CCNet 101/2000 - 6 October 2000


     big boulders resting on a curving plain.
     nothing unusual in that - we've seen it on the Moon and Mars
     - but wait a minute, something's wrong!
     what holds those rocks in place?  gravity?
     sure, but only if they formed right where we see them now!
     If they had fallen, or been blasted from some impact crater
     they'd be gone to orbit by themselves around the Sun.

     so here's a paradox for us to solve.
     In several billion years, how many fragments, boulder-sized,
     encounter Eros and are captured by its feeble pull?
     If such a rock did land, would it have no transverse motion
     to score long grooves across the surface? Could it touch down
     lightly as a feather here on Earth and cheekily
     defy us for an explanation?

     Malcolm Miller

     "ANY day now, a gargantuan wave could sweep westwards across the
Atlantic towards the coast of North America. A mighty wall of water 50 metres
high would hit the Caribbean islands, Florida and the rest of the eastern
seaboard, surging up to 20 kilometres inland and engulfing everything in its
        -- New Scientist, 5 October 2000

     "I told [BBC] Horizon that a La Palama landslide would not be a
     threat to the U.S. -- that did not fit into their script!"
     -- Tsunami expert Charles Mader, 4 October 2000

    NATURE NEWS SERVICE, 5 October 2000

    BBC Online News, 4 October 2000

    Ron Baalke <>

    Andrew Yee <>

(5) ASTEROID 2000 SM10
    Francesco Manca <>

    Richard Godwin <>

    Andrew Yee <>

    Michael Paine <>

    Brian G. Marsden <>

     David Morrison <>

     Larry Robinson <>

     John Nuckols <>

From NATURE NEWS SERVICE, 5 October 2000


Meteorites are rare enough to change hands for impressive sums.
But it has been a long-standing puzzle that the rain of meteorites
onto the Earth is greater than that predicted by simple models of
how they get here. Now researchers offer a solution: the path of a
chunk of extraterrestrial rock can be long, slow and subtle before
it falls to Earth.

Most meteorites are lumps of asteroids. Stretching in a broad belt
between the orbits of Mars and Jupiter, asteroids are the unclaimed
detritus of the Solar System: huge rocks that never became
incorporated into planets when the Solar System formed.

Asteroids mostly tread their repetitive paths around the Sun in lonely
isolation. Occasionally these paths will cross, and two asteroids will
collide and break up. Some of the fragments find their way into the
inner Solar System, and get pulled by gravity towards the Earth and the
other inner planets.

Even when asteroids are shattered by collisions, there is no reason why
the pieces should not continue to circulate in orbits within the
asteroid belt. Unless, that is, they happen to pass into either of two
narrow zones called resonance orbits, where they can acquire unstable

Once trapped in a resonance, a rock can be gently nudged by the
gravitational fields of the giant planets Jupiter and Saturn so as to
take up a new, eccentric orbit that could carry it close to Mars or Earth.
The resonance orbits are thus a kind of escape zone, and the material
within them is ejected relatively quickly and efficiently.

Yet most asteroids are a long way from these zones -- there are simply
not enough nearby to explain how so many fragments are ejected towards
Earth. Two years ago, Paolo Farinella of the University of Trieste in
Italy and co-workers suggested that a long-known but little-regarded
effect might alter the trajectories of asteroid fragments.

A Russian engineer named I. O. Yarkovsky first described the effect
a century ago. He realised that if a body moving in space were to emit
heat asymmetrically, its motion would be altered.

In the 1970s, Charles Peterson suggested that this 'Yarkovsky effect'
could operate on chunks of asteroid, which would be warmer on the sunward
side. This would create a force that deflects the rock very slightly from
its initial course. Over time, the Yarkovsky effect would cause the bodies
to drift.

Now Farinella and David Vokrouhlický of Charles University in Prague have
carried out computer simulations of the behaviour of millions of collision
fragments influenced by the Yarkovsky effect. In particular, they considered
what might happen to the pieces of three existing asteroids -- 6 Hebe, Flora
and Vesta -- if they were to be smashed by collisions.

