. et al. Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis. Nat. Astron. 3, 341–351 (2019).

    Article  ADS  Google Scholar 

  28. Bardyn, A. et al. Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta. Mon. Not. R. Astron. Soc. 469, S712–S722 (2017).

    Article  CAS  Google Scholar 

  29. Kawaguchi, J., Uesugi, K. & Fujiwara, A. The MUSES-C mission for the sample and return—its technology development status and readiness. Acta Astronaut. 52, 117–123 (2003).

    Article  CAS  ADS  Google Scholar 

  30. Kieffer, H. H., Chase, S., Minor, E., Muench, G. & Neugebauer, G. Preliminary report on infrared radiometric measurements from the Mariner 9 spacecraft. J. Geophys. Res. 78, 4291–4312 (1973).

    Article  ADS  Google Scholar 

  31. Chase, S. C. et al. Mariner 10 infrared radiometer results: temperatures and thermal properties of the surface of Mercury. Icarus 28, 565–578 (1976).

    Article  ADS  Google Scholar 

  32. Kieffer, H. H., Martin, T., Peterfreund, A. & Jakosky, B. Thermal and albedo mapping of Mars during Viking primary mission. J. Geophys. Res. 82, 4249–4291 (1977).

    Article  CAS  ADS  Google Scholar 

  33. Christensen, P. R. et al. Mars Global Surveyor Thermal Emission Spectrometer experiment: investigation description and surface science results. J. Geophys. Res. 106, 23823–23871 (2001).

    Article  CAS  ADS  Google Scholar 

  34. Paige, D. A. et al. The Lunar Reconnaissance Orbiter Diviner lunar radiometer experiment. Space Sci. Rev. 150, 125–160 (2010).

    Article  ADS  Google Scholar 

  35. Fukuhara, T. et al. LIR: longwave infrared camera onboard the Venus orbiter Akatsuki. Earth Planets Space 63, 1009–1018 (2011).

    Article  ADS  Google Scholar 

  36. Fukuhara, T. et al. Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera onboard Akatsuki. Earth Planets Space 69, 141 (2017).

    Article  ADS  Google Scholar 

  37. Arai, T. et al. Thermal imaging performance of TIR onboard the Hayabusa2 spacecraft. Space Sci. Rev. 208, 239–254 (2017).

    Article  ADS  Google Scholar 

  38. Saunders, P. Dealing with the size-of-source effect in the calibration of direct-reading radiation thermometer. AIP Conf. Proc. 1552, 619–624 (2013).

    Article  ADS  Google Scholar 

  39. Veverka, J. et al. NEAR Encounter with Asteroid 253 Mathilde: overview. Icarus 140, 3–16 (1999).

    Article  ADS  Google Scholar 

  40. Rubincam, D. Radiative spin-up and spin-down of small asteroids. Icarus 148, 2–11 (2000).

    Article  ADS  Google Scholar 

Download references


We thank all the members of the Hayabusa2 Project and supporting staff for their technical assistance and scientific discussions, and especially S. Matsuura of Kwansei Gakuin University the use of the cavity blackbody and collimator to calibrate TIR. This research is supported in part by the JSPS KAKENHI (numbers JP26287108, JP17H06459, JP17K05639, JP19H01951 and JP19K03958), and the JSPS Core-to-Core programme “International Network of Planetary Sciences”. M.D. acknowledges support from the French space agency CNES. T.G.M. received funding from the European Union’s Horizon 2020 Research and Innovation Programme, under grant agreement number 687378, as part of the project “Small Bodies Near and Far” (SBNAF). A.H. was supported by STFC grant ST/S001271/1.

