Asteroidal Vulnerability
Threads - Asteroid
Removalists, Asteroid Threat (Aug, 2003)
On Friday 28 June 2002 23:53, Ian Musgrave
& Peta O'Donohue wrote:
Not really, no. The idea that we
know enough to use nukes efficiently for this purpose is a gross
generalization. While we don't know enough about asteroid structure in
general to make sweeping generalizations, a lot of asteroids are
"rubble piles", relatively lose agglomerations of rock and
regolith. Exploding nukes next to these ones would not be very
productive.
I would have thought that it
would provide adequate diversion.
No, most of the energy of the
blast goes into pushing the rubble pile apart rather than
pushing it in the direction you want it to. Some bits will be
move sufficiently to avoid the earth, but most won't
If it is a 'rubble pile' then the nuke
would break it up and (presumably) the resulting pieces of rubble
would be too small to do significant harm
Think again, think Shoemaker Levy, is
being hit by 12 1km rocks _really_ any better than being
hit by 1 12Km rock? We might avoid outright extinction but
it wouldn't be very nice. If it is a rubble pile, it
is most likely a carbonaceous chondrite. That means it should be like
Tunguska and break up easily and those fragments should further
fragment on entering atmosphere. Tunguska was a million ton
"fragment" of carbonaceous chondrite... I found the following at:
http://www.totse.com/en/technology/space_astronomy_nasa/tunguska.html
<QUOTE The Tunguska meteor,
whose mass has been estimated at a million tons, apparently did the
same thing. "There are strong parallels here with 40503,"says Doug
ReVelle of Northern Arizona University. Yet the means by which a
million-ton object destroys itself in an instant are not
understood. At the most basic level it appears that objects of
the size estimated for Tunguska -- about 100 meters across -- are too
big to be slowed gradually by the atmosphere but too small to survive
passage. Instead, Sekanina says, they continue to plummet virtually
unimpeded until resistance from the air becomes so great that "the
atmosphere acts as a wall." </QUOTE>
Tunguska was rated at the 100 meter
mark - call that birdshot. Your 12 by 1 km fragments - call them
grapeshot. Your 12km monster - that's elephant shot. So, a rubble
pile would be nasty, but it seems like a nuke or three would at least
reduce the intensity of the event. Basicly, I'd prefer birdshot
to an elephant shot... I don't think the grapeshot would leave much of
anything.
Zero Sum responded:
On Friday 26 July
2002 06:11, Jim Edwards wrote:
> How does all
this discussion apply to asteroid 2002 NT7?
It doesn't. I
don't even know what type it might be.
> This is supposed
to be about 2 km long. If a nuclear explosion near it
would not be enough to deflect it, how about attaching a couple
of rocket motors to it and steering it away from its
collision course?
If a 100 meter meteor
weighs a million tons then I think that a 2000 meter meteor is
going to take one hell of a lot of rocket propellant to move it
out of the way.
> Could this be
achieved in the next 17 years?
In summary, "no
bloody way".
> If it does
collide, how would its effects compare with the famous K/T
boundary event?
K/T is hypothesised
as being a 15KM diameter comet. This makes it likely to be
a carbonaceous chondrite and less damaging than solid
rock. A 2km rock would likely cause more damage than
a 2 km comet. It would probably have less impact than K/T but
"less" is a *bad* word in this case.
> So many
questions, so little time!
Plenty of
answers too, but you have to take the time to look for them.
At 06:11
26/07/02 +1000, Jim wrote:
>How does all this discussion apply to asteroid 2002 NT7? This
is supposed
>to be about 2 km long. If a nuclear explosion near it would
not be enough
>to deflect it, how about attaching a couple of rocket motors to it
and
>steering it away from its collision course? Could this be
achieved in the
>next 17 years? If it does collide, how would its effects
compare with the
>famous K/T boundary event?
Ian Musgrove replied
While 2002 NT7 comes close
to Earth, a "collision" is unlikely, it's current risk factor is rated
as 0.
See the NEO page for a
sober assessment of the risk:
http://neo.jpl.nasa.gov/
While the asteroid is
unlikely to hit earth, and is sufficiently massive that a simple rocket
burn would do anything useful, the continuous application of small
thrusts over 17 years could alter the orbit of an impactor
sufficiently. See the BBC site for a musing on this topic
http://news.bbc.co.uk/1/hi/sci/tech/2148924.stm
Of course we currently don't have the technology to get either atom
bombs or decent thrusters onto an asteroid at the moment, so the
question is moot.
The K-T even was due to an
object between 10-12 km in size, 2002 NT7 is closer in size to an
impactor that hit around 2 million years ago, equivalent to around 1
million megatons of TNT, while it didn't seem to cause mass
extinctions, it would have had a significant effect on costal organisms
sending tsunami's racing up the coast of Chile and produced a few hard
years of climate change.
