[meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Fri, 9 Jul 2010 20:30:44 -0700 (MST)
Message-ID: <9f510032f26dcce3d7dde99c4e52dd65.squirrel_at_webmail.lpl.arizona.edu>

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 Fri 09 Jul 2010 11:30:44 PM PDT