[meteorite-list] Future Planetary Collision?

Thu Jun 11 17:35:03 EDT 2009

Hi, Charley, List,

    I'd just spotted the same press release
(it turns out) on Space.com:
http://www.space.com/scienceastronomy/090610-planets-colllide.html

    The wobbly behavior of the Inner Solar System
is not a new discovery. Here's a movie of the inner
solar system's actual orbital evolution over the last
3,000,000 years:
http://muller.lbl.gov/pages/innerplanets.html
It can be downloaded directly from here: 
http://muller.lbl.gov/images/inner.mov

    The movie that you can view or download from
this page is about 12 Mbytes long, and in .mov
format. I used QuickTime Player (.mov is its native
format) because you can step through it frame by
frame (right/left arrows). Real Player and Windows
Media Player (10) will also play it. You can open it
in a browser window if you have the Quick Time
plugin (takes a bit to download).

    The scale of the animation is not exaggerated
or amplified. If you could sit in space and watch the
inner solar system trace each orbit with a visible
line, this is what you'd see. The units on the edges
of the background plane are AU's. The site rather
modestly says, "Even if you are an expert, you may
be surprised at what you see!"

    "Drunk drivers at NASCAR track" would be
a good title, if you pasted in some little cartoon
racers with sponsor patches. Is this the renowned
"stability" of the solar system we hear so much about?

    And, of course, it IS stable. Nothing has gone
wrong in the last three million years nor for a long
time before that (or has it?). Still, everything just
wobbles like crazy...

    Currently, Venus can approach as close as 24.7
million miles and Mars as close as 34.7 million miles,
but it would seem that in the past (and future too)
their close approaches could be as near as roughly
half that distance.

    I found this movie to be utterly fascinating (could
be just me). After a few times through it, I would
concentrate on just watching one planet at a time:
Mercury slides back and forth like it was shifting the
Sun from one elliptical focus point to the other; Mars'
orbit expands and contracts; Venus and the Earth
pull up close and flirt with resonance lock; they
all rock back and forth.

    Venus is the one that worries me. The orbit of
Venus has peculiarities, too. Venus's "year" is 224.7
Earth days. Venus's "day" is 243.01 Earth days. But
because Venus's axial rotation is backward measured
against the Sun and stars, the Venusian "solar day"
 is only 116.75 Earth days long. Of course, we could
just as well not describe Venus's rotation as "backward,"
but just consider that Venus rotates "normally" but
with its axis turned completely upside-down, by 177.4
degrees! However you look at it, Venus is the only body
of any size in the solar system to rotate "backward."

    If you regard "normal" rotation as required, as it is,
by most theories of solar system formation, then you
have to invoke a Big Whack to turn Venus upside-down!
That would have to be one heck of a whack, too. The
energy transfer would be so great it's hard to imagine
the planet could have survived it.

    So, there's another theory: that the solar tides on the
thick atmosphere have braked Venus down to a standstill
and are now spinning'er up in the backward direction.
Myself, I think the atmospheric torque is just not big
enough to do the job, and since what little we know about
the surface of Venus suggests that there are virtually no
winds at all at the surface (and you have to have wind to
apply atmospheric torque to the surface), I think it's hooey.
The math is complex and not entirely convincing.

    The position of Venus in the Earth's sky cycles in the
time it takes Venus to lap the Earth in its orbit, 593.92
Earth days, the synodic period. Oddly, that period is
almost 5 Venus days, to be exact, 5.0014 Venus days.
This means, annoyingly, that when you're trying to
radar map Venus from the Earth at the close approach
when you have the highest resolution, you're looking
at almost exactly the same patch of Venus you were
looking at the last time! Over and over again.

    The synodic period of Venus, 593.92 Earth days,
is almost exactly 8/5ths of an Earth year, so that every
eight Earth years the positions of Earth and Venus line
up very closely with only a tiny amount of drift in position
from cycle to cycle. Every 152 Venus synodic cycles of
593.92 Earth days, the line-up returns to its original
precise positions, creating a long cycle of precise
repetitions of the positions of Venus and the Earth.
This long cycle takes 243.01 Earth years.

    Now, if the number 243.01 seems familiar, it's
because it "happens" to be the length of Venus's axial
rotation in Earth days, the sidereal period! The extreme
regularity of this cycle of Venusian positions with respect
to the Earth creates the long and precisely repeating
cycle of Venus's transits of the Sun, meaningless except
that these mark the timing of the Sun's passage across
the nodes of the mutual plane of Earth's and Venus's orbits.

