PLEASE NOTE:
*
CCNet DIGEST, 27 August 1998
----------------------------
(1) SPACEGUARD ASTEROID SURVEY: HOW ARE WE DOING?
David Morrison <dmorrison@arc.nasa.gov>
(2) PEPE TO INVESTIGATE COMPOSITION OF ASTEROIDS & COMETS
Andrew Yee <ayee@nova.astro.utoronto.ca>
(3) SPACE/DEV'S NEAP PROJECT FEASIBLE
Jim Benson <Jim@SpaceDev.Com>
(4) ASTEROID HUNTERS TARGET NEREUS
MSNBC Space News
http://www.msnbc.com/news/190529.asp
(5) MORE TSUNAMIS AROUND THE CORNER: WHEN IT HITS, CLIMB A TREE
ABCNEWS.com
http://www.abcnews.com/sections/science/DyeHard/dye70.html
(6) BAD NEWS FOR PLANETARY DEFENSE: SECOND ROCKET IN A MONTH
EXPLODES
AFTER TAKE-OFF
BBC Online Network
http://news.bbc.co.uk/hi/english/sci/tech/newsid_159000/159299.stm
(7) THE SPIN RATE OF M-TYPE ASTEROIDS
C.I. Lagerkvist et al., ASTRONOMICAL
OBSERVATORY
(8) YARKOVSKY EFFECT & ASTEROIDAL MOBILITY
D. Vokrouhlicky, CHARLES UNIVERSITY, PRAGUE
==================
(1) SPACEGUARD ASTEROID SURVEY: HOW ARE WE DOING?
From David Morrison <dmorrison@arc.nasa.gov>
The goal of the Spacewatch Survey is to find 90% or more of the
Near
Earth Asteroids (NEAs) within a decade. It is also a goal of
NASA,
stated in the NASA Office of Space Science Strategic Plan, to
discover 90% of the NEAs within the next decade. This is a
summary,
prepared in collaboration with Alan Harris of JPL, to see where
we
stand today (mid-1998) in this effort. We will try to make
further
updates at 6-month intervals and post them on the NASA Impact
Hazard
website.
For purposes of this discussion, NEAs with D > 1 km are
equated to
asteroids with absolute H magnitude less than or equal to 18.5,
with
perihelion distances less than 1.3 AU. There are approximately
2000
NEAs estimated to exist that fit this definition. Other
definitions
of NEAs (or ECAs, Earth-Crossing Asteroids, or PHAs, Potentially
Hazardous Asteroids) are more restrictive and also require more
detailed analysis of their orbits. The present definition of an
NEA,
however, together with the estimate of 2000 total NEAs, is
sufficient
to asses the current performance of the survey.
Note that many of the NEAs discovered are smaller than 1 km (H
>
18.5). Any survey system will discover as many or more small
asteroids as large ones. But we will consider only asteroids
larger
than 1 km, since these are the most dangerous, and the metric for
success of the survey is defined in terms of objects with D >
1 km.
The three most successful searches during the past year have been
LINEAR (the Lincoln Laboratory NEA survey of the US Air Force),
NEAT
(the NEA survey carried out jointly by JPL and the USAF), and
Spacewatch (the search carried for more than a decade at the
University of Arizona). Together they accounted for more than 85%
of
the discoveries. LINEAR has dominated the recent growth,
going from
0 to 4 to 16 NEAs (D > 1 km) in the last three 6-month
periods.
The table below shows 38 NEAs larger than 1 km discovered from
July
1997 through June 1998. There was a marked increase in discovery
rate
over this period, from 11 NEAs discovered in the first 6 months
to 27
in the most recent 6 months.
In a ten-year survey expected to detect 90% of this NEA
population,
we must discover just over 20% in the first year, with the rate
declining exponentially thereafter as greater completion is
reached
and more of the objects found are rediscoveries. Therefore, to
achieve the stated Spaceguard goal of finding 90% of the 2000
NEAs in
a decade, we must increase the discovery rate by approximately a
factor of 12 over the average for the past 12 months, or a factor
of
9 over the average of the past 6 months. Searchers have just now
pulled within an order of magnitude of the required discovery
rate,
with another factor of 10 needed to implement the Spaceguard
Survey.