See Spaceguard Australia and Tusnami's (scroll to the end of the page)
http://www1.tpgi.com.au/users/tps-seti/spacegd7.html
On Friday 26 July
2002 22:57, Ian Musgrave wrote:
>If it is a rubble
pile, it is most likely a carbonaceous chondrite.
Zero replied
No, rubble piles
are just as likely to be stoney or stoney iron asteroids as
carbonaceous chondrites. Eros, a stoney asteroid is thought to be a
rubble pile Wouldn't vacuum cementing take care of that when make of
relatively homogenous material (asteroidal rock)?
>No. While vacuum
cementing will keep sub surface regolith more or less stable, impact
>disruption and stresses fro gravitation interactions with the
planets (mainly Jupiter) will >overcome vacuum cementing for the
larger bodies.
That means it should be like Tunguska and break up easily and
those fragments should further fragment on entering atmosphere. A 12
kilometer rock travelling at _interplanetary_ speeds does a heap of
damage whether it is intact or as lots of 100 meter chunks. No
question.
But birdshot does
less lethal damage than elephant shot.
This is a misleading analogy,
in that the masses of birdshot and elephant shot are different, and
birdshot undergoes significant aerobraking, this is far less true of
100 meter diameter asteroids.
Being hit by 12 kg of lead moving at 40 km/sec is lethal, irregardless
of whether it is as 1 chunk, 1 kg lumps, or birdshot sized lumps.
No matter the number of pieces, the same amount of kinetic energy
gets dumped into the atmosphere, generating ozone busting nitrogen
compounds and cooling aerosols
Tunguska was a
million ton "fragment" of carbonaceous chondrite... [snip]
Tunguska was rated at the 100
meter mark - call that birdshot. At roughly 100-300 Megatons of TNT,
which devastated 10,000 square kilometers, that's mighty nasty bird
shot. You want 120 of those? (actually you get more like a thousand 100
m fragments from a 12 km parent body) You would have the equivalent of
a modest nuclear war with nuclear winter and ozone layer destruction
thrown in. Not a very good prospect I'm afraid. But a lot better than
larger fragments, surely?
You just get the kinetic energy of the 12 km original distributed over
a larger area. This is not a definition of better with which I am
familiar. Is it better to be hit by one 1000 megaton nuclear bomb, or
10 x 100 Mt nuclear bombs?
Cheers! Ian
On 29/9/2002,
Donald Lang added:
We have recently had a thread about what to do if there is a small body
threatening, well down the track, to thump this planet.
I am interested to know if there is a collection of wisdom on how to
give it an appropriate nudge and end the problem.
It is easy to think of a special case or two. Going to an extreme,
imagine that the collision to be avoided is not in our comparatively
crowded bit of the solar system but out where the comets are reputed to
lurk before they move in on us. Avoid relativity definition
problems by measuring all speeds relative to a system that rotates with
the sun around the centre of the galaxy.
If two bodies out there are going to collide in a century or five, you
would first examine the collision parameters. The speeds at a distance
of, say, a light month from the sun should be of the order 75 times
slower than here. If you discover that two bodies and their velocity
vectors are in the same plane, you need to examine whether somewhere
down their track they will coincide.
I fancy you would try to shift the velocity of the lighter one. If the
distance still to travel is long enough you might try and make that one
leave the shared plane with a sufficient margin to go beyond the
boundaries of the heavy one. That is a push perpendicular to the plane.
You could also give a push at right angles to the velocity vector in
that plane, but still in that plane. You are then making it pass across
the track of the other a bit in advance or to the rear of the spot of
the threatened collision. You can also give the lighter body an impulse
in its current direction of travel so that it goes through the
threatened impact point but sooner or later than the nearest edge of
the other body.
This problem looks comparatively straight forward. The velocities may
well be considered as constants over a considerable time. A small
impulse has a much greater angular effect than that same impulse
administered to velocities that prevail a lot further down the gravity
well. If you are worried about destroying the lighter body with the
impulse you give it, you may have more time to push and therefore a
lesser risk. You can also choose a suitable combination of the three
distinct components of the push as set out above.
Things get a lot more complex 'down' in the middle of the solar system.
The first thing to remember would seem to be the extra built in speed.
If you want to deflect the velocity vector by a particular angular
displacement you need approximately seventy five times the impulse,
with corresponding extra risk of disruption. Standard wisdom would
appear to be that there are more bodies crowded together and with much
higher relative speeds.
There is a problem that made teachers very happy in the days when
'relevance' could include problems above space rendezvous situations.
