PLEASE NOTE:
*
CCNet DIGEST, 21 December 1998
------------------------------
(1) ASTEROID 1998 XB AND ITS IMPLICATIONS FOR IMPACT PROBABILITY
ESTIMATES
Andrea Milani Comparetti <milani@copernico.dm.unipi.it>
(2) 1998 XB NOT SUCH A SURPRISE AFTER ALL
Alan W. Harris <awharris@lithos.jpl.nasa.gov>
(3) THE NEAR BRIEFING (AND A SHORT CHAT WITH DON YEOMANS)
E.P. Grondine <epgrondine@hotmail.com>
(4) DEFINING THE EFFECTS OF SUB-CRITICAL COSMIC IMPACTS ON THE
EARTH
A few thoughts from Jay Tate <fr77@dial.pipex.com>
of
Spaceguard UK.
(5) PLATINUM-GROUP ELEMENTS AND COMETS.
Jeremy B. Tatum <UNIVERSE@UVVM.UVic.CA>
(6) NEAR'S FIRST RENDEZVOUS BURN
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
(7) NASA NEWS SERVICE
Louis Friedman <tps.ldf@mars.planetary.org>
(8) SEARCHING FOR EXTRATERRESTRIAL LASER SHOWS
Andrew Yee <ayee@nova.astro.utoronto.ca>
========================
(1) ASTEROID 1998 XB AND ITS IMPLICATIONS FOR IMPACT PROBABILITY
ESTIMATES
From Andrea Milani Comparetti <milani@copernico.dm.unipi.it>
Pisa, December 19, 1998.
Dear Benny,
I would like to contribute to the discussion on the implications
of the discovery of 1998 XB, started on CCNet DIGEST, 15/12/98 by
David Morrison, including remarks by Alan Harris, Ted Bowell and
David Rabinowitz.
The fact that such a large Near Earth Asteroid has been
discovered
is indeed remarkable, and can suggest a change to population
models of the NEO. How this changes the estimated probability of
collisions with Earth is, however, less obvious, and depends upon
the time span considered.. If you allow me, I would like to use
this remarkable example to illustrate the complexity of these
`risk estimate' arguments.
We (myself and my coworkers S. Chesley, G.F. Gronchi and A.
Rossi)
have redetermined the orbit of 1998 XB and computed its long term
evolution. The orbit was fitted to all observations, including
the
very recent ones (taken until December 17), by using the same
free
software advertised on CCNet DIGEST, 16/12/98, and available at
ftp://copernico.dm.unipi.it/pub/orbfit.
We found that a solution
with semimajor axis close to 1 AU is not any more compatible with
the observations: the best value is now 0.907 AU. (The new best
fit solution is enclosed.) Thus the discussions about the
observing conditions of this asteroid (both in the future and in
the past) have to be revised.
We have propagated this best fit orbit for the time span between
3,000 BC and 3,000 AD (that is, for the time span of the JPL
ephemerides DE406) and found that it does not undergo close
approaches to Earth until after the year 2,800, and very shallow
approaches (0.095 AU) even then. To the contrary, our sample
integration contains 76 close approaches to Venus (0.087 to 0.1
AU), beginning around 400 AD and until the end of our
propagation.
Of course the orbit, as it is known from the short observed arc
we
have now, cannot be used to predict in a deterministic way
specific close approaches, but the frequency and depth of the
close approaches have a meaning. The question is, why there are
no
close approaches to Earth for a time span of at least 5,800
years?
The answer is simply that the nodes of this orbit, the points
where the ellipse intersects the plane of the orbit of the Earth,
are now at 0.601 and at 1.178 Astronomical Units (AU) from the
Sun. Thus approaches to Venus, orbiting the Sun at a distance of
approximately 0.7 AU, are now possible within 0.1 AU, although
very close approaches are excluded; close approaches to Earth at
about 1 AU are not currently possible.
From this simple geometrical argument, as well as from the
numerical integration, it is possible to conclude that the
contribution of 1998 XB to the risk of impact on our planet in
the
near future (a few hundred years) is zero. It is important to
stress this, since an impact by an object of the size of 1998 XB
belongs certainly to the civilisation threatening class, and
probably also to the exctinction threatening class. It is our
responsibility as scientists to keep the public aware of the
dangers, but certainly we should not raise unjustified
alarm.
