Incredible video shows Hayabusa2 pogo-bouncing off asteroid
A new paper analyzes what we know about the sample the probe grabbed last year.
The surface of Ryugu as Hayabusa2 made its approach.
The following series of events is not fiction: fly a probe to an asteroid, bounce off that asteroid while grabbing a piece of it, and fling that sample back to Earth. This series of activities is exactly what JAXA’s Hayabusa2 mission is in the middle of doing. (And by the way, NASA’s OSIRIS-REx mission is working on the exact same thing, just on a timeline a year behind Hayabusa2.)
Hayabusa2 grabbed its first sample from the surface of a near-Earth asteroid named Ryugu in February 2019. (A second sample was collected in July after it blasted a small crater to expose sub-surface material.) A new study published by the team this week details what the probe saw at the sampling site—including remarkable video of the touchdown itself.
We have touchdown
As the probe touched Ryugu’s surface, it fired a small projectile into it, catching some of the debris in an open “sampling horn” before bouncing away and re-establishing its orbit. These samples will return to the Earth later this year, but for now, the scientists are establishing what we know about the areas they came from.
Ryugu has a somewhat angular shape and is a little less than a kilometer across, and its equator bulges out in a distinctive ridge that makes the asteroid resemble a spinning top. Up close, it looks like a rubble pile. But there are some interesting color patterns within that jumble.
A video of the touchdown. Be patient, the screen will eventually fill with the view from different cameras.
Ryugu’s surface is a mix of dark material with slightly reddish or bluish tinges. The blue dominates the poles and equator, and the red dominates the mid-latitude regions in between. But zoom in close and you’ll see that both are represented.
Boulders are primarily bluish—including where you see one cracked open—with reddish patches on the surface. And the spot that Hayabusa2 sampled was initially bluish but was coated in red after the dust settled. Putting everything together, the reddish color seems to be the mark of alteration or weathering—like rust forming on the surface of steel.
Seeing red
Oxidizing isn’t a thing in the vacuum of space, but there are other processes that can create this appearance. Interactions with the charged particles of the solar wind can drive chemical reactions on the surface of a body, for example, but that tends to form a microscopic skin—Ryugu’s layer appears thicker. So the conclusion the researchers draw is that this is actually the product of a closer pass with the Sun earlier in Ryugu’s life. That heat metamorphosed a surface layer of the asteroid, which was then pulverized and redistributed by small impact events. And given that the poles and the equator are topographic highs, that material would tend to settle into the mid-latitude lowlands
Caption
Enlarge/ This illustration shows the researchers’ proposed timeline for Ryugu’s evolution.
The team can actually guess at the timing of Ryugu’s flirtation with the Sun by examining its impact craters. By looking at how craters and their ejected debris overlap, you can work out which are older and which are newer. The inside of the older craters are reddish, but there are younger craters that still appear bluish (more evidence that the reddening was a past episode rather than an ongoing and recent process).
Based on established estimates for the average rates of impact collisions over time, the researchers calculate that the reddening episode occurred somewhere between 300,000 years and 8 million years ago (depending on when Ryugu left the asteroid belt for its near-Earth orbit). They also calculate that Ryugu probably formed (from the breakup of some larger body) around 9 million years before that. That’s quite recent compared to some well-studied breakups of major asteroids, which implies Ryugu is a grandchild, at least—the product of progressively smaller breakups.
The team is hopeful that Hayabusa2 caught some of the reddened material as well as the bluer rock in its sample grab, so we’ll hopefully get a much closer look at this stuff when the shipment comes in later this year.
If you simply can't wait that long for another asteroid sampling fix, NASA’s OSIRIS-Rex probe is currently eyeballing its target on the asteroid Bennu. Its touch-and-go sampling lunge is set for August of this year.
Quelle: arsTechnica
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'Sunburn' discovered on asteroid Ryugu hints at past orbit close to sun
This diagram provided by a research team led by University of Tokyo associate professor Tomokatsu Morota shows the spectrum of colors on the surface of the asteroid Ryugu, which can be observed due to differences in the way sunlight reflects off each part. The reddish areas are believed to have been exposed to extreme solar heat. "TD" marks the area where space probe Hayabusa2 made its first landing. The color of the surface cannot be distinguished and appears all dark to the human eye.
