PLEASE NOTE:
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Date sent: Wed, 23 Apr 1997 16:20:52 -0400 (EDT)
From: HUMBPEIS <B.J.PEISER@livjm.ac.uk>
Subject: Halt Friend, or PHO?
To: cambridge-conference@livjm.ac.uk
Priority: NORMAL
From: Duncan Steel <dis@a011.aone.net.au>
SPACEGUARD AUSTRALIA
This is a brief commentary on the message just circulated
by Bob Kobres.
I agree with Bob's general comments (or bewilderment?) over
acronyms, although I am not sure that it is something to
get too upset about.
Generally, people in the game use the following terms:
Near-Earth asteroid (NEA): the Apollos, Atens & Amors.
That brings up the question of the definition of an Amor:
why the arbitrary cut-off at q < 1.3 AU? The answer is just
history. Myself, I'd like to adopt an
astronomically-defined cut-off, so in the past I've
suggested q less than the q of Mars (at present: something
like 1.33 AU).
Near-Earth object (NEO): NEAs plus similarly-defined comets
(at least that's what I think it means). That would include
_all_ comets with q < 1.3 AU, so that one gets into a
tangle: long-period comets don't spend much time "near"
the Earth. And, in my view, the difference between
"asteroids" and "comets" is moot, with
transitions
_in_both_directions_ occurring on time-scales of
decades and centuries.
Then one has to go back to the definition of Apollos, or
indeed "Earth-crossing" in general. A few years back
there
was a letter in the JBAA from Jan Meeus complaining that,
in this context, one (i) Could not use the Earth's orbit as
circular at 1 AU; and (ii) That meant that one had to
account for the longitude, in deciding whether an orbit was
"Earth-crossing". Other people have made similar
comments
elsewhere (but I have to ask "So what?", for reasons I
mention below). Myself, I usually take any object with q <
1.0167 AU as being "Earth-crossing".
The problem is that everything changes with time. The
Earth's eccentricity changes (slowly), altering the 1.0167
AU distance (or indeed the Q > 0.9833 AU limit for any Aten
asteroid: are there asteroids entirely interior to the
terrestrial orbit? I expect so: we could call them
Vulcans, or Cytherean asteroids, or Hermian asteroids if
the orbits are smaller still). But the most significant
change is the change in the orbits of the NEOs (or call
them what you will). For example, 433 Eros is an Amor, but
the paper in Nature by Michel et al. last year showed that
it is likely to evolve to become an Apollo on a time-scale
like 10^5-10^6 years (in fact that general point was
already well-known, but they did it specifically for Eros).
On the other hand, though, whilst 433 Eros is of interest
to us for scientific reasons, and as the target of the NEAR
spaceprobe, it is not a hazardous object on the time-scale
of interest.
In fact, rather than orbital evolution of orbits causing
drops in perihelion distance q so as to make an impact
possible, on the time-scale of interest to us (the next few
centuries) the important matter are either (i) The rotation
of the line of apsides of orbits (with other orbital changes
occurring at the same time; e.g., orbital evolution of
2P/Encke) under secular perturbations, for objects which
cross the terrestrial orbit but have nodal distances rather
close to 1 AU: the question is when the node is going to be
brought to 1 AU; and (ii) The chaotic evolution of the
orbits of objects which currently have nodes very close to
1 AU. A good example in the latter class is 1991 JX
(now numbered & named something like Goletva): even with
the radar data from the close approach in 1995 (which led
to the actual name it now carries), although we know that
it will miss the Earth healthily in 1999 and 2003, the
circumstances of the close approaches anticipated for 2007,
2011 etc. are _impossible_ to predict with confidence
(that's chaos for you). This all comes about from 1991 JX
having, at present, (a) A node very near 1 AU; (b) Passing
that node when the Earth (itself) happens to be nearby; and
(c) Being rather stably (for time scales like millennia) on
the periphery of the 3:1 mean-motion resonance with
Jupiter, giving it an orbital period almost dead on four
years.
Now, that's all pretty complex, and may be intelligible to
us (shouts of protest in the background), but it is not to
the even above-average person in the street. All that they
can fathom are lumps of rock (& ice) in egg-shaped orbits,
and now and again one happens to zonk into our planet.
For all of the above reasons I tend to agree with Brian
Marsden (and others) that our first cut on finding a new
object is to decide whether it is potentially hazardous
(i.e., a potentially-hazardous object, or PHO).
