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bison

Asteroid 1998 QE2

26 posts in this topic

Having heard nothing more about the work to determine the density of asteroid 1998 QE2, a key piece of information in determining its composition, I enquired about this to a JPL representative. I heard back promptly from David Agle.

I quote his response in full- - "A very preliminary density estimate ~ 1g/cm^3. So a very high porosity object." An interesting response. The object appears to fall neatly between the average density of 'primitive', dark, 'rockpile' asteroids (type C) which this object appears to be, and comets. The former average ~ 1.4 g/cm^3, and the latter about 0.4 g/cm^3.

Type C asteroids typically have a porosity, empty spaces between rocky fragments, of about 27%. In order for this object to have such a low density, it might need to have as much as half its volume taken up by such spaces.

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One is tempted to suggest that this object is a short period (extinct) comet. The exhaustion of most of its light, volatile ice, could increase its bulk density to the stated figure.

The only problem with this explanation is that most comets have much longer orbits. There are some short period comets, though. Those the might be likely to pass near Earth are in the Jupiter Family of Comets. The gravity of Jupiter converts a long period comet to a much shorter orbit.

A combination of orbital factors are used to produce a Tisserand Parameter. In Jupiter Family Comets these are typically less than 3. In asteroids, greater than three. The Tisserand parameter of 1998 QE2 is given as 3.24, indicating it's probably an asteroid, not a comet. Could we have a slightly anomalous comet anyway? I looked at a list of nearly 500 Jupiter Family Comets. All but a dozen were under 3. The highest value, in a single instance, was 3.212.

An unusually porous asteroid looks likelier than a short period comet.

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Bison, what do you make of the 1998 QE2's moon and the extreme difference in albedo? Why is the moon bright but the main object dark?

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It was quite intriguing that the asteroid's moon appeared so much brighter than its primary. The initial suspicion was that it must be much more reflective of the radar waves used to make the image. As it turns out, the real reason for the moon's brightness is that it is spinning much more slowly than the asteroid, once in about 32 hours, as opposed to about 5 hours. A feature of the way that the radar images are made is that the faster an object spins, the more spread out it appears. The slower spinning moon appears less spread out, and so its radar returns are more crowded together, and they appear brighter.

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It was quite intriguing that the asteroid's moon appeared so much brighter than its primary. The initial suspicion was that it must be much more reflective of the radar waves used to make the image. As it turns out, the real reason for the moon's brightness is that it is spinning much more slowly than the asteroid, once in about 32 hours, as opposed to about 5 hours. A feature of the way that the radar images are made is that the faster an object spins, the more spread out it appears. The slower spinning moon appears less spread out, and so its radar returns are more crowded together, and they appear brighter.

Okay, thanks. Is the calculated average density of the moon the same as the primary?

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Posted (edited)

Okay, thanks. Is the calculated average density of the moon the same as the primary?

It's been said by some sources that the density of both the asteroid and its moon could be calculated, other sources neglect mentioning the density of the moon at all. It's not clear if estimating the density of the moon could use the mutual movements of the two object, as they do in determining the mass of the main asteroid, or not. So far, I've only heard of the very preliminary density estimate for the main asteroid. Edited by bison

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It's been said by some sources that the density of both the asteroid and its moon could be calculated, other sources neglect mentioning the density of the moon at all. It's not clear if estimating the density of the moon could use the mutual movements of the two object, as they do in determining the mass of the main asteroid, or not. So far, I've only heard of the very preliminary density estimate for the main asteroid.

Don't you think it's odd that the two-body system can be seen quite clearly but only the average density of the primary can be determined?? I suspect NASA are hiding something personally.

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Don't you think it's odd that the two-body system can be seen quite clearly but only the average density of the primary can be determined?? I suspect NASA are hiding something personally.

The way the situation was explained to me: they have only a very preliminary estimate of the density of the main asteroid. It obviously needs further work before the most accurate possible figure is obtained. This work is presumably based on the effect the main asteroid has on the orbit of its moon. Determining the reciprocal case-- the effect the moon has on the asteroid-- will probably be much more difficult, and require some very exacting work. This latter effect is obviously far smaller than the former.