TOKYO -- Researchers in Japan have discovered dark red "sunburned" areas on the surface of the asteroid Ryugu, suggesting it previously orbited much closer to the sun.【
The discovery, made by a team of researchers from the Japan Aerospace Exploration Agency (JAXA) and the University of Tokyo, was published May 8 in the U.S. journal Science.
JAXA's space probe Hayabusa2, which is believed to have successfully collected material samples from Ryugu, observed sunlight reflecting off the surface of the asteroid from above, and found areas that had turned a blackish red color, widely dispersed across the surface. The surface of craters that were relatively new, however, did not show such coloring. This and other factors led the team to conclude it is highly likely the asteroid was exposed to extreme solar heat for a short period in the past.
Tomokatsu Morota, an associate professor at the University of Tokyo and a member of the research team, said the traces of charring on the surface of Ryugu suggest that the asteroid was exposed to a high temperature of around 600 to 800 degrees Celsius. Although the path of Ryugu's orbit currently lies between Earth and Mars, the team estimates that the asteroid orbited between the sun and Mercury at a certain point between 300,000 and 8 million years ago. The trajectory of an asteroid's orbit can be changed due to gravitational forces from massive planets like Jupiter.
When Hayabusa2 collected rock samples from the surface of Ryugu in February 2019, it landed in a part that contained both the material altered by the sun and the original substance, and it is likely that the probe was able to collect both. Researchers look forward to uncovering the effects of solar heat on the asteroid by closely examining the samples when they are brought back to Earth.
Seiji Sugita, a planetary science professor at the University of Tokyo who is also a member of the team, commented, "Although it has been known that the orbits of asteroids can largely change, this is the first time that material evidence of this has been obtained, and we are astonished."
Quelle: The Mainichi
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JAXA: Asteroid Ryugu may have shifted orbit
Japan's space agency says it has found that the asteroid Ryugu may have orbited between the Sun and Mercury in the distant past. That's different from its current orbit, which passes between Earth and Mars, but not between the Sun and Mercury.
The Japan Aerospace Exploration Agency, or JAXA, says it analyzed images of the asteroid taken by a camera on its probe Hayabusa2.
JAXA says much of the sand and rock on the surface of Ryugu turned red when exposed to heat of more than 600 degrees Celsius. It says such high temperatures are inconceivable even when the asteroid is at its closest point to the Sun in its current orbit.
JAXA says the sand and rock in a recent crater on Ryugu is blue, as it has not been heated to high temperatures. It says this is because the asteroid's orbit moved farther away from the Sun in the relatively recent past.
JAXA says these findings suggest that Ryugu's shortest distance from the Sun about 300,000 to 8 million years ago was roughly one-third to one-fifth the current one.
JAXA adds that Ryugu's orbit at the time may have passed between the Sun and Mercury before shifting to the present trajectory. Mercury is the closest planet to the Sun in our solar system.
JAXA says it remains unknown what caused the change in Ryugu's orbit, but that gravity could be one reason.
University of Tokyo Associate Professor Morota Tomokatsu, who conducted the analysis, says he thinks this is the first time a change in an asteroid's orbit has been studied geologically.
He says he is looking forward to examining sand and other particles to be brought back by Hayabusa2.
Hayabusa2 is now on its way back to Earth after completing its mission on Ryugu last November. The asteroid was about 300 million kilometers from Earth when the explorer reached it in 2018.
Quelle: NHK
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Update: 13.05.2020
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Thermal images from the TIR suggest Ryugu boulders may be "fluffy
The first photographs from Hayabusa2 of the surface of asteroid Ryugu revealed a treacherous landscape, with large boulders carpeting the asteroid to form a rugged topology. Yet when the spacecraft turned on its thermal infrared imager (TIR), it saw a surprisingly homogenous surface in the thermographic images.
How effectively a planetary surface conducts and stores heat is measured by a property known as the thermal inertia. A surface with a high thermal inertia will gain and lose heat slowly, resulting in smaller temperature differences between night and day and summer and winter.
