TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the ...thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m−2 s−0.5 K−1, which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.
•Thermophysical model using a shape model of rough surface reproduces observations.•We obtain a global thermal inertia of 225 ± 45 J m−2 s−0.5 K−1.•We obtain a global RMS surface slope of 47 ± 5°.•We confirm Ryugu's low global thermal inertia owing to porous constituent boulders.
Ejecta velocity distribution is an important property for controlling asteroid surface evolution and for changing the size frequency distribution of asteroids and planetary dusts. Recent asteroid ...explorations revealed that boulders on an asteroid surface had a wide size frequency distribution. On the other hand, many studies on ejecta velocity distribution for cratering experiments used fine-grained homogeneous targets. Thus, to study the ejection process of various-sized boulders on rubble-pile asteroids, we conducted impact experiments using gas guns at impact velocities of 100 m s−1 to 4 km s−1 on targets with various-sized glass beads, and analyzed boulder trajectories in three dimensions to clarify the effect of grain size on ejection velocity distribution. The results showed that the ejection velocity, v0, decreased as the bead size increased, and the ejecta velocity scaling law was improved to v0gR=k2′r0+aR−1μ′ including the bead radius, a; r0 is the initial position of the bead, g is the gravitational acceleration, R is the crater radius, and k2′ and μ′ are, respectively, 0.58 ± 0.02 and 0.62 ± 0.02 for the low-impact velocity range (<200 m s−1) and 0.61 ± 0.07 and 0.57 ± 0.04 for the high-impact velocity range (>1 km s−1). Using our improved ejecta velocity scaling law, we calculated the landing points of ejected boulders and concluded that boulders with radii >0.34R could not be ejected outside the final crater. Moreover, when the Urashima crater on asteroid 162173 Ryugu was formed on the surface, boulders up to 64 m in diameter may have been ejected beyond the escape velocity of Ryugu to become tiny monolithic asteroids.
•Cratering experiments were done on simulated rubble-pile surface on asteroids.•Ejecta velocity distributions for various sized boulders were obtained.•Crater scaling law for ejecta velocity was improved considering the boulder size.•The size of largest boulder ejected outside of the crater is 0.34 crater radius.•Urashima crater on Ryugu might eject a boulder <70 m to be a tiny asteroid.
An impact experiment was performed on the surface of the C-type asteroid (162173) Ryugu using an instrument called the Small Carry-on Impactor (SCI) carried by the Japan Aerospace Exploration Agency ...spacecraft Hayabusa2. Using a small camera separated from the spacecraft, we observed the development of the ejecta curtain formed by the SCI impact in situ. Based on the patterns appearing in the ejecta curtain, we evaluated the physical properties of large boulders and regolith grains on Ryugu. We found that the large boulders on the surface near the SCI impact point had a compressive strength consistent with the previous estimation of a tensile strength based on low thermal inertia observed for boulders on Ryugu. Furthermore, the ejecta curtain consisted of the boulders with a maximum size of several decimeters. Moreover, the filament structure was formed in the SCI ejecta curtain, strongly suggesting that the SCI impact ejecta curtain particles had a size range greater than one order of magnitude; a characteristic size of the regolith grains in the curtain was estimated to be several centimeters. These regolith grains primarily originated from the subsurface layer. We propose three different hypotheses for the geological process that formed the subsurface structure at the SCI impact point consistent with the above observations. Because these hypotheses have different predictions for materials in SCI ejecta, the samples obtained from the second touchdown will be able to constrain which hypothesis is the most likely.
The resurfacing process on Ryugu accompanying the artificial impact crater formation by Hayabusa2's Small Carry-on Impactor (SCI) was studied by comparing pre- and post-impact images of this region ...captured by an optical navigation camera. Three different aspects of the resurfacing process were examined: the crater rim profiles, the motion of boulders and the appearance of new boulders, and the motion vectors of Ryugu's surface around the SCI crater. The averaged crater rim height, h, was derived as follows: h = hr exp −(r/Rrim − 1)/λrim, where Rrim is the SCI crater rim radius of 8.8 m, the fitted parameter, hr, is 0.475 m, and the λrim is 0.245. The ejecta blanket thickness of the SCI crater was thinner than that estimated from both the observation of natural craters and the crater formation theory. However, this discrepancy of the ejecta blanket thickness was resolved by taking into account the new boulders appearing in the post-impact images in the volume. The motion of the discovered boulders could be classified by its mechanisms as follows: a dragging motion created by excavation flow during the crater formation, a pushing motion created by falling-back ejecta, a dragging motion created by the slight motion of the Okamoto boulder, and a motion caused by seismic shaking induced by the SCI impact itself. The seismic shaking caused boulders to move farther than 3 cm from the original site in most of the region within 15 m distance from the SCI crater center, where the maximum acceleration of the impact induced seismic waves 7 times larger than the surface gravity of Ryugu based on the laboratory experiments (Matsue et al. 2020 Icarus, 338, 113520), and the evidence of the seismic shaking for boulders with a movement of >3 cm was detected in about 10% of the boulders in the region between 15 m and 30 m from the crater center, which region was inferred to experience acceleration larger than the Ryugu's surface gravity based on previous laboratory experiments (Matsue et al. 2020 Icarus, 338, 113520).
