If an asteroid is broken up in a collision but some of the pieces remain bound by gravity a gentle collision between two of the fragments could occur.

I believe that is what happened to Itokawa.

English version is finally available
http://www.hayabusa2.jaxa.jp/en/topics/20180905e/

The 'Haya2Now' website is now in English! Just rollover each panel to find out more: Link

and two recent short stories (in English):

Boulders on the surface of asteroid Ryugu
Thermography of Ryugu by the TIR

It looks like the operation has started. This page (link below) is updating occasionally with new NavCam images, I've attached the most recent as of the time of this post.

http://www.hayabusa2.jaxa.jp/en/galleries/onc/nav20180911/

it's not the actual sampling operation, yet. it's the first rehearsal:
http://www.hayabusa2.jaxa.jp/en/topics/20180911e/

We're about 1 km away now (if not less). The most recent image as of this post has a major data drop-out, but here's the most recent good image from UTC 2018-09-12 02:32.
Edit: A new image from UTC 2018-09-12 02:51. I'm pretty sure we can now see the spacecraft's shadow on the asteroid -- a dark smudge centered in the opposition surge that appears to be visible in both of the images in this post.

Good eye! I don't think it was visible in previous images, was it?

Indeed. As far as I can tell, the two images in that post are the only two images where it is clearly visible, and I've not seen it in any previous image.
The site is giving me 404 errors right now, unfortunately.

Edit:
From https://twitter.com/haya2e_jaxa/status/1039723761728217089

According to update in japanese Hayabusa2 descended to 600 m above sufrace and then autonomous system stopped the descent and began to rise, probably due to unabillity to measure distance by LIDAR because Ryugu's surface have very low reflectivity.

As for the spacecraft's shadow - it's clearly visible in several frames:


/how can i reduce the size of inserted image?

The reason for unexpected rise during H2 descent is as follows. My translation of the relevant description in Japanese.

We have been conducting a rehearsal for the first touch down since 10 September. The spacecraft started descending on 11 September.

Today (12 September) H2 was going down to the lowest height intended, but its autonomous system ordered H2 to to rise and
H2 started rising back to its HP.

It is thought that the reason for this is LIDAR could not cope with the low reflectivity of Ryugu and could not measure the seperation
distance well enough.

H２ is now back at its HP ad is healthy. We will be looking again at the constants that had been given to H2 and also looking at
possibility of improved sequence.

P

3D Visualization of Asteroid Ryugu

https://www.youtube.com/watch?v=DDTBBggihiw

CONGRATULATIONS Roman

The spacecraft shadow and opposition effect
http://www.hayabusa2.jaxa.jp/en/topics/20180915e/

A couple interesting tweets from their twitter feed. One about landing site imagery acquisition during the recent rehearsal attempt, and another about MINERVA-II1 rover deployment schedule.

Cool.... but very uncomfortable, I feel like being driving (flying?) in reverse! Wouldn't it be better a rotation downwards? Or even better, from right to left, which probably gives a better 3d effect?

Something has been bothering me a lot. It is as follows.

If an asteroid is made of hard metal and somewhow it is covered with sands and pebbles and rocks and if someting collides with it very
hard resulting schock wave must be cataclysmic to shake them beyond escape velocity, leaving nothing but the metal asteroid.

Craters on Ryugu seem to suggest that Ryugu is like a metal, soft metal, soft enough to leave crater holes but hard enough not to break
up on impact. So, why are there regolith and boulders still left on Ryugu? Where did those boulders come from in the first place?

I am trying to persuade myself that those shaken off Ryugu travelled into deep space and came back to where they started from after
billions of years in the solar system. Am I going mad?

P

There's probably no metal at all in Ryugu, it's more of a 'rubble pile', of low density rocks (just like Itokawa), with many voids and empty spaces inside. I'd guess the inside is as broken and fractured as the surface the cameras can show us.
The craters we see are all very soft, with rounded edges, and every impact just breaks off more pieces, which either don't have enough velocity to escape and come back as boulders (I'd guess the enormous one at the pole is one), or, if they have enough velocity, fly off as separate bodies, permanently. The ones we see now were all probably part of the original body when it first broke off in turn from its parent body eons ago, and have just been broken up and reformed, probably many times since.

The impact projectile Hayabusa 2 is carrying is the perfect experiment to demonstrate this when they fire it at Ryugu. It should replicate the natural process on a smaller scale.

Ryugu's extremely low gravity probably corresponds to it being a rocky rubble pile. If it were a solid metal asteroid, it would have a much stronger density and gravity, right?

the collision will result in huge fragmentation, generating a kind of cloud of material, which will collide each other and dissipate the energy, to the point their relative speed is reduced and accretion starts again.

close observation of Ryugu and its reaction to the impactor is hence part of the evaluation of the theoretical descriptions.

