Starship Asterisk*

alter-ego wrote:
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)

neufer wrote: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.

http://www.universetoday.com/102964/earth-passing-asteroid-is-an-entirely-new-beast/#more-102964 wrote:

[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.>>

neufer wrote:“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.>>

Ross wrote: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?

http://echo.jpl.nasa.gov/asteroids/1998QE2/1998QE2_planning.html wrote: 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.)

Ross wrote: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.

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.