The suggestion is that you are in the same circular orbit around the
Earth as the body with which you are to dock. You are however five
minutes behind it. Naturally you sight along the pointy end of your
craft and fire the rocket at the other end briefly. This is supposed to
give you a nudge equivalent to closing the gap in one orbit. You then
return to your other tasks until it is time to dock one orbit later.
When you get ready one orbit later you discover that you are now ten
minutes behind your target.
Now in the real world your possible nemesis may actually go close
enough to be pushed about a bit by Mars or Venus. It would seem that
there might be some way of profiting by that. I would like to know if
there are obvious ways to profit by nudging the object at a particular
part of its orbit.
I suppose I should also ask humbly for quiet correction of any too
awful errors.
Karyn added:
I was reading a theory on
Space Daily that by painting the asteroid the solar winds would change
the path enough to miss earth.
As for ways to profit by
nudging ... then unless it was early enough perhaps to get a bit of
asteroid belt from jupiter/saturn (which I think is high in minerals?)
then I have no idea :)
Chris Luke
replied:
Painting the asteroid might work if it was within earth's orbit - the
intensity of light would be high enough
The problem is that we probably don't know, because of disruptions from
Jupiter (the gorilla on the block as far as affecting orbits) whether
an object will come close or miss until the last minute, and even what
action to take to nudge it away.
ie - it might be missing, and we move it so it hits earth - bad move
dudes!
Ian Musgrave responded:
At
06:11 26/07/02 +1000, Jim wrote:
>How does all this discussion apply to asteroid 2002 NT7? This
is supposed
>to be about 2 km long. If a nuclear explosion near it would
not be enough
>to deflect it, how about attaching a couple of rocket motors to it
and
>steering it away from its collision course? Could this be
achieved in the
>next 17 years? If it does collide, how would its effects
compare with the
>famous K/T boundary event?
While 2002 NT7 comes close to Earth, a "collision" is unlikely, it's
current risk factor is rated as 0.
See the NEO page for a sober assessment of the risk http://neo.jpl.nasa.gov/
While the asteroid is unlikely to hit earth, and is sufficiently
massive that a simple rocket burn would do anything useful, the
continuous application of small thrusts over 17 years could alter the
orbit of an impactor sufficiently. See the BBC site for a musing on
this topic http://news.bbc.co.uk/1/hi/sci/tech/2148924.stm
Of course we currently don't have the technology to get either atom
bombs or decent thrusters onto an asteroid at the moment, so the
question is moot.
The K-T even was due to an object between 10-12 km in size, 2002 NT7 is
closer in size to an impactor that hit around 2 million years ago,
equivalent to around 1 million megatons of TNT, while it didn't seem to
cause mass extinctions, it would have had a significant effect on
costal organisms sending tsunami's racing up the coast of Chile and
produced a few hard years of climate change.
See Spaceguard Australia and Tusnami's (scroll to the end of the page)
http://www1.tpgi.com.au/users/tps-seti/spacegd7.html
On
the 12/8/2003, this topic was reopened with a post from Nick:
I
was just watching the ABC and an interesting story is airing this week
on catalyst about an asteroid 1km wide on a direct heading for earth,
which is set to touch down in 2880. According to the article on the
catalyst website (http://www.abc.net.au/catalyst/stories/promo.htm)
nuclear weapons may not even be effective at stopping such an asteroid,
for reasons listed in the article. Given the seriousness of this
threat, it does leave me wondering what we would do if a large asteroid
were to pop up around the corner in the not to distant future.
Ashley Tracey
responded:
I thought there
might have been panic in the streets the way they advertised this,
anyway if we, ( not me!) can't stop this one in 800 years maybe we
don't deserve to continue.
Rob Geraghty
commented:
I'm surprised
that deflecting the asteroid is rejected so easily. I wouldn't
have thought it was so hard for a nuclear detonation near even a low
density asteroid to deflect it enough to miss the earth or at least
skim the atmosphere instead of being a direct strike. Better still, a
series of small detonations. The main requirement would be to
make the deflection a long time in advance of the impact rather than at
the last moment, so the deflection required would be much
smaller. Blowing the asteroid into pieces would be more likely to
cause problems than deflecting it.
It would be
interesting to write a simulation.
Peter
Macinnis wrote:
I
was asked yesterday to put together a simple guide to ways our world
may end (that is, things that might bring about the end of our
civilisation as we understand it, wiping out 95% of the population,
losing vast amounts of knowledge and technology). So far I have,
in more or less descending order of probability as I see it, but I
would be interested in additions or reasons for changing the order.
My
next task will be to explore what we ought to pack into a knowledge
lifeboat. I have been down this track before, and will probably go
there again, but it will mostly involve principles and methods, recipes
for measuring and recovering, stuff like that.
