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Mission: Hayabusa 2
Blue Sky
post Jun 28 2018, 09:22 PM
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QUOTE (Therion @ Jun 28 2018, 04:43 PM) *
Or what else could shaped Ryugu like this except the fast rotation?


A glancing blow from some other object could explain both the fast rotation as well as the fact that it is rotating backwards. Maybe something similar to why Uranus has such a wierd axial tilt. Perhaps that impact left more stuff around the equiator. Somebody could do some math modelling of spinning rubble piles to see if they can assume a shape like this..

On the comparison with Itokawa, my idea is that Itokawa is actually two small rubble piles stuck together, without enough total mass to collapse into a more uniform ball. Ryugu is not so much larger that it could have collapsed more, but maybe it is older? Or was not made up of multiple already-formed asteroids in the first place, so is more uniform. That fact that it has more and larger craters might suggest that it is older, using the method developed by William Hartmann to date areas of Mars. Or maybe, being more massive, it just attracted more impacts.
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Phil Stooke
post Jun 28 2018, 09:31 PM
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I would say it has more and larger craters because it's bigger rather than older. There isn't room on Itokawa for big craters.

I am more interested in seeing whether there will be smooth areas (covered with small particles instead of big rocks), as we saw on Itokawa or like the 'ponds' on Eros. So far I don't see any, but we only have one side of it at good enough resolution.

Phil


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Gerald
post Jun 28 2018, 11:23 PM
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QUOTE (Therion @ Jun 28 2018, 10:43 PM) *
Or what else could shaped Ryugu like this except the fast rotation?

My very first intuition was that of a bipyramidal "double shatter cone", as a result of the collision of two objects of similar size.
After initial torque-free precession, rotation would have dampened down to a stable spin axis.
Anything from collisions over outgassing to YORP could have changed its angular velocity afterwards.
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pandaneko
post Jun 29 2018, 09:56 AM
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I popped into a local library today, just for browsing weekly magazines to catch up with goshipps and all that.

I then picked up "Newton", which is a local science for laymen sort of monthly magazine. In it was an article
about Hayabusa 2 and it contained something I did not know.

Apparently, Hayabusa2 was conceived because Hayabusa was deemed lost and dead, even before they knew
Hayabusa was limping, but alive.

I, on the other hand, have been under the impression that H2 was given a go-ahead because Hayabusa managed
to come back...No wonder they asked for donations.

P

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wildespace
post Jun 29 2018, 03:49 PM
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Phil Plait - What would it be like to stand on Ryugu - http://www.syfy.com/syfywire/what-would-it...urface-of-ryugu

Very low gravity + fast rotation = strong centrifugal force, especially at the equator. Hence the equatorial bulge. Impacts with the asteroid would have dislodged a lot of material which then slipped towards equator.

BTW, here's Ryugu with colour information from that tiny colour image. Saturation increased to hopefully show colour variations in the terrain.

Attached Image


And with levels adjusted to reflect its dark albedo:

Attached Image


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Gerald
post Jun 29 2018, 11:26 PM
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QUOTE (wildespace @ Jun 29 2018, 05:49 PM) *
Phil Plait - What would it be like to stand on Ryugu - http://www.syfy.com/syfywire/what-would-it...urface-of-ryugu

Very low gravity + fast rotation = strong centrifugal force, especially at the equator. Hence the equatorial bulge.

I doubt, that this mechanism can explain the bulge, unless you have two very unlikely synchronous impacts.
A rubble pile temprorarily acting like a liquid would approximate a Maclaurin spheroid, if rotationally symmetric to its spin axis. (Faster rotation could result in a Jacobi ellipsoid, or an according rubble-pile approximation)
The equatorial bulge is IMO too prominent for the Maclaurin scenario.
Instead, besides a possible but not very likely central collision of two almost equally-sized bodies, a merged moonlet or ring settled down along the equator would be a plausible scenario, especially considering the frequency of asteroid binaries. This is inspired by one of the scenarios considerd for the equatorial ridge on Iapetus.
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wildespace
post Jun 30 2018, 04:37 AM
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QUOTE (Gerald @ Jun 30 2018, 12:26 AM) *
I doubt, that this mechanism can explain the bulge, unless you have two very unlikely synchronous impacts.
A rubble pile temprorarily acting like a liquid would approximate a Maclaurin spheroid, if rotationally symmetric to its spin axis. (Faster rotation could result in a Jacobi ellipsoid, or an according rubble-pile approximation)
The equatorial bulge is IMO too prominent for the Maclaurin scenario.
Instead, besides a possible but not very likely central collision of two almost equally-sized bodies, a merged moonlet or ring settled down along the equator would be a plausible scenario, especially considering the frequency of asteroid binaries. This is inspired by one of the scenarios considerd for the equatorial ridge on Iapetus.

Looking at the image now, might it have had to do with that large crater? It's very near the equator, and would have kicked up a lot of material. So it a) caused the reverse spin of the asteroid, and cool.gif kicked up the material that settled along the equator.


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pandaneko
post Jun 30 2018, 11:10 AM
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Is Rugu's surface very hot? I believe average temp. is more or less the same as the temp.s anywhere on the
surface as it is spininng very fast.

P
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Therion
post Jun 30 2018, 02:13 PM
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As i understand it, 7.6 hours isn't actually considered fast rotation.
For asteroids this size it's rather slow. That's why i was proposing escaped-satellite scenario.
Here are some ryugu-shaped fast rotators:



Of those, only Bennu and 2008 EV5 have no known satellites. (And they have slowest rotation of this selection.)

Didymos | 1994 CC | 1999 KW4 | 2001 SN263 | 2004 DC

(Source of above image is Hayabusa2 press release from June 21.)
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alex_k
post Jul 3 2018, 04:29 PM
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Attached Image
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alex_k
post Jul 5 2018, 01:01 PM
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QUOTE (Phil Stooke @ Jun 28 2018, 11:34 PM) *
"a graben-like feature"

Or we could say a channel-like feature, or a rolling boulder track-like feature, or even a chain of subdued craters. But until we know more about it, it's probably better to use generic terms like 'valley' or 'trough' which don't imply a particular mode of origin. A graben is a depressed area between two parallel fractures where the surface has dropped down, and that seems unlikely here if the asteroid is what it appears to be, a mass of unconsolidated rubble.

This is really just following the mapping work by Gene Shoemaker, Don Wilhelms and others at USGS who pioneered lunar and planetary geological mapping in the 1960s, it's not my idea.

EDIT: here's an alternative interpretation to make that point graphically.

Phil


Trying to focus on that features

Attached Image


upd: for estimating detalization and certainity of my approximations (focused on center)

Attached Image

some boulders match smile.gif
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wildespace
post Jul 6 2018, 04:16 PM
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QUOTE (alex_k @ Jul 5 2018, 02:01 PM) *
Trying to focus on that features

Attached Image


upd: for estimating detalization and certainity of my approximations (focused on center)

Attached Image

some boulders match smile.gif

Looks like a huge boulder that had been sliced through, revealing much brighter insides.


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Fran Ontanaya
post Jul 9 2018, 01:19 PM
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If you look at Atlas on Saturn, you have another mechanism for equatorial bulges i.e. accretion from within a disk. The bulge in that case would be older than the rubble it captured later on.


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Paolo
post Jul 10 2018, 05:36 AM
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after a few days (too many) of hiatus, here is some new images of Ryugu. A stereo rotation animation
http://www.hayabusa2.jaxa.jp/topics/20180710je/index.html

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MahFL
post Jul 10 2018, 06:18 AM
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QUOTE (Paolo @ Jul 10 2018, 06:36 AM) *
after a few days (too many) of hiatus, here is some new images of Ryugu. A stereo rotation animation
http://www.hayabusa2.jaxa.jp/topics/20180710je/index.html



Amazing. Also looks like that rock was cleaved.
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