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(meteorobs) Excerpts from "CCNet 132/2000 - 14 December 2000"
Finally, an issue of CCNet packed with juicy meteor-related news!
One question on item #10, however: in what sense can the Poynting-
Robertson effect - which I'd understood to be a relativistic effect
due to the absorption and re-radiation of solar energy - be called
"direct radiation pressure". It seems, by definition, as though it
should be called *indirect*. In any case, it's interesting to see
a paper questioning the long-held assumption that these forces near
and dear to the hearts of meteoricists do not affect larger bodies.
As for item #11 - maybe some day, our descendants may watch meteors
from Titan's surface to help confirm the authors' final predictions.
Meantime, they highlight approaches that were new to this amateur. :)
Clear skies,
Lew Gramer <owner-meteorobs@jovian.com>
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 14 December 2000
Date: Thu, 14 Dec 2000 10:44:40 -0000
CCNet 132/2000 - 14 December 2000
--------------------------------
[...]
(6) ROSETTA MISSION TO COMET WIRTANEN ON TARGET
Andrew Yee <ayee@nova.astro.utorontodot ca>
(7) MISSION OVERVIEW: ROSETTA'S PURPOSE
http://sci.esa.int/rosetta
(8) NEAR SHOEMAKER ON TRACK FOR FINAL MONTHS IN ORBIT AROUND EROS
Ron Baalke <baalke@jpl.nasadot gov>
[...]
(10) SOLAR RADIATION PRESSURE ON SMALL NEA
D. Vokrouhlicky & A. Milani
(11) INTERPLANETARY DUST DISTRIBUTION
H. Ishimoto
============================================================
(6) ROSETTA MISSION TO COMET WIRTANEN ON TARGET
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
ESA Science News
http://sci.esa.int
11 Dec 2000
766 Days to Launch ... And Counting!
Representatives of the Rosetta science instrument teams came together from
all over Europe and the United States this week for the 7th meeting of the
Rosetta Science Working Team.
The purpose of the gathering at the premises of Alenia Spazio in Turin was
to familiarise everyone with the latest status of the programme. However,
the primary consideration for everyone was the limited time remaining to
complete the spacecraft Assembly, Test and Verification programme -- 766
days to launch ... and counting.
Members of the ESA project team for Rosetta explained to the 80-strong
audience that the Electrical Qualification Model (EQM) test programme on the
Orbiter and Lander is now half completed, although considerable work remains
to be done.
All of the Orbiter's EQM instruments have been delivered and are currently
undergoing functional testing. This phase of the intensive EQM programme is
scheduled for completion by the end of January 2001, after which the
instrument compatibility tests will begin.
"Progress is being made, but not quite as fast as we originally hoped," said
ESA Project Manager John Ellwood. "However, engineers at Alenia are working
double shifts and we are confident that we will be able to catch up with our
tight schedule."
Meanwhile, all of the experiments on the Orbiter have passed their Final
Design Reviews after several months of discussions about the results of
tests on prototype instruments that have been carried out at the various
institutes. This is an important step towards the Mission Critical Design
Review in April 2001.
The meeting was also informed that the Assembly, Integration and
Verification programme for the Rosetta Orbiter Flight Model will take place
in parallel to the EQM tests. The first step will be the delivery of the
Flight Model structure from Finland to Italy in mid-January, followed
immediately by the delivery of key subsystems such as the harness and the
Reaction Control System.
The scientists were able to see for themselves the current state of play in
the EQM programme during a visit to the giant clean room at Alenia, where
they could inspect at close quarters the Rosetta Orbiter and Lander EQMs (as
well as the Flight Model of ESA's Integral gamma ray observatory which is
scheduled for launch in 2002).
However, the main message for the gathered assembly was pronounced early on
the first day by John Ellwood.
"No major technological risks are outstanding, but we must all continue to
make every effort to meet our deadlines," he said.