David Morrison
NEA DISCOVERY SUMMARY (D > 1 km) FOR JULY 97 THRU JUNE 98
Discoverer 12 month 1997-2 1998-1
-----------------------------------------------------------------------
LINEAR 20 4 16
NEAT 9 4 5
Spacewatch 4 1 3
Other 5 2 3
----------------------------------------------------------------------
Total 38 11 27
(Total required to implement the Spaceguard Survey: >400
discoveries/year)
===================
(2) PEPE TO INVESTIGATE COMPOSITION OF ASTEROIDS & COMETS
From Andrew Yee <ayee@nova.astro.utoronto.ca>
Los Alamos National Laboratory
CONTACT: Ternel Martinez, 505-665-7778
LAB SHIPS SPACE INSTRUMENT PEPE FOR EVENTUAL LAUNCH ON NASA'S NEW
MILLENNIUM DEEP SPACE ONE MISSION
LOS ALAMOS, N.M., Aug. 25, 1998 -- Scientists at Los Alamos
National
Laboratory recently finished testing their part of a space
instrument
designed to give a better understanding of the compositions of
comets
and asteroids, and delivered the instrument for eventual
integration
with the NASA spacecraft.
Los Alamos' Plasma Experiment for Planetary Exploration
instrument is
a joint effort with Southwest Research Institute that is part of
NASA's New Millennium Deep Space One mission, whose primary role
is
to validate new technologies that will allow NASA to better
conduct
future science missions.
PEPE combines two spectrometers into one package. The first
spectrometer analyzes the energy and mass of ions and the
direction
in which they are traveling; the second spectrometer analyzes the
energy and angular distribution of electrons coming from all
directions in space. Together, the measurements give scientists a
better understanding of the state and composition of plasma the
spacecraft encounters.
"PEPE is so versatile that it can be used almost anywhere in
our
solar system. It can analyze everything from comet and asteroid
composition to the compositions of planetary bodies and moons. It
could even tell you if a nuclear or chemical explosion took place
in
the upper atmosphere or in space," said PEPE Project Leader
Beth
Nordholt of the Space and Atmospheric Sciences Group. Seventeen
researchers from this group and the Space Engineering Group
worked on
the PEPE project.
The Laboratory was responsible for constructing the
"guts" of PEPE,
including its accelerator region, front-end electronics board,
ion
microchannel plate and time-of-flight mass analysis section, said
Nordholt.
The Deep Space One mission also will determine if the
spacecraft's
innovative ion propulsion system will have any adverse affects on
the
instruments' ability to make space physics measurements.
"Ion
thrusters allow spacecraft to go as much as ten times faster than
spacecraft using chemical thrusters. They're also more efficient,
so
you need less fuel," explained Nordholt.
"The problem is, the ion thrusters will introduce plasma
into the
surrounding environment and interact with the solar wind, and no
one
knows for sure what that may do to the spacecraft or PEPE,"
she
added. Ion propulsion systems typically are used by
communications
companies to reposition their communications satellites in orbit,
but
until now never have been used for any extended space flight.
To study this effect, PEPE features three
micro-electromechanically
machined calorimeters, provided by Stanford University, that will
measure contaminant deposition on the spacecraft's surface and
help
researchers better understand plasma/surface interactions.
The force of the ion propulsion system that will be used on Deep
Space One is so small that the push it gives the spacecraft is
analogous to the weight of a single piece of notebook paper
resting
in a person's hand. However, the spacecraft accelerates very
gently.
To go from 0 to 60 mph would take the spacecraft two-and-a-half
days.
Nordholt and her colleagues recently shipped PEPE to Southwest
Research Institute, which now will put the finishing touches on
PEPE
and eventually ship it to NASA's Kennedy Space Flight Center for
a
scheduled Oct. 15 launch date. PEPE should by July 1999 make
contact
with the first of three objects -- an asteroid and two comets --
to
determine their compositions.
