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(meteorobs) Excerpts from "CCNet, 049/2000 - 14 April 2000"
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 14 April 2000
Date: Fri, 14 Apr 2000 11:03:34 -0400 (EDT)
CCNet, 049/2000 - 14 April 2000
-------------------------------
(2) IMPACT AT HIGH NOON
Ron Baalke <BAALKE@kelvin.jpl.nasadot gov>
[...]
(4) 1996 PW & 1997 SE5: EXTINCT COMETS OR D-TYPE ASTEROIDS?
M.D. Hicks et al., CALTECH,JET PROP LAB
(5) COLLISIONAL EVOLUTION IN THE VULCANOID REGION
S.A. Stern & D.D. Durda, SW RES INST,DEPT SPACE STUDIES
[...]
(7) RADAR MICROMETEORS
D. Janches et al., PENN STATE UNIVERSITY
[...]
============================================================
(2) IMPACT AT HIGH NOON
>From Ron Baalke <BAALKE@kelvin.jpl.nasadot gov>
NEAR image of the day for 2000 April 13
http://near.jhuapldot edu/iod/20000413/index.html
For people who live in mountain valleys, daylight can be a short-lived=20
experience. Mountains to the east make the Sun rise later in the=20
morning, while mountains to the west make the Sun set earlier in =20
the evening. The brevity of daytime in a valley is accentuated during=20
the shortened days of winter.
The floor of the 5.5-kilometer (3.4-mile) diameter impact =20
crater that dominates one face of Eros experiences the=20
ultimate in short daylight hours. Three factors conspire to make =20
this true. Firstly, Eros rotates rapidly, once every 5.27 hours. =20
Secondly, the crater's 0.9-kilometer (0.56-mile) high walls tend to
block direct sunlight from the floor, even while the outside of=20
the crater is illuminated. Thirdly, during the current season on =20
Eros, the Sun never makes it high in the sky in this location.
This NEAR Shoemaker image, taken April 1, 2000, from a range of 209=20
kilometers (130 miles), caught the crater near local noon with the Sun=20
highest in the sky. On that day, a hardy astronaut standing at the=20
bottom of the crater would have experienced 1 hour and 45 minutes of
daylight.
- -----------------
NEAR image of the day for 2000 April 12
http://near.jhuapldot edu/iod/20000412/index.html
The Record of What Came Before
The most common, pervasive, and scientifically informative =20
landform on Eros is impact craters. This image from NEAR=20
Shoemaker, taken March 20, 2000, from a range of 206 kilometers =20
(128 miles), shows a highly cratered region of the asteroid. The=20
whole scene is 7.4 kilometers (4.6 miles) across. Craters are formed=20
by the explosive impacts of smaller asteroid fragments, which
constantly rain onto the surface over the eons. A freshly=20
exposed surface will have fewer craters than a surface exposed to=20
space for a longer time, because it hasn't experienced the rain of=20
projectiles for as long a duration. The great number of impact =20
craters in the region shown here shows that it has been an=20
extremely long time since this region was wiped clean of craters,
or "resurfaced," by a geologic process such as the chipping =20
off of part of the asteroid. Also, the much greater abundance of=20
smaller craters than larger craters says that small asteroid=20
fragments have impacted Eros much more frequently than large=20
asteroid fragments.
- --------------------------------------------------------
Built and managed by The Johns Hopkins University Applied Physics=20
Laboratory, Laurel, Maryland, NEAR-Shoemaker was the first spacecraft=20
launched in NASA's Discovery Program of low-cost, small-scale
planetary missions. See the NEAR web site for more details=20
(http://near.jhuapldot edu).
============================================================
(4) 1996 PW & 1997 SE5: EXTINCT COMETS OR D-TYPE ASTEROIDS?
M.D. Hicks*), B.J. Buratti, R.L. Newburn, D.L. Rabinowitz: Physical=20
observations of 1996 PW and 1997 SE5: Extinct comets or D-type=20
asteroids?.ICARUS, 2000, Vol.143, No.2, pp.354-359
*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,MS 183-501,PASADENA,CA,91109