This discovery might imply that some previous estimates of the
completeness of our searches were optimistic, but in itself the
discovery of such an object, in an orbit which can not result in
an impact in the near future, does in fact decrease the overall
risk of impact over the next few centuries.
I do not know if you are keen about adding multimedia
capabilities
to your mailing list [not yet, BJP]; in case you are, I am
enclosing a figure (gif encoding) showing the future evolution of
these nodal points. Alternatively, your readers can access this
figure at http://copernico.dm.unipi.it/neo/XB98.gif
.
The figure shows the evolution of the nodal points of 1998 XB
on
the (mean) ecliptic plane; crosses indicate the present
positions.
The continous lines are from a numerical integration over 6,000
years; the dotted lines are from the averaged integration, and
show the approximate future evolution for 25,000 more years.
To extend our study of the evolution of the orbit of 1998 XB
beyond 3,000 AD we have used, rather than a full numerical
integration, an averaged (semianalytic) computation. To give an
idea of the power of this method, note that we have used a
stepsize of 1,000 years, somewhat reduced only near the node
crossing singularities. The long term orbits obtained in this way
are only approximate; on the other hand a full numerical
integration would give an illusory precision, since the initial
conditions are still poorly known. The theory is in 3 papers by
Gronchi and myself now in press; preprints can be obtained at
http://copernico.dm.unipi.it/milani/preprints/preprint.html.
This averaged integration shows that 1998 XB will eventually
undergo a node crossing with the Earth, and thus very close
approaches, even impacts, will become possible, but this will
happen only somewhere between 9,900 and 10,000 AD. If the
asteroid
escapes from collision, and also from approaches so close that
the
orbit would be completely changed, it will undergo a node
crossing
with Venus about 2,800 years later. Note that the node crossing
point is now approaching the Earth from the night side, while the
other node crossing point is approaching Venus from the day side.
This has also some implications on the discussion, reported by
Morrison (also by Boattini and Carusi on CCNet DIGEST, 16/12/98)
on the strategies for detecting all the dangerous asteroids, in
particular Atens.
The conclusion is that 1998 XB indeed contributes significantly
to
the collision rate with the Earth for asteroids of its size
class,
if this rate is averaged over a long time span (e.g. longer than
10,000 years), while it does not contribute at all to the impact
risk for the near future.
This asteroid has also a significant probability of impact on
Venus, again only in the remote future. Because of our
chauvinistic geocentric attitude, we actually hope that it hits
Venus, thus never coming back to a node crossing with our planet.
On the other hand, why do we care about asteroids hitting the
Earth 8,000 years from now? By that time, asteroid deflection
will
be a petty solar system maintenance operation (unless another one
has hit much earlier).
Yours Andrea Milani
Dipartimento di Matematica
Via Buonarroti 2
56127 PISA ITALY
tel. +39-50-844254 fax +39-50-844224
E-mail: milani@dm.unipi.it
WWW: http://virmap.unipi.it/~milani/homemilani.html
1998XB
Keplerian elements: a, e, i, long. node, arg. peric., mean
anomaly
KEP 0.906337
0.351125
13.524
75.688 202.817
247.511
MJD 51200.0000 TDT
MAG 14.200 0.150
RMS 1.336E-03 8.591E-05
6.676E-02 2.056E-01 2.320E-01
1.723E-01
COV
1.78531712E-06
6.21949246E-08
8.91546110E-05
COV
2.73852251E-04
-3.09785443E-04
2.30061620E-04
COV
7.38050507E-09
3.25733117E-06
1.06906372E-05
COV
-1.14565324E-05
8.40898215E-06
4.45661998E-03
COV
1.37089373E-02
-1.54893703E-02
1.15003193E-02
COV
4.22612049E-02
-4.76654254E-02
3.53764033E-02
COV
5.38386605E-02
-3.99704581E-02
2.96766124E-02
COR
1.0000
0.5418
0.9995
COR
0.9970
-0.9992
0.9995
COR
1.0000
0.5680
0.6053
COR
-0.5747
0.5682
1.0000
COR
0.9989
-1.0000
1.0000
COR
1.0000
-0.9993
0.9989
COR
1.0000
-1.0000
1.0000
==========================
(2) 1998 XB NOT SUCH A SURPRISE AFTER ALL
From Alan W. Harris <awharris@lithos.jpl.nasa.gov>
Dear Benny,
It now appears, based in part on the revised orbit (cf. Milani's
message) that 1998 XB is not really so large after all. The
current
best estimate of its absolute magnitude H is 16.0 +/-0.5.