"The major benefits of PEPE are that it can provide nearly
the same
kind of performance as the equivalent instruments on board
Cassini
for approximately 30 percent of the mass and 25 percent of the
power
and cost," said Nordholt. Cassini was a joint NASA/European
Space
Agency effort; it launched last October on a mission to study the
composition of the rings of Saturn and the moons orbiting within
its
magnetic field.
PEPE has no moving parts and weighs about 13 pounds, which
translates
into greatly reduced thrust and power requirements, said
Nordholt. By
comparison, Cassini's plasma spectrometer alone weighed about 50
pounds. Another major benefit is PEPE's versatility, for its
instruments can measure the energy and mass per charge and
velocity
distributions of ions and electrons simultaneously. Mass
spectrometers separate and thus identify ions or ionic fragments
of
the substance being measured based on their mass-to-charge
ratios.
PEPE cost about $3.6 million and took about two years to build
and
test. It is one of two principal instruments on the Deep Space
One
mission, the other instrument being a miniature integrated camera
and
spectrometer, provided by NASA's Jet Propulsion Laboratory.
Twelve
advanced technologies overall make up the mission.
Los Alamos National Laboratory is managed by the University of
California for the U.S. Department of Energy.
===================
(3) SPACEDEV'S NEAP PROJECT FEASIBLE
From Jim Benson <Jim@SpaceDev.Com>
Greetings,
Here is the summary of the other half of Tony Spear's NEAP
findings.
We owe a great debt of gratitude to all the scientists, engineers
and
NASA personnel who have been and continue to be so supportive of
this
pioneering and hopefully historic venture.
Sincerely,
Jim Benson
---------------
Project Assessment Team Declares SpaceDev NEAP Feasible
SAN DIEGO, Aug. 26 /PRNewswire/ -- SpaceDev (OTC Bulletin Board:
SPDV
- news), the world's first commercial space exploration and
development company, announced that a team of independent
reviewers
has concluded that the Near Earth Asteroid Prospector (NEAP)
mission
design, spacecraft design, and project budget are feasible.
The Project Assessment Team was led by Tony Spear, Mars
Pathfinder
Project Manager, who recently retired from NASA's Jet Propulsion
Laboratory (JPL) after thirty years of successful deep space
science
missions. The team consisted of Spear's hand picked deep space
experts from various organizations including JPL.
``We commissioned Mr. Spear's study because we believe his team
consists of some of the most respected and accomplished
deep-space
experts in the world,'' said Jim Benson, president and chief
executive officer of SpaceDev.
The team concluded that the NEAP mission conceptual design is
sound
and the mission could be flown within $50 million, including
launch
cost. ``We are pleased with their findings and we intend to use
Mr.
Spear's report as a roadmap to complete this mission on time and
within our original $50 million estimate announced in September
of
last year,'' said Benson.
Spear's team made several specific recommendations for optimizing
the
NEAP spacecraft to the Nereus carbonaceous asteroid target, the
new
NEAP target selected as a result of the Spear study. Mission
recommendations included possibly adding revenue-producing lunar
or
near-Earth payloads that could be accommodated in the early parts
of
the mission, simplifying the avionics architecture, and designing
a
schedule based on Spear's years of deep space science mission
experience.
``Mr. Spear recommended a pre-project phase to initiate detailed
project planning and design, project team forming, and long lead
procurements,'' Benson commented. ``This pre-project phase would
extend from September 1998 to April 1999. At that time a two-year
development phase would start, supporting the planned launch date
in
April 2001.''
One member of the team, Dr. Robert Farquhar of Johns Hopkins
University's Applied Physics Laboratory (APL), recommended
fundamental simplifications to the propulsion system that could
result in a more reliable and less expensive solution. Dr.
Farquhar
is the Mission Director of APL's Near Earth Asteroid Rendezvous
(NEAR) project. He validated and refined the mission and
trajectory
design work that was pioneered by Dr. Alan Schneider at the
University of California, San Diego (UCSD) in the early design
phases
of the NEAP mission. Dr. Farquhar's input reduced deep space
cruise time by four months.