Thermal inertia depends on composition and also density. Boulders like those scattered over Ryugu are expected to be dense and should therefore have a high thermal inertia, changing temperature more slowly than the surrounding material. On the dayside of Ryugu, this would make the boulders stand out on the TIR image as cold spots. But this was not what was seen.
A paper published in the journal, Nature, this week (March 16, 2020) led by Tatsuaki Okada (ISAS & University of Tokyo) revealed the first global thermal image of an asteroid during a complete rotation, in addition to high resolution images taken close to the surface. This data captured by the TIR on Hayabusa2 showed little temperature difference between the majority of boulders and the surrounding material on the surface of Ryugu, pointing to a similar thermal inertia value of about 300 Jm-2s-0.5K-1 (300 tiu).
Left: "Ryugoid", the expected thermal distribution of the asteroid before arrival. The assumed thermal inertia was 1600 tiu for the boulders and 300 tiu for the regolith. Right: Observed Ryugu, from the global TIR observations at an altitude of about 5km (~4.5 m / pixel). The derived thermal inertia of both boulders and surroundings is about 300 tiu.
The implication is that Ryugu is covered with low-density, porous boulders that are surrounded by similarly porous fragments greater than 10 cm in size. Since the initial expectation was the asteroid should have dense boulders embedded in a layer of fine regolith, this is a startling discovery. The reverberations have important implications for how planets are formed.
Ryugu is an example of a C-type or Carbonaceous asteroid. C-types are primitive asteroids consisting of material that has experienced little change since the formation of the Solar System 4.6 billion years ago. This makes them important time capsules for the early days of planet formation, revealing the material that built the Solar System.
It is suspected that the carbonaceous chondrite meteorites found on Earth are fragments of C-type asteroids. The accessibility of meteorites makes these much studied objects for understanding the Earth’s earliest days. But these meteorites are significantly less porous that the majority of rocks on Ryugu.
The close-up TIR images captured during the Hayabusa2 descent operations reveal a few colder boulders with thermal inertia values corresponding to 600 - 1,000 tiu; a value more typical of the carbonaceous chondrites. But these dense additions are in the minority on the asteroid surface.
Cold spots discovered during close-up thermal images. These images were captured during the TD1-R1-A operation on the 15 October, 2018 from an altitude of 78.8m. The temperature profile along the yellow line is shown in the adjacent panel. The cold spots are colder by more than 20K, indicated a dense and consolidated boulder with high thermal inertia. Only a few cold spots (boulders) are found on the surface of Ryugu: other boulders have the same temperature as their surroundings. Scale bar is 5m in panel a and 1m in panel c.
One possibility is that C-type asteroids may have a far less consolidated nature than expected, consisting of a loose conglomerate of fluffy dust and pebbles. Ryugu would then be a fragment from such a porous parent, with a handful of denser boulders that were formed in the deepest part of the parent asteroid, or even had a separate origin and were delivered to the asteroid during meteoritic impacts.
These fluffy asteroids would struggle to survive the high temperatures and shockwaves experienced during a descent through the atmosphere of Earth, resulting in meteorites that consist of only the dense minor components of the asteroid.
Formation scenario for Ryugu from a porous parent body. (1) Formation begins with the fluffy dust in the Solar nebula. (2) Porous planetesimals were formedr through accretion of dust or pebbles. (3) The parent body of Ryugu may have remained porous owing to a low degree of consolidation. A clear boundary at the inner core is illustrated here, but a gradual increase of consolidation by depth might also have occurred. (4) Impact fragmentation of the parent body, with a few large fragments forming the boulders on Ryugu. (5) Fragments re-accrete to form Ryugu, with porous boulders and sediment on the surface and a small number of dense boulders originating from the parent’s inner core. (6) Re-shaping during a rapid rotation phase created a double-top-shape.
If this fluffy composition was typical of the material that built the planets, then the growth process that took the Solar System from dust to planets might be very different from models that assume a more rigid building material. For example, the result of impacts between porous planetesimals may prove to result more often in coagulation than fragmentation, allowing for a swifter growth and altering the time-scale for planet formation.
It is also possible that Ryugu's porosity may be due to a composition of completely different materials to the carbonaceous chondrite meteorites. For example, the asteroid could consist of carbon-rich material similar to that discovered on comet-67P/Churymov-Gerasimenko. This will ultimately be resolved when Hayabusa2 returns to Earth at the end of 2020 with samples of both surface and subsurface material.
