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

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 14 Jul 2011 23:36:56 -0500
Message-ID: <C1074C7DDDAD4F1C93E28A6A4B5862BF_at_ATARIENGINE2>

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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Received on Fri 15 Jul 2011 12:36:56 AM PDT