Author information

Authors and Affiliations



T.O. led TIR development and experiments, including interpretations of TIR data. TIR development and calibrations: T.O., T.F., S. Tanaka, M.T., T.A., H. Senshu, N.S., H.D., Y.O., T. Sekiguchi, T.K., J.T., T.M., T. Imamura, T.W., S. Hasegawa, J.H., T.G.M. and A.H. TIR data acquisitions and reductions: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., H.D., Y.O., K. Suko and T.K. Thermophysical modelling and discussions: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., M.D., J.B., M.G. and M.H. Shape modelling contributions: Naru Hirata, Naoyuki Hirata, Y. Yamamoto, K.M. and A.M. Landing site selection discussions: K.W., C. Honda, R.H., Y.I., K.M., M.M., T.M., A.M., T.M., H.N., R.N., K.O., K. Shirai, E.T., H. Yabuta, Y. Yokota, H. Yano and M. Yamada. Science operations of spacecraft: M. Abe, M. Hayakawa, T. Iwata, M.O., H. Yano, S. Hosoda, O.M., H. Sawada, T. Shimada, H.T., R.T., A.F., C. Hirose, S.K., Y.M., N.O., G.O., T.T., Y. Takei, T.Y., K.Y., F.T., T. Saiki, S.N., M. Yoshikawa, S.W. and Y. Tsuda; Project administration: S.S., N.N., K.K., T.O., M. Arakawa, S. Tachibana, H.I., M.I., S. Tanaka, F.T., T. Saiki, S.N., M. Yoshikawa, S.W., and Y. Tsuda, Interpretation and writing contribution: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., H.D., J.H., J.B., M.G., M.D., T.G.M., A.H., E.T., T.M. and S.S. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Tatsuaki Okada.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 A modelled thermal image of asteroid Ryugu before the arrival of Hayabusa2.

An example thermal image of the asteroid reference model ‘Ryugoid’ produced by the Hayabusa2 Landing Site Selection Data Preparation team shows the surface covered with granular regolith (with thermal inertia 300 tiu) and dense boulders (with thermal inertia 1,600 tiu), with most of the boulders identifiable as ‘cold spots’.

Extended Data Fig. 2 Temperature calibration of TIR using multiple apparatuses.

DN, digital number. a, An example of the TIR pixel response of a centre pixel at (164, 124) obtained by the ground-based calibration tests. The purple dots and line show the data for the blackbody plate and their linear regression. The blue and green dots show the data for the cavity blackbody and the collimator, respectively. bd, Thermal images from TIR using the different apparatuses for the 100 °C target: b, the blackbody plate (total area coverage), c, the cavity blackbody (simulation to Mid-Altitude, 5 km), and d, the collimator (simulation to Home Position).

Extended Data Fig. 3 Effective diameter dependence of the LUT for TIR calibration.

This figures show the values of the slope (a) and the intercept (b) at the centre pixel at (164, 124) in terms of the effective diameter D of the target. For D ≤ 300 pixels, the red lines are derived from the data of the collimator and the cavity blackbody sources. For 300 pixels <D ≤ 322 pixels, the green lines are derived from the blackbody (BB) plate source.

Extended Data Fig. 4 A formation scenario of Ryugu from a porous parent body.

(1) Formation began with fluffy dust in the solar nebula. (2) Porous planetesimals were formed by accretion of dust or pebbles. (3) The parent body of Ryugu might have remained porous owing to a low degree of consolidation. A clear boundary of the inner core is illustrated but a gradual increase of consolidation by depth might be expected. (4) Impact fragmentation of the parent body occurred. Some large fragments are the boulders on Ryugu. (5) Part of fragments re-accreted to form Ryugu, with porous boulders and sediments on the surface, and some dense boulders originating from the inner core. (6) Re-shaping caused by a change in rotation rate to form a double-top-shape.

Extended Data Table 1 Ryugu site information for diurnal temperature profiles
Extended Data Table 2 TIR major operations at Ryugu before mid-October 2018 used in this study

Supplementary information

Supplementary Information

This file contains additional information about Supplementary Videos 1 and 2.

Supplementary Data

This zipped file contains source data for Figures 1-4 and the Shape Model.

Video 1: Global one rotation thermal image set at HP

Video showing one-rotation global thermal images by Hayabusa2 TIR from Home Position.

Video 2: Global one rotation thermal image set at Mid-Altitude

Video showing one-rotation global thermal images by Hayabusa2 TIR from Mid-Altitude.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Okada, T., Fukuhara, T., Tanaka, S. et al. Highly porous nature of a primitive asteroid revealed by thermal imaging. Nature 579, 518–522 (2020).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing