[X] My Assistant

[Phil Stooke]

I don't think it needs one big crater to erase all the others. I think every impact digs a shallow crater in the existing rubble, and (a) shakes everything a bit, jostling rubble everywhere including on its own rim, and ( throws debris out at low speeds into loose looping trajectories that eventually reaccrete, further degrading any crater rims.

Ejecta fragments leave a surface at a range of speeds, fast at first in the impact process, then slower as energy diminishes during the process. The faster stuff is just lost from an object like this, but the slower stuff can reaccrete. Itokawa should eventually erode away, I suppose, but now we see it covered with the slow-moving and reaccreted ejecta.

The seismic shaking model is going to work a lot better with a solid (maybe fractured but still basically monolithic) asteroid such as Eros is believed to be, than with a true rubble pile. A lot has been said about rubble piles in the past, but everything we've seen at high resolution has looked more like a monolith (surface expressions of linear fractures etc.) Reported low densities on those bodies are probably due to open fractures rather than interstitial pore space in rubble. Itokawa might be our first close look at an object consisting at least 50 percent of true rubble.

Well, that's my take on it up to now...

Phil

[Rakhir]

New update : Solar Radiation Pressure Force acting on Hayabusa Station Keeping

http://www.isas.jaxa.jp/e/snews/2005/1025.shtml

Rakhir

[edstrick]

Phil: My current take which has just been reinforced by the rotating image from 8 km is that this 'oid is a 2-potato fused binary. One large potato making up 2/3 of the length and a smaller one at an angle to the bigger one, with a small "chunk" making a shim (showing up as a ridge) between the two in that "valley" in the north that contains most of the smooth "plains"

Everything's covered with rubble making this fantastic hackly texture, but I think 80% maybe of the mass is in two monolithic chunks

[Phil Stooke]

edstrick, I think that's very reasonable as well.

Phil

[dvandorn]

I don't know if each of the larger pieces is monolithic. Each seems to be "prickly" with large boulders sticking out -- at least in those places where the regolith doesn't bury everything.

It's possible that the two larger bodies, and whatever mid-sized bodies create the "bridge" between them, are all themselves rubble piles. This could be a loose accretion of three or four rubble piles... with the seismic "shaking" needed to smooth out the regolith surfaces being provided by the very slow contact/accretion process that has loosely glued the pieces together.

In other words, three or four rubble piles have accreted, and the "collision" (I hate to call it an impact, it's so slow and low-energy) that causes each new rubble pile to "stick" to the overall body has shaken the material enough to redistribute and smooth out the fines-sized particles. Thus, we have no "one large crater" that accounts for the shaken-smooth quality of the regolith.

-the other Doug

[Phil Stooke]

http://www.isas.jaxa.jp/e/enterp/missions/...usa/today.shtml

Hayabusa has swooped down to 4.8 km above the north pole (or nearby high latitudes) to map the pole better. (clutches throat and gasps, while crawling through a desert devoid of pictures) - "images - awk - images!"

Phil

[dilo]

New update : Solar Radiation Pressure Force acting on Hayabusa Station Keeping

http://www.isas.jaxa.jp/e/snews/2005/1025.shtml

Rakhir

"This acceleration is calibrated to 1.261x10-10[km/s2]" ...as predicted! :
http://www.unmannedspaceflight.com/index.php?
showtopic=870&view=findpost&p=22525
my figures were accurate within 5% error
Assuming an Itokawa density of 2 g/cm2, gravitational acceleration was about 1.2x10-11Km/s2 and remaining solar pressure is about 10 times higher than gravity!
In fact, descent/sample collection should carefully account for this tiny force!

[RNeuhaus]

That density is twice than the ones of water and less than half of ones of stone. So it seems that the mass of Itokawa is alike a rubber.

Rodolfo

[ElkGroveDan]

So it seems that the mass of Itokawa is alike a rubber.

Well the shape is almost right

[dilo]

That density is twice than the ones of water and less than half of ones of stone. So it seems that the mass of Itokawa is alike a rubber.

Rodolfo

Pay attention Rodolfo, I was talking about accuracy of my estimate of overall acceleration, based on 15-19 September arc.
Unfortunately, estimation of asteroid mass rely on the small amount of residual acceleration, which is 1.34x10-7 (my estimate) - 1.26x10-7 (calibrated solar pressure) = 8x10-9 m/s2, which correspond to a mass of 1.6E10 Kg.
If my figures are right, this should bring to a density of only 1.3 g/cm2 if based on radar model of Itokawa (an ellissoid with 550x305x275 meters); however, the press release states that Itokawa gravity account for about 1/10 of total acceleration, so probably my total acceleration is slightly underestimated (it should be 1.40x10-7 m/s2, which is the 5% error I was talking about).

In conclusion, actual density should be close to 2 g/cm2, reasonable for a moderately porous rock.
These figures can change (toward higher densities) if actual asteroid dimensions are slighly lower as suggested by first Hayabusa images (can someone confirm this?).

[Phil Stooke]

"Well the shape is almost right"

Good one, ElkGroveDan!

Phil

[ElkGroveDan]

"Well the shape is almost right"

Good one, ElkGroveDan!

Phil

I'm so ashamed. That was my big post #100 and it was just a wisecrack.

[Phil Stooke]

"Good one, ElkGroveDan!

Phil

I'm so ashamed. That was my big post #100 and it was just a wisecrack."


Tsk! I should have said "bad one!"

Phil

[Toma B]

"Opposition surge" on Itokawa...
New update...

Update

[ljk4-1]

"This acceleration is calibrated to 1.261x10-10[km/s2]" ...as predicted! :
http://www.unmannedspaceflight.com/index.php?
showtopic=870&view=findpost&p=22525
my figures were accurate within 5% error
Assuming an Itokawa density of 2 g/cm2, gravitational acceleration was about 1.2x10-11Km/s2 and remaining solar pressure is about 10 times higher than gravity!
In fact, descent/sample collection should carefully account for this tiny force!

So does this also help the case for solar sailing as well - at least until Cosmos 2 gets up there?