This
corresponds to a diameter of around 2.5 km, and makes is one of
the
larger NEAs, but by no means largest, discovered in the last
couple
years. At 16.0, it is unremarkable. Using the latest orbit,
I computed
an ephemeris for it a year ago, when it should have been even
more
favorably placed for discovery. It was in as sense, mag. 13.8 and
only
10 degrees or so from the opposition point. But it was dead
center in
the milky way and the moon interferred by the time it got out of
the
milky way. Before that, no good apparitions for about 10
years. So it
is in fact not even remarkable that it wasn't discovered before.
Alan Harris
===================
(3) THE NEAR BRIEFING (AND A SHORT CHAT WITH DON YEOMANS)
From E.P. Grondine <epgrondine@hotmail.com>
Benny -
This last Wednesday a briefing on the NEAR probe was held at NASA
Headquarters, and afterwards I had an opportunity to speak with
Don
Yeomans on how things are going at the new NASA NEO Office. Here
goes...
The briefing started with an introduction by Dr. Carl Pilcher,
who set
forth NASA's reasons for funding NEAR, in what he identified as
their order of importance. First in Dr. Pilcher's list came
the
probe's value to NASA's Origins program, which is that it will
throw
light on the fundamental building blocks of the Solar System,
including
the organic chemicals from which life formed. This is in keeping
with
the current NASA emphasis on the search for life, including life
on
Mars. Second in Dr. Pilcher's list came the value of NEAR for
planetary
defense, and Dr. Pilcher placed a limit on the threat to
asteroids 1 km
in diameter. This limit probably reflects current NASA
emphasis in
this area, with Sub-Critical Impactors ignored, at least for the
time
being. Third came the possible future resource use of asteroids
in
manned space development.
Dr. Pilcher noted that the results from Mathilde had come as a
surprise, in that the large crater on Mathilde showed that some
asteroids could absorb the force of a massive collision without
shattering. He emphasized that it was clear from this that
a better
understanding of the physical properties of the asteroids would
be
necessary if we ever had to stop one of them.
Bob Farquhar of John Hopkins APL spoke next on the actual
mechanics of
scheduled rocket firings for the Eros rendezvous. The
science
experiments are timed with these burns so as to maximize the
science
return for the probe at any time in the event of failure, so more
on
these later.
Andy Cheng of the APL, the mission's primary scientist, spoke
next on
the basic classes of asteroids, which he grouped in laymen's
terms into
differentiated and undifferentiated. The undifferentiated
asteroids he
described as the basic building blocks, while the differentiated
asteroids he described as the remains of planetisimals which
formed,
differentiated, and then broke apart under collision. Cheng went
on to
discuss how it was hoped that the spectrometry results from NEAR
would
allow S type asteroids to confirmed as the source for common
chondrite
meteorites.
Cheng also discussed the low density of asteroids, using Phobos
as
an example this time instead of Mathilde, and compared Phobos'
collisional history with Mathilde's.
Don Yeomans spoke next emphasizing the necessity of getting good
density data for Eros. The first rocket burn will put NEAR
in range of
Eros' gravity, and the first images and radar data will be
available at
that time, so an initial density calculation should be possible.
In
laymen's terms Don divided the asteroids into monolithic and
rubble
pile, and it is hoped NEAR will provide better information to
refine
models of interior voids.
Don emphasized the problems that are likely to be encountered
trying to
put NEAR into orbit around Eros. Since Eros is likely to be
non-spherical in shape, it will not be possible to leave NEAR in
one
fixed orbit around Eros, but instead it will be necessary to
modify
NEAR's orbit every week to week and an half to prevent it from
colliding with Eros. In turn, orbital data will then be used to
refine
density models for Eros.
Joe Veverka of Cornell spoke next on the camera and spectrometer.
The
first images of Eros against its star field are currently being
used
for navigation, as well as to map Eros' rotation . As NEAR
approaches
Eros, additional images will be used for navigation and initial
science. The researchers are concerned about companions and the
threat
they may present for NEAR. Additional images at a
resolution of 250 m
per pixel will be taken by NEAR before it orbits Eros, and these
will
be assembled into a movie and released sometime around 14
January.