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The way the situation was explained to me: they have only a very preliminary estimate of the density of the main asteroid. It obviously needs further work before the most accurate possible figure is obtained. This work is presumably based on the effect the main asteroid has on the orbit of its moon. Determining the reciprocal case-- the effect the moon has on the asteroid-- will probably be much more difficult, and require some very exacting work. This latter effect is obviously far smaller than the former.

Do you think that the exact dynamics of the two body system will be resolved with an accurate simulation model or is NASA 'too busy'?

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Posted (edited)

They may eventually do a computer simulation of the shape and motions of the asteroid and its moon. It's not unreasonable that this would take some time. An accurate density figure for the asteroid should come first.

I was also interested in the spectroscopic analysis of the object. This will be key to determining what the object is made of. I asked Alessondra Springmann, an astrophysicist working on 1998 QE2, what had been learned. She replied that the asteroid appears to be transitional between asteroid classes D and X.

Class D asteroids are essentially metallic objects, and quite dense. Class X asteroids are made of mixed silicates, carbon compounds, and possibly ice. Its very interesting that an object described as 'primitive'-- meaning largely untouched since its initial formation, should seem to show signs suggesting the complete mergence of two very different sorts of objects, into one, essentially spherical, body

Edited by bison

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They may eventually do a computer simulation of the shape and motions of the asteroid and its moon. It's not unreasonable that this would take some time. An accurate density figure for the asteroid should come first.

I was also interested in the spectroscopic analysis of the object. This will be key to determining what the object is made of. I asked Alessondra Springmann, an astrophysicist working on 1998 QE2, what had been learned. She replied that the asteroid appears to be transitional between asteroid classes D and X.

Class D asteroids are essentially metallic objects, and quite dense. Class X asteroids are made of mixed silicates, carbon compounds, and possibly ice. Its very interesting that an object described as 'primitive'-- meaning largely untouched since its initial formation, should seem to show signs suggesting the complete mergence of two very different sorts of objects, into one, essentially spherical, body

Yes, I agree, it *is* intriguing. Thanks for the detailed reply. All the best.

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The average density of class X asteroids is about 2.9 g/cm^3, that of class D around 9.6. Interesting that an object that, by dynamical methods, seems to have a density of approximately 1 g/cm^3 should appear to be made of materials,that, if combined, could make for a density of 6 or 7. Although 1998 QE2 left the vicinity of Earth over a month ago, it seems that it may still hold some surprises for us.

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The average density of class X asteroids is about 2.9 g/cm^3, that of class D around 9.6. Interesting that an object that, by dynamical methods, seems to have a density of approximately 1 g/cm^3 should appear to be made of materials,that, if combined, could make for a density of 6 or 7. Although 1998 QE2 left the vicinity of Earth over a month ago, it seems that it may still hold some surprises for us.

Agreed.

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After being told that 1998 QE2 has a spectrum combining characteristics of class X and D asteroids, I began to wonder where I had gotten the idea that it was class C. I then found that spectra of the object had been made on June 5th with the 5 meter Hale telescope, at Palomar observatory. This was only a few days after the nearest approach of 1998 QE2. It was still quite close. They found that the asteroid most closely matched class C. When or where the information suggesting class X and D was obtained, I couldn't say, as yet.

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Bison,are you ever actually going to get to the point?

Please make your point otherwise your posts could be considered thread bumping.

From the rules:

1d. Thread bumping: Do not post 'bump' messages solely to return a thread to the top of the topic index.

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I am pursuing the story of the continuing, and still unresolved scientific investigation of the object 1998 QE2. I have again contacted one of the scientists involved in this work, in order to resolve a seeming discrepancy in reports of the taxonomy of the object. I propose to report her response, if and when one is received. I also propose to post other information about 1998 QE2 as I become aware of it, and to comment on it.

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Erratum to my post, # 12, July 3rd. Figures for density of class D and class X asteroids inadvertently reversed. It should read: Class D average density ~ 2.9 g/cm3, Class X density ~ 9.6.