Quelle: JAXA
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Creating a crater to constrain the age of an asteroid’s surface
An important science goal for the Hayabusa2 mission is to map the history of asteroid Ryugu. As a primitive carbonaceous asteroid from the early days of the Solar System, Ryugu’s life traces the movement of ices and organics; the ingredients for habitability.
With the majority of information about the asteroid gathered from detailed observations, a key question for constructing a chronology is the age of that observed surface. Dating of surfaces of celestial bodies is done by counting the size and frequency of craters. Comparing such counts with the expected rate of impacts yields a value for the surface age.
However, these age estimates can be very uncertain. The first assessment of the Ryugu surface suggested the asteroid had stayed in the asteroid belt (where collisions mainly occur) anywhere between 6 million and 200 million years: a range covering over an order of magnitude. This uncertainty is due to not knowing how the material of Ryugu’s surface responds to an impact. In particular, what force stops a crater growing?
When a crater cavity forms, it must fight against the mechanical strength of the rock to break it apart, and also gravity that resists material being thrown outwards. Which force ultimately determines where the evacuation of the surface stops affects the size of the final crater. The sizes of craters that cover a surface therefore depend on if crater generation is strength limited or gravity limited and an accurate assessment of age requires knowing which is dominating.
On Earth, craters with diameters more than a few 10s of metres are gravity dominated. Ryugu’s gravity is much lower at only approximately 0.00001 G (1 G = Earth gravity), but its surface material may also be weak, resulting in no obvious winner between the two forces.
ONC images of the SCI crater. (A) The area prior to the impact, taken from an altitude of 1.72km. MB marks the Iijima boulder, SB is the Okamoto boulder. (B) Same location after the impact. (c) SCI crater is marked with the yellow semi-circle and the white box is the region shown in (D) with the small pit marked. (Figure 1 in Arakawa et al, Science 2020.)
Models and simulations of crater formation highlight differences in the velocity and distribution of ejecta generated in both the strength and gravity limited scenarios. But to observe this, the crater-forming process has to be caught in action. This was achieved on 5 April, 2019 when Hayabusa2 released the small carry-on impactor (SCI).
In a paper published today (19 March, 2020) in Science Magazine, a team led by Masahiko Arakawa (Kobe University) scrutinised the formation of the SCI-generated crater for evidence of the forces dominating the evacuation.
The SCI is a 2kg copper projectile that struck the surface of Ryugu at a speed of 2km/s. While the main spacecraft moved away from the impact site to avoid the resulting debris, the collision was observed by a deployable camera, DCAM3. Three weeks later, the new crater was imaged by the onboard telescopic optical navigation camera (ONC-T) as the spacecraft descended to 1.7km over the impact site.
DCAM3 images of the SCI impact at different elapsed times after the collision (Figure 3 in Arakawa et al, Science 2020).
The movie of above images. The images were taken 185s before the impact and 3s, 5s, 36s, 100s, 192s, 396s and 489s after the SCI collision. The right image is an enlargement of the left.
The heavily bouldered surface of Ryugu prevented the creation of a symmetrical crater. The SCI impact moved dozens of these rocks, including the 5m Iijima boulder, which was shifted 3m to the north-west of its original position. However, the Okamoto boulder to the south remained immobile and seems to have blocked crater growth. The result was a semi-circular crater with the ejecta thrown towards the north-west. Observed via the DCAM3, the ejecta itself was split into rays by boulders that blocked the evacuation flow of material.
Measured across at the original surface elevation, the new crater was 14.5m in diameter and 17.6m when measured across the raised rim. This is seven times larger than the crater generated in a similar impact on Earth, due to the much weaker gravity and surface strength. A small pit was also formed close to the impact site, implying that Ryugu may have a layered structure, with a harder layer containing the pit overlaid with a fluffier surface material.
(A) DCAM3 image of the ejecta created during the SCI impact, with four separate rays of material. (B) ONC optical image of the impact site, with numbers corresponding to the ejecta rays in (A). (C). Change in reflectivity before and after the SCI impact. Darker regions are thought to indicate material ejected from below the surface. (Figure 4 in Arakawa et al, Science 2020.)