Patrick Michel is part of the Hayabusa scientific team, and he is an expert in simulation of such processes,

movies illustrating the various mechanisms here : simulations of asteroid collisions

OrbitrapInSpace , thank you for this. My thinking was too simplistic. I would have thought that all bits and pieces on the asteroid surface
will move in the same way upon impact, each with enough escape velocity. Mutual collisions in confusion after impact leading to energy
dissipation, no, never thought about it that way.

My guess is that they will then fall into an equilibrium state for some time, just like gas molecules in a container, forming, presumably
clouds around the asteroid, then eventually falling back down to its surface, making up regolith and boulders as we see now.

Yes, it makes sense.

P

Highly detailed paper on interior mechanisms of 4th rover, MASCOT:
https://elib.dlr.de/104842/1/Bachelor%20The...%20Hass_red.pdf

Hayabusa 3d/HD realtime simulator soon available? (maybe)
https://www.nhk.or.jp/corporateinfo/english...df/20180906.pdf


Next operations in schedule:
MASCOT operation: October 2 ~ 4 (MASCOT separation October 3)
Touchdown 1 rehearsal 2: mid-October
Touchdown 1: late-October

Press briefings:
Sept. 27, Thursday 15:30-16:30 JST (06:30-07:30 GMT)
Oct. 11, Thursday 15:30-16:30 (TBD)

http://global.jaxa.jp/projects/sat/hayabus...hayabusa2_e.pdf

3D Web Visualization Tool to support HAHAYBUSA2-MASCOT Landing Site Selection Process
Imagine if it was made available to the public! :-)

It is possible to calculate H2 altitude in real time from value ONC_A contained in this file (used by http://haya2now.jp/):

Altitude (km) = 354 / sqrt(ONC_A)


It's a pity there is not such a page for rovers!

Here, I am not talking about gaseous planets and icy comets.

Having seen NASA photos of Mars over the years and those from Itokawa, and this time from Ryugu it makes me think that the materials
we see on these cellestrial bodies remarkably look like the soil on which, for instance my house stands. When Bennu photos reach us
showing another soild surface like earth soil that will be enough to convince me that our solar systmes is made up of common soils,
common to everybody.

Am I right in thinking that way. That is to say that our earth soil is not unique. My previous thinking was that soils were produced by
rivers.

P

Soils on Earth have a high biological content and well as water, rock fragments, minerals etc. The biological element makes them very different from the soils on all other bodies, as far as we know.
I believe the soils (more accurately regolith) of the Moon, asteroids, comets, Venus, Mars etc are made up solely of rock. Ice fragments are also in the mix in the case of Martian poles and comets, and possibly lunar and Mercury polar areas. The rock fragments are produced by collision -- meteoroids hitting the surface of the Moon or an asteroid, breaking up larger rocks and splattering melt around, which hardens into lumps. On planets with atmospheres and water there is also wind and water erosion creating sands and gravel, frost-shattering, and perhaps rocks ejected from volcanoes. So it is all quite complex and varied.

Remember that appearance does not equate to composition. Relatively fine-grained material of all types on any body will look quite similar, but it originated via a variety of processes. For example, the 'sand dunes' of Titan would quickly melt or sublimate under terrestrial conditions.

Does the Sun spin?

P

I am trying to build up of an image of our solar system formation in my own small way.

P

It does, and does rotate faster near the equator than at the poles. Speed can be estimated from sun spot motion. See, for instance this wikipedia article on "Solar rotation"
Thank you for all your contributions to this forum.
Fernando

Just for fun... what if Ryugu was on Paris

Click to view attachment

This really helps get scale. The little rock on the surface are not very little.

Useful for upcoming MASCOT delivery: you can know in realtime Hayabusa 2 altitude by reading this file:
http://haya2now.jp/data/data.json

I reverese-engineered the source, and after several calculations it appears that H2 altitude in km is given by this formula:

A = 354 / sqrt(ONC_A)

File is updated in realtime during operations, and ONC_A value is used to set the size of the disk which represents Ryugu in Hayabusa2 simulator.

ONC_A represents the area of the disk.

You can then use this page to figure out how big the rovers would appear:
http://win98.altervista.org/telescopio.html

(MINERVA-II rovers have a 0.00018 km diameter and 0.00007 km height.)
(MASCOT is around 40x30x20cm, hence maximum dimension is 0.00040 km)

I'm trying to setup a page which automatically determines pixel size of the rovers depending on altitude, but I am encountering difficulties with cross-site data access.

Here is my simplistic view of the formation of our solar system.