Anyhow,
here is my bang and whimper list:
*
asteroids
*
ice age triggered by climatic effects
*
warfare over either religion or water, escalating
*
desertification and famine
*
loss of biodiversity and ecosystem collapse
*
crop diseases that affect grasses and cereals
*
massive vulcanism (unlikely to be worldwide)
*
plagues previously unknown (they will happen, they won't end anything,
going on past experience)
*
alien invasion
*
massive bioterrorism (likely but not totally fatal)
*
chemical weapons (making the huge amounts needed is almost impossible)
*
GM crops (there is a potential for harm, but no likelihood of total
devastation)
I
have ruled out unnatural phenomena like the world going mad because
they are forced to listen to Margaret Thatcher before they have brushed
their teeth, cohabit with John Howard, or take master classes with
George Bush.
Toby Fiander
responded:
Peter said:
>
I was asked yesterday to put together a simple guide to ways our world
> may end (that is, things that might bring about the end of our
> civilisation as we understand it, wiping out 95% of the population,
> losing vast amounts of knowledge and technology).
I know Peter
wanted a simple guide, but I don't think that the factors that would
lead to a collapse of civilisation would be simple. I also think
societal effects need to be mentioned in any analysis. Commonly,
society is spoken of in terms of the economy due to the
predominance of greed, so that is what I have done below.
The combination
effects of the list seem much more likely to occur than any single
event. For example, loss of biodiversity and climatic change
(not necessarily an ice-age) could jointly lead to ecosystem changes
(not even collapse), and the economic collapse leads to
wars for complicated reasons involving food water and energy... and
eventually to widespread nuclear and biological
conflict. There is an argument we are on the top of this slope
but the bottom is not visible..
Alternatively,
some minor natural event (not necessarily as disasterous as a large
asteroid, but that is an extreme example) which precipitates
sufficient hardship and thus political change to bring a world economic
collapse ... with similar effects... and so on.
There are even
some alternative later pathways in which there is economic warfare of
the kind we have seen a small Australian example of
recently. Examine carefully why the Government would suddenly
impose a tariff on the importation of alcohol affecting small liquid
fuel suppliers only. No wonder Trafigura did not want to talk
about MTBE and the alternatives when I wrote to them about it - but I
digress.
My point is that
there is a linkage between all of the matters Peter has listed - except
perhaps the invasion of aliens - and so when the cataclysm
occurs, there will be no one single factor. Alas, the linguistic mood
and tense are deliberate.
As to what would
be put in any bank of knowledge to assist in the reconstruction of
civilisation, I am uncertain. I think the important matters
probably relate to how not to get into the same predicament again,
which is more of a socialogical matter than a scientific one. But
the following science-related matters are probably of greatest
importance:
- expert knowledge in physiology, diagnosis in humans and,
possibly, surgery; this would need to be more than a computerised
paper record to be useful,
- the compiled wisdom in engineering, mathematics and information,
and communication technology
- biological understanding relating to ecosystems and whatever
genetic biological wisdom is then available,
- skills in farming, and the major trades....
Look, I don't
know - culture is so complicated. We, the humans not as
individuals but as a collective, are the knowledge bank. Destroying or even
ignoring any of it seems silly and preserving any of it without a
society to do it seems quite difficult and in a way pointless.
Gary-Peter
Dalrymple wrote:
Two
points
Firstly,
Apocalypse stories are a fairly common sub-genre in Science Fiction.
The
Sydney Futurians have discussed the topic and a websearch of panels at
World Science Fiction conventions will reveal more. I.e Russell
Braddon has the world ending due to a virus that makes sheep carnivores
and there are stories of giant mutant funnel web spiders (by an Irish
Author!) and a giant mutant rabbit plague etc. many are ridiculous, but
the writers cover most concievable 'angle'.
Secondly,
I would not be so relaxed about the 'Asteroid threat'. To
the best of my knowledge there is no table or model for an accurate
estimation of the size distribution of asteroids.
I.e. most of the ones that we know of are 'bright' and regular, having
been observed at advantageous times (near Earth Crossing, near
opposition) by an irregular array of telescopes.
If
the current Spaceguard survey can just see a 10km (dinosaur killer) at
Mars orbit, how many 5km or 1 km rocks are there that cannot be seen,
twice as many or ten times as many?. An much smaller rock
than this splashing down into the North Atlantic would not need to kill
more than 5% of the World's population to have a much greater economic
effect that the Black Death.
To
be 'discovered' an asteroid has to be observed at two or more points on
it's orbit, some time apart for an orbit to be calculated.
It is possible to catch an asteroid once and fail to recover it,
indicating an irregular orbit (uncalculatable risk of Earth crossing)
or just bad luck.
Wishing
you well and thinging that some of that Manildra millions could be
funding a decent Antarctic Asteroid spotting program.
Paul Williams
added:
<snip>
> Anyhow, here is my bang and whimper list:
>
<snip>
>
> * crop diseases that affect grasses and cereals
>
<snip>
A real
possibility if we end up totally relying on only a few genotypes.
