CCNet SPECIAL, 10 October 2000


From Benny J Peiser <>

While the UK Government is currently considering whether or not to
fund, as recommended by the UK Task Force on NEOs, the building of a
3m survey telescope for location in the Southern Hemisphere, David
Morrison has stirred up a public row about the need for such a telescope.

In a comment on 5 October, David Morrison, the chairman of the IAU
Working Group on NEOs, questioned whether there are any grounds
for such an instrument to be located in the Southern Hemisphere.
Not only would such a location be 'less profitable' because "it is
more often cloudy" in the Southern Hemisphere. What is more,
such a telescope would be superfluous "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."

In response to the criticism from within the NEO community,
David Morrision has responded (see below) by saying that his critics
have misinterpreted his comments. David claims that it was "not a
commentary on the excellent UK Taskforce Report, but rather on
misconceptions one sees in some press coverage of NEO issues."

Blaming the media is, however, is just another red herring. The
fact is that the media simply reported the reasons for a Southern
Hemisphere telescope as presented in the Task Force Report. In fact,
I am not aware of any media report that did not present the Task
Force's recommendation for a SH telescope accurately.

To make matters worse, David Morrison has now circulated further
arguments against a Southern Hemisphere surevy telescope.
According to Alan Harris, there is no scientific evidence
whatsoever that such a telescope would benefit the Spaceguard
survey because there would be no enhancement in the rate or timing
of completion. "I too would like to be able to advocate
a southern hemisphere telescope more strongly, but that's not
what comes out of the calculation." And, in applauding Morrison's
decision to make his own personal doubts public, Harris appeals
directly to UK decision makers:

"Your piece summarizing misconceptions about Spaceguard is excellent.
I hope your distribution gets around to enough of the media to make a
difference in their reporting, and as well to the various agencies
and committees that make decisions regarding telescope placement,
space missions, etc."

I don't think the UK Government will be much influenced by these
direct or indirect attacks against a Southern Hemisphere telescope
coming, as it were, from a minority of U.S. researchers. The fact,
however, that this embarrassing controversy has been instigated
by the chairman of the IAU WGNEO is more worrying. His handling of
this row, I regret to say, as well as the bad timing during a period
of delecate Government deliberations, is damaging to NASA, the
IAU and the NEO community as a whole.

Benny J Peiser


From David Morrison <>

NEO News (10/9/00) Dialog about Spaceguard

Dear Friends and Students of NEOs:

This edition of NEO News distributes dialog on the Spaceguard
strategy stimulated by my last e-mail message, which dealt with four
misconceptions that one sees frequently in the press. Some of this
discussion is fairly technical, but I hope it will be of interest to
some of you.

To my surprise, a few individuals accused me of criticizing the
recent report of the UK NEO Taskforce in my last NEO News. I thought
I made it clear that the misconceptions I was discussing are those
that one sees in some popular press coverage of the NEO hazard
issues. The UK Taskforce has written an outstanding report. I
previously circulated their executive summary and recommendations,
and I urge everyone to read their full report on the Internet.  I am
sorry for anyone who mistakenly connected my "common misconceptions"
with the UK NEO Taskforce Report.

David Morrison



A few comments on your misconceptions:

>  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.

But if those close passes are almost always on the daytime side of
the Earth, you still wouldn't have had much of a chance to find it
with an opposition search.

>  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.

There are NEO orbits that favor discovery from southern hemisphere
sites. Although it is true that an object will sooner or later be
visible from the northern hemisphere, it may never be as bright when
placed in the northern hemisphere, and therefore it may never be
discovered from a northern hemisphere site.  NEOs are not routinely
being found anywhere in their orbits.  Rather, they are being found
while they are in a portion of their orbit that brings them close to
the Earth.  If that portion of the orbit lies below the ecliptic
plane, then a southern hemisphere site has a much better chance of
discovering it.  I wouldn't go so far as to say that Spaceguard
REQUIRES a southern hemisphere survey telescope, but it certainly
makes sense to not put all our eggs in the northern hemisphere
basket.  And it certainly doesn't make sense to locate any such
telescope at a cloudy site, be it northern or southern hemisphere, so
that part of your argument looks to me like a red herring. I haven't
read the UK report in its entirety yet (just the Executive Summary so
far), but I wouldn't call it a misconception if the report was
recommending that the southern hemisphere be given priority over the

>  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.

