[X] My Assistant

[Guest_BruceMoomaw_*]

Remember, the skinniest dimensions of Itokawa are only about 250 meters -- half a pixel in ISS's best images of Titan! It's about 1/50th the size of Eros! It's tiny.

Look at the limb of Itokawa in the JAXA releases and think of the horizon in snapshots Apollo astronauts took on the Moon -- those are similar scales. You don't see many craters on the near-horizon of the Apollo photos, either.

I'm not sure if a world Itokawa's size *can* show much in the way of craters. It may be in that all asteroids this small will have surfaces completely flattened by to saturation, like a World War I battlefield.

I don't really see why -- look at the closeup views of the Moon's surface from various descending spacecraft (from the Rangers on) and they're speckled with clearly visible craters down to just a meter or two in diameter. After all, the size distribution of meteoroids -- and thus of the craters they gouge out -- is fractal; the relative number of craters of different relative sizes remains the same on any size scale. No; something has genuinely caused a shortage of craters on Itokawa -- either seismic shaking like that which obliterated all the smallest craters on Eros (which could be more extreme in this case, if Itokawa also has a regolith), or else the simple fact that Itokawa is a relatively fresh fragment of a bigger asteroid and so just hasn't had time yet to accumulate many craters.

[Bob Shaw]

Bruce:

Yup, quite right - and even if the surface *was* a WWI battlefield, the *last* craters would show up - and generally you'd start with bigger, older ones, and move down to smaller, more recent ones (well, more-or less). So, as you say, either something wipes the slate clean, or the surface is fairly fresh.

The devil will be in the detail...

Bob Shaw

[Toma B]

What has hapened to regular (daily) updates and NEW IMAGES?!?

[dvandorn]

I think that, while Itokawa is quite small, it's also massively uneven in its mass distribution. It took the guys controlling NEAR quite a while to be able to perform maneuvers in which they had any confidence of the resulting trajectory -- all because Eros' gravity field was extremely uneven.

At 25km, the small mass of Itokawa won't have much of a measurable effect on Hayabusa. As it approaches, the gravity effects become more noticeable -- and as the uneven field passes through Haybusa as Itokawa rotates, the *cumulative* accelerations (in all different vectors) will be difficult to predict.

I think they'll likely approach slowly and try to model the gravity field as they close in, so that they don't end up getting swatted badly out of position as they get *really* close.

-the other Doug

[tedstryk]

I don't really see why -- look at the closeup views of the Moon's surface from various descending spacecraft (from the Rangers on) and they're speckled with clearly visible craters down to just a meter or two in diameter.  After all, the size distribution of meteoroids -- and thus of the craters they gouge out -- is fractal; the relative number of craters of different relative sizes remains the same on any size scale.  No; something has genuinely caused a shortage of craters on Itokawa -- either seismic shaking like that which obliterated all the smallest craters on Eros (which could be more extreme in this case, if Itokawa also has a regolith), or else the simple fact that Itokawa is a relatively fresh fragment of a bigger asteroid and so just hasn't had time yet to accumulate many craters.

If it is very loosely packed, it might just knock a bunch of material off rather than make a crater. Also, there would, I imagine, be a lack of ejecta formations because so little would come back.

[tedstryk]

I think that, while Itokawa is quite small, it's also massively uneven in its mass distribution.  It took the guys controlling NEAR quite a while to be able to perform maneuvers in which they had any confidence of the resulting trajectory -- all because Eros' gravity field was extremely uneven.

At 25km, the small mass of Itokawa won't have much of a measurable effect on Hayabusa.  As it approaches, the gravity effects become more noticeable -- and as the uneven field passes through Haybusa as Itokawa rotates, the *cumulative* accelerations (in all different vectors) will be difficult to predict.

I think they'll likely approach slowly and try to model the gravity field as they close in, so that they don't end up getting swatted badly out of position as they get *really* close.

-the other Doug

I wonder if there is any debris around it. It couldn't hold it there, but if the thing has had so much of a bump recently, I wonder about small objects overing around it.

[JRehling]

I don't really see why -- look at the closeup views of the Moon's surface from various descending spacecraft (from the Rangers on) and they're speckled with clearly visible craters down to just a meter or two in diameter.  After all, the size distribution of meteoroids -- and thus of the craters they gouge out -- is fractal; the relative number of craters of different relative sizes remains the same on any size scale.  No; something has genuinely caused a shortage of craters on Itokawa -- either seismic shaking like that which obliterated all the smallest craters on Eros (which could be more extreme in this case, if Itokawa also has a regolith), or else the simple fact that Itokawa is a relatively fresh fragment of a bigger asteroid and so just hasn't had time yet to accumulate many craters.

I'm not quite sure if it makes sense to say that the distribution is fractal. I have a sense of what you're saying, but only a vague sense. The distribution fits a power law, but that does not mean that the presence of impact craters looks the same at all scales.



Here are some pictures from airless worlds that show what I'm talking about. Craters absent? No. But on the small scale, they start to be hard to find.

http://spaceflight.nasa.gov/gallery/images...s11_40_5903.jpg

http://nssdc.gsfc.nasa.gov/imgcat/hires/a11_h_37_5458.gif

http://www.nasm.si.edu/collections/imagery...12-47-6898.html

http://nssdc.gsfc.nasa.gov/planetary/image...t_157415053.jpg

Put a piece of paper with an Itokawa-shaped hole cut into it over any of those images, and you won't see more than two craters. The Eros descent pictures make this phenomenon stand out.

If it were a fractal, you'd see craters in images like this:

http://nssdc.gsfc.nasa.gov/planetary/missi..._157416593.html

You don't.