6 Hebe is close to an orbital resonance, but the other two are not. Yet the
researchers found that, given long enough (a billion years or so, less than
a quarter of the Solar System's age), between 40 and 90 per cent of the
pieces got to the two resonances in all cases.

Only a small fraction of the ejected material then finds its way to Earth --
1.18 per cent from one resonance, 0.23 per cent from the other. But this is
enough to make up much of the shortfall that earlier calculations, which
neglected the Yarkovsky effect, incurred.

Farinella would surely have delighted to see his earlier hunch vindicated
in Nature(1). He died in March from a heart problem.


Vokrouhlický, D. & Farinella, P. Efficient delivery of meteorites to the
Earth from a wide range of asteroid parent bodies. Nature 407, 606-608

© Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE

From BBC News Online, 4 October 2000

By BBC News Online science editor Dr David Whitehouse

The reason why so many space rocks rain down on Earth is due the uneven
heating they experience from the Sun when they are still millions of
kilometres away from our planet.

Almost all meteorites are chipped off asteroids during collisions, but the
way this debris manages to reach us has always been a puzzle.
The answer, according to David Vokrouhlický, of Charles University in
Prague, Czech Republic, and the late Paolo Farinella, of the University of Trieste,
Italy, depends upon a tiny force that acts on the rocks as they move through

The force, caused by Sun heating up just one side of the rocks, steers the
debris into one of two particular orbits where the gravity of Jupiter and
Saturn will then fling it towards the Earth.

Nearly all the asteroids reside in a belt between the orbits of Mars and
Jupiter, about 330 million to 480 million kilometres from the Sun.

It was thought fragments from asteroid collisions only escaped the belt if
they passed into one of two narrow regions called resonance orbits, where
Jupiter and Saturn's gravity could hurl them towards the inner Solar System.

But there is a big problem with this idea. There are not enough asteroids
near the resonances to explain the number of fragments that enter the Earth's
atmosphere. About 1,000 tonnes of rock strikes the Earth each year and ends
up as meteorites on the ground.

Writing in the journal Nature, the researchers suggest an explanation can
be found in a tiny effect first described a century ago by the Russian
engineer Yarkovsky. He said that a rotating body in space that was warmed
by the Sun would emit heat unevenly.

This results in a small force that causes asteroids to drift from their
original orbits, directing more material than expected towards the resonance
ejection point. Most will fall into the Sun, but a surprisingly large 0.5%
will reach the Earth.

Copyright 2000, BBC


From Ron Baalke <>

News Services
University of Arizona
Tucson, Arizona

Contact Information:

Robert McMillan, 520-621-6968,
Tom Gehrels, 520-621-6970,

Oct 4, 2000

New Spacewatch Telescope Detects Its First Asteroids

By Lori Stiles

KITT PEAK, Ariz. -- University of Arizona Spacewatch Project founders
just realized a 20-year dream.

Spacewatch astronomers led by Tom Gehrels and Robert McMillan have used
a 36-inch (0.9-meter) UA telescope on Kitt Peak to electronically scan
the skies for asteroids throughout the solar system since 1984. Before
Spacewatch, astronomers used photographic plates to hunt asteroids.

Spacewatch has been a striking technological and scientific success. But
Gehrels' and McMillan's original hope in 1980 was to use a 72-inch
(1.8-meter) telescope in their electronic asteroid survey.

Two weeks ago, perseverance and hard work paid off. The new 72-inch
Spacewatch telescope captured its first light from an asteroid, asteroid
2000 RD 53, on Sept. 14. The Spacewatch team took first digital data
with the telescope on the same very fast moving near-Earth object on Sept.

Last Thursday, Sept. 28, the Spacewatch team made its most interesting
observations yet. Telescope-drive software tracked the fast-moving
asteroid 2000 SM10 for more than three hours.

Like happy new parents, Spacewatchers provide information, images and
video of the newborn and its accomplishments on the web.

"I think the 1.8-meter will be the biggest telescope in the world
dedicated full time to asteroid discovery and astrometry," McMillan,
Spacewatch director, said. (Astrometry is a branch of astronomy that
measures the positions and movements of celestial bodies.)