I agree that spaced-based telescopes are not cost effective.  The
threat posed by asteroids with orbits entirely interior to the
Earth's orbit is the same as the threat posed by asteroids with
orbits entirely exterior to the Earth's orbit, namely zero, at least
in the short term. Long term perturbations can change those orbits
into Earth crossing, and potentially Earth intersecting, orbits.  So
if there is reason to find Amors, then there is reason to find
Apohele as well.  But even if we focus our attention on Atens and
Apollos, it needs to be emphasized that as the aphelion distance
decreases, the amount of time spent beyond the orbit of the Earth
goes down.  Opposition searches are biased against the discovery of
Atens, particularly those with the smaller aphelion distances.  Your
example of Venus isn't a very good one, simply because it is
magnitude -4, whereas a 1 km asteroid would be about magnitude 21 at
the same location in the Solar System.  Venus is already at a airmass
of 3 as of sunset tonight (as seen from Mauna Kea), so if you wait
for just a half hour for the sky to get partially dark, you're
already at an airmass of 4.4, so extinction has taken a half
magnitude from you, even at a high altitude site like Mauna Kea, and
more like three quarters of a magnitude for a typical NEO survey
site.  By the end of astronomical twilight, Venus is at an airmass of
9.6 and an altitude of 5 deg.  How many telescopes are designed to
point that low?  How much time do you have to revisit the field and
detect motion?  Keep in mind that the surveys revisit the same field
three, four, or five times to detect moving objects.  Just because it
is easy to see Venus in the evening sky does not mean it is easy to
find asteroids in that part of the sky. Certainly it gets easier as
the solar elongation increases, but the probability of finding an
object interior to the Earth's orbit goes down as the solar
elongation goes up.

>  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.

The most dangerous objects are those that have tangential orbits with
low inclinations.  They spend more time in the Earth's capture cross
section than an object that crosses through the Earth's orbit more
obliquely.  Objects that are tangential at aphelion pass through the
Earth's capture cross section more frequently than objects that are
tangential at perihelion, simply because their orbital periods are
shorter, and objects that are tangential at aphelion will not be
discovered by opposition searches at all.  I think you are
downplaying the significance of the Apohele population too much.



To Duncan [Steel],

I hope you will provide a response on the notes that David Morrison
has just circulated. While I agree mostly with what he has said I
think some items could have been better worded. My concerns are set
out below.

'Any NEA that hits the Earth will [most likely] pass close by the
planet [dozens, if not] thousands of times in advance, and it is on
one of those passes that we expect to find it.'

'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.' This seems
dismissive and without justification (eg your comments to me about
the suitability of the Flinders Ranges). It certainly won't assist
efforts to have the UK and Australian governments take some
responsibility for NEO detection. Don Yeomans made the point that,
although not strictly essential to the US goal, a southern hemisphere
site would add to the internationality of the search. He also pointed
out that the current US sites are subject to regional weather
problems (monsoons?) and both longitudinal and latitudinal diversity
were wise approaches.



Nice discussion. I noted that Michael Paine copied his message to me
on to you. I don't see what he's upset about. Everyone in the field
would find something to disagree with in amongst what any of us might
say or write. So what? That's healthy. I think he interpreted:

"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" meaning that the SH in general is more cloudy than the NH,
which is a pretty extreme interpretation on his part. The basis of
your statement is entirely correct. (In fact one could go further: I
have no idea why the UK NEO TF stipulated a southern site for a 3-m
search telescope; I recommended to them such a beast but pointed out
that as it would be larger than any present search tool it could go
anywhere, without affecting sky coverage as it would go deeper, and
thus said that it should go in the cheapest - for the UK - good
location, which implies the Canary Islands.)

Of course Chile and southern Africa provide excellent sites in the
SH, and the NEO TF had one of those in mind: either at ESO as part of
UK membership, or in South Africa as foreign aid and/or a buy-in to
SALT. Michael is of course wedded to the Australian case which, due
to the intransigence of the government there, does not rank WRT the
UK initiative. My previous comment to him about the Flinders Ranges
was simply connected with recent Australian media reports (as seen in
CCNet/Peisergrams): the empty arguments were pivoting on Siding
Spring versus Woomera, whereas the Flinders were not mentioned. The
Flinders Ranges - about 400 miles north of Adelaide, 150 miles west
of Woomera (where the USAF has a base) - represent by far the best
location for an observatory within Australia, as was known even
before the Anglo-Australian Telescope was put at SSO (decision made
in the late 60s). But that is irrelevant unless the Australian
government decides to do something itself.