The minor planets 1996 PW and 1997 SE5 are two of the few known=20
asteroids with orbital elements typical of long-period and=20
Jupiter-family comets and as such represent strong candidates=20
for extinct cometary nuclei. We obtained filter photometry of 1996 PW=20
and filter photometry and medium-resolution CCD spectroscopy of 1997=20
SE5 during their discovery apparitions. We also observed a suite of=20
D-type asteroids as possible spectral analogs of cometary nuclei. Both=20
1996 PW and 1997 SE5 have moderately red, featureless spectra typical=20
of the D-type asteroids, cometary nuclei, and other extinct cometary=20
candidates. The photometry for 1997 SE5 was fit by a triple-peaked=20
lightcurve with a period of 9.050 +/- 0.005 h and an amplitude of 0.4=20
magnitude, suggesting a relatively complex and elongated shape. With=20
this work, 1997 SE5 and 1996 PW join the ranks of 3552 Don Quixote and=20
944 Hildago as established candidates for extinct comet nuclei. (C)=20
2000 Academic Press.
============================================================
(5) COLLISIONAL EVOLUTION IN THE VULCANOID REGION
S.A. Stern*), D.D. Durda: Collisional evolution in the Vulcanoid=20
region: Implications for present-day population constraints. ICARUS,=20
2000, Vol.143, No.2, pp.360-370
*) SW RES INST,DEPT SPACE STUDIES,BOULDER,CO,80302
We explore the effects of collisional evolution on putative Vulcanoid=20
ensembles in the region between 0.06 and 0.21 AU from the Sun in order=20
to constrain the probable population density and population structure=20
of this region today. Dynamical studies have shown that the Vulcanoid=20
Zone (VZ) could be populated. However, we find that the frequency and=20
energetics of collisional evolution this close to the Sun, coupled with =
the efficient radiation transport of small debris out of this region,=20
together conspire to create an active and highly intensive collisional=20
environment that depletes any very significant population of rocky=20
bodies placed in it, unless the bodies exhibit orbits that are circular =
to similar to 10(-3) or less or highly lossy mechanical properties that =
correspond to a fraction of impact energy significantly less than 10%=20
being imparted to ejecta. The most favorable locale for residual bodies =
to survive in this region is in highly circular orbits near the outer=20
edge of the dynamically stable Vulcanoid Zone (i.e., near 0.2 AU),=20
where collisional evolution and radiation transport of small bodies and =
debris proceed most slowly. If the mean random orbital eccentricity in=20
this region exceeds similar to 10(-3), then our work suggests it is=20
unlikely that more than a few hundred objects with radii larger than 1=20
km will be found in the entire VZ; assuming the largest objects have a=20
radius of 30 km, then the total mass of bodies in the VZ down to 0.1 km =
radii is likely to be no more than similar to 10(-6) M+, <10(-3) the=20
mass of the asteroid belt. A 0.01-AU-wide ring near the outer stability =
boundary of the VZ at 0.2 AU would likely not contain over a few tens=20
of objects with radii larger than 1 km. Despite the dynamical stability =
of large objects in this region (Evans, N. W., and S. Tabachnik, 1999,=20
Nature 399, 41-43), it is plausible that the entire region is=20
virtually empty of kilometer-scale and larger objects. (C) 2000
Academic Press.
============================================================
(7) RADAR MICROMETEORS
D. Janches*), J.D. Mathews, D.D. Meisel, V.S. Getman, Q.H. Zhou:=20
Doppler studies of near-antapex UHF radar micrometeors. ICARUS, 2000,=20
Vol.143, No.2, pp.347-353
*) PENN STATE UNIVERSITY,COMMUN & SPACE SCI LAB,316 EE=20
EAST,UNIVERSITY PK,PA,16802
A 'radar micrometeor' is the radar-scattering signature from the free=20
electrons in the plasma generated by entry of a dust-sized meteoroid=20
into the atmosphere. We report the first direct Doppler measurements,=20
made using the Arecibo Observatory 430-MHz radar, of the so-called=20
meteor head echo. Our observations demonstrate that this region is=20
moving with the speed of the meteoroid as determined from the meteor=20
head-echo altitude-time trajectory and that this radar return is=20
distinct spatially and in velocity from the much more commonly observed =
trail echo. We also report the first observations of near-antapex=20
micrometeors which are characterized by the very slow atmospheric=20
speeds expected from low-ecliptic-inclination objects entering the=20
atmosphere from behind Earth's orbital path. Of the 32 meteors observed =
during four early evening hours of observations on 10 January 1997,=20
velocities were determined for 18 of the meteors of which 7 were at or=20
just below Earth escape velocity (11.2 km/s), We give heliocentric=20
orbits for the 11 meteor events with speeds greater than the escape=20
velocity and present a detailed analysis of these orbital parameters=20
and their possible origins. One particle was determined to be=20
interstellar: a preliminary analysis indicates that the ecliptic=20
coordinates of the radiant relative to the local standard of rest (LSR) =
(with the solar motion relative to the nearby stars removed) are lambda =
=3D 43.02 degrees, beta =3D -43.28 degrees, V =3D -25.11 km/s or, in =
system=20
II galactic coordinates l(II) =3D 219.8 degrees, b(II) =3D -52.4 =
degrees, V=20
=3D -25.1 km/s. (C) 2000 Academic Press.
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