Astronomers refer to brightness in terms of "magnitude," with larger
magnitudes corresponding to dimmer objects. The unaided human eye when
dark-adapted under clear, dark sky sees objects at about six-and-half
magnitude brightness. The 36-inch telescope detects objects down to 21.7
magnitude. (That's roughly equivalent to photographic film rated at ASA
one million, McMillan noted.)

The 72-inch will detect objects down to 22.7 magnitude, or
two-and-a-half times fainter than the 36-inch can detect. The bigger
telescope will discover twice as many asteroids as the smaller telescope
now finds, McMillan said.

Plans are to upgrade -- not retire -- the 36-inch telescope. McMillan
said. Now that the 72-inch telescope is coming on line, the 36-inch can
be temporarily shut down late next year so new detectors can be
installed. The new detectors are 10 times larger than the detector that
has been used in the telescope since 1989. "That upgrade alone will boost
our discovery rate by a factor of 6 to 10, depending on how we use it."

"The telescopes will be complementary. The smaller telescope, when
upgraded, will get a much wider field of view, or cover 10 times as much
sky. The 1.8-meter will concentrate on finding the very faint objects,"
McMillan said. Faint targets for the new telescope include the small
Near-Earth Asteroids, some of the bigger and brighter Trans-Neptunian
Objects in the Kuiper Belt, and Near-Earth Asteroids that have
previously flown by Earth as these objects usually appear fainter on
successive swings by the planet, he added.

The 72-inch telescope looks radically different from its white, single-
barreled 36-inch elder sibling.

UA originally acquired the 72-inch, f/2.7 fused silica mirror blank from
the military for an asteroid telescope, but the mirror blank was loaned
to the Multiple-Mirror Telescope on Mount Hopkins, Ariz., until 1993,
Gehrels noted. The mirror is mounted in altitude-azimuth type mount in a
mirror-support cell contributed by the UA/Smithsonian MMT Observatory.

The telescope itself was built at the UA's University Research
Instrumentation Center. Telescope designers used "folded prime focus"
rather than a straight prime-focus for a more compact telescope that
could be housed in a smaller, less expensive dome. The telescope support
structure is painted black to reduce light scattering, prompting
engineers and astronomers to dub it the "Stealth" telescope.

Contributions from foundations, corporations and private individual
donors, and grants from NASA and the U.S. Air Force Office of Scientific
Research paid for the roughly $5 million telescope.

The venerable 36-inch Spacewatch telescope, which was originally sited
on the UA campus in 1921, moved to Kitt Peak in 1962. Among its
distinguished accomplishments:

* First to use CCD scanning routinely in astronomy
* First to use CCDs to survey the sky for comets and asteroids
* First near-Earth asteroid detected with a CCD (1989 UP)
* First astronomical group to develop automated, real-time software for
  moving-object detection
* First to discover a near-Earth asteroid by software (1990 SS)
* First automatic discovery of a comet (C/1992 J 1)
* Detected smallest known asteroid (1993 KA2, about 4 - 9 meters
* Detected closest known approach of an asteroid to the Earth (1994 XM1,
  at 105,000 km)
* Identified two new asteroid populations -- small near-Earth asteroids
  and distant Centaurs (objects in unstable orbits between Jupiter and
* Discovered fastest rotating and most accessible asteroid at time of
  discovery (1998 KY 26)
* Continues to detect 20 to 30 near-Earth asteroids annually
* Smallest telescope in the world for Trans-Neptunian Object
  (Trans-Neptunian Objects, or TNOs, are primordial objects orbiting the
  sun beyond Neptune.)

Related Links

* Spacewatch Project

[ ]

[Image 1]
Spacewatch Project Director Robert McMillan with the 1.8-meter
Spacewatch telescope (PHOTO: Lori Stiles)

[Image 2]
McMillan fills the "dewar" that chills the 36-inch telescope detector
(PHOTO: Lori Stiles)

[Image 3]
The 72-inch Spacewatch dome (left), the 36-inch Spacewatch dome (PHOTO:
Lori Sitles)


From Andrew Yee <>

[Extracted from inScight, Academic Press.]
[ ]

Monday, 2 October 2000, 5 pm PST


More and more supposedly single asteroids are turning out to be pairs
traveling through space together. Only a few weeks ago, astronomers reported the third
and fourth of such twins, orbiting out in the main asteroid belt between Mars
and Jupiter (ScienceNOW, 21 September ). Now planetary scientists have for the
first time found twin asteroids near Earth. Although no immediate threat to our
planet, this is the first pair close enough to have created the double
craters found around the world.