I was glad to see you clearing up these misconceptions about
Spaceguard. In particular, I agree with your assessment of the
importance of a southern hemisphere survey site. The fact is that for
surveying it doesn't matter where you look as long as you are looking
at new sky. On the other hand, with the Japanese coming online and
the American surveys continually improving, I think it makes the most
sense to put any new survey instruments in the southern hemisphere if
a site with suitable weather and seeing can be made available.

That said, I think you did promulgate a new misconception when wrote:
>  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.

Your conclusion that the Atens are not much of a threat is completely
at odds with the arithmetic you give for comets. An Aten such as 1999
KW4, with a period of about six months and a MOID of 0.014 AU,
represents a very dangerous _class_ of object, approaching close to
the Earth's orbit twice a year. Furthermore, note that such objects
are moving relatively slowly during a potential encounter, thus
significantly increasing the likelihood that they will be in the path
of the Earth when it passes by.

The point is that the Atens should be the focus of _special_ effort
from the surveys. Dave Tholen and his colleagues are making
productive efforts to search at low solar elongations, but they need
more support, and the surveys could do more. (But I do see that also
LONEOS has some recent coverage in this region.) I suspect that
survey (over?)emphasis on the opposition region is due in large part
to the way they are "scored." Under present guidelines a harmless
Amor and a very threatening Aten are rated as equally significant
<>, so there is little
incentive to search in the low yield regions far from opposition. I
think a change of criteria may be warranted to encourage the surveys
to spend more time at low solar elongations.



I'm afraid I have to object to some of David Morrison's comments
regarding 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

MORRISON COMMENT: I hope that Benny is alone in misinterpreting my
remarks so completely. My essay on the Spaceguard Strategy was not a
commentary on the excellent UK Taskforce Report, but rather on
misconceptions one sees in some press coverage of NEO issues. I did
not criticize the Taskforce recommendation for a new 3-m survey
telescope, nor did I write that the conditions for astronomy in the
Southern Hemisphere are inferior to those in the north. As an
astronomer I am especially aware of the superb observatory sites in
Chile, which are second to none in the world.

David Morrison



Your piece summarizing misconceptions about Spaceguard is excellent.
I hope your distribution gets around to enough of the media to make a
difference in their reporting, and as well to the various agencies
and committees that make decisions regarding telescope placement,
space missions, etc.

Some of the comments made in response to your list of "misconceptions
of the Spaceguard Strategy", in particular relating to the need for
surveying from the southern hemisphere or from space, approach the
level of "junk science".  Rather than making intuitive appeals to
"the world as it might be," we would do better to look at
quantitative calculations of "the world as we think it is."   Further
criticism should then be based on the model representation used in
the calculation rather than intuitive wishes of how thing "should be."

I have reported several times the results of my modeling of "all sky
surveys."  In one simulation reported a couple years ago, I
explicitly tested the benefit of one-hemisphere surveying versus
two-hemisphere. Actually the latter was modeled assuming no horizon
limit but just a solar elongation limit, with a space survey in mind,
but it should be, if anything, over-optimistic compared to a
both-hemispheres from the ground simulation.  The answer is that
completion with time advances along about 10-15% faster than when the
survey is confined to a single site at +35 deg. latitude.  This
result can be understood from other geometrical modelings that
clearly indicate that survey efficiency is to first order
proportional to the sky area covered each month.  By missing the
southern polar cap of sky, say south of -40 deg. declination, you
miss approximately one steradian of sky, or about 10-15% of what's
otherwise visible.  Thus the model result is completely consistent
with what one would expect.  Of course one should not be too
complacent when you get a plausible answer (like it or not), but
further study should be directed to checking the model.  I too would
like to be able to advocate a southern hemisphere telescope more
strongly, but that's not what comes out of the calculation.

Regarding interior asteroids, in my simulation models I have "salted"
the set of orbital elements of synthetic NEAs with some asteroids
that never quite reach out to the Earth's orbit, even though no such
objects have yet been discovered with certainty.  I did this by just
allowing for smooth dispersions of elements matching the statistics
of known Atens to continue past the break we see at Q > 1.0, down to
Q = 0.95.  this allows for totally interior objects down to the limit
of anything that can come within 0.05 AU of the Earth, that is, the
PHAs.  Of the 1000 NEAs in my simulation, 26 have such elements.  I
have always maintained that these 26 objects are no harder to
discover from an all-sky survey than any other objects.  I confess
that this was based on a very casual look at results, so having heard
this gripe about needing to go into space to find them, I ran a
simulation with only these 26 objects to see how we would do in a
ten-year all sky survey.

