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Rosetta flyby of Asteroid Steins, 5th September 2008
Hungry4info
post Sep 7 2008, 05:06 PM
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If we assume (do we know?) that the Steins' axis of rotation is perpendicular to the line of the crater chain, we may also say that the impactors that made the chain were in a line and impacted Steins one by one in different locations as the asteroid rotated. It looks to me like the first and last craters are separated by about 104 degrees (I note that there might be additional craters in this chain on the other side of the body). Division by 6 gives ~17.3 degrees. So I'm going to say that each crater is separated by an average of 17.3 degrees.

Steins has a rotation period of 6.052 hours as stated here:
http://sci.esa.int/science-e/www/object/in...fobjectid=43356

So from the time of the first impact to the last would have been 6.052 * (104/360) = 1.74836 hours. Again dividing by six (not 7, as we assume the first impact is at t=0), each impact was separated by ~17.4835 minutes.

17 minutes is far too high for the breakup of the body to have been caused by Steins itself. I don't know Steins' mass, so I don't know what the roche lobe might be, but I'm going to guess it's a lot closer to Steins than what would allow seven fragments to drift apart from each other that much. If a breakup of the impactor did indeed occur, I think it needs to be by another source like siravan said.

Alternatively, there is always the prospect that it's not a crater chain, like volcanopele is suggesting. Didn't they measure the spectrum of the asteroid? Can we expect compositional maps?


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ugordan
post Sep 7 2008, 05:09 PM
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I'm not buying the broken-up impactor idea one bit. Think of what would have to happen: a body would have to be torn up by another massive body (say Jupiter), travel some vast distance and then the fragments still be clumped together enough to produce a crater chain only some 3 kilometers in length. After travelling millions of kilometers I'd expect the broken up fragments to be separated by hundreds or thousands of kilometers.

Hungry4info: you're neglecting that both the asteroid and the putative fragmented impactor would have been travelling during the course of ~2 hourcs, it's not like the asteroid would come to the mutual orbit crossing point, stop and rotate while collecting the fragments and then move on. There would have to be an amazing coincidence of fragment dispersal so that each one has a slightly different orbit just enough to hit the asteroid crossing point at the same time the asteroid hits it, for each of the fragment orbits. You might as well win the lottery, too.


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centsworth_II
post Sep 7 2008, 05:59 PM
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If an elongate, rubble-pile asteroid like Itokawa (but the size of Braille) and Steins collide. Wouldn't that make a chain of impact craters?
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Phil Stooke
post Sep 7 2008, 06:04 PM
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Look at a page of text from a bit too far away to read the words, and you will see irregular white lines running over the page - "rivers" in printer jargon. They consist of chance alignments of spaces between words. This crater chain is just a chance alignment of craters. Proof - they are of different ages, as shown by the different degrees of sharpness and regularity.

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volcanopele
post Sep 7 2008, 06:56 PM
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Very good point, Phil. If these are impact craters, that would be most likely explanation. These can't be secondary impacts, Steins has too little gravity to pull ejected material back in. Any material from that north polar impact crater would be long gone, except very slow moving ejecta...and they wouldn't form craters, those would show up as boulders on the surface (resolution too low to resolve these). As others have explained effectively, these likely aren't a crater chain caused by a broken-up impactor, Steins is too small, and rotates too slowly. And it would certainly NOT break up an in-coming body, Steins is not Jupiter. I'm willing to bet that Steins just about fills up its Roche lobe.

So you are back to either a pit chain, a la Phobos, or just a coincidental arrangement of craters.


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ugordan
post Sep 7 2008, 07:46 PM
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QUOTE (volcanopele @ Sep 7 2008, 08:56 PM) *
I'm willing to bet that Steins just about fills up its Roche lobe.

Um... how do you calculate the Roche limit if it's the only isolated body around?


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volcanopele
post Sep 7 2008, 08:17 PM
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.... don't ask math questions on a Sunday....


