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(meteorobs) Excerpts from "CCNet DIGEST, 26 May 1999"
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 26 May 1999
Date: Wed, 26 May 1999 10:01:27 -0400 (EDT)
CCNet DIGEST, 26 May 1999
=========================
[...]
(2) PROGRESS REPORT ON NASA'S NEO SEARCH PROGRAM
David Morrison <dmorrison@arc.nasadot gov>
(3) GAINING INSIGHTS ON SHOOTING STARS
BERGEN RECORD CORP
http://wwwdot bergen.com:80/morenews/science24199905248.htm
(4) TEMPERATURE & GAS PRODUCTION ON A MODEL COMET NECLEUS
A. Enzian et al., CALTECH, JET PROP LAB
(5) OPTICAL PROPERTIES OF COMETARY DUST
P.A. Yanamandra Fisher & M.S. Hanner, CALTECH,JET PROP LAB
==================================================
(2) PROGRESS REPORT ON NASA'S NEO SEARCH PROGRAM
>From David Morrison <dmorrison@arc.nasadot gov>
NEO News (5/25/99): NASA NEO Search Program
Dear Friends and students of NEOs:
[snip snip snip...]
"We briefly note that our ground-based survey work on NEOs is but a small part
of NASA's total program of studying the comets and asteroids that comprise the
NEO population. Space-based efforts include NASA's Near Earth Asteroid
Rendezvous (NEAR) mission, which will spend a year closely studying the
near-Earth asteroid Eros. The Deep Space-1 spacecraft, an exciting technology
mission, will study the asteroid 1992 KD. The recently launched STARDUST
spacecraft will return cometary dust samples to the Earth in early 2006. NASA
has selected another mission, the Comet Nucleus Tour (CONTOUR) mission, to
investigate three diverse cometary nuclei. In addition, NASA is a partner on two
non-US missions, the ROSETTA mission which will perform a landing on a cometary
nucleus and the Japanese MUSES-C mission which will return a sample from a near
Earth asteroid. The data returned from these missions on the physical and
chemical nature of the target bodies will be absolutely vital if we are
presented with a future need to modify the orbit of an Earth-threatening NEO."
[snip snip snip...]
==================================================
(3) GAINING INSIGHTS ON SHOOTING STARS
>From BERGEN RECORD CORP
http://wwwdot bergen.com:80/morenews/science24199905248.htm
Gaining insights on shooting stars=20
Monday, May 24, 1999
By ALEXANDRA WITZE
Special from The Dallas Morning News
Halfway between the two most anticipated meteor showers of the decade,
astronomers are finding that the more they learn about shooting stars,
the less they seem to understand.
Last November, scientists locked their collective gaze on the Leonid meteors.
Using specially outfitted instruments on airplanes and on Earth, researchers
gathered the most data ever on a single meteor shower.
"It really was the Leonids up close and personal," says Peter Jenniskens, an
astronomer at the NASA Ames Research Center and the Search for Extraterrestrial
Intelligence Institute, both in Northern California. Round two will come this
November, when the Leonids are expected to put on a particularly good show, as
they did in 1998. Scientists are glad for this second chance; the data they
gathered last year seem to raise more questions than answers.
The 1998 Leonids campaign revealed details about the temperature, speed, and
chemical makeup of meteors as they streak through Earth's atmosphere, scientists
reported at the Ames center last month. But they didn't detect organic material
within the meteors or debris trailing after them as expected, Jenniskens says.
Like all meteor showers, the Leonids happen annually when Earth passes through a
trail of debris left by a passing comet. The dust particles, no bigger than a
Rice Krispie, burn up in the atmosphere as blazing light streaks. The Leonids
occur every Nov. 17 and are named because they appear to shoot outward from the
constellation Leo.
The Leonids are pieces of Comet Tempel-Tuttle, which sheds debris as it swings
through the solar system every 33 years or so. So roughly every 33 years,
skywatchers expect a particularly good Leonids show. Comet Tempel-Tuttle last
visited in February 1998; it may create good Leonid displays through 2001, some
astronomers think.
But other astronomers have found that it wasn't the 1998 visit of the comet: but
rather one six centuries ago -- that made last year's shower so vivid. A team of
Irish and Russian astronomers has calculated that the 1998 show was caused by
debris left by Tempel-Tuttle in 1333.
The peak of the 1998 shower came more than half a day earlier than expected:
which suggested it wasn't that year's comet debris burning up.
"We thought if we searched through all the possibilities for the last 1,400
years, we'd find one in particular that gave the timing just right," says David
Asher of the Armagh Observatory in Northern Ireland.
Asher and colleagues calculated that debris from the 1333 passage would have
been in just the right place to create last year's display.
For a different perspective, other astronomers took to the skies last November.