"We are 'Go' for launch on 12 January 2003 -- but Comet Wirtanen will not
wait!"
USEFUL LINKS FOR THIS STORY
* More about Rosetta
http://sci.esa.int/rosetta
IMAGE CAPTIONS:
[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=13&cid=12&ooid=1
2155]
The faint, moving image of the nucleus of Comet Wirtanen (in the circles),
as observed by the 8.2-m VLT KUEYEN telescope (formerly UT2) and the VLT
Test Camera on 17 May 1999, during the commissioning phase. Photograph
courtesy ESO.
[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=13&cid=12&ooid=2
4951]
Rosetta Orbiter EQM at Alenia Spazio in Turin, Italy, 11 October 2000.
[image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=13&cid=12&ooid=1
2176]
Rosetta rises to meet the challenge. A three colour image of Comet Wirtanen,
taken from observations on Pik Tersko, which shows the three cometary
components -- dust, neutral gas and ions. Using different filters on the
telescope, the cometary water (H2O) ions appear red, the dust is green and
neutral CN gas is blue.
This shows Wirtanen's ion tail (here H2O+) for the first time. It appears as
a straight, red diffuse band to the left side (anti-sunward direction). The
blue sphere is the very extended neutral CN coma. In contrast to this, the
dust is much more concentrated and dominates the near nucleus region, here
seen as a yellowish green colour. This image illustrates that 46P/Wirtanen
is dust poor and about 2-3 times less dusty than Comet Halley.
Photo Max-Planck-Institut fur Aeronomie, courtesy T. Credner, J. Jockers,
T.Bonev.
============================================================
(7) MISSION OVERVIEW: ROSETTA'S PURPOSE
http://sci.esa.int/rosetta
The International Rosetta Mission was approved in November 1993 by ESA's
Science Programme Committee as the Planetary Cornerstone Mission in ESA's
long-term space science programme. The mission goal is a rendezvous with
comet 46 P/Wirtanen. On its eight-year journey to the comet, the spacecraft
will pass close to two asteroids, (Otawara and Siwa are now the planned
targets)
Rosetta will study the nucleus of comet Wirtanen and its environment in
great detail for a period of nearly two years, the near-nucleus phase
starting at a heliocentric distance of about 3.25 AU, with far-observation
activities leading ultimately to close observation (from about one km
distance).
Rosetta operations will be carried out from ESA's Operations Centre (ESOC)
in Darmstadt. Orbit determination for all mission phases will also be
performed by ESOC.
Rosetta will be launched in January 2003 by an Ariane-5 from Kourou, French
Guiana. To gain enough orbital energy to reach its target, one Mars and two
Earth gravity assists will be required. The long mission duration required
the introduction of extended hibernation periods. The mission falls into
several distinct phases:
Major event Nominal date
Launch 12 January 2003
Mars gravity assist 26 August 2005
First Earth gravity assist 21 November 2005
Otawara flyby 11 July 2006
Second Earth gravity assist 28 November 2007
Siwa flyby 24 July 2008
Rendezvous manoeuvre 29 November 2011
============================================================
(8) NEAR SHOEMAKER ON TRACK FOR FINAL MONTHS IN ORBIT AROUND EROS
>From Ron Baalke <baalke@jpl.nasadot gov>
http://www.jhuapldot edu/public/pr/001213.htm
NEAR Shoemaker Engine Burn Puts Spacecraft on Track for Final Months in
Orbit
December 13, 2000
An engine burn at 3:15 p.m. (EST) today put the NEAR Shoemaker spacecraft in
orbit just 22 miles (35 kilometers) above Eros' center of mass in
preparation for low altitude operations in January and February, just prior
to the mission's end. The orbit correction maneuver lasted a minute and a
half and pushed the spacecraft from an elliptical orbit approximately 120
miles (200 kilometers) above Eros at its farthest point, into its current
circular orbit around the tumbling space rock.
