Mars and Jupiter colliding I was going to post a comment on my own theory about Mars and Jupiter colliding, but I just read the rules and it breaks it...badly. I was looking for informal peer review.
So my question is, has there been any mainstream published work about the possibility of planets colliding or interacting greatly? 
(Velikovsky doesn't count obviously, I'm thinking an actual scientist)
(besides the Earth-Moon of course)
I can't seem to find any. 
 A: There is tons of work about planets (and asteroids, moons, dwarf planets, Kuiper Belt Objects...) interacting greatly.
For example, the Nice model of the formation of our solar system states that the gas giants were originally closer together than they are now.  Interactions with small bodies caused the planets to spread apart until Jupiter and Saturn's orbital periods were in a 1:2 ratio with each other (whole number ratios of orbital periods cause interesting things to start happening).  This "orbital resonance" causes a significant change in the orbits of the gas giants.  Some realizations of this model even have Neptune originally being closer than Uranus and the two planets end up changing their order.
Pluto (as well as many other icy bodies) is in a 2:3 orbital resonance with Neptune: the influence of Neptune's gravity has locked Pluto's orbit to have this 2:3 orbital resonance.
Jupiter's three innermost moons have a 1:2:4 resonance, which is thought to keep Io in an orbit where tidal forces from Jupiter keep its interior hot and the moon is volcanically active as a result.
As far as collisions, it is thought that the Moon was formed as the result of a collision between the young Earth and a roughly Mars sized object.
And while a reference currently escapes me, I remember reading that Venus and Jupiter affect various modes of precession of Earth's rotation with potentially significant effects on the habitability of the planet over the millenia. 
A: The distance between the Earth and the Moon is increasing.
The distance between the Earth and Mars is Increasing.
The distance between the Sun and every planet is increasing.
So the distance between each planet and the next planet is increasing.
The Sun and all of the Planets, because they are relatively large,
all collect space materials, so they gain mass.  The rate is comparatively
low compared to the rate when the planets were first forming between
roughly 4.6 and 3.8 Billion years ago, but it is still there.
The strangest part is that when Planets, and especially the Sun, 
gain mass, the biggest partner of any Pair accelerates the smaller
partner to a higher orbit, and a higher orbital velocity, while at the same 
time decelerating itself to a lower velocity and a lower orbit.
Both Orbits are relative to their combined Barycenter ( point where both
planets orbit about ).
The Earth-Moon distances increase by 38.2 +/- 0.7 mm per year.
This is the net change in distance, measured by Lunar Laser Ranging.
The distance between the Earth and Mars increases by several meters
per year, measured by the increasing amount of time in milliseconds
for signals to be received and responded to by Machines on the Mars surface.
Planets act like gravitational vacuum cleaners.  They clean a pathway,
around the Sun, and they bend trajectories of larger objects, that will
eventually collide with planets, thus allowing the planet to gain mass.
What is interesting is that you can estimate the age of the Earth-Moon
system by taking a ratio of the change in the average distance per 
year divided by the average distance between the two Planet Centers.
Both must be in the same units ( millimeters ). The change is one part 
in 9.08876 E 9 units ( mm / mm ), so the age is 1/2 of the denominator.
That is 4544.38 Million years. It is 1/2 because 1/2 of the mass of the 
Big Planet ( Earth ) goes to moving the Moon up, while the other half goes to
lowering the Earth down to a lower orbit.
This mass change is roughly 1.3 x 10^15 kg per year increase in Earth mass,
but the surface area of the earth is so enormous, that the increase is only about
1.03 mm per year of increased radius.  most of this increase gets washed away,
and ends up in the bottoms of the oceans mixed with what ever gets eroded off
continents, and deposited on the ocean floors.
So to answer your question, no Mars and jupiter should never collide.
They should continue to gradually move farther apart as both move farther away from the 
Sun.
Mike Clark
Golden, Colorado, USA
