[meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 10 Jul 2010 00:56:59 -0500
Message-ID: <15A09A54FCEE48E6A4308331EA2F7203_at_ATARIENGINE2>

Hi, All,

If I was foolish enough to look into my crystal
ball and make predictions (I am), I would say
that there is a likelihood of Lutetia having a
very large crater "on" it.

Vesta has such a crater, the Great South Polar
Crater, 480 km across, or nearly 80% of Vesta's
diameter!

Why would I think that? Well, that 85-89 degree
axial tilt would require, by today's orthodoxy, a
major impact. Laying a 100-kilometer body over
on its side to rotate like a fallen top is not a small
job.

Such an impact would surely leave a crater at least
as large as Mimas "Death Star" crater and perhaps
as relatively large as Vesta's Polar Giant. Of course,
because of the fast fly-by, we have only a 50%-50%
chance of seeing it at all!

The likelihood of such a giant crater also implies
something else about Lutetia. It's a strong body. It
would have to be to survive a turn-over impact. It
can't possibly be a porous, friable, crumbly body
(like a carbonaceous). To withstand the transfer of
that much axial torque changing force, I think it
would have to be the metallic object that its density
suggests that it is.

So, if it's a metallic object, why the hydrates and the
 silicates on the surface? Well, if the impact that tilted
the axis was slow enough, Lutetia would end up tilted
but capturing much of the impactor's material.

If the impactor was weak and Lutetia was strong, the
tilty impact slow, wouldn't Lutetia be covered with
perhaps a miles-thick layer of regolith from the disrupted
impactor?

An iron heart under carbonaceous cover? If Rosetta
could measure Lutetia's moment of inertia we'd know!


Sterling K. Webb
-------------------------------------------------------------------------
----- Original Message -----
From: <lebofsky at lpl.arizona.edu>
To: "Jason Utas" <meteoritekid at gmail.com>
Cc: "Meteorite-list" <meteorite-list at meteoritecentral.com>
Sent: Friday, July 09, 2010 10:30 PM
Subject: Re: [meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia


Hi Jason:

The asteroids that are spectrally similar to carbonaceous chondrites (CI
and CM) are B-, C-, and G-class asteroids. The density of Ceres is about
2
grams/cc and I think that they range up to about 2.5 grams/cc. Many Cs
have densities lower than 2, which probably indicates that they are
rubble
piles.

I think that the CM grain density is something like 2.7 grams/cc (Britt
et
al.)

While there are a number of M-class asteroids that are spectrally linked
to a metallic composition (fairly flat visible spectra), there is a lot
of
evidence that they may not be metallic. I do not remember the infrared
spectral properties of 16 Psyche, but its density is around 2.0
grams/cc.
Lutetia has a 3-micron feature indicative water of hydration (as seen in
CI and CM meteorites) and also has a silicate feature in the 10-micron
region.