Big bang, then elementary particles collied with one another, producing light atoms. Light atoms in turn collied amongst themselves and
accretion started all over the univere.

Here, I turn my attetion to our own. Sun was being formed, but smaller accretions were also taking place all over in our solar system.

There must have been porous asteroids, already hardened, rock solid asteroids, and those somewhere between. They kept colliding over
astronomical time scale.

If, as I have been reminded, there had been post collisional accretions then most of healthy (not bombarded by other asteroids) and
unhealthy (affected but still kept accretion) asteroids must be like Ryugu, with an equatorial vulge.

I bet Bennu will be looking like Ryugu and perhaps, Ikokawa was one of the odd asteroids?

P

The sun is a second generation star so the solar system was not part of the first accretion events in the universe. We can study primordial objects in our solar system but unfortunately this will not be the same as studying the original primordial objects of the universe.

Apologies. I will stop talking about solar systems. Good timing because I was going to add more to mine. Thanks.

P

I'm having fun with Ryugu, putting it on major cities and capitals to give a sense of scale:

Paris


New York City


London


Tokyo


Rio de Janeiro

Problem is - if you did that - it would collapse under terrestrial gravity into a conical rubble pile.

Oooops from a diamond shape to a cone, that's less impressive.

Angle of repose be dammed, your work helped me get a grasp of the size of Ryugu... Having a terrestrial reference has always helped for scale (like @fredk's mystery man in all the MER pans). Though, somewhere on the internets, someone will probably use on of those images to illustrate a NASA doomsday conspiracy cover up theory.

Well...early on in Cassini they kept putting out an image of Enceladus hovering over a MODIS image of the UK for scale.

I kept wondering....why do they want to crash it into Birmingham?

🤣

Looking at those wonderful images of asteroidal rubble, I can imagine many of those boulders slowly descending out of the sky over millennia, and just settling there quietly... do we think that's how Ryugu accumulated, gently?

No, I'm afraid its history has been a lot more violent.

Most likely, it began life as part of a much larger body which was eventually blown apart by a devastating collision. This and many other fragments dissipated and took on their own identities. Ryugu was itself hit by many small impactors which ejected some fragments altogether, but threw out others at lower speed, which fell back to build up a layer of fragments. Then one day a big impact shattered the whole body. Again, quite a lot was lost for ever but enough stuff was thrown out more slowly and collected again under its own cumulative gravity to form the pile of rubble you see today. Since then a few other impacts have made the craters we can see, ejecting some fragments and redistributing the rest.

The last part of that could be said to match your gentle accumulation description, but it was just a part of a brutal past.

Phil

This is one reason I am excited for New Horizons at Ultima Thule. Getting to see a hopefully unaltered body up close should help answer a lot of questions on small body growth at the birth of the Solar System. I'm betting, based on what we saw with Rosetta, that it will be covered in 2-5m scale boulders that gently gathered together, perhaps as a few larger bodies merged over time. I'm betting against a shattered rubble pile!

For Ryugu, the lack of dust is fascinating. Is it too small to hold onto any generated during impacts? Does the dust settle inwards as the rubble pile redistributes itself after each impact? Hopefully the rovers and sample return has some answers!

I'm going to assume that's well informed speculation and deduction, rather than you having a blue police box parked in the corner of your garage 😁 My poor sense of humor aside (sorry man, you just give such a strong sense of witnessing the history of Ryugu) , such a violent history strike me as consistent with the apparent lack of hydration in the surface material, which surprised JAXA (I guess that's part of your reasoning? ) . But given such a well mixed object, might there not still be hope of finding comparatively unaltered material?

No... it's the police box.

Phil

I knew it!!!

Hi everybody, I'm doing a little forum maintenance. I've been remiss in organizing these asteroid mission subforums. There's now a separate Hayabusa2 subforum. I have split off the MASCOT posts into their own thread and also the MINERVA-II-1 posts into their own thread since it looks like people will be playing with those photos for a while. We'll keep this original thread open for general discussion. When it comes time to do the first sampling maneuver, we'll start a new thread for that, too.

I found an official DLR log for H2 position, expressed in meter distance from center rather than from ground:
https://www.dlr.de/dlr/en/portaldata/1/scri...nt_decimals.csv
Ryugu radius is around 450 meters.

This will be instead useful during return to Earth (if updated):

https://www.dlr.de/dlr/en/portaldata/1/scri..._english_KR.txt

These data are used by the DLR simulators:
https://www.dlr.de/dlr/en/portaldata/1/scri...busa2/tall.html
https://www.dlr.de/dlr/en/portaldata/1/scri...abusa2/box.html

But it looks like they don't work properly.