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Posted (edited)

Can you please reveal the point of this thread?

Of course there are discrepancies when studying distant objects. It does not mean that the explanations are malicious. Are you suspicious also when something curious has a perfect and convenient explanation?

Edited by Timonthy

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Posted (edited)

Reveal the point of this thread? I already did just that. (see post #16). I have accepted everything I have learned about this object at face value.

I asked astrophysicist Alessondra Springmann about the seeming contradiction in the taxonomic type of this asteroid. She gave a very helpful reply. She suggested that one side of the object may be of one mineral type, like carbonaceous chondrite meteorites, and the other side a mixture of silicates and metal.

This suggests a very interesting history for this object. It may have been impacted by two asteroids, one of D and one of X class. Alternately, One side of the object may have collected the debris of a nearby collision of two such asteroids.

There is no current asteroidal classification, as far as I am aware, describing a transitional type between classes D and X, or any report, to my knowledge, telling of an asteroid with two hemispheres with very different types of mineral composition. It appears that this is a very unusual object, one worthy of our continued attention. Despite the fact that this object passed by Earth some five weeks ago, these are still very 'early days' in a scientific sense, as far as this object is concerned.

Edited by bison

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There have been fifteen years of observations of this object. It has consistently been assigned to the group of C class asteroids. Why, one wonders, has no one ever managed to note that one side of the object has, instead, a mixture of D and X class spectra, as is now seems to be the case? If the D and X class features are inconspicuous, and only became detectable upon the object's close approach, why was the spectrum still not dominated by the overall C class spectrum of the object?

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Dr. Marina Brozovic, head of the JPL radar investigations into 1998 QE2, stated that it had not been expected that this object would prove to have a moon. Most near Earth asteroids with moons have rotation periods of about 2 to 3 hours. With the currently estimated spin of 1998 QE2 in the area of around 4 hours, it appears to be exceptional.

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Dr. Marina Brozovic, head of the JPL radar investigations into 1998 QE2, stated that it had not been expected that this object would prove to have a moon. Most near Earth asteroids with moons have rotation periods of about 2 to 3 hours. With the currently estimated spin of 1998 QE2 in the area of around 4 hours, it appears to be exceptional.

I think you're on to something here Bison. I wonder what u-turns they will make with Pluto and Charon coming into simulation modelling view?

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The situation of Pluto and its moon Charon is quite different from that of 1998 QE2. The former are mutually tidally locked, in the latter, only the moon is. The moon of QE2 presumably lacks the mass to tidally lock its primary, even thought they are separated by only around 6 kilometers. The moon is also quite large in relation to its primary, as moons go-- about 22 % as large. We have no firm figures for the mass or density of QE2 or its moon, only the 'very preliminary' figure of ~1 g/cm^3 for the primary. Six and a half weeks, now, since the closest approach of the object. Still waiting for the refined figures. My latest inquiry about this to JPL has gone unanswered.

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The situation of Pluto and its moon Charon is quite different from that of 1998 QE2. The former are mutually tidally locked, in the latter, only the moon is. The moon of QE2 presumably lacks the mass to tidally lock its primary, even thought they are separated by only around 6 kilometers. The moon is also quite large in relation to its primary, as moons go-- about 22 % as large. We have no firm figures for the mass or density of QE2 or its moon, only the 'very preliminary' figure of ~1 g/cm^3 for the primary. Six and a half weeks, now, since the closest approach of the object. Still waiting for the refined figures. My latest inquiry about this to JPL has gone unanswered.

It will remain unanswered whether your right or wrong on this I imagine.

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Posted (edited)

Perhaps my inquiry will not be answered. I put the question to: questions@neo.jpl.nasa.gov, so it seems the appropriate place. I trust I hadn't 'worn out my welcome', having never asked a question at that address before. I merely asked for the updated density figure, something they had already proposed to provide, once it was discovered the object had a moon. I asked my question in a polite manner. Even the admission that a better figure than the very preliminary ~ 1 g/cm^3 still wasn't available, and perhaps a sense of when that information might be available would have been appreciated.

Edited by bison

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