That's pretty easy to decide very quickly: an asteroid is
found with q near 0.8 AU, Q near 3 AU, inclination 20
degrees, argument of perihelion such that a rotation of the
line of apsides of 30 degrees is needed to shift the
node to 1 AU; this is not a PHO, because the time-scale for
making an Earth-impact possible is of order a thousand
years (close approaches to Mars notwithstanding). On the
other hand, if it was found to have Q near 5 AU, then much
faster (and quite likely chaotic) orbital evolution under
the influence of Jupiter would be possible, and we would
need to get a good enough orbit so as to determine (i) The
orbital evolution through to the next plausible encounter
with Jupiter; and (ii) Quite apart from that, the behaviour
of the nodal distance under secular perturbations.
So, it all gets a bit complicated. But from the _hazard_
perspective, I am just noting that we do have ways to
determine whether an object (asteroid or comet) is a friend
(usable for science/raw materials?) or a PHO within a short
time of discovery, always assuming that the necessary
astrometry is collected. Brian Marsden maintains a list of
PHOs on the WWW pages of the Minor Planet Center.
Dr Duncan Steel
dis@a011.aone.net.au
Spaceguard Australia P/L
P.O. Box 3303
Rundle Mall
Adelaide, SA 5000
Australia
Phone: +61 8 8232 6133
Fax +61 8 8232 6103
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Date sent: Wed, 23 Apr 1997 15:42:36 -0400 (EDT)
From: HUMBPEIS <B.J.PEISER@livjm.ac.uk>
Subject: Re: EOCO
To: cambridge-conference@livjm.ac.uk
Priority: NORMAL
WHAT'S IN A NAME?
If some day in the future we discover well in advance that
an asteroid that is big enough to cause a mass extinction
is going to hit the Earth, and then we alter the course of
that asteroid so that it does not hit us, it will be one of
the most important accomplishments in all of human history.
-Sen George E. Brown, Jr.
The Earth-Moon System (EMS) and its inhabitants are in
danger. It is not the kind of danger that most people are
familiar with such as disease, pestilence, or the threat of
nuclear war, but one that is rapidly moving to the
forefront of scientific research, exploration, and
analysis-the very real hazard of a large earth-crossing
object (ECO) impacting on the EMS. As the Earth revolves
around the Sun, it periodically passes close to orbiting
asteroids and comets, producing near-earth-object (NEO)
situations. When asteroid or comet orbits intersect the
orbit of the earth, they are referred to as ECOs. Clearly, a
global effort is needed to deal with this problem and to
provide perhaps the only means of preserving the human race
from possible extinction.
----
So begins the Introduction of, Planetary Defense:
Catastrophic Health Insurance for Planet Earth, a research
paper presented to Air Force 2025 by Col John M.
Urias, Ms. Iole M. DeAngelis, Maj Donald A. Ahern, Maj Jack
S. Caszatt, Maj George W. Fenimore III, and Mr. Michael J.
Wadzinski in October 1996. See:
http://www.au.af.mil:80/au/2025/volume3/chap16/v3c16-1.htm
The point I wish to make about this introductory paragraph
is in regard to the acronyms used. What is an
Earth-orbit-crossing object to be called?
Personally I favor EOCO, as this is about the least
ambiguous set of letters not presently in use to define
something else. NEA is in trouble due to funding
cutbacks--Oh--sorry wrong subject. NEO is a pretty good
term for `new' and if you do an Internet or subject search
you'll find that a lot of people use it a lot.
Neocatastrophism in the NEO News--hmmm. ECO might work
better if it had wings like: potential-ECO-busters, but if
one seeks a short term to describe threatening cosmic
debris ECO just sounds too Earth friendly. Not only that,
but ECO it is so liberally used and abused as a descriptor
that it is useless as keyword search term. Now in spite of
the fact that EOCO is a four letter acronym and uses more
ink than its more widely used rivals it is rather straight
forward and does have a use history which goes back over a
decade. In fact, within the above paper is a reference to
my editorially misnamed, 1987, article--Meteor Defense--in
which I used the acronym EOCO. This was published
in an internationally distributed magazine--The Whole Earth
Review.
A three letter device I earlier (1980) used to describe
these things was OFU for `object falling unguided.' There
was of course some humor intended with this choice but the
term also served to point out that we could, if we acted
soon enough, turn a threatening OFU into a guided object
long enough to avoid unwanted repercussions on Earth.
Another aspect I noted about this acronym was related to
the question of what do you say when you realize your about
to be blitzed by a cosmic intruder--Duck? No--if English
is your tongue and the fireball is upon you, you're more
likely to only have time to utter an incomplete statement
of alarm.!
I don't know--maybe we should just find a new name like:
Potential Earth Popper (PEP). I can see the funding
proposals now: Don't let Earth get PEPpered--spend
money here!
Bob Kobres
email= <bkobres@uga.cc.uga..edu>
url= http://abob.libs.uga.edu/bobk
phone= 706-542-0583