[meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Sat, 10 Jul 2010 03:23:58 -0700 (MST)
Message-ID: <ee610a9edfbb82e0249472dfcdc4e5de.squirrel_at_webmail.lpl.arizona.edu>

Hi Sterling:

But remember, that the M classification is based on is visible spectrum.
Overall, it would have to have a relatively featureless visible spectrum
that is redder than yur "typical" C-class asteroid.

However, I think that there here been more detailed observations that show
that there may be C-type patches on its surface. So, we may be looking at
an M asteroid with carbonaceous patches on its surface from impacts. While
I do not have access to the paper, I think that these observations are not
consistent with the opposite: a carbonaceous surface with impact features
that expose the metallic core of the asteroid.

Guess we will know soon. I am still concerned with the high density. Even
Vesta is only 3.5 grams/cc.

Larry

> 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 06:23:58 AM PDT