The team hopes to place NEAR into orbit around Eros by January
10, with
an initial orbit planned for 1,000 kilometers from Eros's center.
The
next orbit is planned for 200 kilometers, with camera data in 32
meter
pixels and spectrometry data in 1 kilometer pixels. It is hoped
that
the differing orbits will allow spectrometry data to gathered on
the
same scale as was obtained for camera data on earlier orbits.
Thus
NEAR's orbit will then be lowered to 35 kilometers (15 to 20
kilometers
from Eros' surface), with camera data in 2.5 - 4.5 meter pixels,
and a
last orbit planned for 100 meters above Eros' surface, with
spectrometry
data in 2.5 - 4.5 meter pixels.
Finally, assuming that NEAR survives, and that sufficient fuel
remains
onboard, (and NEAR has a factor of 3 times the minimum fuel
estimated
for nominal operations), hovering operations and/or a possible
soft
landing are contemplated.
Lidar data and infrared or gamma ray spectrometry data will be
taken in
all orbits. All of these orbits are subject to change
depending on how
irregular Eros actually turns out to be. Deep Space Network
communication time is currently not scheduled beyond February,
2000.
During questioning, Don spoke on the usefulness of NEAR
rendezvous
operations for possible future asteroid interception. In
this regard
it should be noted that NEAR's instrument mass is 56 kilograms,
and
fuel mass is 325 kilograms. 56 kilograms is barely the working
mass of
a back pack nuclear weapon, with a charge of say 10 kilotons; and
even
then part of NEAR's instrument suite is used to aid navigation.
The
fuel safety factor of 3 might leave say 200 kilograms available
for a
nuclear charge; enough for a boosted fission or fusion charge of
125
kilotons or so, but I don't believe enough for a really large
fusion
charge. (N.B.-I'm sure others on the list have more accurate
numbers for
the masses of nuclear charges.)
NEAR was launched aboard a Delta 2 in February 1996, with a
travel time
to Mathilde of 4 months, and a travel time to Eros of 22 months.
A CHAT WITH DON
After the briefing, I spoke with Don Yeomans about the new NASA
NEO
Office. The initial plans are working their way through the
NASA
planning process, and this review is being done in co-ordination
with
the Air Force. Don has some good news for us, that there will be
additional funding for the MPC, and he hopes that he will have
some more
good news for the Conference members shortly.
Well Benny, that's it for now. Until next time...
Best wishes -
Ed
=================
(4) DEFINING THE EFFECTS OF SUB-CRITICAL COSMIC IMPACTS ON THE
EARTH
A few thoughts from Jay Tate < fr77@dial.pipex.com > of
Spaceguard UK.
On 11 December 1998 a meeting entitled "Defining The Effects
Of
Sub-Critical Cosmic Impacts On The Earth", jointly sponsored
by the
Royal Astronomical Society, the British Interplanetary Society
and the
Geological Society was held at the London headquarters of the
Geological Society. The event was expertly organised by
Richard L. S.
Taylor, Julian A. Hiscox and David Hughes.
The definition of a Sub-Critical Impactor (SCI) is based on the
work of
Morrison et al., who determined that the impactor diameter
threshold
for a globally threatening impact was 1.5 to 2 kilometres. In the
light of the impact of Comet Shoemaker-levy 9 this estimate was
revised downwards to 1 kilometre. An SCI therefore is an object
that
impacts with the surface of the Earth, or has an effect on that
surface, that has a diameter of less than 1 kilometre.
Benny has recently published the abstracts of the papers
presented at
the meeting, so I don't intend to plough the same furrow, bit I
think
that it is worth examining some overall impressions of the
discussion, and to consider the overall significance of the
event.
The first thing to note is that the meeting happened at
all. As
recently as three years ago it is unlikely that anyone would have
considered the subject of "sub-critical impacts" worthy
of a dedicated
conference! Events over the past few years have brought the
theme of
impact studies to the attention of the scientific community and
the
general public, and the interest generated is spreading.
The second important aspect of this gathering was the
multi-disciplinary nature of the speakers and audience. The
threat of
asteroidal and cometary impact is often considered to be in the
province of astronomy, but it is becoming increasingly clear that
there
are multi-disciplinary ramifications. Indeed, it was geological
and
palaeontological data that led to the linking of the Chicxulub
feature
with the K-T boundary event. The multi-disciplinary nature of
impact
studies was emphasised at the Spaceguard 2 conference at the
Royal
Greenwich Observatory in July 1997, and it was most encouraging
to see
that the message is spreading through the scientific community.
