[meteorite-list] Dawn Spacecraft to Enter Asteroid's Orbit on July15

From: Galactic Stone & Ironworks <*meteoritemike_at_meteoritecentral.com*> Date: Sat, 16 Jul 2011 20:26:05 -0400 Message-ID: <CAKBPJW_Aex2TziYpvKxAF7Mx1QPWYGcUbiRdsGNkaQag1Cz1EA_at_mail.gmail.com> · meteoritemike_at_meteoritecentral.com

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On 7/15/11, Sterling K. Webb <sterling_k_webb at sbcglobal.net> wrote:
> List,
>
> This is the first image of Vesta to show significant
> detail on the surface -- the Dawn Framing Camera
> view of July 9th of Vesta. It can be found full-sized at:
> http://www.nasa.gov/images/content/569937main_dawn-image-070911-43_full.jpg
>
> No information is given about the orientation of
> Vesta in the image, i.e., where are the poles?
> Please, let's assume an oblate spheroid (north
> and south poles) and not a tri-axial (with north and
> south poles, east and west poles, and front and
> back poles -- too confusing. Meet you at the
> Back Pole on Tuesday?
>
> It is NOT what I was expecting. How about you?
> Everyone assumes "giant basins" on Vesta. One
> radar team even produced a "map" of the basins
> on Vesta's surface. The radar studies show a very
> "rough" surface compared to most asteroids. Big,
> rough ejecta, in other words, which implies craters,
> lots and lots of them.
>
> http://observatory.space.edu/research/gaffeyResumePDFs/1997/Gaffey%20%281997%29%20Vesta%20Surface%20Lithology.pdf
> "The background surface of Vesta -- they say -- "is a
> relatively dark howardite or polymict eucrite (pyroxene-
> plagioclase) assemblage with several compositionally distinct
> bright regions clustered in one hemisphere viewed around the
> maximum in Vesta's lightcurve. These include what appears to
> be an olivine-bearing unit (suggested name ''Leslie Formation'')
> located near Vesta's equator which probably represents an impact
> basin (and/or its associated ejecta) that penetrated through
> the basaltic crust. Other high-albedo compositional units
> including an apparently low-calcium eucrite region and several
>  diogenite (pyroxenite) regions, at least one located near the
> southern pole, may be smaller, shallower impact basins. By
> analogy to the eucrite meteorites, which represent surface flows
> or shallow intrusions and which constitute the major compo-
> nent of the regolith-derived howardites and polymict eucrites,
> we conclude that the howardite/polymict eucrite units represent
> regolith-gardened original surface of Vesta. It is probable
> that the low albedo of the background surface on Vesta is due
> to an age-related darkening effect similar to that inferred from
> the Galileo images of Gaspra and Ida. This mechanism is conindeed
> sistent with the correlation of absorption feature intensity with
> the lightcurve. Vesta appears to have an old eucritic surface,
> darkened with age and represented among the meteorites by
> the regolith-derived howardites and polymict eucrites, on which
> several impacts on one hemisphere have exposed fresher
> brighter diogenite and olivine-bearing material... discrete
> circular features were used on the plausible assumption
> that impacts have been the most important geologic process
> on this surface for most of the age of the solar system."
>
> Impacts, basins, craters -- where are they?
>
> The image shows less than a dozen 15-mi craters,
> very sharp, high rimmed, conical, obviously "recent"
> and un-degraded. There are no other craters and
> certainly no larger recognizable ones.
>
> In the upper left there is a feature that might be the
> remains of a 50-mi crater, indistinct and virtually
> "filled in," that has two 5-mi craters in it. Below it
> is a meandering dark formation several hundred
> miles long and perhaps 40 miles wide. Lava flow?
>
> And that is the question. Vesta can't be uncratered.
> Therefore the surface is covered or "mantled" with
> something. Lava flows are unlikely as Vesta's core
> must have been "cold" (and solid) for 3 billion years
> (at least). Unless, of course, for some mysterious
> reason, it isn't... And if Vesta was covered with lava
> flows billions of years ago, those plains should have
> a full statistical range of crater sizes on them now.
> And they don't...
>
> Yes, all the "big basins" could be shy, hiding on the
> side away from the camera. But if there are 15-mi
> craters and 200-mi basins, where are all the sizes
> in-between?
>
> http://echo.jpl.nasa.gov/asteroids/mitchell+1996+CPV_icarus.pdf
> says "Lee et al. (1996) estimate that blocks up to
> 1 km in size could be produced during the formation
> of the large impact basins inferred to be present on
> Vesta's surface." This image is roughly 440 pixels
> across and Vesta is a (mean) diameter of 530 km.
> If there are 1-km blocks lounging about, they should
> show up in the next jump-up in image size...
>
> They also say the gravity is great enough to prevent
> ejecta from being distributed evenly around the whole
> planet(oid). Yeah... A regolith "tens of meters deep"
> is suggested. What about one ten kilometers deep?
>
> And, while we're at it, what are those Ganymede-like
> "grooves" curving around in parallel. Not tectonic.
> Some of the groove sets turn and cross over each
> other at 90 degrees.