Larry
> Hola,
> We seem to have a bit of a problem...
> The article seems to suggest that carbonaceous chondrites have a
> density between nearly 4 and 5 grams per cubic centimeter.
> Might anyone on the list be willing to comment on this slight
> discrepancy?
>
>>A team of researchers used the VLT
> and Keck telescopes to estimate Lutetia's bulk density, finding it to
> be
> in the range 3.98 to 5.00 g cm^-3 , depending on the model that is
> adopted. Although no precise value could be determined this range of
> density would support a carbonaceous composition (see Drummond et al.,
> [2010]).
>
> -As opposed to:
>
> http://www.meteorites.com.au/odds&ends/density.html
>
> The numbers in the article simply stuck me as out of place - yes the
> asteroid appears to be less dense than an iron meteorite, but it's a
> difference of only 20-30%. If you take a look at the following paper
> -
>
> http://www.dnp.fmph.uniba.sk/etext/40/text/MAPS36Welten2.pdf
>
> They assume the density of the given mesosiderite to be 5 grams per
> cubic centimeter - a value the authors say is at the more dense end of
> the spectrum for even stony-iron meteorites.
>
> Drummond points out in his paper that Lutetia is in fact likely not a
> carbonaceous chondrite.
>
> http://arxiv.org/pdf/1005.5353
>
> He suggests that it is most likely an enstatite chondrite, but also
> notes that the density body as a whole might be less than its
> constituents, as it may be a rubble-pile asteroid (a mix of solid
> chunks of matter and empty space). As such, I would have to say that
> it is most likely composed primarily of stony-iron or iron material.
> A dense stony body would also be a possibility, but as Drummond et al.
> note, this body is apparently more dense than your average chondrite
> of *any* type.
>
> -And the recently calculated values showed it to be more dense than
> earlier estimates!
>
> Regards,
> Jason
>
>
> On Fri, Jul 9, 2010 at 9:25 AM, Ron Baalke
> <baalke at zagami.jpl.nasa.gov>
> wrote:
>>
>> http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=47389
>>
>> Rosetta flyby of asteroid (21) Lutetia
>> Euoprean Space Agency
>> July 9, 2010
>>
>> Discovered in Paris by Hermann Goldschmidt in November 1852, asteroid
>> (21) Lutetia has been a cosmic riddle for astronomers. In an attempt
>> to
>> pin down its properties once and for all, ESA's Rosetta spacecraft
>> will
>> fly past Lutetia within an estimated distance of close to 3170 km, at
>> a
>> relative speed of 15 km/s on 10 July 2010 at approximately 15:45 UT
>> (spacecraft event time), 18:10 CEST (ground event time).
>>
>> Follow the flyby live via the webcast
>> <http://www.livestream.com/eurospaceagency> from ESA/ESOC: 10 July
>> 2010
>> starting at 18:00 CEST.
>>
>> Frequent updates on activities leading up to the flyby can be found
>> on
>> the Rosetta blog <http://webservices.esa.int/blog/blog/5/page/1>.
>>
>> Details of the spacecraft preparations leading up to the flyby,
>> including images of Lutetia acquired during the navigation campaign,
>> can
>> be found in the status reports
>> <http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31523&farchive_objecttypeid=30&farchive_objectid=30930>.
>>
>> This asteroid flyby will address a number of open questions about
>> Lutetia; in particular, the observations and measurements obtained by
>> instruments on board Rosetta will:
>>
>> * Attempt to settle the ongoing debate as to the asteroid's true
>> composition. In particular to ascertain if it is a C-type or
>> M-type asteroid.
>> * Determine the mass and density of the asteroid with unprecedented
>> precision.
>> * Search for an exosphere around the asteroid and determine its
>> composition.
>> * Provide ground-truth for the better calibration of existing
>> observations obtained by ground-based telescopes.
>> * Test out the scientific instruments on board Rosetta as it
>> continues to travel to its final destination: comet
>> 67P/Churyumov-Gerasimenko.
>> * Carry out a close-up study of a primitive building block of the
>> Solar System, with the intention of using it to decode how our
>> solar neighbourhood formed.
>>
>> Situated in the main asteroid belt and with estimated dimensions of
>> 132
>> x 101 x 76 km (see Belaskaya et al., [2010]), asteroid (21) Lutetia
>> has
>> been subjected to intense ground-based scrutiny since it was
>> announced
>> as a target for Rosetta in 2004. Initial observations recorded a high
>> albedo, suggesting a high metallic content, and led to the body being
>> classified as an M-type asteroid (see Bowell et al., [1978]). Should
>> (21) Lutetia indeed turn out to be M-type, the Rosetta flyby would be
>> the first close encounter of a spacecraft with this class of
>> asteroid.
>>
>> However, Lutetia's true nature has always been far from clear-cut.
>> One
>> difficulty in unambiguously classifying Lutetia is the lack of clear
>> features in the spectrum of this asteroid. Recent visual
>> spectroscopic
>> studies, reported in Belaskaya et al., and Perna et al., have noted
>> different spectral slopes at different rotation phases. This has been
>> interpreted as arising from inhomogeneities in the asteroid's make
>> up,
>> perhaps caused by local differences in mineralogical or chemical
>> content
>> of the surface.
>>
>> Some researchers have suggested the closest analogue to Lutetia's
>> surface is a type of carbonaceous chondrite meteorite (see Barucci et
>> al.). When Lutetia was at opposition in 2008/2009 the opportunity was
>> taken to test this theory further. A team of researchers used the VLT
>> and Keck telescopes to estimate Lutetia's bulk density, finding it to
>> be
>> in the range 3.98 to 5.00 g cm^-3 , depending on the model that is
>> adopted. Although no precise value could be determined this range of
>> density would support a carbonaceous composition (see Drummond et
>> al.,
>> [2010]).
>>
>> The ground-based observations in preparation for the flyby have also
>> allowed astronomers to construct Lutetia's light curve. Most
>> asteroids
>> tend to be irregularly shaped and therefore different amounts of
>> sunlight are reflected towards the Earth as they rotate. Hence the
>> ratio
>> between the three major axes defining the asteroid as well as its
>> rotational properties can be determined from measuring how this
>> reflected light changes with time. Assuming a certain reflectivity
>> (albedo) the dimensions of the asteroid can also be estimated.
>> Knowing,
>> from this preparatory work, that Lutetia rotates with a period close
>> to
>> 8.17 hours was of great help in planning the scientific measurements
>> for
>> the flyby.
>>
>> The encounter of Rosetta with asteroid (21) Lutetia is key to
>> understanding the true nature of this puzzling member of the main
>> asteroid belt. Only with the close inspection that is possible with a
>> flyby can the riddles of Lutetia be solved, as this provides the
>> opportunity to measure and analyse many of the asteroid's properties
>> including its shape, density, composition and surface topography. The
>> instruments on board Rosetta have been designed specifically for such
>> tasks and will be able to provide the answers that are sought.
>>
>> The flyby at Lutetia will be the second time Rosetta has studied an
>> asteroid up-close. In 2008 the spacecraft flew past asteroid (2867)
>> Steins at a distance of just 802.6 km, only 2.6 km further out than
>> baselined. However, these two asteroids are just stepping stones on
>> the
>> journey to Rosetta's ultimate goal, the rendezvous with comet
>> 67P/Churyumov-Gerasimenko, scheduled for 2014. The Rosetta team hopes
>> that with this rendezvous they can decipher the enigmas of the
>> formation
>> of our Solar System, just as its namesake helped unscramble ancient
>> Egyptian hieroglyphics.
>>
>> Orbital and physical characteristics of asteroid (21) Lutetia
>> based on pre-Rosetta observations
>>
>> Semimajor axis, a (AU) 2.44*
>> Orbital eccentricity, e 0.16*
>> Orbital period (y) 3.8*
>> Inclination (deg) 3.07*
>> Dimensions (km) 132 x 101 x 76 (From Drummond et al., 2010)
>> Taxonomic type C or M
>> Sidereal rotation period (h) 8.168270 (from Carry et al., 2010)
>> Albedo 0.1-0.22 (estimates vary according to the technique used; see
>> Belskaya et al., 2010)
>>
>> /(* Source: IAU Minor Planet Center
>> <http://www.cfa.harvard.edu/iau/Ephemerides/Bright/2000/00021.html>.)/
>>
>>
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Received on Sat 10 Jul 2010 01:56:59 AM PDT