(the Potato Institute in Peru holds/grows thousands of potato
cultivars - it would be wise if all important food crops had this
safety fallback facility)
>
> * alien invasion
Alien invasion
may be down the list a little...
Who/what would
bother?
Using an
anti-matter/matter propulsion system we could do a round trip to Beta
Centauri in about 20 years or so.
If we set up a
most efficient self-replicating automatic probe system, using
anti-matter/matter propulsion, physicist Lawrence Krauss calculates
that we could explore the galaxy in about 10 million years. It is
*only* about 90,000 light-years in diameter.
Why aren't alien
probes here now?
Or maybe they
are....
Or maybe we're
alone - which perhaps makes it even more important that we don't stuff
up.
Peter
Macinnis responded:
At
10:50 13/08/03 +1000, Toby wrote:
>Peter said:
>
>> I was asked yesterday to put
together a simple guide to ways
>I know Peter wanted a simple guide, but I don't think that the
No,
the client wants a simple guide. I will be following a path much
like yours, arguing that most of them (other than the loopy ones are
likely to feed into each other: surges from an asteroid blast would
release clathrate methane, the old Chicxulub asteroid - Deccan basalts
flows linkage is still to be tested, and so on.
That
said, it should be possible to dissect them out, deal with each
separately, examine the causes, and then show how even small
interferences can blow out -- and I am not talking random chaos and
Brazilian butterflies here, but serious knock-on effects.
I
can see few better ways of reminding people of the essential
interconnectedness of things, if I may be forgiven a bit of quiet New-Ageism
there . . .
Nick replied:
Yes, the
way it was advertised could have led you to believe the human race was
doomed any time into the near future unless you went and did a bit more
further research, but it's all about getting the viewer interested in
the scheme of things I suppose
David
Maddern added:
Emails
into my machine on this subject have not seemed to mention that a thing
expected to cross our path in 800+ years must have an enormous error in
that expected path, for instance if tomorrow it deviated 1mm by then it
will be outside the solar system by squillions of K.
Rob Geraghty
replied:
>
Emails into my machine on this subject have not
> seemed to mention that a thing expected to cross
> our path in 800+ years must have an enormous error
> in that expected path,
Possibly.
I imagine they checked their calculations reasonably thoroughly.
The earth is a pretty big target.
>
for instance if tomorrow it deviated 1mm by
> then it will be outside the solar system
> by squillions of K.
I seriously
doubt that a 1mm deviation would impart solar escape velocity.
1mm per day as a continuous acceleration over 800 years would have that
sort of effect, but I don't think the asteroid has an engine attached
to it?
There's the
solution to the problem - just whack an ion engine on it sufficient to
divert it by 1mm per day for 800 years... :)
Seriously
though, I think the thing which people are missing is that we've only
just started looking and have already found an object which has a
trajectory which will intersect that of the earth. It increases
the likelihood of finding something which may hit us sooner, I would
think. Or we could stick our heads in the sand and say "Que sera
sera".
Tony
Legg wrote:
>
Anyhow, here is my bang and whimper list:
>
> <snip>
> * alien invasion
>
> <snip>
Peter,
alien invasion has no credibility whatever, unless you are referring to
microbes or viruses from space á la Hoyle.
Nick replied:
Assuming aliens
do exist however (which i believe is what your questioning here), but
assuming they do, if we look at earth and look at the split of nations
as peaceful and hostile, wouldn't it be a reasonable assumption that
there'd very well also be a race of hostile species and unhostile
species, or even further- hostile portions of a species and unhostile
portions of a species. Then permitting the fact they actually had the
technology, whcih if they do exist and can get here the civilisation is
obviously thousands of years more advanced then ours and could have
very well proven everything we 'know' about space travel (ie-faster
then speed of light etc) wrong in the process, then who is to say they
very well couldn't be hostile and invade us from space.
Perhaps more
fiction then science, but hey, you never know what the seti@home client
may find any one of these days.
Peter
Macinnis added:
I
rather think and hope that I will live to see the first evidence of
life evolving in another system -- then we can definitely speak of a
law of evolution.
There
is a LONG tradition of depicting Utopias at increasingly greater
distances, and the first "men in the moon" and Martians were typically
well-meaning and friendly, but Imperialism came into SF in the late
19th century as Belgian, Briutish, German, French and other hoons began
stealing countries, and suddenly we had the notion of the
technologically superior race enslaving us.
When
imperialism and hegemonism fades away, perhaps we will assume peaceful
aliens once again, but for as long as technological superiority is
assumed to confer the right to impose one's will, that will not happen.
Technological
superiority confers the right to help others to attain the same level,
but not the right to force them to attain that same level.
