The QE2

[neufer]

Well the new results are in after more radar measurements, and your inclination towards a lower density was correct, but surprisingly, still ~2x too high!
The orbital period and "maximum" radius are listed as 32hr and 6.4km respectively, which sets QE2's density ≤ 1gm/cm³. A fluffy asteroid indeed! I was expecting both a lower density and faster orbital velocity than my original calculations, but it turns out the new velocity is about the same (only 9% faster) which means the density is much lower. If this density is correct, it must be a record holder. (Mathilde density = 1.3gm/cm³)

Not a record if it is a captured comet (; Halley's Comet density = 0.6 gm/cm³)

Congratulations on getting the calculations right even if your density guess was more reasonable for Amor asteroids (i.e., 433 Eros) than for an old comet. However, I simply couldn't jive the occasional 3 pixel Doppler width of the satellite with something that was supposed to rotate ~30 times slower than the primary. Now it is only ~7 times slower than the primary (assuming that it orbits roughly around the equator).

[alter-ego]

Not a record if it is a captured comet (; Halley's Comet density = 0.6 gm/cm³)

Bingo!

Congratulations on getting the calculations right even if your density guess was more reasonable for Amor asteroids (i.e., 433 Eros) than for an old comet.

Thanks!

[neufer]

"It takes all the running you can do, to keep in the same place."

[b][color=#FF0000]Radar images of asteroid 1998 QE2 and its satellite on June 7. Each frame in the animation is a sum of 4 images, spaced apart by about 10 minutes. (Arecibo Observatory/NASA/Ellen Howell)[/color][/b]

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Earth-Passing Asteroid is “An Entirely New Beast”
by Jason Major on June 17, 2013

<<On the last day of May 2013 asteroid 1998 QE2 passed relatively closely by our planet, coming within 6 million kilometers… about 15 times the distance to the Moon. While there was never any chance of an impact by the 3 km-wide asteroid and its surprise 750 meter satellite, astronomers didn’t miss out on the chance to observe the visiting duo as they soared past as it was a prime opportunity to learn more about two unfamiliar members of the Solar System.

By bouncing radar waves off 1998 QE2 from the giant dish at the Arecibo Observatory in Puerto Rico, researchers were able to construct visible images of the asteroid and its ocean-liner-sized moon, as well as obtain spectrum data from NASA’s infrared telescope in Hawaii. What they discovered was quite surprising: QE2 is nothing like any asteroid ever seen near Earth.

Both Arecibo Observatory and NASA’s Goldstone Deep Space Communications Complex in California are unique among telescopes on Earth for their ability to resolve features on asteroids when optical telescopes on the ground merely see them as simple points of light. Sensitive radio receivers collect radio signals reflected from the asteroids, and computers turn the radio echoes into images that show features such as craters and, in 1998 QE2′s case, a small orbiting moon.

QE2′s moon appears brighter than the asteroid as it is rotating more slowly; thus its Doppler echoes compress along the Doppler axis of the image and appear stronger.

Of the asteroids that come close to Earth approximately one out of six have moons. Dr. Patrick Taylor, a USRA research astronomer at Arecibo, remarked that “QE2′s moon is roughly one-quarter the size of the main asteroid,” which itself is a lumpy, battered world.

Dr. Taylor also noted that our own Moon is a quarter the size of Earth.

QE2′s moon will help scientists determine the mass of the main asteroid and what minerals make up the asteroid-moon system. “Being able to determine its mass from the moon helps us understand better the asteroid’s material,” said Dr. Ellen Howell, a USRA research astronomer at Arecibo Observatory who took both radar images of the asteroid at Arecibo and optical and infrared images using the Infrared Telescope Facility in Hawaii. While the optical images do not show detail of the asteroid’s surface, like the radar images do, instead they allow for measurements of what it is made of.

“What makes this asteroid so interesting, aside from being an excellent target for radar imaging,” Howell said, “is the color and small moon. Asteroid QE2 is dark, red, and primitive – that is, it hasn’t been heated or melted as much as other asteroids,” continued Howell. “QE2 is nothing like any asteroid we’ve visited with a spacecraft, or plan to, or that we have meteorites from. It’s an entirely new beast in the menagerie of asteroids near Earth.” Spectrum of 1998 QE2 taken May 30 at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea was “red sloped and linear,” indicating a primitive composition not matching any meteorites currently in their collection.

Source: Universities Space Research Association press release.>>

[geckzilla]

Rose rock?