Dr. Farquhar and Dr. Donald Yeomans, a senior research scientist
at
JPL, first identified Nereus as one of the most scientifically
interesting objects that NEAP could visit. The pair also
performed a
preliminary analysis of the possible trajectory available to NEAP
to
reach Nereus and concluded that such a mission would need
relatively
little fuel and little time. Both features lead to a simpler,
smaller
spacecraft than was first anticipated.
``We were truly fortunate to have so many of the world's leading
scientists and engineers provide their expertise to this mission
design, and we fully intend to utilize the advice they have
provided
us,'' added Benson. ``Overall, these recommendations, and the
choice
of Nereus as the 'quintessential' target asteroid have allowed us
to
simplify and miniaturize the spacecraft, which we believe leads
to a
less expensive and even more feasible mission. As an example, the
smaller NEAP spacecraft now being designed for Nereus can be
launched
by a much wider variety of commercially available rockets, giving
us
greater flexibility in selecting a low-cost launch provider.''
The new NEAP orbits could include close lunar swing-bys that
could
serve as excellent practice runs for operations at Nereus. The
company plans to calculate as many lunar swing-bys as possible
for
the nine months available, including half-month ``backflips'' and
double-lunar swing-by orbits similar to those used by the ISEE-3
spacecraft in 1983. Some of the swing-bys will be designed to fly
as
close to the moon as is safely possible, and perhaps fly low over
polar craters where lunar water is believed to exist. It is
intended
that the injection from the parking orbit will be directed into a
high- altitude ``phasing'' orbit whose apogee would be only
slightly
beyond the moon's orbit.
SpaceDev, the world's first commercial space exploration and
development company, intends to launch the first privately
financed
spacecraft to land on another planetary body. SpaceDev is selling
rides for scientific instruments to governments and companies to
transport their instruments and experiments through deep space to
a
near Earth asteroid. SpaceDev intends to sell the data acquired
by
its instruments as commercial products. Colorado-based SpaceDev
has
offices in San Diego, CA and Washington, DC.
The foregoing press release includes numerous forward-looking
statements concerning the company's business and future prospects
and
other similar statements that do not concern matters of
historical
fact. The federal securities laws provide a limited ``safe
harbor''
for certain forward-looking statements. Forward-looking
statements in
this press release relating to product development, business
prospects and development of a commercial market for
technological
advances are based on the company's current expectations. The
company's current expectations are subject to all of the
uncertainties and risks customarily associated with new business
ventures including, but not limited to, market conditions,
successful
product development and acceptance, competition and overall
economic
conditions, as well as the risk of adverse regulatory actions.
The
company's actual results may differ materially from current
expectations. Readers are cautioned not to put undue reliance on
forward-looking statements. The company disclaims any intent or
obligation to update publicly these forward-looking statements,
whether as a result of new information, future events or for any
other reason.
Note: News releases and other information on SpaceDev can be
accessed
at http://www.SpaceDev.com
or http://www.ctaonline.com/spdv
on the
Internet.
SpaceDev -
NEAP (Near Earth Asteroid Prospector)
-o- Commercial Space Exploration & Development of Space
Resources -o-
http://www.spacedev.com
-o- Info@SpaceDev.Com
==================
(4) ASTEROID HUNTERS TARGET NEREUS
New plan calls for private firm to send probe in 2002
By Alan Boyle
From MSNBC Space News
http://www.msnbc.com/news/190529.asp
Aug. 25 — Drawing on advice from a Mars Pathfinder veteran,
a San
Diego company has shifted its focus for a privately financed
mission
to an asteroid. SpaceDev’s new target is called Nereus, and
the new
time frame for the encounter would be 2002. The company’s
founder
also hinted that the new trajectory would allow for lunar
observations.
IM BENSON, president and chief executive officer of SpaceDev,
couched
his statements cautiously Tuesday because of questions raised by
the Securities and Exchange Commission about some of SpaceDev’s
past
statements.
But he voiced enthusiasm about the revised mission plan. “From
an
energy point of view and a science point of view, it’s a
pretty
exciting mission,” Benson said.