The answer is, every single one of the 26 NEAs passes into the field
of detection at least once in ten years, so potentially the
completeness could be 100% with a faint enough survey.  The faintest
one comes up to a sky magnitude of 20.6, assuming an H value of 18.0.
Thus my nominal survey to V = 20.5 would catch 25 of the 26 objects
in a ten-year survey, or 96% completion.  Recall that for the full
set of 1,000 objects, my model predicts just about 90% completion for
all objects of H = 18.  Thus it seems that my previous claim is
correct: interior objects are as easy, indeed a little easier, to
detect than the average NEA.  90% completion of the interior objects
requires a 20.0 mag. survey threshold.

It is worthy of note that in the dispersion of elements I computed,
based on reasonable extrapolations from discovered bodies, only 26
out of 1000, or 2.6%, turned out to be interior asteroids.  To the
extent that my distribution of elements is reasonable, interior
asteroids may not be a very big fraction of the total.  If we now
believe the total number (to H = 18) is only around 750, there may be
fewer than 20 large interior asteroids to be found.  On the other
hand, if you believe the present survey is getting around 40%
complete to H = 18, then we should have found half a dozen interior
asteroids already, yet we have found none.  To be sure, only about
half of the present discoveries have come from "all sky" coverage, so
maybe the expectation should only be 3.  But unless someone can find
a gross error with my model, I am inclined to believe this lack of
any discovery to date tells us more about the number of such objects
than it does about the difficulty of finding them.



 From David Morrison

Michael Paine questioned the statement that a NEA is likely to pass
close by the Earth thousands of times before it hits. He suggested
that it would be more accurate to say "dozens of times". This is
partly an issue of semantics, of what I meant by "close" to the
Earth. When I wrote "thousands of times", I referred to roughly the
distance to the Moon, since the cross-section of a circle the size of
the Moon's orbit is about 3000 times greater than the impact
cross-section of the Earth. If by nearby we mean 12 times the
distance of the Moon (what the press has tended recently to call a
"near miss"), then it is likely that the typical NEA will pass this
close to the Earth roughly a million times before it hits.

Clark Chapman asked the origin of the estimate that Spaceguard
surveyed out to a distance of about 100 million km.  This is
approximately the distance from Earth at which a 1-km diameter NEA
with average reflectivity (albedo) will be detected with current
systems.  Alan Harris provides the following more detailed analysis:
Sky magnitude relates very simply to distance, phase angle and H
magnitude. For the present surveys, we are seeing to sky magnitude 19
approximately, and the detection criterion is that an H = 18 object
should appear V = 18 or brighter.  In the opposition direction, that
means solar phase angle is zero so that correction is zero, and the
distance factor 5log(r*d) should be no more than 1.0.  At opposition,
r = d+1, so all this means d = 0.85 and r = 1.85.  So looking
straight out, the maximum distance is around 130 million km, and it
is out just barely into the main belt.  Looking 90 deg. off the
opposition point, you have a significant solar phase angle but the
solar distance is less for a given Earth distance, so the numbers are
not so different.  The numbers work out to approximately d = 0.7 AU,
r = 1.22 AU, solar phase angle = 35 deg, which gives you 5log(r*d) =
-.34 and solar phase angle effect = +1.3 mag.  so 90 deg. off the
opposition point the maximum range for detection is 0.7 AU = 105
million km.  So Dave's "100 million km" is not bad, a bit of an


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


CCNet 102/2000 - 10 October 2000 (LETTERS TO THE MODERATOR)

     New faces in Orion

     By Malcolm Miller

     Motes swirling in a vacuum amount to nothing much;
     magnitudes of absorbtion, dark nebulae, or a reddish glow
     for Malin's photographs or Hubble's CCDs.
     Look closer! Behind their blue-lit curtains or black velvet tabs
     there's action -
     accretion discs that spin, bipolar flows, staccato jets of Herbig-Haro stuff,
     new stars of modest size in clouds of dust.
     All this was new not long ago and yet it seemed to make good sense.
     Complacency! But then the Demon King appeared to stir
     our pantomime -
     shockingly naked characters came in,
     huge planets without stars that glowed white-hot
     devoid of nuclear reactions, too small for stars,
     refuting all hypotheses of how a planet forms!
     The audience sits stunned by this burlesque -
     what happened to the dignified dance of rocky moons
     and banded Jupiters, each in its place,
     keeping strictly to the cosmic beat we understand?
     schräge Musik bursts out, jazz joins the music of the spheres,
     and we may need to learn new melodies and rhythms!