>
> And one more thing... what is a mountain 125 miles
> across doing there, just sticking out of the side of
> Vesta? Or is it the high edge of a buried crater rim?
> There is a lot of "gouged out" terrain.
>
> Another oddity is the very narrow range of contrast
> in the "albedo" features (light and dark). Variations
> are subtle (play with the histogram). Interestingly,
> all the papers on Vesta refer to the surface as "dark,"
> while in fact the albedo of Vesta is considerably
> higher than the Earth's. Shiny basalt everywhere.
> Dark rock, yes, but bright dark rock...
>
> No doubt, all questions will be answered with bigger
> images... or more mysteries revealed. There's nothing
> like a new planet for fun! (OK, ok, "planet-oid," kill-joy.)
> I should sign this "Puzzled" or "Perplexed," because I am.
>
>
> Sterling K. Webb
> ------------------------------------------------------------------------
> ----- Original Message -----
> From: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>
> To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
> Sent: Thursday, July 14, 2011 10:23 AM
> Subject: [meteorite-list] Dawn Spacecraft to Enter Asteroid's Orbit on
> July15
>
>
>>
>> http://www.jpl.nasa.gov/news/news.cfm?release=2011-208
>>
>> NASA Spacecraft to Enter Asteroid's Orbit on July 15
>> Jet Propulsion Laboratory
>> July 14, 2011
>>
>> [Image}
>> Asteroid Vesta NASA's Dawn spacecraft obtained this image of the giant
>> asteroid Vesta with its framing camera on July 9, 2011. Image credit:
>> NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
>>
>> PASADENA, Calif. -- On July 15, NASA's Dawn spacecraft will begin a
>> prolonged encounter with the asteroid Vesta, making the mission the
>> first to enter orbit around a main-belt asteroid.
>>
>> The main asteroid belt lies between the orbits of Mars and Jupiter.
>> Dawn
>> will study Vesta for one year, and observations will help scientists
>> understand the earliest chapter of our solar system's history.
>>
>> As the spacecraft approaches Vesta, surface details are coming into
>> focus, as seen in a recent image taken from a distance of about 26,000
>> miles (41,000 kilometers). The image is available at:
>> http://www.nasa.gov/mission_pages/dawn/multimedia/dawn-image-070911.html .
>>
>> Engineers expect the spacecraft to be captured into orbit at
>> approximately 10 p.m. PDT Friday, July 15 (1 a.m. EDT Saturday, July
>> 16). They expect to hear from the spacecraft and confirm that it
>> performed as planned during a scheduled communications pass that
>> starts
>> at approximately 11:30 p.m. PDT on Saturday, July 16 (2:30 a.m. EDT
>> Sunday, July 17). When Vesta captures Dawn into its orbit, engineers
>> estimate there will be approximately 9,900 miles (16,000 kilometers)
>> between them. At that point, the spacecraft and asteroid will be
>> approximately 117 million miles (188 million kilometers) from Earth.
>>
>> "It has taken nearly four years to get to this point," said Robert
>> Mase,
>> Dawn project manager at NASA's Jet Propulsion Laboratory in Pasadena,
>> Calif. "Our latest tests and check-outs show that Dawn is right on
>> target and performing normally."
>>
>> Engineers have been subtly shaping Dawn's trajectory for years to
>> match
>> Vesta's orbit around the sun. Unlike other missions, where dramatic
>> propulsive burns put spacecraft into orbit around a planet, Dawn will
>> ease up next to Vesta. Then the asteroid's gravity will capture the
>> spacecraft into orbit. However, until Dawn nears Vesta and makes
>> accurate measurements, the asteroid's mass and gravity will only be
>> estimates. So the Dawn team will need a few days to refine the exact
>> moment of orbit capture.
>>
>> Launched in September 2007, Dawn will depart for its second
>> destination,
>> the dwarf planet Ceres, in July 2012. The spacecraft will be the first
>> to orbit two bodies in our solar system.
>>
>> Dawn's mission to Vesta and Ceres is managed by JPL for NASA's Science
>> Mission Directorate in Washington. Dawn is a project of the
>> directorate's Discovery Program, which is managed by NASA's Marshall
>> Space Flight Center in Huntsville, Ala. UCLA is responsible for
>> overall
>> Dawn mission science. Orbital Sciences Corp. of Dulles, Va., designed
>> and built the spacecraft. The German Aerospace Center, the Max Planck
>> Institute for Solar System Research, the Italian Space Agency and the
>> Italian National Astrophysical Institute are part of the mission team.
>>
>> For a current image of Vesta and more information about the Dawn
>> mission, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov
>> .You also can follow the mission on Twitter at:
>> http://www.twitter.com/nasa_dawn .
>>
>> Priscilla Vega/Jia-Rui Cook 626-298-3290/818-354-0850
>> Jet Propulsion Laboratory, Pasadena, Calif.
>> priscilla.r.vega at jpl.nasa.gov / jccook at jpl.nasa.gov
>>
>> Dwayne C. Brown 202-358-1726
>> NASA Headquarters, Washington
>> dwayne.c.brown at nasa.gov
>>
>> 2011-208
>>
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