SpaceDev first announced its intent to launch a Near Earth
Asteroid
Prospector almost a year ago, with the idea of gaining revenue
from
the sale of slots for scientific packages as well as data. Benson
said three teams of researchers are seeking NASA funding for
experiments that would fly on the probe for a price.
The initial plan called for the NEAP probe to be launched
sometime
during 2000 toward one of several asteroids, but a study
conducted
for SpaceDev by Mars Pathfinder project manager Tony Spears
concluded
that Nereus was a much more attractive target, Benson said.
Benson cited “the potential of finding water and carbon
compounds on
the asteroid, and the unique chance to compare on-site
measurements
with ground-based measurements from Nereus’ close approach
to Earth.”
Nereus, which is less than a mile in diameter, is due to pass
within
about 2.5 million miles of Earth in January 2002. SpaceDev’s
new plan
calls for NEAP to be launched on April 3, 2001, remain in the
Earth-moon system until January 2002, then rendezvous with Nereus
in
April 2002. Benson said the new trajectory could allow for “lunar
flybys,” perhaps enabling researchers to learn more about
the moon’s
water reserves.
DATA AND SPACE FOR SALE
SpaceDev says it intends to gather data using a multiband CCD
camera,
a neutron spectrometer and perhaps an alpha X-ray spectrometer
aboard
the probe — then sell that data to researchers.
The company is also offering space aboard NEAP for additional
instruments. Benson said Carnegie Mellon University and NASA’s
Jet
Propulsion Laboratory have proposed sending a shoebox-sized
“nanorover” down to the asteroid’s surface, Utah
State University
researchers are seeking to place a laser altimeter and 3-D mapper
aboard the probe, and the University of California at Berkeley
has
proposed including a gamma ray burst detector.
NASA is considering such proposals as part of a months-long
review
process. If NASA gives the go-ahead, SpaceDev would get a cut of
the
money provided to the researchers.
Benson said the fixed prices charged by SpaceDev are a third of
the
cost of what NASA is currently paying for similar data from
similar
missions. “We’re bringing home the bacon for a third of
the cost,” he
said.
UNCERTAINTIES REMAIN
There are still a number of uncertainties surrounding the
mission,
including the SEC’s claims that SpaceDev made misleading
statements
about its future earnings and its arrangements with NASA. Benson
has
disputed the agency’s allegations and said the matter would
be
considered during a future hearing before an administrative law
judge.
“Hopefully we’re going to get through this thing pretty
quickly,” he
said. Benson and Spears aren’t the only ones who have
identified
Nereus as an attractive target for study. The asteroid also has
been
mentioned as a potential target for Japan’s MUSES-C probe,
which
would land on an asteroid and bring a sample back to Earth
sometime
in the next decade. The most recently announced mission plan
calls
for NASA’s Jet Propulsion Laboratory to provide a nanorover
for
MUSES-C.
Nereus isn’t the only potential target for MUSES-C: Mission
planners
are also considering sending the Japanese probe to Asteroid 1989
ML.
SpaceDev engineer Terrance Yee indicated that his company was in
contact with MUSES-C planners and that the two missions appeared
to
be complementary. “We’ll proceed with our mission in
any case,” Yee
said.
Copyright 1998, MSNBC
===============
(5) MORE TSUNAMIS AROUND THE CORNER: WHEN IT HITS, CLIMB A TREE
Special to ABCNEWS.com
http://www.abcnews.com/sections/science/DyeHard/dye70.html
It rolled ashore thundering like a jet aircraft taking off, and
people watched in horror as the wall of water erased their homes
and
swept thousands to their deaths.
Some thought they were being punished for their sins as the
tsunami
swept across Papua New Guinea on July 17, leaving only shattered
boards where a village once flourished.
Others thought a giant explosion caused the wave, which towered
more
than 40 feet. Some survivors reported seeing sparks fly from the
water and thought the ocean was on fire.
