PLEASE NOTE:
*
CCNet 132/2000 - 14 December 2000
--------------------------------
"The asteroid belt between Mars and Jupiter may not be alone
in
harbouring debris left over from the formation of the planets.
New
calculations hint there could be similar asteroid hoards
associated
with planets nearer the Sun. This lends added urgency to
scientists'
eager scrutiny of the belt once regarded as the trash heap of the
solar
system, now deemed a potential source of rare minerals or of
bodies on
a collision course with Earth."
--Philip Ball, Nature, 12 December 2000
(1) MORE ASTEROIDS IN SOLAR SYSTEM THAN EXPECTED
Ron Baalke <baalke@jpl.nasa.gov>
(2) RESEARCHERS CLAIM COMPELLING EVIDENCE OF ANCIENT LIFE ON MARS
Andrew Yee <ayee@nova.astro.utoronto.ca>
(3) POSSIBLE MARTIAN MAGNETOFOSSILS
Geochimica Et Cosmochimica Acta Vol. 64 (23)
pp. 4049-4081
(4) NEW IMAGES SUGGEST PRESENT-DAY SOURCE OF LIQUID WATER ON MARS
NASA Astrobiology Institute, 12 December 2000
(5) ARE EXTA-SOLAR PLANETS MISINDENTIFIED SMALL STARS?
Scientific American, January 2001
(6) ROSETTA MISSION TO COMET WIRTANEN ON TARGET
Andrew Yee <ayee@nova.astro.utoronto.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.nasa.gov>
(9) THE LINCOLN NEAR-ASTEROID RESEARCH PROGRAM (LINEAR)
G.H. Stokes et al.
(10) SOLAR RADIATION PRESSURE ON SMALL NEA
D. Vokrouhlicky & A. Milani
(11) INTERPLANETARY DUST DISTRIBUTION
H. Ishimoto
============
(1) MORE ASTEROIDS IN SOLAR SYSTEM THAN EXPECTED
From Ron Baalke <baalke@jpl.nasa.gov>
From Nature Science Update, 12 December 2000
http://helix.nature.com/nsu/001214/001214-6.html
More asteroids caught in Trojan force traps
PHILIP BALL
The asteroid belt between Mars and Jupiter may not be alone in
harbouring
debris left over from the formation of the planets. New
calculations hint
there could be similar asteroid hoards associated with planets
nearer the
Sun. This lends added urgency to scientists' eager scrutiny of
the belt once
regarded as the trash heap of the solar system, now deemed a
potential
source of rare minerals or of bodies on a collision course with
Earth.
Wyn Evans and Serge Tabachnik of the University of Oxford, UK,
have worked
out that there are probably other asteroid pockets in the inner
Solar
System, which are being shepherded by Mars, Earth, Venus and
Mercury. And
the duo suggests where, when and how astrophysicists should look
for them.
The asteroids associated with the giant planet Jupiter are known
as 'the
Trojans'. They owe their existence to a peculiarity of
gravitational
interactions. Isaac Newton showed in the seventeenth century how
gravity
holds a planet in orbit around the Sun. But the equations to
describe the
motion of three mutually gravitating bodies, rather than just
two, are
fiendishly difficult to solve.
In the eighteenth century, the French mathematician Joseph Louis
Lagrange
simplified things. He showed that if one of the bodies orbits
around
another, like a planet around the Sun, a third small body can get
trapped at
any of five specific points -- Lagrange points -- relative to the
orbiting
planet.
In 1906, astrophysicists detected the first of Jupiter's Trojan
asteroids at
one of its Lagrange points. Now, 470 Trojans are known and there
may be as
many as 2,500, some with diameters that could exceed 15
kilometres.
Saturn too has some small moons captured at its Lagrange points.
And in
1990, a Trojan-like asteroid called '5261 Eureka' was discovered
on Mars'
orbit, showing that you don't have to be a giant planet to secure
your own
asteroids.
But in the Monthly Notices of the Royal Astronomical Society 1,2,
Evans and
Tabachnik ask: how many asteroids are associated with the inner
planets, and
how easy would it be to spot them, given that they'd be
proportionately
smaller than those of Jupiter?
These questions can only be answered through computer simulations
of the
planetary motions. This involves scattering 'candidate Trojans'
around the
orbits of the inner planets and then seeing whether the
simulations trap
them at Lagrange points or send them hurtling off elsewhere.