DP107, as the single asteroid was known, was just 7 million kilometers from
Earth -- a 40th the distance to the asteroid belt -- when its true nature
was found. That nature emerged during a long-running survey of near-Earth
objects conducted by Steven Ostro of the Jet Propulsion Laboratory in Pasadena,
California, and his colleagues. By bouncing radar pulses off a near-Earth
object and using the Doppler effect induced by its rotation, they can make a
picture of sorts. Observations using NASA's Goldstone radar in Southern California on
22 and 23 September revealed that DP107 was two separate objects traveling at
least a kilometer apart at times, according to the International Astronomical
Union Circular announcing the discovery.

Theoreticians already have a plausible explanation for binaries like DP107.
In 1996, planetary dynamicists William Bottke of the Southwest Research
Institute in Boulder, Colorado, and Jay Melosh of the University of Arizona in Tucson
proposed that Earth's gravity could split an asteroid in two if it passed
nearby, assuming that numerous collisions with other asteroids had already
reduced it to a flying pile of rubble.

How often does this happen? Astronomer Petr Prevac of Ondrejov Observatory
near Prague recently estimated that about one-sixth of near-Earth objects may
actually be twins. Based on the abundance of double craters pocking the
planets, Bottke and Melosh came to the same estimate for asteroids crossing Earth's
path. That could make it more difficult to send future spacecraft to orbit them,
notes Ostro; and diverting a pair on a collision course with Earth would make
Bruce Willis's job in the movie Armageddon look like child's play.

Related links from the article above:

* Item No. 2, The IAU circular announcing the discovery

Copyright © 2000 by the American Association for the Advancement of Science.

[Extracted from inScight, Academic Press.]

(5) ASTEROID 2000 SM10

From Francesco Manca <>

Dear Dr. Peiser,

small objects  (H > 22.0) like 2000 SM10 are listed in the our SAEL page
available at

In the past years many other asteroids have come closer less
than 0.010 A.U.

Best Regards

Francesco Manca
Sormano Astronomical Observatory (CODE 587)
Localita' Colma del Piano
I-22030 Sormano (Co) - Italy

CORRECTION: In my editorial on 4 October, I stated that "objects have come
closer than 2000 SM10 (miss distance 1.7 km) earlier..." Obviously, this
should read "miss distance 1.7m km"! BJP


From Richard Godwin <>

Dear Benny:

THE WATCH and The Space Frontier Foundation Conference 9 in Los Angeles.

A quick note to advise you that this conference will be commencing on
Thursday Oct 19th at the Marriott Hotel, Manhattan Beach LA.
On Saturday 21st, there will be the Asteroid (NEO) session which is
sponsored by The Watch a project of the Space Frontier Foundation designed
to raise funds for the follow-up work needed to meet the 1km discovery

At the meeting, (which can be researched at
there will be presentations by some of the leaders in the field, From Dr.
Brian Marsden to Dr Eleanor Helin, as well as Jay Tate from the UK and
Andrea Carusi of SpaceGuard.

We hope on the Saturday afternoon to not only be hosting some of the
worlds' foremost authorities on the subject of NEO's but to also have
present some of the people that will, hopefully, help us financially to meet our

We will also be putting together a "Shopping list" of what is needed in
terms of equipment and manpower to reach the goals of both SpaceGuard and

We also hope to agree on a policy statement that will be presented to
the US Congress in March of 2001 asking for extra funds to achieve our

I would invite anybody who is interested in the field of NEO's, to the
conference, where you will meet some of the most committed space
pioneers in the world.

Look forward to meeting anybody who can make it to LA this month.