•We studied resurfacing processes caused by the artificial impact on asteroid Ryugu.•Pre- and post-impact images enabled us to determine the disturbed area.•The impactor disturbed the surface within a 40 m radius from the crater center.•The ejecta blanket thickness was approximated by the exponential equation.•More than 50% of boulders within a 15 m radius was moved by seismic shaking.
Hayabusa2 deployed two artificial landmarks called “target markers (TMs)” on the asteroid Ryugu for autonomous landing control. To achieve precise deployment on target landing sites, the TMs were ...designed to dissipate kinetic energy and released near the asteroid surface (with an altitude of less than 40 m). This study evaluates the performance of the ballistic deployment in the actual microgravity environment by reconstructing the trajectories of the TMs from optical, altimetric, and radiometric data. In addition, based on the reconstructed trajectories, low-velocity impacts of the TMs on the surface of Ryugu are characterized with dynamical parameters, such as dissipated energy and a coefficient of restitution. The physical implications of the impact analysis are discussed in comparison with on-ground experimental data. Furthermore, the gravitational environment is investigated using the reconstructed trajectory data and a shape model of Ryugu, providing information on the local gravity anomaly. Consequently, this paper demonstrates the usefulness of deployable artificial landmarks for small-body landings and further offers insight on surface conditions and internal structures near the Hayabusa2 landing sites where samples of Ryugu were collected.
•Hayabusa2 deployed two target markers (TMs) on the asteroid Ryugu as artificial landmarks for autonomous landing control.•TM trajectories were reconstructed from flight data, demonstrating the performance of the ballistic deployment under microgravity.•Beanbag-like structure of the TM successfully dissipated kinetic energy and prevented a rebound after the impact.•Impact and gravimetric analyses based on the reconstructed trajectories provided insights on the surface and internal structures of Ryugu.
Asteroid 162173 Ryugu has numerous craters. The initial measurement of impact craters on Ryugu, by Sugita et al. (2019), is based on Hayabusa2 ONC images obtained during the first month after the ...arrival of Hayabusa2 in June 2018. Utilizing new images taken until February 2019, we constructed a global impact crater catalogue of Ryugu, which includes all craters larger than 20 m in diameter on the surface of Ryugu. As a result, we identified 77 craters on the surface of Ryugu. Ryugu shows variation in crater density which cannot be explained by the randomness of cratering; there are more craters at lower latitudes and fewer at higher latitudes, and fewer craters in the western bulge (160°E – 290°E) than in the region around the meridian (300°E – 30°E). This variation implies a complicated geologic history for Ryugu. It seems that the variation in crater density indicates that the equatorial ridge located in the western hemisphere is relatively young, while that located in the eastern hemisphere is a fossil structure formed during the short rotational period in the distant past.
•We examined the spatial distribution of impact craters on Ryugu.•We completed a global impact crater catalogue of Ryugu (D > 20 m).•Crater density variations cannot be explained by the randomness of cratering.•More craters are seen at lower latitudes and less at higher latitudes.•There are fewer craters in the western bulge and more around the meridian.
An artificial impact experiment is scheduled for 2018–2019 in which an impactor will collide with asteroid 162137 Ryugu (1999
JU
3
) during the asteroid rendezvous phase of the Hayabusa2 spacecraft. ...The small carry-on impactor (SCI) will shoot a 2-kg projectile at 2 km/s to create a crater 1–10 m in diameter with an expected subsequent ejecta curtain of a 100-m scale on an ideal sandy surface. A miniaturized deployable camera (DCAM3) unit will separate from the spacecraft at about 1 km from impact, and simultaneously conduct optical observations of the experiment. We designed and developed a camera system (DCAM3-D) in the DCAM3, specialized for scientific observations of impact phenomenon, in order to clarify the subsurface structure, construct theories of impact applicable in a microgravity environment, and identify the impact point on the asteroid. The DCAM3-D system consists of a miniaturized camera with a wide-angle and high-focusing performance, high-speed radio communication devices, and control units with large data storage on both the DCAM3 unit and the spacecraft. These components were successfully developed under severe constraints of size, mass and power, and the whole DCAM3-D system has passed all tests verifying functions, performance, and environmental tolerance. Results indicated sufficient potential to conduct the scientific observations during the SCI impact experiment. An operation plan was carefully considered along with the configuration and a time schedule of the impact experiment, and pre-programed into the control unit before the launch. In this paper, we describe details of the system design concept, specifications, and the operating plan of the DCAM3-D system, focusing on the feasibility of scientific observations.
