Starship Asterisk*

http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20111203 wrote:

The QUE 97053 (left) and EET 90020 (right) eucrites Image Credit: Hap McSween (University of Tennessee), and Andrew Beck and Tim McCoy (Smithsonian Institution)

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217 gram HED meteorite recovered in 1942 in Africa Image Credit: Hap McSween (University of Tennessee), and Andrew Beck and Tim McCoy (Smithsonian Institution)

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<<The HED (howardite, eucrite and diogenite) meteorites are a large group of meteorites believed to originate from Vesta, a hypothesis that is consistent with current Dawn observations. The eucrites are crystallized lavas that have the composition of basalt, the most common lava type on the Earth. The QUE 97053 (left) and EET 90020 (right) eucrites, pictured here, were recovered in Antarctica. These images are of thin slices of the meteorites as viewed through a polarizing microscope. The white bars in the images, each 2.5 millimeters long, indicate the scale. When polarized light passes through thin slices of rock, different minerals have different colors. QUE97053 (left) consists mostly of elongated gray crystals of feldspar (calcium aluminum silicate) and brightly colored grains of pyroxene (magnesium iron silicate). The texture of this rock is what would be expected from crystallization of a molten magma. EET90020 (right) has similar mineralogy but a recrystallized texture of equant grains formed by later heating. Equant grains have the same or roughly the same dimensions in all directions. Eucrites like these comprise some fraction of Vesta’s surface. Their compositions can be compared with observations from various instruments aboard Dawn. They can be compared with the VIR (Visible and Infrared Imaging Spectrometer) spectra to determine mineralogy and with the GRaND (Gamma Ray and Neutron Detector) observations to calibrate and interpret the GRaND instrument’s responses.>>

The HED (howardite, eucrite and diogenite) meteorites are a large group of meteorites believed to originate from Vesta, a hypothesis that is consistent with current Dawn observations. Howardites are regolith breccia rocks, meaning that they formed through the grinding and fusion of rock and dust that occurs during meteor impacts on the surface of Vesta. Howardites are comprised of fragments of eucrite and diogenite of varying grain sizes, which can be seen in this picture of the Bununu howardite. This sample weighs 217 grams and was recovered in 1942 in Africa. Along with fragments of eucrite and diogenite, some howardites also contain solar wind implanted noble gasses, which confirms that they once resided on the surface of their parent body. This makes howardites a good laboratory analog for spectral and chemical measurements that will be made of the Vestan surface by Dawn.>>

http://www.areavoices.com/astrobob/?archive=2010-02 wrote:
<<To figure out what asteroids are made of astronomers use an instrument called a spectroscope that dissects light into very fine gradations of color. They point it at an object like Vesta and study the sunlight it reflects. Minerals absorb and reflect specific colors of light and imprint it with their individual chemical "signature". Scientists studying Vesta find the very same signatures in its light as they do in a group of igneous meteorites found here on Earth. The meteorites fall into three categories:

• 1) eucrites (YOU-crytes), formed in surface lava flows,
  2) diogenites (DYE-uh-JEN-ites), from magma that solidified deeper inside the asteroid and
  3) howardites, created by the mixing of the two through impact bombardment on the asteroid's surface.

A slice of the eucrite meteorite NWA 3147 that's likely originated in a lava flow on the surface of Vesta.

The rock is composed of small mineral crystals feldspar and pyroxene. Photo: Bob King

This is the howardite NWA 3149.

You can see it's made of broken pieces of other rocks that were cemented together by the heat and pressure of impact. Photo: Bob King

The match between these types and different areas of Vesta's surface is excellent. Amazing that these chips from an asteroid were delivered right into our laps. Astronomers suspect they got here through a huge impact on Vesta. Take a look at the photo above by the Hubble Space Telescope. The piece missing on the asteroid's bottom is a crater 270 miles in diameter and eight miles deep that was excavated by an asteroid impact long ago. It knocked off tons of material and created not only the vestoid family of asteroids but shot pieces across the solar system, some of which found new homes on our planet.>>

http://www.areavoices.com/astrobob/?archive=2010-02 wrote:


Thanks to the bright, naked eye star Gamma Leonis, Vesta will be easy to find in binoculars over the next few weeks. The circle represents about one binocular field of view. The yellow crosses show the asteroid's position each night. Use the wide-angle map below to help you find Leo and Gamma. Vesta is just to the right of a somewhat fainter star tonight. Created with Chris Marriott's SkyMap software.

