[meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Fri, 9 Jul 2010 15:49:59 -0700
Message-ID: <AANLkTincHaCAhu7c0fLjdwcF1FylELw2tO88djigItsR_at_mail.gmail.com>

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 Fri 09 Jul 2010 06:49:59 PM PDT