Donald Lang
wrote:
Before I get
left too far behind on this item, and with apologies for not doing
homework in advance, you might add risks of a "nearby" supernova.
The homework
would presumably require working out how near a supernova needs to be
to represent a risk to our level of technology and how much closer
before it might wipe out life on Earth. The risks are I think at
longest range an extra flood of cosmic rays that would overwhelm the
magnetc field of the Earth and release the dreaded flood of mutations
on the surface. A
bit closer and
still without homework I can see a threat to the atmosphere just
because the amount of visible and other radiation would boil it away.
The next step is to get close enough to boil the oceans as well.
A quick estimate
and I will duck for cover. A supernova gets as bright as a whole galaxy
- briefly. Say 10^12 times as bright as our sun. That is eight light
minutes away. Shove in an inverse square law. A supernova eight million
light minutes away would then be as bright as our sun, as seen from
Earth. That is roughly five thousand light days, or about a dozen light
years. Such an item would give us a considerable scare. I don't think
there is anything in a considerably larger radius, say a couple of
hundred light years, that is considered a warming up candidate as a
supernova. So my guess is that we don't need to be alarmed yet and we
need not even be super alert. But if one did stick its nose up in the
form of a suddenly expanding red giant, quite a lot of humanity might
decide that they really had learned a lot in Sunday School after all.
Those who think this would be a revolting
spectacle might
as well pray that it does not happen.
Jim
Thornton added:
We
know Earth is teaming with life. So, if we put all the scientists
on the moon, would they be able to communicate with any life form other
than mankind?
So
what hope is there of communicating with anything further afield
regardless of being peaceful or hostile; not to mention the naive
expectation something hostile would even admit it? One thing is
almost certain, they won't have signed the Geneva Convention.
Ian Musgrave
replied to Rob:
>Possibly.
I imagine they checked their calculations
>reasonably thoroughly. The earth is a pretty big
>target.
Space is really
REALLY big. You may think that it's a long way to the shop if you want
a sausage roll, but compared to space that's nothing.
In reality,
Earth is a small target, and the asteroid a much smaller dart.
Working out if one hits the other is no small matter. Especially
when minute observational uncertainties can mean the orbit may be out
by hundreds of thousand kilometers.
What typically
happens is when an unknown asteroid is discovered it is watched over a
few nights and its position measured as accurately as possible on that
system. An orbit is the derived and checked to see if it intersects
with Earths. The orbit isn't a cut and dried thing, but a probability
estimate (this is never mentioned in popular reports). If the orbit
intersects, a call is sent out via the asteroid observers network for
more observations for different stations, to improve the accuracy of
the orbit estimation, and the orbit is recalculated, more observations
are taken. the orbit recalculated again etc.
What also
typically happens is that reporters monitor the asteroid network, pick
up the first "call for measurement" and write a "killer asteroid to hit
in year xxxx" before the asteroid watchers have revised their orbits.
While
preliminary observations may indicate that there is an earth crossing
in 880+ years, small (and I mean small) errors in
the astrometry means there is a large uncertainty about this, also, the
contributions of minor gravitational tugs from Jupiter etc. will affect
the orbit over this time frame, and they are harder to model when these
sorts of time frames are considered (as small errors add up).
As an example, A
few weeks ago a relatively well known asteroid, Benjamina, occulted a
star. Despite being well known, corrections to the astrometry were
being made right down to the wire. If we need to correct an orbit for a
well known asteroid days before an event, imagine the corrections
needed for a newly discovered asteroid.
Rob
Geraghty wrote:
Whether
an asteroid hits the earth, skims it or misses in 800 years is really
of no importance to anyone alive today. The more important point
- as I mentioned earlier - is that we've only just started looking, and
we've already found an object which has a trajectory
that will coincide (roughly, if you prefer) with that of the
earth. Doesn't that make it more worthwhile making sure there
isn't one that could turn up sooner?
Gary Ruben
replied:
One thing I
wasn't really sure about on the Catalyst asteroid story, which someone
may be able to help with was the argument that a porous asteroid would
not be deflected as much by an explosion as would a solid asteroid. It
seems to me that a porous asteroid of the same size would have a much
lower mass and therefore be easier to deflect. Then they showed a NASA
guy shooting
a chunk of porous rock with a big gun and claimed that it absorbed the
pellet without disintegrating. Doesn't this fact mean that it would
absorb the energy of a nearby explosion more efficiently which should
make deflection easier again?
As for Peter's
list,
Should comets be
in the list as a separate entry?
What about
'Scientists create mini-black hole in accelerator which expands to
engulf the Earth'. Actually, after I wrote this line I noticed that
Paul's link to The Age article has Martin Rees raising this one.
How about rogue
black hole sails into the Earth.