The maneuver is the latest in the mission's five-year history that has taken
the NEAR Shoemaker spacecraft on a 2-billion-mile journey and provided a
unique 150,000-image photo-op since it began its orbital approach in
January.
"The next two months will be the most challenging time of the entire mission
for the operations team," says Dr. Robert W. Farquhar, mission director for
NASA's Near Earth Asteroid Rendezvous (NEAR) program at The Johns Hopkins
University Applied Physics Laboratory (APL), Laurel, Md. "We're working very
closely with the navigation team at the Jet Propulsion Laboratory to ensure
the success of each maneuver. The final controlled descent on Feb. 12 is one
of the most complicated maneuvers to date, but the return will be worth it.
We expect to get images that are 10 times better in resolution than anything
we've taken so far."
NEAR Shoemaker will stay in a 22-mile (35-kilometer) orbit until Jan. 24,
when three more engine burns will push it first to within 12 miles (19
kilometers) and then back to a circular 22-mile orbit, by the end of the
month. During the lower orbits the spacecraft will come within 1.9 miles (3
kilometers) of the asteroid's ends.
The mission team will use the progressively lower orbits and low flyovers to
collect valuable data. "This will give us an excellent opportunity for the
gamma ray spectrometer to measure element abundances at low altitudes," says
Project Scientist Dr. Andrew F. Cheng of the Applied Physics Laboratory.
"These measurements will help us clear up some questions we have regarding
how closely Eros' composition fits the pattern of ordinary chondrites. The
spectrometer will also give us composition measurements from 10 centimeters
below the surface and will give us a reading of natural radioactivity on the
asteroid. At the same time the imager and laser rangefinder will be giving
us additional low-altitude, high-resolution data to complete a global
mapping of Eros."
On the final day of the mission, Feb. 12, 2001, the spacecraft will execute
a series of maneuvers that will enable NEAR Shoemaker to gather
high-resolution images from only 1,640 feet (500 meters) above the
asteroid's surface.
NEAR Shoemaker has been in orbit around Eros since Feb. 14, 2000, conducting
the first in-depth study of an asteroid. APL manages the NEAR mission and
built the spacecraft. For more information on the mission, and for daily
images of Eros, visit Web site: (near.jhuapldot edu).
__________________
Media contact:
JHU Applied Physics Laboratory:
Helen Worth Michael Buckley
Laurel, MD 20723 Laurel, MD 20723
Phone: 240-228-5113 Phone: 240-228-7536
E-mail: E-mail:
helen.worth@jhuapldot edu michael.buckley@jhuapldot edu
============================================================
(10) SOLAR RADIATION PRESSURE ON SMALL NEA
D. Vokrouhlicky, A. Milani: Direct solar radiation pressure on the orbits of
small near-Earth asteroids: observable effects? ASTRONOMY AND ASTROPHYSICS
362: (2) 746-755 OCT 2000
We consider the perturbations of Near-Earth Asteroid orbits due to direct
solar radiation pressure (both the absorption and the reflection
components). When the body is spherical and the surface albedo homogeneous
the effect is small (and only short-periodic). However, when at least one of
these restrictive and unrealistic assumptions is relaxed,long-term orbital
effects appear and they may potentially lead to observable displacement of
the orbit. We illustrate this conclusion by computing the orbital
perturbations due to radiation pressure for objects with an odd-zonal
distribution of albedo and for objects with ellipsoidal shape. Especially in
the first case the effects are large, due to the long-term perturbations of
the semimajor axis. For high-eccentricity orbits observed over a long
interval of time, the (v/c)-correction of the direct radiation pressure,
known as Poynting-Robertson effect, should be also considered. As an example
we demonstrate that for the asteroid 1566 Icarus, during its next close
approach to the Earth, the orbit displacement due to the direct solar
radiation forces might be, under reasonable assumptions. comparable to the
orbit determination uncertainty, thus potentially observable.
