[meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Fri, 9 Jul 2010 20:24:03 -0700
Message-ID: <GOEDJOCBMMEHLEFDHGMMMEGBEEAA.mojave_meteorites_at_cox.net>

Hi Sterling/Jason/List,

Back in 2008 I worked closely with Jim Baer to help refine
the masses of many of the largest asteroids by searching for
pre-encounter archive imagery of "target asteroids". The technique
is a pretty clever one: whenever any asteroid (usually it's
a small one) has a very close encounter with a large asteroid
whose mass you'd like to compute, the small asteroid's orbit
will be gravitationally perturbed by an amount that depends on
the closest approach distance, the velocity of the encounter,
and the mass of the large asteroid. Since the encounter velocity
and closest approach can be accurately determined, the amount of
the deflection can be used to solve for the mass.

(21) Lutetia was one of the asteroids for which I submitted
archive astrometry for a number of target asteroids. I was able to
find and measure over 130 positions in the archive images of six
target asteroids: 11362, 36409, 38772, 56641, 60272 and 61507.
I don't happen to recall if my astrometry helped further refine
Lutetia's mass or not, but my efforts *did* refine the masses
for quite a few large asteroids: (2) Pallas, (3) Juno, (4) Vesta,
(6) Hebe, (7) Iris, (8) Flora, (9) Metis, (10) Hygiea,
(11) Parthenope, (13) Egeria, (15) Eunomia, (16) Psyche and
(19) Fortuna. The work for each asteroid is quite labor-intensive,
and usually the deflections are too small to get a good estimate
of the mass. But the occasional successes made the work quite
rewarding, and I expect Jim and I will resume this work at
some point in the future when our schedules allow.

--Rob

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com]On Behalf Of
Sterling K. Webb
Sent: Friday, July 09, 2010 6:06 PM
To: Jason Utas; Meteorite-list
Subject: Re: [meteorite-list] Rosetta Flyby of Asteroid 21 Lutetia


The density of Lutetia in the most recent
determination is 5.55 +/- 0.88, or between
4.67 and 6.43. Complete data can be found at:
http://home.earthlink.net/~jimbaer1/astmass.txt

The dimensions of Lutetia have been calculated
by some as 115 x 96 x 80 and by (more) others as
120 x 100 x 80, but whatever way you slice the
data it comes to a volume equivalent to a 95.76
kilometer sphere.

There have been two reported stellar occultations
by Lutetia, observed from Malta in 1997 and Australia
in 2003, with only one chord each, roughly agreeing
with IRAS measurements of a spherical shape roughly
~100 km.. See:
    "Tedesco, E.F., P.V. Noah, M. Noah, and S.D. Price.
IRAS Minor Planet Survey. IRAS-A-FPA-3-RDR-IMPS-V6.0.
NASA Planetary Data System, 2004."
http://sbn.psi.edu/pds/resource/imps.html

At this size and the given flyby distance, the image of
Lutetia should subtend about 90 arc-minutes. I expect
plenty of detail. )The Steins image was 60 pixels across,
yet 23 craters were identified and a chain of seven
craters in a row.)

    "There are various indications of a non-metallic
surface: a flat, low frequency spectrum similar to that
of carbonaceous chondrites and C-type asteroids and
not at all like that of metallic meteorites, a low radar
albedo unlike the high albedos of strongly metallic
asteroids like 16 Psyche, evidence of hydrated materials
on its surface, abundant silicates, and a thicker regolith
than most asteroids."
http://en.wikipedia.org/wiki/21_Lutetia
(lots of references)

Oddly, its axis is tilted over at 85-89 degrees, so it rotates
(in 6+ hours) on its side like Uranus. It has been whacked
but good. Did somebody say "breccia"?

Lutetia was discovered on November 15, 1852 by Hermann
Goldschmidt from the balcony of his apartment in Paris.

Now, THAT'S the way to do science!

Maurice, more Champagne, s'il vous plait!


Sterling K. Webb
Received on Fri 09 Jul 2010 11:24:03 PM PDT