Red scoria?

[alter-ego]

“What makes this asteroid so interesting, aside from being an excellent target for radar imaging,” Howell said, “is the color and small moon. Asteroid QE2 is dark, red, and primitive – that is, it hasn’t been heated or melted as much as other asteroids,” continued Howell. “QE2 is nothing like any asteroid we’ve visited with a spacecraft, or plan to, or that we have meteorites from. It’s an entirely new beast in the menagerie of asteroids near Earth.” Spectrum of 1998 QE2 taken May 30 at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea was “red sloped and linear,” indicating a primitive composition not matching any meteorites currently in their collection.

Source: Universities Space Research Association press release.>>

The first peep:
http://www.unexplained-mysteries.com/fo ... pic=250176

"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."

[bison]

I contacted astrophysicist Alessondra Springmann, who is doing work of 1998 QE2. I asked her what had been learned about the composition of the object. She replied that it had not been definitely sorted into a particular class, but that it appeared to have characteristics of class X and class D asteroids. Class X asteroids are made of mixed silicates, carbon compounds, and possibly ice in their interiors. Class D asteroids are essentially metallic objects. An interesting combination of two very different sorts of objects.

[mjimih]

Any similarity with low density moons in the solar system yet, like for example Hyperion from Saturn?

[Ross]

Hyperion, like most of Saturn's moons, is probably largely ice. The bulk densities range from about 0.5 to 1.5 g/cm^3. Given a very preliminary density figure for 1998 QE2 of 1 g/cm^3, it's tempting to think it mostly ice, too. That, of course, suggests an extinct comet nucleus. The Tisserand parameter is high, 3.24. Most comets show less than 3. A few above three, but still lower than this object. The orbit appears asteroidal, rather than cometary.
It seems odd that the dynamically determined density is at odds with the the apparent taxonomy. A new, transition type of asteroid, with features of the very dissimilar D and X classes, with respective densities of about 9.6 g/cm^3, and 2.9 ? That would seem to suggest a far higher density-- 6 or 7. To bring this down to ~ 1g/cm^3 would seem to require the object to be so porous as to be virtually hollow.

[mjimih]

new beast eh? may i imagine for a moment that a dieing comet, captured by the asteroid belt, subsequently attracted rocks and dirt over the eons and is a hybred object. Comet on the inside and compacted rocks on the outside.

[Ross]

Its usual for comets to have a mineral 'slag' coating. If it becomes too thick, it can shut off ice sublimation altogether, causing the comet to become extinct. Could a comet pass close enough to an asteroid to have its orbit altered to one like an asteroid? Given the low mass of asteroids, this seems somewhat unlikely. Still, we apparently have an unusual specimen in QE2, so who can say? By the way-- an erratum to my last post: I inadvertently reversed the density figures for the D and X class asteroids. It should read average density of D class ~ 2.9 g/cm^3, and X class 9.6. This means, of course, that it is class X that is metallic and class D made of silicates, etc.

[Ross]

The mineral composition of QE2 appears to have yet another layer of complexity. Besides the odd mixture of asteroid Class X (metallic) and D (mixed silicates) indications, we now have more. Spectra taken at the Hale telescope, Palomar observatory, give a 'best fit' for class C, similar to carbonaceous chondrite meteorites.
Astrophysicist Alessondra Springmann responding to my follow-up question, suggests that different sides of the object have these different compositions, thus explaining the seeming contradictory results. If this turns out to be the correct explanation, QE2 will presumably have to have an asterisk after it taxonomic type, in the asteroid lists, and a bottom-of-the-page note explaining its oddly mixed and partitioned composition.

[Ross]

It almost looks as if a C class asteroid received debris on one side, from the nearby collision of a D and an X class asteroid. Odd, though, that such debris didn't persist long enough to cover both sides of QE2. It only takes a couple of hours for it to turn its opposite face in any given direction. Has anyone a better explanation?

[Ross]

There have been 15 years of observation of QE2. These have consistently seen it classified as C class, similar to carbonaceous chondrite meteorites. Odd that at least one recent observation has yielded mixed D and X class spectra. If one side or portion of the object has the indicated composition, why had this not be noticed before? If these features were inconspicuous, so that they could only be detected when QE2 was nearby, why did the overall C class spectrum still not dominate, or at least figure prominently in the results?