Addresses:
Vokrouhlicky D, Charles Univ, Astron Inst, V Holescovickach 2, CR-18000
Prague 8, Czech Republic.
Charles Univ, Astron Inst, CR-18000 Prague 8, Czech Republic.
Univ Pisa, Dipartimento Matemat, I-56127 Pisa, Italy.
Copyright ) 2000 Institute for Scientific Information
============================================================
(11) INTERPLANETARY DUST DISTRIBUTION
H. Ishimoto: Modeling the number density distribution of interplanetary dust
on the ecliptic plane within 5AU of the Sun. ASTRONOMY AND ASTROPHYSICS 362:
(3) 1158-1173 OCT 2000
We have used the relationship, consistent with observational data, between
the radial dependence of the dust supply and the mass dependence of the
number density distribution, to consider the parent bodies of interplanetary
dust. We examine the number density distribution of the interplanetary dust
within 5AU of the Sun on the ecliptic plane.
For the model calculations, the number density equations for the ecliptic
plane are solved directly by taking into account collisional destruction
between particles and the Poynting-Robertson effect, and by assuming a state
of equilibrium and axial symmetry in the interplanetary dust cloud. Typical
models for the radial dependence of the dust input on the ecliptic plane are
considered. For three typical dust groups that are characterized by their
orbits-i.e., bound particles, hyperbolic particles of collisional origin,
and interstellar particles-a variety of simple models of the physical
parameters are considered. These include the particles' optical properties,
the mean sweep-out velocities of the dust clouds, the power law distribution
of mass in the collisional fragments, the maximum size of particles, and the
inner/outer boundaries.
>From the model calculations, the existence of the three characteristic
particle groups and their input radial dependencies are found to play
important roles in determining the environmental conditions of
interplanetary dust and the number density distribution of the particles.
The roles played by comets and asteroids are estimated by analyzing the
relationship between the radial dependence of the dust input and the
resultant number density distribution at 1AU. To simulate the flux curve of
interplanetary meteoroids at 1AU (e.g., Grun et al. 1985), a source that
directly supplies the interplanetary dust is required. It is found that the
simulated number density distribution fits that observed at 1AU well, if the
mass production rate of dust sources outside 1AU increases with a radial
index of -3 similar to -4 as the solar distance decreases. Such dust sources
are more likely to be comets rather than asteroids.
The numerical results indicate that, at 1AU, cometary dust is the major
component of particles with masses m greater than or equal to 10(-6) g, and
almost comparable in number to asteroidal particles with masses 10(-12) g
less than or equal to m less than or equal to 10(-7) g. Furthermore, we can
expect that within IAU the contribution of cometary particles increases as
the solar distance decreases, due to the direct input of cometary particles.
In order for the results to be consistent with the observed r(-1 similar to
-1.3) radial dependence in the number density distribution of the zodiacal
cloud inside IAU, the mass production rate by the dust source should be
almost constant or decreasing as the solar distance decreases.
Using a possible model for the dust sources and for the radial dependence of
dust input, the number density of hyperbolic particles of collisional origin
at 1AU is estimated to be similar to 1.8 x 10(-4) m(-2)sec(-1).
Hyperbolic particles and the influx of interstellar particles (m similar to
10(-13) g) inside 5AU increase the number density of interplanetary dust
particles in the medium-sized range (10(-15) g less than or equal to m
greater than or equal to 10(-6) g). Interplanetary dust beyond 3AU of the
Sun will, therefore, maintain a flat radial distribution of medium mass
particles if the interstellar flux is significant.
Addresses:
Ishimoto H, Japan Meteorol Agcy, Meteorol Res Inst, Nagamine 1-1, Tsukuba,
Ibaraki 3050052, Japan.
Japan Meteorol Agcy, Meteorol Res Inst, Tsukuba, Ibaraki 3050052, Japan.
Copyright ) 2000 Institute for Scientific Information
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