Costas Synolakis, professor of civil engineering at the
University of
Southern California, led a team of U.S. scientists to Papua New
Guinea to see what they could learn from a tsunami that,
according to
the textbooks, never should have happened.
“It was an eye-opener,” Synolakis says. Synolakis had
led other
post-tsunami expeditions in the past, but this one was different
because the villagers spoke English so the team didn’t have
to rely
on the filtered testimony of government officials. “Now we
could
understand the horror directly,” he says. But when it was
all over,
Synolakis was left with questions of his own.
A Surprise
The earthquake that preceded the tsunami had a magnitude of 7,
which
scientists didn’t think was powerful enough to trigger such
a
devastating wave. Underestimating the threat of tsunamis from
even
moderate earthquakes could mean much of the west coast of North
America is more vulnerable than had been thought.
His team, sponsored by the National Science Foundation, went on a
purely scientific expedition, but the mission soon included
educating
the locals. One top government official pleaded with the
scientists
to help his people understand they were not being punished “for
the
impiety of some people,” as he put it.
So they took time off from the difficult task of reconstructing
the
tsunami and tried to explain to the locals they weren’t to
blame.
Particularly troubling was the question from a 15-year-old
schoolgirl
who asked about a “mountain of water with fire sparkles.”
Many of the
victims had what appeared to be severe burns, suggesting that the
water was on fire.
The tsunami hit about an hour after sundown, and the waters of
the
South Pacific are rich with dinoflagellates and other
bioluminescent
microorganisms that light up when disturbed.
Sparkle Like Fire
“In the darkness, it may well have caused the water to
sparkle like
fire,” he says.
And the burn marks were caused by friction, not fire, as the
victims
were swept through the turbulent waters.
“I found that most people felt more reassured to hear that
this is a
natural event and it will happen again sometime in the future”
rather
than being fed an empty promise that it’ll never happen
again,
Synolakis says.
In a report to the science foundation, team member Emile Okal,
professor of geophysics at Northwestern University, notes that
there
are about 10 magnitude 7 earthquakes every year. Normally they
don’t
produce tsunamis.
“Of the nine large tsunamis that have occurred in the past
six years,
only the New Guinea one resulted from an earthquake as small as
magnitude seven,” Okal says.
The wave hit five to 10 minutes after the quake. “It is
possible the
wave was breaking as it attacked” the narrow spit of land
between the
villages of Arop and Sisano, Okal says.
But the earthquake was on the beach, not offshore, so what could
have
caused such a large displacement of water that produced a
tsunami?
Okal believes the quake triggered a huge landslide offshore.
Kick the Bucket
“A tsunami is like kicking a bucket of water,” he says.
“You can make
ripples at the surface by kicking or deforming the bottom of the
bucket. This is what happens during an earthquake. The ocean
floor is
deformed and the water is kicked around.”
An offshore landslide could do the same thing by “kicking
the bucket”
and changing the contour of the bottom, “which gives rise to
extremely turbulent motions.”
If that scenario is correct—“we cannot explain it
otherwise,” Okal
says—then other areas of the world are more vulnerable than
had been
thought. He cites, for example, the west coast of Canada, and
particularly the Fraser River basin. That large river has
deposited a
mountain of sediments along the coast.
“All that sediment just piles up in structures that are
probably
quite unstable,” he says.
A moderate earthquake might be all it would take to cause the
mountain to slump, generating a powerful tsunami.
The same scenario could also play out in Southern California,
according to Synolakis, who lives in the coastal community of
Venice.
A moderate quake could cause an offshore landslide, which could
send
a wall of water crashing into the thousands of people who visit
those
beaches on summer days.
One of the Deadliest
The death toll in Papua New Guinea may never be known precisely,
but
it is expected to approach 3,000, making the tsunami one of the
most
deadly of the century.
The coastal land there is so flat that the victims “had no
place to
run,” Okal says.
Their best hope is to build their homes farther back from the
beach,
but that message never seems to catch on, even in sophisticated
parts
of the world like Southern California.
So what’s left to do?