During the remainder of this month and into early March, sky watchers with binoculars have a great opportunity to see the brightest asteroid of them all -- 4 Vesta. Vesta was the fourth asteroid discovered (hence the number) and was found by astronomer Heinrich Olbers on March 29, 1807. It's 329 miles across and orbits within the asteroid belt at an average distance from the sun of 220 million miles. This month Vesta is at opposition to the Earth (making a close approach) and shining at 6th magnitude. From a dark sky site, it's very faintly visible to the naked eye but any pair of binoculars will show it easily.

Gamma Leonis is the bright star in the bend in the "Backwards Question Mark" of Leo that you'll point your binoculars at to seek Vesta. Right next to it is 40 Leo. You can use the Big Dipper to get you there. This map shows the sky about 9-9:30 p.m. as you look east. Created with Stellarium>>

The most prominent asteroid in the sky is currently yours for the perusing with binoculars -- and perhaps even the naked eye.

Tomorrow night, Wednesday, Feb. 17, Vesta, the second most massive object in the asteroid belt, reaches what astronomers like to call "opposition." An asteroid (or planet or comet) is said to be "in opposition" when it is opposite to the sun as seen from Earth. In other words, if you were to stand outside with the sun directly above you at high noon, Vesta would be directly below your feet some 211,980,000 kilometers (131,700,000 miles) away. With Vesta at opposition, the asteroid is at its closest point to Earth in its orbit.

Wednesday night, the asteroid is expected to shine at magnitude 6.1. That brightness should make it visible to interested parties brandishing telescopes or binoculars, and even those blessed with excellent vision and little or no light pollution or clouds in their vicinity. Vesta will be visible in the eastern sky in the constellation Leo.

What makes this space rock so prominent these days? Along with its relative proximity at this point, a full half of the asteroid is being bathed by sunlight when seen from Earth, making it appear brighter. Another attribute working in the observer's favor is that Vesta has a unique surface material that is not as dark as most main belt asteroids - allowing more of the sun's rays to reflect off its surface.

If spotting Vesta in the night sky has whetted your appetite for mega-rocks, all we can say is, stay tuned. NASA's Dawn spacecraft, currently motoring its way through the asteroid belt, will begin its exploration of Vesta in the summer of 2011.

http://en.wikipedia.org/wiki/HED_meteorite wrote:
<<The HED meteorites are a grouping of about 60% of all Achondrites meteorite types, the:

* Howardites
* Eucrites
* Diogenites
* Dunite
* Olivine Diogenites

These are all thought to have originated from the crust of the asteroid 4 Vesta, their differences being due to different geologic histories of the parent rock. Their crystallization ages have been determined to be between 4.43 and 4.55 billion years from radioisotope ratios. HED meteorites are differentiated meteorites, which were created by igneous processes in the crust of their parent asteroid. They are a relatively common type except for the dunite which is represented by only one meteorite, NWA 2968. The HED meteorites account for about 5% of all falls, which is about 60% of all Achondrites.

[ The "Lorton" Chondrite Meteorite: http://asterisk.apod.com/vie ... IN#p115300 ]
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It is thought that the method of transport from 4 Vesta to Earth is as follows:

1. An impact on 4 Vesta ejected debris, creating small (10 km diameter or less) V-type asteroids. Either the asteroidal chunks were ejected as such, or were formed from smaller debris. Some of these small asteroids formed the Vesta family, while others were scattered somewhat further. This event is thought to have happened less than 1 billion years before now. There is an enormous impact crater on 4 Vesta covering much of the southern hemisphere which is the best candidate for the site of this impact. The amount of rock that was excavated there is many times more than enough to account for all known V-type asteroids.

2. Some of the more far-flung asteroid debris ended up in the 3:1 Kirkwood gap. This is an unstable region due to strong perturbations by Jupiter, and asteroids which end up here get ejected onto far different orbits on a timescale of about 100 million years. Some of these bodies are perturbed into near-Earth orbits forming the small V-type near-Earth asteroids such as e.g. 3551 Verenia, 3908 Nyx, or 4055 Magellan.

3. Later smaller impacts on these near-earth objects dislodged rock-sized meteorites, some of which later struck Earth. On the basis of cosmic ray exposure measurements, it is thought that most HED meteorites arose from several distinct impact events of this kind, and spent from about 6 to 73 million years in space before striking the Earth.>>

Throughout February, you'll be able to spot asteroid 4 Vesta easily through binoculars and even glimpse it with unaided eyes under a dark sky. The asteroid reaches opposition, and peak visibility, February 17/18. At that point, it will glow at magnitude 6.1.
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Bright Algieba is a useful guide throughout the month. Asteroid 4 Vesta remains within the same binocular field of view as the star.