The confidence in the results depends on sampling enough
possibilities and
on running the simulations for long enough compared with the age
of the
Solar System. Evans and Tabachnik ran their simulations to cover
up to 100
million years -- ten times longer than most previous studies, but
still
quite short relative to the Solar System's 4.6 billion-year age.
The simulations reveal that, like Mars, Venus and Earth can
shepherd Trojans
at their Lagrange points. For Mercury, very few candidate Trojans
survive in
the traps because the planet is small and has a rather unusual
orbit.
Astrophysicists have already scanned the Lagrange points of Mars
and Earth
and found nothing, aside from Eureka and one other martian
Trojan. But Evans
and Tabachnik have used their results to propose optimal search
strategies
that could be conducted with existing telescopes and might prove
more
fruitful.
1. Tabachnik, S. A. & Evans, N. W. Asteroids in the inner
solar system - I.
Existence. Monthly Notices of the Royal Astronomical Society 319,
63-79
(2000).
2. Evans, N. W. & Tabachnik, S. A. Asteroids in the inner
solar system - II.
Observable properties. Monthly Notices of the Royal Astronomical
Society
319, 80-94 (2000).
© Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE
============
(2) RESEARCHERS CLAIM COMPELLING EVIDENCE OF ANCIENT LIFE ON MARS
From Andrew Yee <ayee@nova.astro.utoronto.ca>
Iowa State University
Contacts:
Dennis Bazylinski, Microbiology, (515) 294-2561
Teddi Barron, News Service, (515) 294-4778
12-12-00
ISU SCIENTIST ON TEAM THAT FINDS COMPELLING EVIDENCE OF ANCIENT
LIFE ON MARS
AMES, Iowa -- An Iowa State University professor is part of a
research team
that has found compelling evidence that Mars once supported
primitive life.
The researchers discovered evidence of bacteria in a Martian
meteorite. Tiny
magnetite crystals -- so called magnetofossils -- embedded in the
meteorite
were confirmed to be the type produced only by a biological
process unique
to magnetotactic bacteria.
Dennis Bazylinski, associate professor of microbiology, was one
of nine
researchers conducting the four-year investigation, which was
funded by
NASA's Astrobiology Institute. A report of their research is in
the December
issue of the scientific journal, "Geochimica et Cosmochimica
Acta."
[ http://www.elsevier.nl/cgi-bin/cas/tree/store/gca/cas_sub/browse/browse.cgi?year=2000&volume=64&issue=23&aid=2555
]
"Finding these type of magnetic crystals in any material
from another planet
is an amazing and important finding," said Bazylinski. He
leads one of the
few labs capable of culturing these magnet-producing bacteria,
which are
common in many freshwater and marine environments on Earth.
The researchers studied the magnetite crystals that were located
in
carbonates in the Martian meteorite. The 4.5 billion year-old
meteorite was
found in Antarctica in 1984. Earlier research has confirmed that
the
carbonates formed on Mars, signaling that the magnetite crystals
also were
formed on Mars.
Magnetite crystals produced by magnetotactic bacteria are
chemically pure
and generally defect free and have a distinctive size and shape.
Their
properties are so unusual that they have only been seen in
magnetite
crystals produced through biological processes by organisms.
The researchers discovered that about one-fourth of the
magnetites in the
meteorite are identical to the magnetites produced by a strain of
magnetotactic bacteria called MV-1, which have been isolated and
studied
extensively by Bazylinski.
"There is currently no known chemical means of producing
these magnetite
crystals with their unique morphologies," Bazylinski said.
"The significance
to astrobiology and geobiology is that many scientists have been
searching
for 'biomarkers' for life, that is, chemical, isotopic, and/or
mineral
indications that life was present, either in extreme habitats or
in ancient
materials on Earth and, of course, now in extraterrestrial
materials. The
need for biomarkers is obvious and these magnetite crystals might
prove to
be an excellent biomarker."
Since the team began the research in 1996, observations from the
Mars Global
Surveyor have indicated that Mars had a strong magnetic field at
about time
that the carbonate containing the unique magnetites was formed.
"Now we are trying to answer the question of whether
magnetotactic bacteria
could have actually lived on Mars," Bazylinski said.
"And we have found
certain aspects of their metabolism which suggest that they might
have been
able to do so."
The journal "Science" recently published research
showing evidence of
widespread sediment layers on Mars, which the researchers
interpret to be
the product of many lakes. Because these lakes may have provided
a habitat
for magnetotactic bacteria, this finding supports the possibility
that the
bacteria may have existed on Mars, Bazylinski said.