    So, how many 243.01 Earth day sidereal periods of
Venus does the transit cycle take? Why, 365.24 of them,
which "happens" to be the number of days in the Earth
year, just as 243.01 is the number of Earth days in the
Venus day.

    Personally, I find that just plain spooky. Officially, these
coincidences are just that: coincidences. The Earth and
Venus are not in an 8:5 resonance, officially, yet when you
either regress or progress the orbits, these regularities
do not go away. They drift in and out of greater or lesser
regularity for as far as the floating point calculations can
go, for millions of years, without any divergence. It is an
extremely stable configuration.

    There was a lot of argument in the 1960's about whether
this was "really" a resonance or not, and by the 70's, it was
branded an annoying coincidence. Personally, I think it's
too neat to be a coincidence, so I was cheered last year when
I ran across a AGU paper that calculated that the differences
between the atmospheric tidal torques and the solid tidal
torques generated by Venus's tiny eccentricity acted to push
Venus back and forth  and avoid the adjacent two planets
falling into a recurring perfect face-to-face lock, Venus with
the Earth, by minutely altering the length of Venus' "day"!

    I stand on "spooky" as the best description of the
universe. Oddly, shifting from one dangerous situation
to another makes life in the universe fairly safe. The "danger"
in a dangerous situation is that you stay in that situation.
If two planets have a resonant lock and keep meeting
face-to-face, the two will increase the eccentricity (but
not the period) of each other's orbit by the slow repetition
of tugging at each other every close pass, making each
successive pass closer and closer and closer... So it's a
good thing that something always messes up that doomed
arrangement, like the meddling of Jupiter.

    These articles about simulations are always interesting,
but there are always things that can get overlooked. Take
the solar wind. Among other things, when you calculate
"backwards," you have to keep changing the mass of the
Sun! The solar wind carries mass away from the Sun so
that it becomes progressively lighter over long timespans.
You have to work out the rate of mass loss and keep
adding that mass back into the Sun as you go back
millions, even billions, of years in time! This constantly
changes the central force in your motion equations.

    I don't doubt the calculations... exactly. The principal,
Laskar, has been doing these simulations for more than
20 years. He says an Earth-Venus bump is the most likely
bad news (and that's obvious without a super-computer).
They say their model is more accurate "because Laskar
and Gastineau's model relies on non-averaged equations and accounts
for general relativity." Well, their model is not the first to
account for general relativity. The reason that they present
the results of many runs of their model is this: even though
the math is now accurate enough to calculate long periods,
every few million years (about 12) you run into a chaos
bottleneck, a "divergence," which is a fork in the road with
a 50%-50% chance that one of two paths is correct.

    What they have done is flipped a coin every 12,000,000
years or so and gone on, then repeated the run and gone
the other way this time, with each divergence. So they
get statistics, not certainty. 12 times out of 2500 times,
the Earth did such and so, they say. What does that
"prove"? Anything at all?

    There is no certainty. On the micro-scale, the universe
is quantum chaos. Particles tunnel right through force
barriers by de-materializing and re-materializing on the other
side. Electrons act like waves one minute and then turn into
particles the next instant. God plays dice with the universe;
matter transforms at random. It's a mess, Lord.

    On the macro-scale of everyday life, the universe is
deterministic. My computer works (most of the time); my
car engine runs; gravity always makes things fall at the
same accelerated rate. Objects that act like particles never
turn into waves and vanish -- Phfft! It's so orderly.

    But on the super-macro-scale of deep space and deep
time, the universe goes back to being in a state of quantum
chaos at a long slow pace, seemingly deterministic until it
goes just as random and whacky as the micro-scale universe.

    As for the solar system, enjoy it while it lasts.

Sterling K. Webb
---------------------------------------------------------------------------------
----- Original Message ----- 
From: "Charley" <cmb62 at columbus.rr.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, June 11, 2009 11:49 AM
Subject: [meteorite-list] Future Planetary Collision?

> Hi List,
>
> Maybe a bit off topic although lots of meteoroids would be created.
>
> A French researcher says we may have a collision with Venus or Mars in 
> 3.5 billion years.
>
> http://www.chinadaily.com.cn/world/2009-06/11/content_8271159.htm
>
>
> Best regards,
>
> Charley
>
> "Well, squids don't work. Hey! Let's
>  try elephants !"
>
>                    Hannibal
>
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