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3488
post Sep 7 2008, 10:44 PM
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I like that VP, perhaps best not to ask ANY questions that require an intelligent answer on a Sunday. laugh.gif

I have been during quite a bit of gemming up on 2867 Šteins today, but not from Wikipedia.

2867 Šteins although not originally a part of 4 Vesta as per my first theory, does appear to be a chunk knocked off Asteroid 434 Hungaria, so my theory that 2867 Šteins is part of a mantle from a larger body, still stands.

Apparently some Hungaria asteroids do come close to the orbit of Mars, though 2867 Šteins, does not & remains a core member that stays well within the main Asteroid Belt, though E Type main Belt asteroids are incredibly rare, so Rosetta by some very remote & lucky chance encountered one, on only the eigth close asteroid pass (ninth including 243 Ida's moon Dactyl).

I wonder if someone knows. Does 2867 Šteins rotate prograde or retrograde? The Press Conference stated that 2867 Šteins has virtually no axial tilt, but did not mention direction of axial rotation.

If anyones interested, view of Sun & Earth from 2867 Šteins @ time of Rosetta Closest Approch.

Attached Image


Andrew Brown.


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Hungry4info
post Sep 8 2008, 12:33 AM
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QUOTE (ugordan @ Sep 7 2008, 02:46 PM) *
Um... how do you calculate the Roche limit if it's the only isolated body around?


http://en.wikipedia.org/wiki/Roche_limit

What he was saying is that the point at which tidal forces would tear an asteroid into a string of asteroids SL9-style would probably be closer to the centre of the asteroid than the surface is.


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Decepticon
post Sep 8 2008, 02:26 AM
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Can someone clarify if the camera safe mode cost the flyby of Hi-Res images?
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elakdawalla
post Sep 8 2008, 03:02 AM
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I suppose it depends upon what you mean by Hi-res. If you mean "would we have higher-resolution images of Steins if the camera had not safed?" the answer is, I believe, yes. My understanding was that OSIRIS NAC was also to be used to image Steins throughout the encounter, so, in theory, we could have had images with 5x better resolution. Such is life. The WAC images are still pretty great, with the special treat of going through zero phase, which is probably making the spectroscopists drool.

I have a nagging memory that there was a New Horizons instrument that safed during the Jupiter flyby for a similar reason (safety tolerances set very conservatively, given the fact that Jupiter was not the prime target). Is this right? Does anybody remember which one?

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mchan
post Sep 8 2008, 04:59 AM
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QUOTE (Hungry4info @ Sep 7 2008, 05:33 PM) *
http://en.wikipedia.org/wiki/Roche_limit

What he was saying is that the point at which tidal forces would tear an asteroid into a string of asteroids SL9-style would probably be closer to the centre of the asteroid than the surface is.

This might make sense if the first use of "asteroid" in the sentence refers to an object apart from the second use of "asteroid" (singular) in the sentence.

Jason used the term "Roche lobe" instead of "Roche limit". The wikipedia describes the former in the context of a binary star system. This is still an effect between two bodies. Gordan was asking what Roche meant for a single body in isolation.
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lyford
post Sep 8 2008, 05:58 AM
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QUOTE (elakdawalla @ Sep 7 2008, 07:02 PM) *
I have a nagging memory that there was a New Horizons instrument that safed during the Jupiter flyby for a similar reason (safety tolerances set very conservatively, given the fact that Jupiter was not the prime target). Is this right? Does anybody remember which one?

PEPSSI?


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volcanopele
post Sep 8 2008, 08:50 AM
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I used Roche lobe because I listen to the small icy moons people on the imaging team a little too much. It is used in context of small, fluffy moons that can't really accrete anymore.


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ugordan
post Sep 8 2008, 08:55 AM
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That was my point exactly, an isolated object isn't prevented by outside tidal forces from continually accreting material as one would be if it were in say Saturn's orbit. Granted, there's a weak tidal effect by the Sun, but it's insignificant in this case.


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