The third overwhelming impression was that the fact that cosmic
impacts
play and have played a significant part in the geological and
biological evolution of our environment now appears to be widely
accepted. The only controversy seems to be the extent to which
these
events have had an effect when stacked up against other natural
processes such as volcanism and continental drift. This paradigm
shift
is fertile ground for debate and research, with a frisson of
urgency
given the risk of future catastrophic events.
Although there were a number of areas where interpretations of
data
were matters of debate, the only major area of disagreement to
emerge
during the meeting's deliberations concerned the rate at which
impacts
(in this case, sub-critical impacts) occur on the Earth. I
presented
the "party line", developed by individuals such as
Shoemaker, Muinonen,
Rabinovitz, Steel and Bailey (amongst many others). Dr. David
Hughes
presented a view that would involve a substantial reduction in
the
impact flux, and supported his contention with expertly derived
data.
In discussions after the presentations it became clear that there
is a
distinct need for better empirical data to reduce the
uncertainties in
the estimates presented by both sides of the argument. The
available
data sets are very limited, and are thus open to different
interpretations. Only when reliable population figures are
available
and the dynamical properties of Earth threatening objects are
better
understood will the estimated impact rates derived by different
methods
converge towards the true figure.
The following two speakers addressed geological evidence of past
impacts, but I fear that much of what they had to say was quite
beyond
me! I have no doubt that details of their papers will be
available from
the Geological Society in due course, but the bottom line
appeared to
be that there is considerable work to be done in the field, and
once
again the multi-disciplinary nature of the required research was
emphasised. It was interesting to note that one of the suggested
reasons that impact evidence has not been found before was that
"existing concepts in geology do not permit reconstruction
of these
abrupt events".
Professor Chandra Wickramasinge described how the stable ice ages
experienced by the Earth in the past could have been ended by the
injection of water into the atmosphere by cometary impacts - a
fascinating perspective given the usual preoccupation with
"cosmic
winter" effects after a significant impact.
Dr Norman MacLeod warned the conference against developing a new,
possibly flawed paradigm concerning mass extinctions. He
clearly
demonstrated that the "big five" mass extinction events
that we are
aware of need not have been caused by single events (such as
large
impacts or volcanism), but were probably the result of a
combination of
circumstances. He again stressed the need for more research
into the
subject on a multi-disciplinary basis.
Julian Hiscox then discussed the bioevolutionary consequences of
sub-critical impacts, describing the probability that the
majority of
the water on Earth was delivered by cometary impacts. He
then
considered the possibility that the range of organic compounds
also
present on cometary nuclei could have played a key role in the
genesis
of life. He pointed out that the rise of mammals was as a
direct
consequence of Chicxulub impact that hastened the demise of the
dinosaurs, and that there are bound to be many other examples of
advantage or disadvantage to species resulting from impact
events.
Moving somewhat closer to home, Dr Marie-Agnes Courty presented
new
findings that appear to link the destruction layers in the Near
East around 2300 BC with an extra-terrestrial impact, rather than
volcanic activity as previously thought. She suggested that
"the
ambiguity lies partly in the weak knowledge of the geoscience
community
of minor collisions with Earth" and the inability of
conventional
science to discriminate between instantaneous and extended
events. Her
evidence points to a major event, somewhere in the Middle East at
around 4000 BP that might have caused global environmental
consequences. This ties in nicely with other work pointing to the
collapse of other civilisations world-wide at that time.
Her data was
(at least to the lay person) extremely convincing, but there is
clearly
more work to be done in the field.
Dr Benny Peiser then considered the multi-disciplinary search for
evidence of recent, i.e. Holocene impacts. He pointed out that
such a
search is significant, not just for historical interest, but as
an
indicator of the current impact flux that reflects the present
hazard
to our own civilisation. He called upon current knowledge
of
historical and pre-historical environmental punctuations that
could
be linked to impacts, and made a convincing case for the role of
sub-critical cosmic events in the development of human societies
throughout history. This theme was continued by Dr Victor Clube
who
traced the human concern about comets throughout history, and the
possible (probable?) reasons for that concern and fear.