Kind regards

Richard Godwin
Executive Director The Watch


From Andrew Yee <>

Instituto de Astrofisica de Canarias
La Laguna, Tenerife, Spain

José Manuel Abad Lińán, +34922605182,

05 Oct 2000

Birth Of Lonely Giant Planets Observed

Spanish and German astronomers are to report today in Science, on their
discovery of isolated giant planets undergoing formation

Researchers from the Instituto de Astrofisica de Canarias (IAC), the
California Institute of Technology and the Max Planck Institut für
Astronomie, co-ordinated by Professor Rafael Rebolo (IAC/CSIC), have
discovered in the Orion region three giant planets and another fifteen
bodies, whose planet status could be confirmed once analyses are
completed. The planets detected are reported to have masses between 5
and 15 times the mass of Jupiter, the largest planet in the Solar System.
The results to be published by the specialised journal Science, include
unprecedented images and spectra of bodies whose planetary masses are
not associated to a given star. The superjupiters examined roam freely
in Orion's Sigma cluster, a very active star formation region located
at approximately 1000 light years from Earth. The age of these
extraordinarily young planets is expected to be less than five million

Images of these solitary planets have been obtained, in the visible range,
with the 2.5-m Isaac Newton Telescope, at the IAC's Spanish Observatorio
del Roque de Los Muchachos (La Palma), and in the infrared, with the 3.5-m
telescope at Calar Alto Observatory (Almeria, Spain). The combination of
these data has allowed identification of a large concentration of very dim,
exceedingly red objects, in a small region surrounding Orion's stellar
system known as Sigma. These features are characteristic of giant
planets currently undergoing a formation process. Subsequently, the
spectra obtained with the world's largest telescope -- the 10-m Keck
telescope on Mauna Kea Observatory (Hawaii)- confirmed these findings.

Although the existence of Jupiter-like bodies orbiting stars has been
known since 1995, images of these giants have not been obtained to date,
essentially because they are as much as one thousand million times
fainter than the stars they are orbiting. The contraction process affecting
these newly detected planets is in full swing -- which means that their
size is diminishing due to gravity -- and they irradiate about ten thousand
times more energy than is to be expected once they reach the size of
Jupiter, i.e. when they become more stable.

To capitalise fully on this circumstance, researches began exploring in
1998, surveying the Orion region -- renowned for hosting huge numbers of
young stars -- in the search for giant planets. The results to be released
today by Science show, for the very first time, images and spectra of
bodies showing planetary masses which oddly enough are not linked to
any of the surrounding stars.

These so-called superjupiters float freely within a star cluster, but at
distances sufficiently large to allow them to avoid the gravitational
attraction of other stars. Of the eighteen candidates detected so far,
three have been scrutinised using spectroscopic techniques and have
been confirmed as gaseous objects with surface temperatures in the
range 1,500 degrees Celsius, as expected for planets slightly less
massive than Jupiter undergoing very early evolutionary phases.

In the words of Prof Rafael Rebolo, "this discovery is a challenge for
current theories. In fact, a definitive explanation is still lacking. These
bodies appear to be far too numerous and young to have formed in
protoplanetary disks and later ejected as a result of the collisions
between stars present in the disks. A more plausible hypothesis is that
they emerged directly from the fragmentation and collapse of clouds
of dust, a process that may well occur in a few million years time".
However, the fragmentation scenario poses difficulties from the
theoretical point of view when attempting to explain the formation
of bodies with masses so close to Jupiter's, and hence a definitive
explanation for their existence is still pending.

The objects detected in Orion will cool down progressively," according
to Víctor Sánchez Béjar, a PhD student and team member at the IAC,
"and in a few hundred million years will reach surface temperatures in
the range 0 to 100 degrees centigrade. They will never develop rocky
regions and temperatures will continue to drop until they fall in the
range of Jupiter's".

It is still premature to affirm how many of these giant planets may be
present in the Galaxy. However, if the statistics inferred for Orion were
representative of the entire Milky Way, hundreds of millions of isolated
superjupiters would be found populating interstellar space. According
to the researchers involved in the study, there are indications that they
could be as numerous as solar-type stars. In the Sun's neighbourhood
(i.e. in a radius of 20 light years) there could be 30 or 40 such objects.
Their discovery is clearly a challenge for current technologies.