[alter-ego]

There have been 15 years of observation of QE2. These have consistently seen it classified as C class, similar to carbonaceous chondrite meteorites. Odd that at least one recent observation has yielded mixed D and X class spectra. If one side or portion of the object has the indicated composition, why had this not be noticed before? If these features were inconspicuous, so that they could only be detected when QE2 was nearby, why did the overall C class spectrum still not dominate, or at least figure prominently in the results?

Though QE2 1998 has been known for 15 years, I'm not sure the spectral characteristics have been known well enough to confidently place it as a C-class. Per the Astronomer's Telegram (June 14, 2013) the asteroid is currently classified within the C-group as a Ch-class (absorption feature at 0.7um), which is relatively uncommon. (Interestingly, one characterized example of this type is asteroid Ate w/density = 1.2 ± 0.3 g/cm³). However, prior to the new observations, the JPL observation planning update (2013) lists QE2's spectral classification as unknown:

Orbital and Physical Characteristics
orbit type Amor
semimajor axis 2.427 AU
eccentricity 0.568
inclination 12.8 deg
perihelion distance 1.049 AU
aphelion distance 3.805 AU
Tisserand parameter 3.240 Not obviously cometary
absolute magnitude (H) 16.4
diameter 2.75 km (Trilling et al. 2010)
optical albedo 0.06 (Trilling et al. 2010)
rotation period 5.28 h (Kevin Hills, pers. comm.)
pole direction unknown
lightcurve amplitude 0.2 mag (Kevin Hills, pers. comm.) This suggests that the shape is not highly elongated.
spectral class unknown, but optically dark (Binzel, Moskovitz, and Polishook, pers. comm.)

It seems remote that such a porous, yet almost spherical, asteroid would be the result of a collision. Perhaps QE2 is the child of a larger asteroid collision? How it came to be with apparently dual spectral signatures, and do they originate from different hemispheres is all very interesting and subject to speculation. I'd like to know more about it's moon, e.g. what spectral class is it?

[Ross]

This link to a page from the Lincoln Laboratory seems to establish that the classification 'Ch' for QE2 had been made as early as 1998, 15 years ago: http://earn.dlr.de/nea/285263.htm Recent observations at Table Mountain and Palomar observatories support this classification. It's still not clear where JPL got the conflicting, mixed D and X classification. The conclusion that the object has different spectra on different sides seemed the best way to reconcile the variant spectral results reported recently, although there are still some serious problems with this.

[Ross]

I inquired to JPL about a possibly updated density figure for this object. We still only have the nearly month-old, 'very preliminary' estimate of ~1 g/cm^3. My query has gone unanswered.
In retrospect, it seems odd that so much was made of the fact that an accurate density figure for the object, and its moon, would we readily obtainable, using centuries-old math, once the object was seen to have a moon.
Now this problem seems to have become a much larger one, involving interminable delays. I wonder what the trouble can be.

[alter-ego]

I inquired to JPL about a possibly updated density figure for this object. We still only have the nearly month-old, 'very preliminary' estimate of ~1 g/cm^3. My query has gone unanswered.
It seems odd that so much was made of the fact that an accurate density figure for the object, and its moon, would we readily obtainable, using centuries-old math, once the object was seen to have a moon.
Now this problem seems to have become a much larger one, involving interminable delays. I wonder what the trouble can be.

There may not be any trouble.
They may be just wanting to refine the results (1.1? 0.9?) and not produce a bunch of piecemeal conclusions. With the unexpected moon, there is additional data that might be extractable, e.g. QE2's wobble about the barycenter which would provide more details about the mass distribution of the system. Shape details would also affect a refined density estimate. Basic physics does reveal a density of ~1g/cm³ as I posted on June 7, but more analysis is needed to refine it, so I'm not surprised (impatient maybe ) that more time is needed. At the time of your communications with JPL, it's possible that newer data was still being assimilated.

Per your earlier post:

This link to a page from the Lincoln Laboratory seems to establish that the classification 'Ch' for QE2 had been made as early as 1998, 15 years ago: http://earn.dlr.de/nea/285263.htm Recent observations at Table Mountain and Palomar observatories support this classification.

I disagree.
I don't see an earlier date associated than with the taxonomic classification anywhere within this link. The discovery MPEC does not reveal a spectral class, and at face value, the only references to the Ch classification (this class was defined in 2002) is for 2013. I can't find any other earlier references for QE2's spectral classification. Unless I've missed something, I believe you'll find those ATel references are the first (definitive) spectral classification. I think JPL's QE2 background statement, claiming the prior classification as "unknown", is correct.