I have deliberately not gone into the subjects discussed in
detail for
a number of reasons; the full papers will no doubt be published
in the
fullness of time, I am not qualified to comment on technical
points and
time and space do not permit. But, there were a number of
recurring
themes throughout the day. I have mentioned them before, but they
bear
repeating.
There is a clear need for a multi-disciplinary approach to impact
studies.
There is a clear need for research into the impact flux on the
Earth.
There is a clear need for research into the effects of impact
events.
There is a clear need for education, both within and without the
scientific
community, on the significance of impact studies.
I believe that the meeting held on 11th December could be the
beginning
of a new era in Earth studies, where researchers of many
disciplines
co-operate to understand one of the basic formative processes of
our
planet, and one of the greatest hazards that face our
civilisation.
The meeting was a first faltering step, but it would be
criminally
irresponsible to let this opportunity slip through our fingers.
Jay Tate
====================
(5) PLATINUM-GROUP ELEMENTS AND COMETS.
From Jeremy B. Tatum < UNIVERSE@UVVM.UVic.CA
>
Since it now seems to be universally accepted that the presence
of
iridium or other platinum-group elements in geological samples is
incontrovertible evidence of cometary impact, can someone please
post
on the peisergrams a list of the comets in which these elements
have
been detected and how their cometary abundance was determined?
====================
(6) NEAR'S FIRST RENDEZVOUS BURN
From Ron Baalke < BAALKE@kelvin.jpl.nasa.gov
>
http://near.jhuapl.edu/
On Dec. 20, at 5 p.m. EST, NEAR's large bipropellant engine will
be
turned on for the first of a possible four engine burns that will
put
the spacecraft at optimum speed and location for its Jan. 10
rendezvous with asteroid Eros. The burn will be executed from the
NEAR Mission Control Center and will last 20 minutes, expending
more
than half of NEAR's onboard fuel.
Presently, NEAR and Eros are traveling in approximately the same
direction. Like a vehicle on an onramp trying to merge onto the
interstate, NEAR is speeding up to merge with Eros, which is
coming
from behind at a speed of 2,180 mph (974 meters per second),
relative
to the spacecraft.
The next burn will occur on Dec. 28.
======================
(7) NASA NEWS SERVICE (CCNet DIGEST 17/12/98)
From Louis Friedman < tps.ldf@mars.planetary.org
>
Did the NASA press release really say "Mars Observer"
spacecraft?!
What an embarassing error for them.
Louis Friedman
===========================
(8) SEARCHING FOR EXTRATERRESTRIAL LASER SHOWS
From Andrew Yee < ayee@nova.astro.utoronto.ca
>
Harvard University
3 December 1998
Researchers Searching for Light from E.T.
By Maria Cristina Caballero and John Lenger, Harvard Gazette
If E.T. won't call, maybe he'll shine a light on us instead.
That's the hope of Harvard researchers involved in the search for
extraterrestrial intelligence (SETI) who have unveiled a new
experiment
that involves scanning the heavens for flashes of laser light.
Professor of Physics Paul Horowitz's laboratory recently
installed the
experiment at the Harvard-Smithsonian Oak Ridge Observatory in
Harvard,
Mass. The optical SETI (or OSETI) experiment has occasionally
registered a signal similar to what one would expect if another
civilization's laser were aimed at us. None of those signals has
shown
the regular repetitions that could indicate an intelligent hand
behind
them; but then, the researchers have just begun looking. Though
such
searches for laser lights from beyond our solar system have been
done
before, in an isolated and sporadic way, the Harvard experiment
is the
first broad-based and systematic search.
Horowitz has had his ears to the skies for interstellar radio
messages
for the past 20 years. He directs Harvard's BETA project, which
for the
past three years has searched 600 million channels for radio
signals
broadcast by an intelligent civilization. Before BETA there was
META,
an 8.4 million-channel searching device that went on-line in 1985
and
was supported in part by funds from E.T. director Steven
Spielberg.
While BETA continues its radio-wave search in full force,
collecting
the equivalent of a compact disc's worth of data every two
seconds,
none of the radio signals collected has yet been shown to be of
intelligent origin. "After 20 years, maybe it's time to try
something
else," Horowitz says.