     Malcolm Miller

    Mark Kidger <>

    Michael Paine <>

    Erik Asphaug <>

    Duncan Steel <>

    Andy Nimmo <>

    Bob Perry <>


From Mark Kidger <>

Dear Benny:

The large number of asteroid "scares" this summer seems to reflect
various things: firstly the increasing efficiency of detection of such
objects, although many, no doubt, still get through, particularly if
they are only observable from the south; and secondly, the increased
public awareness of the ptential asteroid threat.

It is interesting to note though that such passes are, as is being
commented, if not two a penny, at least far more common than is commonly
thought. Taking the statistics for known objects, we have 15 approaches
to within 2.5 times the distance of the Moon. Of these, all but two have
been observed since 1989, but just one of the 2000 close approaches is
on the list - that of 2000 LG6. The distance of 2.5 lunar distances is
rather greater than for the 2028 1997 XF11 "non-event", which puts the
recent close approach hysteria in perspective. None of the recent close
approaches are anything like as important as two events that, in their
day, went almost unremarked: 1996 JA1 and 1989 FC, both very big objects
by the standards of Earth approachers. The former was by far the closest
approach by an object larger than 100-m in diameter but, even so,
something like 1.2 lunar distances. 1999 AN10, which has by far the
closest known approach over the next 100 years, will get a little closer
than this in 2027, but will still be slightly outside the Moon's orbit.

If we take an albedo of 15% as a rough guide for these objects we find
that of the five asteroids known to have passed closer than the Moon,
none is larger than 11-m in diameter. Suppose a 20km/s impact velocity
and a density 3 times water and the biggest of these events are around
100Kton in kinetic energy. In other words, devastating if they fall in a
populated area, but hardly a threat to the planet. However, with the
increasing spread of the Earth's population, the threat from small
objects is going to be increasingly important - not all of them will
fall in uninhabited regions as did the famous Tunguska event.

Within these 15 closest approaches though, there are several that are
far more spectacular. Four have a kinetic energy of 400Mtons or greater.
It is a salutary warning that the very largest, the only object larger
than 1km in diameter to have approached to within 2.5 lunar distances,
is the famous 1937 UB - Hermes - and completely lost. Despite the
greatly increased efficiency of detection, no one has yet been able to
locate this object. If someone wants a good headline, the kinetic energy
of this object is around 100 000Mton. The case of Hermes at least warns
us that we should not be overconfident that most of the big ones have
been found - it's there somewhere, and laughing at us.

2000 SM10 is a real non-event in terms of close approaches, but even
this object has a kinetic energy around 100Mton - equivalent to a pretty
unpleasant nuclear exchange in impact potential. The close approaches of
the recent weeks have not been a threat, but they should serve to remind
us that there is an awful lot of junk flying around the inner solar
system and, sooner or later, even though the Earth is a small target,
and the heavily populated areas of the Eath and even smaller one, we
won't always be lucky.

Mark Kidger


From Michael Paine <>

Dear Benny,

DOUBLE ASTEROID FOUND CLOSE TO HOME (CCNet 6 Oct) caught my interest.
The Inscight article mentioned that the asteorid  (2000DP107) might have
been torn apart during a close encounter with Earth. I asked Andrea
Milani about past close approaches by this object. He kindly did some
calculations (see the NEODys website
)and provide the following advice:

 Close approach to EARTH on 1949/09/05.40027 33164.40027 MJD at
 Close approach to EARTH on 1941/09/19.56265 30256.56265 MJD at
 Close approach to MARS on 1891/09/16.39627 11991.39627 MJD at
 Close approach to EARTH on 1890/09/19.84806 11629.84806 MJD at
 Close approach to MARS on 1871/01/05.52667  4432.52667 MJD at
 Close approach to EARTH on 1847/09/13.62703 -4082.37297 MJD at

"The close approach in 1847 is the most interesting, both because it is
closer and because that far back the uncertainty is larger. I have
performed close approach analysis, including Newton method to find the
closest possible approach (see for an explanation of this
obscure jargon) and found that the close approach could not have been at
less than 1.9296e-2 AU in 1847. Is this encounter at 3M km enough to
split an asteroid? I do not know the answer.