* María Rosa Zapatero Osorio (IAC- California Institute of Technology)
* Víctor Sánchez Béjar (IAC)
* Eduardo Martín (California Institute of Technology - Institute of
  Astronomy, University of Hawaii)
* Rafael Rebolo (IAC-Spanish Council of Scientific Research)
* David Barrado y Navascués (Max Planck Institut für Astronomie -
  Universidad Autónoma de Madrid)
* Coryn Bailer-Jones (Max Planck Institut für Astronomie)
* Reinhard Mundt (Max Planck Institut für Astronomie)
Notes for Editors:

3D animated movies and interview with one of the researchers [and
images] available in avi and mov (Quick Time) format at

For further information contact the IAC Press Office at
+34 922 605 371 / 182 / 293

From Michael Paine <>

Dear Benny,

BBC and New Scientist have articles about a possible tsunami threat the
the US East Coast due to a potential underwater landslide at La Palama.

However, tsunami expert Charles Mader advises "Like asteroids smaller
than 1 km, the La Palama landslide generated wave would have a short
wavelength and a short period (less than 10 minutes) wave that would
rapidly decay to a deep water wave before it got to US coasts. I told
Horizon that a La Palama landslide would not be a threat to the U.S. --
that did not fit into their script!"

"Megatsunami" will be broadcast in Britain on 12 October at 9.30 pm on

Michael Paine
(For more on asteroid tsunami see )



From Brian G. Marsden <>

     Certainly, Bob Kobres is right to point out the close approach of
Lexell's Comet to the earth in 1770, despite the fact that this comet had a
perihelion distance of 3 AU prior to its jovian encounter three years earlier.  I used
a somewhat fictionalized version of the Lexell story in my presentation of
some spoof "eighteenth-century IAU Circulars" at an NEO conference in Sicily
seven years ago.  We must indeed be aware that comets can surprise us in
unusual ways.

     But Bob speaks of PHOs. Appropriately homonymous though this term may
be, I have not advocated its use for comets. At the U.N. NEO conference in 1995
I introduced the term PHA for an asteroid with an orbit coming within 0.05
AU of that of the earth (and of absolute magnitude 22.0 and brighter, although
maybe we need now to extend this to intrinsically fainter objects) but
remarked that the corresponding PHC was not a useful concept. This was
precisely because almost all asteroids would not switch from PHA to non-PHA
status (or vice versa) over the course of a century or so (and it is quite
sufficient to take the earth's orbit to be a fixed circle for this purpose),
whereas Jupiter can frequently and dramatically change the paths of the
rather smaller number of known short-period comets, and we need to--and usually
can--specifically study such changes.

     As Bob indicates, the study of the motions of comets is also
complicated by the effects of the loss of material associated with the sublimation of
ices. One can indeed be suitably wary of active comets.  But, as he says, it is
also clear that there are *inactive* comets, therefore indistinguishable
from asteroids in appearance.  Of course, if they are truly inactive, either
having lost all their ices or having the ices completely smothered by nonvolatile
material, their motions are quite gravitational.  We in fact know of some
comets that are only mildly active, and we have seen that their motions are
affected little, if any, by outgassing. Much cometary outgassing is in fact
rather predictable. 

     Might we be missing such activity on what we think of as asteroids when
they are near perihelion?  That is something I have wondered about from time
to time, particularly when the perihelion distance is small enough that the
object can not then be observed in a dark sky.  What one *can* say is that
the orbits of well-observed, small-perihelion objects like Icarus and Phaethon
are completely compatible with point-mass dynamics (including relativistic
terms, of course), even though the former is just coming up its 46th passage some
28 million km from the sun since discovery.

     This is not to say that an "asteroid" could not surprise us in this
way, to the extent of making the difference between an impact and a miss a
century hence for an object whose orbit we think we know very well. But the
predicted miss would surely be enough of a "near-miss" to interest us, and we still
get to monitor the object, including its physical appearance, between now and
then, thus steadily improving the prediction until the outcome is clear.