After delivering its sample capsule to Earth, the Hayabusa2 spacecraft started its extended mission to perform a flyby of asteroid 2001 CC
21
in 2026 and rendezvous with asteroid 1998 KY
26
in 2031. ...During the extended mission, the optical navigation camera (ONC) of Hayabusa2 will play an important role in navigation and science observations, but it has suffered from optical deterioration after the spacecraft’s surface contact with and sampling of asteroid Ryugu. Furthermore, the sensitivity of the telescopic camera (ONC-T) has continued to decrease for more than a year, posing a serious problem for the extended mission. These are problems that could potentially be encountered by other sample-return missions involving surface contact. In this study, we evaluated the long-term variation of ONC performance over the 6.5 years following the launch in 2014 to predict how it will perform during observations of the two target asteroids in its extended mission (6 and 11 years from the Earth return, respectively). Our results showed several important long-term trends in ONC performance, such as transmission, dark noise level, and hot pixels. During the long cruising period of the extended mission, we plan to observe both zodiacal light and exoplanet transits as additional science targets. The accuracy of these observations is sensitive to background noise level and stray-light contamination, so we conducted new test observations to search for the lowest stray light, which has been found to depend on spacecraft attitude. The results of these analyses and new test observations suggest that the Hayabusa2 ONC will be able to conduct cruising, flyby, and rendezvous observations of asteroids with sufficient accuracy.
Graphical Abstract
An asteroid exploration probe “Hayabusa2”, that was developed by Japan Aerospace Exploration Agency (JAXA), was launched on December 3rd, 2014 to challenge complicated and accurate operations during ...the mission phase around the C-type asteroid 162137 Ryugu (1999 JU3) (Tsuda et al. in Acta Astron. 91:356–362,
2013
). An impact experiment on a surface of the asteroid will be conducted using the Small Carry-on Impactor (SCI) system, which will be the world’s first artificial crater creation experiment on asteroids (Saiki et al. in Proc. International Astronautical Congress, IAC-12.A3.4.8,
2012
, Acta Astron. 84:227–236,
2013a
; Proc. International Symposium on Space Technology and Science,
2013b
). We developed a new micro Deployable CAMera (DCAM3) system for remote observations of the impact phenomenon applying our conventional DCAM technology that is one of the smallest probes in space missions and gained a great success in past Japanese mission IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun). DCAM3 is a miniaturized separable unit that contains two cameras and radio communication devices for transmission image data to the mothership “Hayabusa2”, and it observes the impact experiment at an unsafe region in where the “Hayabusa2” is difficult to stay because of a risk of exploding and impacting debris hitting. In this paper, we report details of the DCAM3 system and development results as well as our mission plan for the DCAM3 observation during the SCI experiment.
Zodiacal light (ZL) is sunlight scattered by interplanetary dust particles (IDPs) at optical wavelengths. The spatial distribution of IDPs in the Solar System may hold an important key to ...understanding the evolution of the Solar System and material transportation within it. The number density of IDPs can be expressed as
n
(
r
)
∼
r
-
α
, and the exponent
α
∼
1.3
was obtained by previous observations from interplanetary space by Helios 1/2 and Pioneer 10/11 in the 1970s and 1980s. However, no direct measurements of
α
based on ZL observations from interplanetary space outside Earth’s orbit have been performed since then. Here, we introduce initial results for the radial profile of the ZL at optical wavelengths observed over the range 0.76
-
1.06 au by ONC-T aboard the Hayabusa2# mission in 2021-2022. The ZL brightness we obtained is well reproduced by a model brightness, although there is a small excess of the observed ZL brightness over the model brightness at around 0.9 au. The radial power-law index we obtained is
α
=
1.30
±
0.08
, which is consistent with previous results based on ZL observations. The dominant source of uncertainty arises from the uncertainty in estimating the diffuse Galactic light (DGL).
Graphical Abstract
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