Could be that there were previously even closer approaches?  I can give
the answer without using the 'brute force' method of numerically
integrating the orbit for hundreds of years. See in the attached figure
[please contact Andrea if you would like a copy] the time evolution of
the MOID of 2000DP107. As you can see, the MOID is decreasing with time,
so in 1847 the encounter was close to the minimum possible, and before
that time the encounters must be shallower [not as close]. If anything,
extrapolating from the figure we should worry about the years of the
next node crossing, around 2570... assuming we have this kind of long
term way of thinking. Nothing excludes the possibility of a closer
encounter during the previous node crossing, which was several thousands
of years ago; but, for how long would the binary be stable?" (end quote
from Andrea)

Andrea poses two pertinent questions:
1.How close would the parent body need to get to the Earth or Mars (or
the Moon?) to break apart?
2.How long would a binary object like this stay together?
There is enough here to keep the dynamicists busy for years. This object
seems to have been playing chicken with Earth and Mars for thousands of
years and maybe once got so close that it split apart. The combined
absolute magnitude is around 18 so if it had been a single asteroid its
diameter would have been about 1km.

Michael Paine


From Erik Asphaug <>


That's an interesting observation, and I don't doubt that if you go
back farther (which you can't, because it becomes indeterministic)
there may be some extremely close encounters with Earth.  However,
an asteroid needs to get inside 0.69 * Rroche to begin to shed mass
(Sridhar and Tremaine 1992). For a density half the density of the
primary, along a parabolic orbit, this is about 1.6 times the radius
of the planet, or 0.6 radii from the surface of the planet. That is
much, much, much closer than 0.02 AU. Note that Toutatis comes within
0.01 AU in Sep 2004.
The argument presented by Asphaug and Benz (1996) is statistical,
comparing cross sections of the "tidal disruption annulus" with the
cross section for planetary collision. We found that most small
bodies will be tidally disrupted before they accrete onto the parent
planet, for the simple reason that the tidal annulus (from
R to 0.69*Rroche, times a gravitational focusing factor) is greater
in area than the collision
disk (from 0 to R, times focusing).

I should specify that the previous set of arguments apply to an initial
strengthless sphere of uniform density! For objects of that nature,
encountering a planet twice as dense, they are somewhat more likely to
disrupt tidally as to strike the planet.

It would be harder (nearly impossible) to tidally disrupt an asteroid
or comet with global cohesion. On the other hand, were a contact binary to
swing near Earth, it would be easier to separate into its individual
lobes than a spheroidal rubble-pile, since its mass centers are already as
separated as they can be, and still be connected. This has been
explored extensively by Melosh and his group, in an effort to understand
doublet craters on Earth and Venus.
Finally, note that all groups working on the tidal disruption of
strengthless aggregate asteroids and comets (e.g. Asphaug and Benz 1996;
Richardson et al.;  Bottke et al.) report the formation of satellite
pairs and swarms in the disruption aftermath. To wit, we proposed that
the late-fragmentation of P and Q, in the S-L/9 chain, was actually the
ejection of unstable satellites as N-body configurations collapsed into
N-1 body configurations. 

It is certainly plausible that some asteroid pairs and dumbell or
binary shapes are the result of tidal disruption.  For the record,
the first I heard of this was a DPS abstract by Hills and Solem, around
1993, where they wanted to explain peanut-shaped asteroids this way. 
It never made it to press so far as I am aware.



From Duncan Steel <>

Dear Benny,

Brian Marsden's nice discussion of the predictability
(or otherwise) of cometary trajectories, and his
differentiation between PHAs and PHCs as opposed to
grouping them together as PHOs, reminds me of the nursery tale
of Jack and the Beanstalk, and the giant saying:
"Fee, Fi, Fo, Fum, [Phee, Phi, Pho, Phum?]
I smell the blood of an Englishman."