How about adding
'Evolution of humans into a separate species' either via the
human->cyborg->robot route or the genetically modify beyond
recognition route.
I note that the
Alien Invasion entry could equally well appear at any position on the
list depending on your personal preference.
Ian Musgrave posted:
> > In reality, Earth is a small
target, and the
> > asteroid a much smaller dart.
>
>Whether an asteroid hits the earth, skims it or misses
>in 800 years is really of no importance to anyone
>alive today.
I think that
worrying about the survival of the human race in the future is of
importance to those alive today. However, the issue addressed was the
certainty of the measurements.
>The
more important point - as I
>mentioned earlier - is that we've only just started
>looking, and we've already found an object which has a
>trajectory that will coincide (roughly, if you prefer)
>with that of the earth.
It's the third
object found so far, the other two generated much media hype until
further measurements showed they would not hit.
>Doesn't
that make it more
>worthwhile making sure there isn't one that could turn
>up sooner?
We worked that
out before we even started looking, this is why things like LINEAR (the
Lincoln Institute Near Earth Asteroid Robotic telescope) and Spaceguard
(http://www1.tpgi.com.au/users/tps-seti/spacegd.html) were set up. To
catch these objects.
I don't
wish to give the impression that there is no threat from these
objects, and I definitely support the search for them, unlike our
short-term orientated politicians. However, I am trying to put the
discovery of potential objects in perspective. They tend to get hyped
out of all proportion while we are in the early phases of determining
their orbits to sufficient accuracy to determine if they are a threat.
and:
At
10:30 15/08/03 +1000, Gary wrote:
>One
thing I wasn't really sure about on the Catalyst asteroid story, which
>someone may be able to help with was the argument that a porous
asteroid
>would not be deflected as much by an explosion as would a solid
asteroid.
>It seems to me that a porous asteroid of the same size would have a
much
>lower mass and therefore be easier to deflect. Then they showed a
NASA guy
>shooting a chunk of porous rock with a big gun and claimed that it
absorbed
>the pellet without disintegrating. Doesn't this fact mean that it
would
>absorb the energy of a nearby explosion more efficiently which
should make
>deflection easier again?
No.
Most of the energy goes into locally compressing the material, rather
than deflecting the asteroid as a whole. Compare the effect of hitting
a billiard ball with a billiard cue, and hitting a lower mass hacky
sack with a billiard cue. The billiard ball caroms of at a rate of
knots, the hacky sac quivers a bit and doesn't roll far at all.
There
was a good article a few months ago in either New Scientist or
Scientific American about this.
Paul Williams
commented:
This New
Scientist article (from last year) touches very briefly upon possible
methods:
http://journals.iranscience.net:800/www.newscientist.com/www.newscientist.com/news/news.jsp@id=ns99992730
A giant 'pillow'
is suggested to cushion the asteroid/comet as thrust is applied by a
fueled-up spacecraft. Long term gentle nudging/pushing appears to
be the way to go. We shouldn't need a 'pillow'. I would think
(but do not know) that larger asteroids would have some spin - this, it
seems, would complicate matters somewhat.
Thinking upon
possible results from an 'encounter' with a large asteroid: The
palaeontologic record indicates that mass extinctions occur at 'long'
time intervals. Some argue that mass extinctions occur every 26
million years or so. The fossil record is far from perfect so it
may be best concentrate on the well documented mass extinctions of
which there have been 6 since the Late Cambrian.
We do not know if these extinctions were caused by asteroids.
Certainly the weight of evidence for the extinction event at the end of
the Cretaceous (65 mya) leans one strongly towards a 'doomsday
asteroid' belief.
The greatest of
all mass extinctions occurred at the end of the Permian (about
245 mya) when about 95% of species disappeared from the fossil record.
This was about 115 million years after the previous mass
extinction event.
We don't know
what caused the Permian extinction event but one does feel some
suspicion when looking up at the night sky...
Jim Thornton added:
The slow motion
image showed the rock moving slightly when hit by the bullet. So the
impact was not totally absorbed. At a large enough distance a
small movement is all that may be required. Providing all the
calculations are correct and it doesn't move it into a collision course.
Paul Williams,
replying to Donald:
> Before I get
left too far behind on this item, and with apologies for not
> doing
homework in advance, you might add risks of a "nearby" supernova.
<snip>
Not doing
homework either...
I think (but do
not know) that the radiation expelled by a supernova, appearing as
bright as our sun, would be very nasty. I realise that 'very
nasty' is not something quantifiable, so may I hazard that this
radiation may destroy the Earth's ozone layer at some multiples of 12
light years distance?
>I
don't think there
> is anything in a considerably larger radius, say a couple of
hundred light
> years, that is considered a warming up candidate as a supernova.
So my guess
> is that we don't need to be alarmed yet and we need not even be
super alert.