The ejecta observed by DCAM3 provided two vital clues to the forces at work. Firstly, the curtain of ejecta remained attached to the ground. Previous laboratory experiments suggest that this is seen when gravity is limiting the crater evacuation, but not in the strength-limited case. The second clue was material in the ejecta was at least partially deposited on the crater rim. Models of the velocity of such evacuated particles suggest this is also a sign of gravity-limited crater formation.
Ryugu history. ※1 : Theoretical calculations and astronomical observations estimate the typical lifetime of near-Earth objects as about 10 million years. (JAXA)
For the weak gravity of Ryugu to be dominating the crater formation process, the asteroid’s surface material must an extremely low strength. A comparison would be a cohensionless material such as sand, whose strength originates from friction that depends on the (weak) surface gravity.
Scaling laws that predict crater size suggest that the SCI should generate a crater with radius 6.9 - 7.7m in the case where gravity is dominating. This agrees well with the apparent radius of 7.3m.
If the size distribution of craters on Ryugu’s surface is determined by gravity, then the asteroid’s surface age is at the younger end of the predicted range. The SCI experiment suggests Ryugu stayed in the asteroid belt for between 6.4 and 11.4 million years. After this time, the orbit of Ryugu would have changed as it entered the inner Solar System to become a Near-Earth Asteroid (NEA). In this more empty part of space, collisions become far less frequent and it becomes hard to judge the passing of time. Roughly speaking, NEAs are estimated to exist for about 10 million years.
There is one caveat. Any major change in surface topology that erases old craters effectively re-sets the surface age clock. It is therefore not possible to tell if 10 million years has passed since Ryugu was first formed, or if this has been the time since a global resurfacing event. Distinguishing between these scenarios will be the subject of future work on this fascinating asteroid.
Quelle: JAXA
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Quelle: JAXA, Twitter
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Update: 24.05.2020
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Hayabusa 2 photos indicate asteroid Ryugu orbited near sun
The asteroid Ryugu used to pass far closer to the sun, even closer than Mercury, an analysis of images shot by Japan's Hayabusa 2 space probe reveals.
The probe snapped the images of the asteroid after it fired a bullet into Ryugu, stirring up sediments, during the Hayabusa 2's first landing attempt in February last year.
A team of researchers, primarily from the University of Tokyo and the Japan Aerospace Exploration Agency (JAXA), noted numerous dark red particles measuring 0.3 millimeters or less flying off the asteroid in the photos.
The team's analysis of the blackish red substances showed they were exposed to heat from the sun between 300,000 and 8 million years ago, the scientists said in findings published May 7 on the website of U.S. scientific journal Science.
"Ryugu may have traveled in an orbit inside Mercury," said team member Tomokatsu Morota, an associate professor of planetary science at the University of Tokyo.
Morota was referring to the possibility that Ryugu initially orbited in the asteroid belt outside Mars, then was pulled closer to the sun by nearby planets' gravity and changed course to pass near the Earth and Mars.
The images of the Hayabusa 2 landing site also show pale white grains of sand that have not turned dark red. Both types of particles are believed to have been collected during the landing mission.
The scientists expect that by comparing the samples following the planned return of Hayabusa 2 will allow them to identify much more detailed processes of how Ryugu was formed and developed.
Quelle: The Asahi Shimbun
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Update: 15.07.2020
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Hayabusa2 to return samples to Earth in Dec.
Japan's asteroid probe Hayabusa2 will return a capsule containing samples from the asteroid Ryugu to Earth in early December.
Japan's education ministry announced that Hayabusa2 is scheduled to drop the capsule onto an Australian desert on December 6.
The probe, which is operated by the Japan Aerospace Exploration Agency, or JAXA, has touched down twice on Ryugu before collecting sand and rocks from its surface. The spacecraft left the asteroid in November last year to return to Earth.
JAXA has already formed a team of scientists to recover the capsule and analyze its contents.
The team will examine the sample for possible clues into the origins of life.
JAXA is now deliberating the asteroid to which Hayabusa2 will next head after it drops the capsule.