As Brian and I are both Englishmen, one might enquire
as to the figurative smell of blood that he did not mention.
He said how Icarus and Phaethon (both small perihelion distance
objects) have observed ephemerides that do not differ from
point mass calculations over the time they've been known
(from 1947 for Icarus, 1983 for Phaethon), making the
case for the predictability of asteroid orbits as opposed
to comets influenced by non-gravitational forces (originating
from the sublimation of ices). He then went on to say that
an 'asteroid' might surprise us, in effect by suffering
non-gravitational perturbations sufficient to turn a calculated
near-miss into an impact trajectory. The question then is:
How do you recognise a weak comet? Rather than saying "It's
a PHA but it won't hit us soon", instead we'd soon be saying
"Oh, PHC."

In this context the case of Phaethon is interesting, because it
has an associated meteoroid stream (producing the Geminid meteor
shower in December, and the Daytime Sextantids in June when the
Earth makes a post-perihelion intersection of the stream). Such
an association is generally characteristic of cometary activity,
although there have been suggestions about how an asteroid might
produce a stream of small debris. Leaving that possibility aside,
the prima facie evidence is that Phaethon was once an active comet.

The timescale for dispersal of the meteoroid stream is of order
10^4 years, meaning that on the astronomical timescale Phaethon
has quite recently made the comet-asteroid transition. (Although
not as recently as 4015 Wilson-Harrington, say.) Or maybe it is
just cross-dressing for a while, and will soon burst forth into
splendid cometary activity, as 2P/Encke seems to have done some
time shortly before its discovery in 1786.

Obviously asteroids show no nebulosity due to possessing comae
or tails, and so simple imaging would not indicate the initiation
of outgassing. One wonders, then, how we should monitor 'asteroids'
that could be potential impactors within, say, a century, *if* they
were subject to moderate non-gravitational forces.

The UK NEO Task Force report has already evinced some confusion
amongst the ranks of astronomers due to its recommendation that
"spectroscopic" follow-up of asteroids should be conducted. Prior
to the finalisation of the report I pointed out that what was
really involved was *spectrophotometry*. That is, photometry
using the standard broad-band filters in order to determine colours
(B-V etc.) and hence provide categorisation of the asteroids into
the various classes (C-type, M-type etc.). The rejoinder to that,
quite correctly, was that the report was aimed at the public and
politicians, who would not understand "spectrophotometry" but might
comprehend "spectroscopy". (And we'll leave "spectrophotopolarimetry",
aimed at deducing surface texture too, out of this discussion.)

My point, though, is that *spectroscopy* might indeed be a useful
technique in our armory of observations of PHAs. Using a suitable
high-dispersion spectroscope on a large telescope we might identify
some characteristic emission line which would show that outgassing
was indeed taking place, without needing to wait years before the
ephemeris residuals indicated tiny non-gravitational accelerations.

When 5335 Damocles was found (as 1991 DA) we obtained such spectral
data using the Anglo-Australian Telescope specifically to look for
weak cometary activity. Likely others have done the same for other
suspicious objects. But should we be trying to do this as a matter
of routine for all PHAs, on a regular (every apparition) basis?

Duncan Steel


From Andy Nimmo <>

Dear Dr Peiser,

The announcement of the discovery of 18 interstellar bodies of large
planetary dimensions in Orion, reminds me of the item of mine you were
kind enough to publish in CCN on 21st June, "A MECHANISM FOR
INTERSTELLAR CREATION". As you will appreciate the clear forecast of
this hypothesis is that there ought to be large numbers of interstellar
bodies created this way, of all sizes from asteroids right up to small
stars. Accordingly, you may say justly, 'It was forecast here!'

Best wishes, Andy Nimmo.


From Bob Perry <>

John Nuckols wrote:

>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. . . .

It depends; how big (i.e., massive) is the NEA or NEO? Is it solid or
frangible? How much advance notice would we have? I've read Peter Tyson's
1995 article in MIT's 'Technology Review' about, among other things, changing the
orbit of an object bound for Earth

and seen discussions of it in several books and TV shows like Discovery
Cannel's "3 Minutes to Impact". My first thought is that it would have to be done
with a manned mission, just because the general public would be more likely to
trust heroes with nukes rather than robots with nukes. My second thought is that
doing it that way would be expensive. My third thought is that not doing
anything would be very expensive.

And remember, many people have told us that changing the phase of the object
in its orbit would be more effective than trying to shift it 'up or down' or
'left or right'. I suppose I should search the CCNet archives for discussions on
the possible techniques; has anyone posted them on a web site for the general
public? Popular magazine? Scientific Journal?

Bob Perry

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