Brian G. Marsden


From David Morrison <>
     [as posted on NEO News 5/10/2000]

In the wake of the release of the report of the UK NEO Taskforce,
there has been a great deal of media attention to the NEO impact
hazard. The UK Taskforce report is well reasoned and constructive in
its approach. The media attention and public dialog have been
largely positive. But I have also noted some misconceptions that are
bubbling to the surface in this dialog. Perhaps it is useful,
therefore, to reiterate the basic strategy of carrying out an
inventory of potentially threatening objects -- that is, the
Spaceguard Survey strategy -- as I understand it.

Spaceguard is designed to discover objects (primarily asteroids,
NEAs) out to distances of roughly 100 million km and gradually build
toward an inventory that is complete at the larger sizes. If any of
these NEAs poses a collision threat, it can be identified decades or
even centuries before the impact. Over time, the degree of
completeness at any size increases. The current NASA objective is to
achieve 90% completeness down to 1 km diameter (absolute magnitude H
= 18) by 2008. Even larger numbers of smaller NEAs are being
discovered as well. The objective adopted in the UK Taskforce report
is to extend completeness to roughly 500 m diameter or about H = 19.

The most important measures of search capability are total telescope
aperture, size of the focal plane detectors, and available hours.
Additional telescopes can be used either to increase sky coverage or
to lengthen exposures, permitting detection of fainter objects.
However, there are other secondary factors that can be important and
that require careful modeling to make optimum use of the survey
telescopes. As we push to fainter objects, it is more efficient to
use larger telescopes and smaller pixels, rather than simply to
lengthen exposures (especially for fast-moving NEAs near the Earth).
We are using 1-meter telescopes for Spaceguard because they are
available, but larger instruments would probably be more efficient.
The UK Task Group noted this when they recommended a 3-meter
telescope to extend the search to absolute magnitude H = 19.
Third-order factors include the geographic distribution of the
telescopes and the sky brightness at the observatory sites.

In contrast to these basic principles, there are several
misconceptions that continue to appear.

Misconception #1: Spaceguard is looking for objects that pose an
immediate impact danger to the Earth. -- We see this when journalists
express dismay that a NEA was not discovered until it passed the
Earth. A similar fallacy leads to a call to monitor NEAs with orbits
interior to the Earth because a significant fraction of NEAs that
will hit will approach the Earth from interior directions. But this
is not what Spaceguard is all about. We are inventorying the
population to guard against future impacts. This is not a last-minute
warning system. Any NEA that hits the Earth will pass close by the
planet thousands of times in advance, and it is on one of those
passes that we expect to find it.

Misconception #2: Spaceguard requires a Southern Hemisphere survey
telescope. -- The most important metric here is not geographic
location but total cumulative sky coverage. Other things being equal,
it would be great to have a southern site. But we would probably
profit more, for example, by another northern telescope in an
excellent site rather than a southern telescope where it is more
often cloudy. There are many factors to consider in addition to
geography, because an NEA that is missed one year because it passed
the Earth at southern latitudes will most probably be discovered in
the northern skies a few years later.

Misconception #3: Spaceguard would be better done with a telescope in
space. Or: We need a telescope in space (or at Venus or Mercury) to
see the NEAs that are interior to the Earth or that approach the
Earth from the sunward direction. -- Telescopes in space are
extremely expensive. I have seen NO proposal over the past decade
that suggests that a survey from space would be cost-effective
relative to ground-based telescopes. As far as the interior NEAs are
concerned, a population of NEAs that stays entirely interior to the
Earth poses no impact threat. Those that are Earth-crossing need to
be surveyed, but that can be done from the ground. Telescopes can
easily look at objects closer than 90 degrees to the Sun (for
example, look at Venus any clear evening these days). Also, an NEA
that passes the Earth this year on the sunward side is likely to pass
on the anti-sun side next time around.