The devil is in the details. First of all, it certainly doesn't seem to be a fractal: For the Apollos and NEAR, you certainly can tell the difference between high-up images and down-lower ones. Itokawa's craterlessness may show something more than the Apollo and NEAR closeups, but those don't look like orbital views, either.

[Guest_BruceMoomaw_*]

(1) All the lunar photos you've shown have been taken from the surface, at a very oblique angle -- which tends to conceal small craters on the Moon. Just take a look at the Moon's surface from ABOVE -- either from descending Rangers and LMs, or high-resolution orbiting Lunar Orbiters and Apollos -- and it's veritably peppered with clearly visible craters down to a meter or less in size.

(2) There's no doubt that there is a striking shortage of small craters on Eros -- and the suspect, as I said, is seismic shaking due to the small size of Eros relative to the Moon. And, as I said, this is also the likely cause for Itokawa's shortage of craters IF it has a regolith. If it doesn't -- if it's bare rock -- then the other explanation, that it's a recently detached fragment, is presumably the explanation.

I tell you, though: when you look at the latest photos of Itokawa, it looks to me as though it's quite densely covered with crap. Those spiky protrusions or "warts" all over it look to me like impact-detached boulders that fell back onto the surface, just as with Eros -- but bigger compared to the total size of the asteroid.

[JRehling]

(1)  All the lunar photos you've shown have been taken from the surface, at a very oblique angle -- which tends to conceal small craters on the Moon.  Just take a look at the Moon's surface from ABOVE -- either from descending Rangers and LMs, or high-resolution orbiting Lunar Orbiters and Apollos -- and it's veritably peppered with clearly visible craters down to a meter or less in size.

On further review, I suspect you are right on this... BUT in lunar descent images from Apollo, the remaining difference is phase. All of the Apollos landed shortly after local dawn, giving stretched out shadows. Looking at the full Moon, you hardly see craters in the highlands, save a ray system or five, which we can't expect Itokawa to have. We're seeing most of Itokawa at a pretty full phase. And I think I see a pale crater rim in one of the Sept. 10 images. I agree with your overall sense given the high-phase images, but it isn't data yet. And I realize that I switched my argument from "closeup, the Moon has no craters" to "maybe Itokawa does"!

[Guest_BruceMoomaw_*]

There are pretty good shadows in the Sept. 11 image ( http://www.hayabusa.isas.jaxa.jp/e/index.html ) -- but I can only see one possible crater (over on the right end). Certainly it has very few of them compared to either the Moon or Eros -- and, given the amount of loose junk on its surface that one can clearly see in that photo, the explanation has got to be seismic shaking. An impactor that would gouge out quite a small crater on the Moon shakes the whole of Itokawa so much that ALL its regolith jumps off the surface in that super-weak gravity, and then slowly sprinkles back evenly all over the asteroid.

I agree with Bell, though, that at the core it's just a single lump of rock with a large chunk knocked out of one side -- I don't see how that peculiar shape could be a contact binary.

[JRehling]

There are pretty good shadows in the Sept. 11 image ( http://www.hayabusa.isas.jaxa.jp/e/index.html ) -- but I can only see one possible crater (over on the right end).

I disagree that there are good shadows there. The weird shape of Itokawa means that the upper middle in this picture is in favorable illumination, but seen obliquely. For the most part, the left end is a "full" spheroid and the right end is a gibbous spheroid. The left end there looks just as washed out as a full Moon, and the rest of it is favorably lit mainly when oblique to the camera.

[jaredGalen]

Here a two frame animated gif that kind of enhances the shadows.

Attached thumbnail(s)

 

[tedstryk]

Well, in a few weeks, Hayabusa is supposed to move from its current spot to look study Itokawa at a lower phase angle. That should resolve this shadow issue.

[dvandorn]

(1)  All the lunar photos you've shown have been taken from the surface, at a very oblique angle -- which tends to conceal small craters on the Moon.  Just take a look at the Moon's surface from ABOVE -- either from descending Rangers and LMs, or high-resolution orbiting Lunar Orbiters and Apollos -- and it's veritably peppered with clearly visible craters down to a meter or less in size.

In fact, people who are paid to investigate these things have found the the Moon is in a complete steady-state saturation state in all craters smaller than about one kilometer. There is a constantly maintained population of craters of all sizes below one kilometer -- including craters only microns in size. Lunar rocks feature "zap pits" which are the result of impacts from very tiny dust-mite-sized particles. So the instance of lunar craters from one meter to only a few microns in size is well-established as being maintained at its saturation point -- there are as many craters evident as is possible, considering the erasure of earlier craters by later cratering events.

The fact that the terminator regions of Itokawa do *not* show a saturation level of cratering, down to the resolution limit, is indeed a telling observation. This is in no way similar to lunar surface cratering in abundance or size distribution, even at the same scales.

-the other Doug

[AndyG]

I wonder if there is any debris around it.  It couldn't hold it there, but if the thing has had so much of a bump recently, I wonder about small objects overing around it.

I suspect there isn't any debris to speak of.

Given a mass for Itokawa of about 4.8*10^10kg (about right for a 2000kg/m^3 body of dimensions 600*290*260m) the escape velocity is only 10 to 12cm/s depending on your starting altitude on the surface. Any impact by another body at crossing-orbit speeds, even if as low as a few tens of metres per second, will knock chunks off Itokawa that are likely never to return to the parent body. And those (few?) remnants that fortuitously don't achieve escape velocity have likely had centuries to land again.

Looking at a simple spreadsheet for orbiting Itokawa, my personal favorite altitude is some 540m from the centre of gravity. An orbit here - albeit perturbed by the weird gravity in orbiting a shape like this - will be asteroid-synchronous, allowing some really good opportunities for data collection to be made for one spot over the course of a local day. I'll take the animated movie, please!

Andy G