In addition to Bazylinski, the scientists are Kathie
Thomas-Keprta, Simon
Clemett, and Susan Wentworth, Lockheed Martin at Johnson Space
Center; David
McKay and Everett Gibson, NASA/JSC; Joseph Kirschvink, California
Institute
of Technology; H. Vali, McGill University, Montreal; and
Christopher
Romanek, Savannah River Ecology Laboratory.
Note to Editors: A jpeg photo of Bazylinksi is available by
e-mailing
tbarron@iastate.edu .
==============
(3) POSSIBLE MARTIAN MAGNETOFOSSILS
From Geochimica Et Cosmochimica Acta Vol. 64 (23) pp. 4049-4081
http://www.elsevier.nl/cgi-bin/cas/tree/store/gca/cas_sub/browse/browse.cgi?year=2000&volume=64&issue=23&aid=2555
Elongated prismatic magnetite crystals in ALH84001 carbonate
globules:
Potential Martian magnetofossils
a Kathie L. Thomas-Keprta
b Dennis A. Bazylinski
c Joseph L. Kirschvink
a Simon J. Clemett
d David S. McKay
a Susan J. Wentworth
e Hojatollah Vali
f Everett K. Gibson, Jr.
g Christopher S. Romanek
a Lockheed Martin, 2400 NASA Rd. 1, Mail Code C23, , Houston, TX
77058, USA
b Iowa State University, Dept. of Microbiology, 207 Science I, ,
Ames, IA
50011, USA
c California Institute of Technology, Div. of Geological and
Planetary
Sciences, 1200 E. California Blvd., Pasadena, CA 91125, USA
d NASA/Johnson Space Center, Mail Code SN, , Houston, TX 77058,
USA
e McGill University, Dept. of Earth and Planetary Sciences, 3450
University
St., , Montreal, PQ H3A 2A7, Canada
f NASA/Johnson Space Center, Mail Code SN2, , Houston, TX 77058,
USA
g Savannah River Ecology Laboratory, Drawer E, University of
Georgia, ,
Aiken, SC 29802, USA
Received 7 December 1999; Revised 30 May 2000; Accepted 30 May
2000
Abstract
Using transmission electron microscopy (TEM), we have analyzed
magnetite
(Fe3O4) crystals acid-extracted from carbonate globules in
Martian meteorite
ALH84001. We studied 594 magnetites from ALH84001 and grouped
them into
three populations on the basis of morphology: 389 were
irregularly shaped,
164 were elongated prisms, and 41 were whisker-like. As a
possible
terrestrial analog for the ALH84001 elongated prisms, we compared
these
magnetites with those produced by the terrestrial magnetotactic
bacteria
strain MV-1. By TEM again, we examined 206 magnetites recovered
from strain
MV-1 cells. Natural (Darwinian) selection in terrestrial
magnetotactic
bacteria appears to have resulted in the formation of
intracellular
magnetite crystals having the physical and chemical properties
that optimize
their magnetic moment. In this study, we describe six properties
of
magnetite produced by biologically controlled mechanisms (e.g.,
magnetotactic bacteria), properties that, collectively, are not
observed in
any known population of inorganic magnetites. These criteria can
be used to
distinguish one of the modes of origin for magnetites from
samples with
complex or unknown histories. Of the ALH84001 magnetites that we
have
examined, the elongated prismatic magnetite particles (~27% of
the total)
are indistinguishable from the MV-1 magnetites in five of these
six
characteristics observed for biogenically controlled
mineralization of
magnetite crystals.
Full text available online (PDF 3085 Kbytes)
© Copyright 1999-2000, Elsevier Science, All rights reserved.
================
(4) NEW IMAGES SUGGEST PRESENT-DAY SOURCE OF LIQUID WATER ON MARS
From NASA Astrobiology Institute, 12 December 2000
http://nai.arc.nasa.gov/index.cfm?page=mars_gullies
In what could turn out to be a landmark discovery in the history
of Mars
exploration, imaging scientists using data from NASA's Mars
Global Surveyor
spacecraft have observed features that suggest there may be
current sources
of liquid water at or near the surface of the red planet.
The images show the smallest features ever observed from martian
orbit --
the size of an SUV. NASA scientists compare the features to those
left by
flash floods on Earth.
Gullies on Mars are divided into three parts: the alcove, the
channel, and
the apron. Water seeps from between layers of rock on the wall of
a cliff,
crater, or other type of depression. The alcove forms above the
site of
seepage as water comes out of the ground and undermines the
material from
which it is seeping. The channel forms from water and debris
running down
the slope from the seepage area. The aprons are the down-slope
deposits of
ice and debris that were moved down the slope and through the
channel.