Misconception #4: Spaceguard may be missing populations of objects,
such as NEAs with orbits that are mostly interior to the Earth's
orbit, or intermediate-period comets. Thus, in effect, we may be
looking in the wrong place and missing lots of potential impactors.
-- It is true that Spaceguard is biased against some classes of
Earth-crossing objects. But are those objects a major part of the
impact threat?  Probably not, although each case should be looked at
individually. The chance of impact of an object is roughly
proportional to how frequently it crosses the Earth's orbit. An NEA
that stays almost all the time interior to the Earth is not much of a
threat. A comet with a 200-year period is about 100 times less likely
to hit the Earth in any given time period than an NEA with a 2-year
period. As an extreme example, consider the Oort cloud comets; there
are probably 100 billion of these larger than 1 km, but they
contribute less to the impact flux on Earth than the 1000 NEAs in
this size range. In general, Spaceguard will be sensitive to the same
populations that threaten the Earth, since it is biased toward
finding objects that pass by the Earth often.

David Morrison, NASA Ames Research Center
Tel 650 604 5094; Fax 650 604 1165 or


I'm afraid I have to object to some of David Morrison's comments regarding
the the UK Task Force Report on NEOs.

David suggests that the main recommendation of the Task Force Report,
i.e. that the UK Government, preferably with European partners, should
build a large survey telescope for location in the Southern Hemisphere,
is in some way a "misconception." In view of the fact that the UK Government
is currently considering the recommendations made by the Task Force,
I don't think it is very wise of him to claim that "we would probably
profit more, for example, by another northern telescope in an excellent
site rather than a southern telescope where it is more often cloudy."
This is a rather dubious and questionable argument. But, coming, as it does,
from a senior NASA source, it sends out all the wrong signals.

Given that the NEO Task Force recommendations are based on sound
scientific advice of the world's leading NEO experts, David's doubts
about the Report's main recommendation are, I regret to say, extremely
unhelpful. I am sure, however, that they neither represent official
NASA nor indeed IAU policies.

Benny J Peiser


From Larry Robinson <>

Dear Benny:

My wife, Judy, and I are avid fans of CCNET and print out and read every
issue as it comes out. Since we live in the Kansas City area, on the Kansas
side, we were pleased to see the article in the Star and also quoted on
CCNET. We spend every available clear night imaging minor planets from our
kitchen breakfast nook using our little telescope and CCD camera in the
backyard. Much of our effort is spent making follow up obwervations on Near
Earth Asteroids and Comets recently discovered by the Surveys. We have even
made a few discoveries ourselves of main belt minor planets. 

Being from Kansas, we are not huge fans of the government solving all of our
problems. We have a proud history of taking care of ourselves. The search
for and tracking of near earth asteroids is no exception. There are
currently four different small observatories in our neighborhood doing this
work. Our reports grace the pages of the Minor Planet Center daily orbit
updates almost every day. Look for observatory codes 739, 734, 849, 649.
In this small way we fill our moments with value and contribute where our
small apertures allow. It is the best we can do to make a real difference in
the race for knowledge before it is too late for mankind and other species
perhaps more worthy of our salvation.

Larry Robinson
Sunflower Observatory
IAUC/MPC Observatory Code 739
Olathe, KS 66062


From John Nuckols <>

Dear Benny,

I enjoyed reading the various opinions regarding keeping and using
nuclear weapons as a defence against asteroids.  This is a comment on
Syuzo Isobe's letter.

I agree with Syuzo Isope that it is of the utmost importance to
determine the orbits and collision times of all NEAs.  Whether or not we
should plan on using nuclear explosions as a possible shield against the
danger of catostropic collisions should depend upon whether or not such
weapons could be used effectively for that purpose.  If they could be
effectively used for that purpose, they should not be banished on the
grounds that they might be misused for some other purpose.

I have wondered if the nuclear shield would really be effective against
the threat of an NEA striking earth.  Unless the nuclear weapon could be
used in some way to divert the orbit of the NEA, I doubt it would be
effective. If the earth were in line to be struck by a large NEA,
breaking it up would not be effective and might even be worse than being
struck by a single object.  Under those horrible assumed circumstances,
to send nuclear weapons against an approaching large NEA would be like
asking a robber to shoot you with the 12-guage shotgun in the corner
rather than the pistor he was holding in his hand.

Keep up the good work, Benny!

John Nuckols

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