Jenniskens, for instance, led a NASA effort that flew two airplanes in parallel
paths to photograph and study the meteors.
Surprisingly, he discovered that most of the Leonids burned up at roughly the
same temperature, regardless of their size or speed. Meteors were also detected
at higher altitudes than ever before -- 120 miles above Earth's surface.
"It's very hard to understand why the meteors light up at that high altitude,"
he says, where there is little atmosphere to create friction. Possibly the
Leonid particles contain volatile chemical components, which burn up more easily
than expected.
The Ames team plans to repeat the two-airplane approach this November, perhaps
substituting a Boeing plane with an infrared telescope on its top. Such a
telescope could better study the heat given off by the meteors and any debris
they leave behind.
Other researchers bounced light beams off the glowing trails left by the
Leonids, discovering turbulent swirls in the smoke. Knowing how those trails are
structured can help scientists understand how meteors dissipate their heat into
cold space, Jenniskens says.
Copyright, 1999 Bergen Record Corp.
==================================================
(4) TEMPERATURE & GAS PRODUCTION ON A MODEL COMET NECLEUS
A. Enzian*), J. Klinger, G. Schwehm, P.R. Weissman: Temperature and gas =
production distributions on the surface of a spherical model comet=20
nucleus in the orbit of 46P/Wirtanen. ICARUS, 1999, Vol.138, No.1,=20
pp.74-84
*) CALTECH, JET PROP LAB, DIV EARTH & SPACE SCI, MS 183-601,=20
PASADENA,CA,91109
A multidimensional comet nucleus model is used to estimate the=20
temperature and gas production distributions on the surface of a comet=20
nucleus in the orbit of of 46P/Wirtanen. The spherical model nucleus is =
assumed to be made up of a porous dust-ice (H2O, CO) matrix. Heat and=20
gas diffusion inside the rotating nucleus are taken into account in=20
radial and meridional directions. A quasi-3D solution is obtained=20
through the dependency of the boundary conditions on the local solar=20
illumination as the nucleus rotates. As a study case, we consider a=20
homogeneous chemical composition of the surface layer which is assumed=20
to contain water ice. The model results include the distributions of=20
temperature and gas production on the surface. For the chosen test case =
of a nucleus spin axis perpendicular to the orbital plane we found that =
the CO gas production on the surface is quasi-uniformly distributed in=20
contrast to the nonuniform water outgassing. The mixing ratio at a=20
specific point on the comet nucleus surface is not representative of=20
the overall mixing ratio which is observed in the coma. (C) 1999=20
Academic Press.
==================================================
(5) OPTICAL PROPERTIES OF COMETARY DUST
P.A. Yanamandra Fisher*), M.S. Hanner: Optical properties of=20
nonspherical particles of size comparable to the wavelength of light:=20
Application to comet dust. ICARUS, 1999, Vol.138, No.1, pp.107-128
() CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91109
Scattering calculations for nonspherical particles have been carried=20
out in order to explain observed optical properties of cometary dust.=20
We focused on two optical properties of cometary dust sensitive to=20
particle shape: negative linear polarization at phase angles less than=20
or equal to 21 degrees and the 11.2-mu m silicate emission feature. The =
discrete dipole approximation (DDA) method was employed to compute the=20
scattering matrix for nonspherical silicate and absorbing particles of=20
size comparable to the wavelength. Silicate particles with a variety of =
shapes and size parameter X-eq similar to 2.5, corresponding to a=20
linear dimension of 0.5-1.0 mu m, can produce negative linear=20
polarization at small phase angles, whereas carbon particles produce a=20
strong positive maximum of polarization near phase angles of 90=20
degrees. Mixtures of silicate and carbonaceous material, on a scale=20
small compared to the wavelength, eliminate the negative polarization=20
in this size range; however, macroscopic mixtures of silicate and=20
carbon could yield the observed negative linear polarization at low=20
phase angles (less than or equal to 21 degrees) and a maximum positive=20
polarization at phase angle of 90 degrees. The position of the 11.2-mu=20
m thermal emission peak observed in comets, attributed to crystalline=20
olivine, depends strongly on particle shape even for particles much=20
smaller than the wavelength and can be matched with anisotropic Mg-rich =
olivine for our model tetrahedra or moderately elongated bricks.=20
Spheres and extreme shapes, such as disks or needles, appear to be=20
ruled out. Approximately 20% crystalline olivine and 80% disordered=20
olivine reproduces the observed spectra of comets with comparable peaks =
at 10 and 11.2 mu m, e.g., P/Halley, Bradfield 1987 XXIX, Mueller, Levy =
1990 XX, and C/1995 O1 (Hale-Bopp). This study is an essential first=20
step toward realistic modeling of comet dust as aggregates composed of=20
nonspherical monomers having dimensions comparable to the wavelength of =
incident radiation. (C) 1999 Academic Press.
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