"We see features that look like gullies formed by flowing
water and the
deposits of soil and rocks transported by these flows. The
features appear
to be so young that they might be forming today. We think we are
seeing
evidence of a ground water supply, similar to an aquifer,"
said Dr. Michael
Malin, principal investigator for the Mars Orbiter Camera on the
Mars Global
Surveyor spacecraft at Malin Space Science Systems (MSSS), San
Diego, CA.
"These are new landforms that have never been seen before on
Mars."
FULL STORY at http://nai.arc.nasa.gov/index.cfm?page=mars_gullies
=============
(5) ARE EXTA-SOLAR PLANETS MISINDENTIFIED SMALL STARS?
From Scientific American, January 2001
http://www.sciam.com/2001/0101issue/0101scicit3.html
LOST WORLDS
Evidence for the maverick view that extrasolar planets are really
small
stars
PASADENA, CALIF.--"It's not even wrong" was physicist
Wolfgang Pauli's
famous putdown for a theory he regarded as implausible and
inconsequential.
For the past several years, it has been most astronomers'
response to the
ideas of David C. Black. The researcher from the Lunar and
Planetary
Institute in Houston is the most outspoken skeptic of the
discovery of
planets around other sunlike stars. He thinks the planets are
actually
misidentified stars, and he has stuck to that position despite
the failure
of his predictions, the weight of scientific opinion and an
almost total
lack of observational support. His colleagues whisper that his
planet
doesn't go all the way around his star.
POSSIBLE PROTOPLANET, hanging on at the lower left from a star
system in
Taurus, has several times Jupiter's mass. Such direct, infrared
views are
needed to determine whether, in other systems, massive planets
are really
brown dwarf stars.[S. TEREBEY Extrasolar Research Corp. AND NASA]
Now, for
the first time, some evidence for Black's view has emerged. At
the Division
for Planetary Sciences conference in Pasadena last October,
veteran planet
hunter George D. Gatewood of the University of Pittsburgh
Allegheny
Observatory presented the results of a study he conducted with
Black and
then graduate student Inwoo Han. They checked whether the parent
stars of
the purported planets swayed from side to side, the sign of a
cosmic
do-si-do with partners too small to be seen directly. In many
cases, the
team concluded, the swaying motion was strong enough that the
partners must
be fairly heavy--brown dwarfs or other smallish stars, it would
seem. At the
least, the group has stirred a debate over selection biases in
the planet
searches and spiced up the broader discussion over what exactly a
planet is.
In the 1980s the name of David Black was practically synonymous
with
extrasolar planets. He was once the head of the National
Aeronautics and
Space Administration's search. But his reputation started to
slide in 1995
when planet hunting became planet finding. None of the new worlds
resembled
anything in our solar system. Black took this as a sign that they
weren't
planets after all. Their mass distribution and orbital
characteristics, he
asserted, look rather like those of stars. But most
astronomers--including
ones who used to share his views, such as William D. Heacox of
the
University of Hawaii at Hilo--now say Black is clinging to
outmoded ideas.
If nature created odd planets, even ones with starlike orbits, so
be it.
Accept it and move on.
To be fair, there was always a loophole in the observations. The
swaying
motion of the parent stars has two components, one along the line
of sight
(the radial velocity) and the other across the sky (the
astrometric motion).
Today's instruments can spot the latter only if the partner is
fairly
massive, like a star, so nearly all planet discoveries rely on
the former.
But radial velocity alone can merely put a lower limit on the
planet masses,
and if the orientation is just right, the true mass might be much
greater.
Han, Gatewood and Black have extended previous work that merged
radial
velocities with astrometric data from the Hipparcos satellite.
They found
that out of 30 stars with companions, 15 showed astrometric
motion, which
implies that the partners are brown dwarfs or stars. "If
that's right, it
sure does make life interesting," Heacox says. The response
from other
planet people has been swift and vigorous. "The claim by
David Black is
completely incorrect," says famed planet finder Geoffrey W.
Marcy of the
University of California at Berkeley. He and others argue that
the inferred
orientations are incredibly improbable. Four of the partners were
said to
orbit within one degree of perfect alignment with the line of
sight. Yet the
chance of any single partner of a given mass having that
orientation is
about 1 in 5,000. Conversely, for every partner with that
orientation, there
should be 5,000 or so with less extreme orientations. No such
bodies are
seen. Marcy is so convinced that he says Scientific American
"will be doing
science a bum steer" simply by mentioning Black's work.