The idea of analyzing light flashes from distant parts of the
galaxy is
not a new one, Horowitz explains. Charles Townes, who shared the
Nobel
Prize in Physics in 1964 for his work on masers and lasers, first
raised the idea in a paper co-authored with R.N. Schwartz,
"Interstellar and Interplanetary Communication by Optical
Masers," that
was published in the journal Nature in April 1961. But technology
developed just within the past five years finally made it a
viable
project. "This is very much an experiment of the '90s,"
Horowitz says.
The beauty of the new experiment, Horowitz explains, is that
flashes of
concentrated light are easy to detect and show up as distinct
from
other sources of illumination. If, for instance, Earthlings aim a
high-intensity pulsed laser at a distant star, anyone watching
from
that star with a moderate-sized telescope will suddenly see a
flash
1,000 times brighter than the light of our Sun -- "an
efficient
interstellar beacon."
And since the brightness of starlight and laser light both
decrease at
the same rate, that particular high-intensity laser beam shot
from
Earth would always be 1,000 times brighter than the light from
our Sun,
no matter how far it travels.
Reversing the direction, any extraterrestrial flash pointed the
way of
Earth would be easily distinguishable from the light of a distant
star.
That could make pulsed light the preferred method for
communicating
across galactic distances instead of radio waves. If you remember
childhood games involving walkie-talkies and messages
communicated by
flashlight beams, you'll recall that flashlight beams were much
more
reliable than static-filled walkie-talkie transmissions, if not
as
dramatic.
Horowitz, who has long been an optimist regarding the idea of
extraterrestrial civilizations, cautions, however, that,
"Maybe they're
using 'zeta rays' to communicate, and the problem is we haven't
discovered zeta rays yet."
Still, the elegance and simplicity of the new laser-detection
experiment is appealing. BETA took four years to build (at a cost
of
hundreds of thousands of dollars), involves enough high-end
computer
equipment to fill a large truck, and uses an 84-foot radio
telescope.
The OSETI equipment, funded by the Planetary Society, the SETI
Institute, and the Bosack-Kruger Charitable Foundation, was put
together in three months by Horowitz and fellow researchers
Jonathan
Wolff, Chip Coldwell, and Costas Papaliolios at a cost of less
than
$10,000.
"It fit in the back seat of my Corolla," Horowitz says,
describing the
monitoring device as being as big as a box "for a large loaf
of bread."
It uses leftover light from a 61-inch telescope that already was
engaged in a survey of 2,500 nearby solar-type stars, an
experiment run
by researchers Joe Caruso, David Latham, Robert Stefanik, and Joe
Zajac.
The simplicity of the new OSETI equipment means the experiment
could
easily be duplicated elsewhere. Researchers at the University of
California at Berkeley, who have done some preliminary looking,
are
setting up an optical SETI experiment along the same lines, and
their
experiment will be operational soon.
A more detailed description of Harvard's optical SETI experiment
is
on-line at http://mc.harvard.edu/hgroup.html, and contains a call
for
the SETI research community "to consider alternative OSETI
strategies
-- choice of wavelength, pulse widths and repetition rates,
revisit
times, etc. -- in an attempt to identify a particularly
compelling a
priori strategy, involving both sender and receiver, that could
be the
basis for major Earth-based OSETI receiving efforts in the near
term."
Optical SETI is an added tool for searching the heavens that has
emerged just as the more traditional searches of radio
frequencies are
getting tougher. Darren Leigh, a recent Harvard Ph.D. in applied
physics who oversees the BETA project, says that cellular phones
in
particular have made it harder to hear signals from outer space.
As
satellite transmissions increase, Leigh says, we are confronted
by the
possibility that our interest in talking with each other might
mean
less chance of hearing a call from extraterrestrials.
Horowitz is optimistic about the optical SETI project, but 20
years of
waiting have made him cautious. "I'll be excited when we get
results,"
he says.
PHOTO CAPTIONS:
[ http://www.news.harvard.edu/science/current_stories/3.Dec.98/seti.3.dec.98.html
]
[Image 1]
Part of the experiment's crew poses with the telescope being used
for
the SETI experiment. From left to right are researchers
Costas
Papaliolios, Chip Coldwell, Paul Horowitz, and Jonathan
Wolff. A more
detailed description of Harvard's optical SETI experiment is
online
[ http://mc.harvard.edu/hgroup.html
].
[Image 2]
The OSETI equipment fit in Horowitz's Corolla; Jonathan Wolff
(above)
built most of the device.
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