When the tsunami hits, the scientists advise, run as far from the
beach as possible. And if that won’t do it. climb a tree,
preferably
one with deep roots. But if Papua New Guinea is like just about
any
place else, by the time it happens again, those warnings will
have
long been forgotten.
Copyright 1998, ABCNews
======================
(6) BAD NEWS FOR PLANETARY DEFENSE: SECOND ROCKET IN A MONTH
EXPLODES
AFTER TAKE-OFF
From BBC Online Network
http://news.bbc.co.uk/hi/english/sci/tech/newsid_159000/159299.stm
A powerful new version of Boeing's Delta 3 rocket exploded
shortly
after blasting off from Cape Canaveral on its maiden flight on
Wednesday.
The $225m mission to put in orbit a private Galaxy 10 US
communications
satellite, operated by PanAmSat Corp., ended in a bright flash of
flame
and burning debris just one minute and 20 seconds after the
rocket
lifted off at 2117 (EDT).
The mission had been delayed for two days because of Hurricane
Bonnie,
and Boeing was eager to get the rocket off the launch pad before
Hurricane Danielle threatened a crucial tracking station on the
island
of Antigua in the Caribbean.
The new booster was designed to haul twice as much cargo as its
predecessor, the Delta 2.
The rocket was designed to compete against Europe's Ariane and
the US
Lockheed Martin Atlas rockets for lucrative commercial launch
contracts.
The Galaxy 10 communications satellite was designed to beam
television
channels to cable operators across the United States and the
Caribbean.
Copyright 1998, BBC
==================
(7) THE SPIN RATE OF m-TYPE ASTEROIDS
C.I. Lagerkvist, I. Belskaya, A. Erikson, V. Schevchenko, S.
Mottola,
V. Chiorny, P. Magnusson, A. Nathues, J. Piironen: Physical
studies
of asteroids - XXXIII. The spin rate of M-type asteroids.
ASTRONOMY &
ASTROPHYSICS SUPPLEMENT SERIES, 1998, Vol.131, No.1, pp.55-62
*) ASTRONOMICAL OBSERVATORY, BOX 515, S-75220 UPPSALA, SWEDEN
The results from photometric lightcurve observations of nine
M-type
asteroids are presented. New rotation periods were determined for
6
asteroids: 217 Eudora (12.54 h), 322 Phaeo (17.56 h), 572 Rebekka
(5.65 h), 757 Portland (6.58 h), 857 Glasenappia (8.23 h) and 872
Holda (7.20 h), B - V colour measurements of seventeen previously
unclassified asteroids add seven asteroids to the known M-type
population. An excess of fast rotators among M-type asteroids
compared to asteroids of other taxonomic types is evident. The
six
asteroids with slow spin rates, but hitherto classified as M, are
shown to have classification parameters untypical for the M-type
population. Copyright 1998, Institute for Scientific Information
Inc.
==================
(8) YARKOVSKY EFFECT & ASTEROIDAL MOBILITY
D. Vokrouhlicky: Diurnal Yarkovsky effect as a source of mobility
of
meter-sized asteroidal fragments - I. Linear theory. ASTRONOMY
AND
ASTROPHYSICS, 1998, Vol.335, No.3, pp.1093-1100
CHARLES UNIVERSITY, INST ASTRON, V HOLESOVICKACH 2, CZ-18000
PRAGUE
8, CZECH REPUBLIC
A linear theory for the heat conduction in a spherical, solid and
rotating body illuminated by solar radiation is developed in
detail.
The principal aim is to compute the recoil force, due to
thermally
reemitted radiation, which is commonly known as the ''Yarkovsky
force''. We concentrate on the thermal effect which depends on
the
rotational period of a body rather than on the period of
revolution
around the Sun and deal with the general case of an arbitrary
obliquity of the spin axis to the orbital plane. This ''diurnal''
thermal effect is considered to be an important source of
mobility
for meter-sized stony asteroidal fragments in the main belt. We
compare our results with those of previous authors and show that
the
results of Peterson (1976) are accurate for meter-sized
asteroidal
bodies (although he used unrealistically long rotation periods).
Copyright 1998, Institute for Scientific Information Inc.
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