Two independent groups have weighed in. Tsevi Mazeh and Shay
Zucker of Tel
Aviv University suggest that the truth lies somewhere in the
middle. They
confirm that two of the bodies indeed have the heft of a
star--but only two.
They see no astrometric motions for the other bodies. Hipparcos
expert
Dimitri Pourbaix of the Free University of Brussels initially got
similar
results but now suspects that the analyses have fallen prey to
subtle
computational biases that overestimate the mass and underestimate
the error
bar. To resolve the dispute, astronomers will need
higher-precision
astrometry (as at least two teams now intend) and direct searches
for
infrared light from the stellar companions (as Mazeh plans this
month at the
Keck Observatory on Mauna Kea in Hawaii).
Although it looks as if Black is wrong, planet hunters can't go
scot-free
just yet. Even two stellar interlopers would be two too many.
Brown-dwarf
expert Gibor Basri of Berkeley and others say it is quite
plausible that
searchers have unwittingly skewed their sample. No matter what,
the
theorists still have their work cut out for them. What could
possibly
account for the amazing diversity of worlds, from the mannerly
ones in our
solar system to the errants traipsing through interstellar space?
Do they
all deserve the label "planet"? Basri quotes from Lewis
Carroll: "'When I
make a word do a lot of work like that,'" said
Humpty-Dumpty, 'I always pay
it extra.'"
--George Musser
Copyright 2000, Scientific American
============
(6) ROSETTA MISSION TO COMET WIRTANEN ON TARGET
From Andrew Yee <ayee@nova.astro.utoronto.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=12155
]
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=24951
]
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=12176
]
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.nasa.gov>
http://www.jhuapl.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.jhuapl.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@jhuapl.edu
michael.buckley@jhuapl.edu
==================
(9) THE LINCOLN NEAR-ASTEROID RESEARCH PROGRAM (LINEAR)
G.H. Stokes, J.B. Evans, H.E.M. Viggh, F.C. Shelly, E.C. Pearce:
Lincoln
Near-Earth Asteroid Program (LINEAR). ICARUS 148: (1) 21-28 NOV
2000
The Lincoln Near-Earth Asteroid Research (LINEAR) program has
applied
electro-optical technology developed for Air Force Space
Surveillance
applications to the problem of discovering near-Earth asteroids
(NEAs) and
comets. This application is natural due to the commonality
between the
surveillance of the sky for man-made satellites and the search
for
near-Earth objects (NEOs). Both require the efficient search of
broad swaths
of sky to detect faint, moving objects. Currently, the Air Force
Ground-based Electro-Optic Deep Space Surveillance (GEODSS)
systems, which
operate as part of the worldwide U.S. space surveillance network,
are being
upgraded to state-of-the-art charge-coupled device (CCD)
detectors. These
detectors are based on recent advances made by MIT Lincoln
Laboratory in the
fabrication of large format, highly sensitive CCDs. In addition,
state-of-the-art data processing algorithms have been developed
to employ
the new detectors for search operations. In order to address
stressing space
surveillance requirements, the Lincoln CCDs have a unique
combination of
features, including large format, high quantum efficiency, frame
transfer,
high readout rate, and low noise, not found on any commercially
available
CCD. Systems development for the GEODSS upgrades has been
accomplished at
the Lincoln Laboratory Experimental Test Site (ETS) located near
Socorro,
New Mexico, over the past several years. Starring in 1996, the
Air Force
funded a small effort to demonstrate the effectiveness of the CCD
and broad
area search technology when applied to the problem of finding
asteroids and
comets. This program evolved into the current LINEAR program,
which is
jointly funded by the Air Force Office of Scientific Research and
NASA.
LINEAR, which started full operations in March of 1998, has
discovered
through September of 1999, 257 NEAs (of 797 known to date), 11
unusual
objects (of 44 known), and 32 comets. Currently, LINEAR is
contributing
similar to 70% of the worldwide NEA discovery rate and has
single-handedly
increased the observations submitted to the Minor Planet Center
by a factor
of 10. This paper covers the technology used by the program, the
operations,
and the detailed results of the search efforts.
Addresses:
Stokes GH, MIT, Lincoln Lab, 244 Wood St, Lexington, MA 02420
USA.
MIT, Lincoln Lab, Lexington, MA 02420 USA.
Copyright © 2000 Institute for Scientific Information
=======================
(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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