I had to cheat
in the end .... found this intriguing article which suggests that a
recent extinction of molluscs (2 million years ago) was the result of a
supernova at a distance of perhaps 120 million light years. This view
is considered speculative:
http://www.aip.org/pt/vol-55/iss-5/p19.html#ref
This led me on
to 'The Local Bubble' - lots of 'recent' supernovae - maybe. A
new satellite - the Cosmic Hot Interstellar Plasma Spectrometer
Satellite (CHIPS) launched earlier this year should enable us to learn
a lot more about 'The Local Bubble':
http://science.nasa.gov/headlines/y2003/06jan_bubble.htm
And then some
possible nasty 'space weather' (gas and dust) coming to a solar system
near you - soon (about 50,000 years):
http://www-news.uchicago.edu/releases/96/960609.solar.sytem.crash.shtml
Gerald
Cairnes posted:
I
also have some reservations about the conclusions as you seem to have.
Some where I have read that if the largest hydrogen bomb we can
currently make were detonated at a kilometre above ground it would
leave a hole as much as 800 m deep. Maybe I have the numbers wrong but
if that is the case then a kilometre wide asteroid IS going to feel the
blast big time and there
is a relative difference between a very small pellet and the rock
samples tested compared with a multi megaton nuclear device and the
asteroid.
Jim Thornton
replied:
I saw the
program last night but can't recall anybody reassessing the damage
caused by a dry sponge rather than a solid rock.
Gerald
Cairnes replied:
There
were two illustrations one of solid rocks and one of porous rocks being
impacted by small high speed pellets. I have no difficulty with the
absorbance of the shock waves by the porous rock, that was anticipated
but my objection is that the rocks were impacted by very small pellets
relative to the rock size. A multi megaton bomb as large as we can make
compared with
the asteroid in question is not a fair comparison even though I do not
challenge the results of the experiment.
On
the point Gary was making I think the reason that there would be a
proportionately reduced impact on the porous rock lies in the way the
shock waves would be diverted along the enormous number of walls of the
cells in a totally random way thus reducing the mass directional
effects and dispersing the impact internally. Would be expected to get
a bit hot though. Porous structures such as this can be enormously
strong but light.
Jim responded:
The point I was
making was in regard to potential impact damage if the rock hits
Earth. Would it be any different being porous?
Gerald
answered:
At
a kilometre wide I don't think we'd notice the difference even if it
could be measured. On a mass for mass basis I should imagine the end
result should be much the same even if the impact characteristics are
different. You are right though because the experiment was about a
small pellet impacting a relatively large porous object where the real
issue is a relatively small porous object (maybe) impacting a large
solid one. Not the same thing.
I
would think in reality a massive nuclear blast close by would shatter a
porous asteroid regardless of the porous nature as the pores would be
too small to have much effect relatively. The resistance in the pore
tracks would be massive and it would be shattered long before any shock
absorbing could take place. The nuclear pressure wave would effectively
be virtually instantaneous.
David Allen
wrote:
Did anyone else
pick that on Crapalyst the pores in the porous object were stated to
contain air? Highly unlikely, I would have thought.
Rob Geraghty replied:
I
missed the episode of Catalyst, but maybe "air" was a poor choice of
words. "gas" maybe would be a better one. I have no
problem with a rock in space containing pockets of gases from when it
formed. Have you ever seen a "thunder egg"? They are formed
when minerals grow into a bubble inside rock.
Or
how about pumice stone? It's got enough bubbles of gas in the
rock structure that it can (in some cases) float on water.
David Allen
replied:
>
I missed the episode of Catalyst, but maybe "air" was
> a poor choice of words. "gas" maybe would be a
> better one.
Maybe I should
have inserted quotes around 'air'. Air is a fairly specific mixture of
gases which, I venture to suggest, is unlikely to be common in the same
proportions elsewhere in the universe other than Earth. Being,
allegedly, a science show I would have expected a more careful use of
words. In any case, I would have found an exact description of the
gas(es) present in the pores to be of considerable interest.
>
I have no problem with a rock in space
> containing pockets of gases from when it formed.
Neither have I.
>Have
> you ever seen a "thunder egg"? They are formed when
> minerals grow into a bubble inside rock.
There was a
tourist attraction devoted to 'Thunder Eggs' a little west of
Rockhampton until a year or so ago when it closed after a fatal
landslide. The answer to your question is, 'yes', dug 'em up in fact.
>Or
how about pumice stone? It's got enough bubbles of gas in the
> rock structure that it can (in some cases)
>float on water.
Not uncommon on
the beaches hereabouts and as a one time resident of New Zealand
something I'm fairly familiar with.
My criticism was
entirely to do with the use of the word 'air'. Had gas or gases been
used I would have had no problem.
