
The random walk of cars and their collision probabilities with planets - sndean
https://arxiv.org/abs/1802.04718
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antognini
Hanno Rein and Daniel Tamayo are pretty legit dynamicists. For anyone
interested in doing calculations like these themselves, they've been working
on this amazing N-body library called Rebound:

[https://github.com/hannorein/rebound](https://github.com/hannorein/rebound)

One of my all-time favorite figures is Fig. 1 of this paper of theirs
describing one of the integrators used in Rebound:

[https://arxiv.org/abs/1704.07715](https://arxiv.org/abs/1704.07715)

~~~
simonbyrne
Thanks for the link, that paper was a great read: clever idea, and very well
written.

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userbinator
_By running a large ensemble of simulations with slightly perturbed initial
conditions, we estimate the probability of a collision with Earth and Venus
over the next one million years to be 6% and 2.5%, respectively_

More fortunately, even if it were to "collide", it would likely burn up in the
atmosphere like the majority of other, even very large[1], objects that
reenter.

[1]
[https://en.wikipedia.org/wiki/Mir#Final_days_and_deorbit](https://en.wikipedia.org/wiki/Mir#Final_days_and_deorbit)

~~~
pavel_lishin
"Free Tacos for Everyone in the Solar System if Mir hits our Target on Venus!"

~~~
eterm
Reference for reference:
[http://news.bbc.co.uk/1/hi/world/americas/1231447.stm](http://news.bbc.co.uk/1/hi/world/americas/1231447.stm)

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fsiefken
2 questions:

> The repeated encounters lead to a random walk that eventually causes close
> encounters with other terrestrial planets and the Sun.

What are terrestrial planets? I thought there was only one terrestrial planet;
earth. Or is meant 'earth like' planets, specifically Venus, Mars and Earth?

> dynamical lifetime of the Tesla to be a few tens of millions of years

What is the dynamical lifetime? I'd think that with all the rocks out there
the Tesla will get some dents from collisions. So many dents that I think in a
million years it'll be toast. How does this square with a dynamical lifetime
of tens of millions of years?

~~~
PeterisP
What collisions? The whole point of this article is that the most likely rock
to hit Tesla is the Earth. Space is very, very, very empty; all the (many)
rocks out there are scattered in extremely huge amounts of empty space.

Even in a dense asteroid belt, almost every path encounters zero rocks; and
the orbit of that Tesla is not in asteroid belts but in interplanetary space
that's quite "clean".

Tesla _would_ be expected to get some abrasion / "wear&tear" from hitting
individual atoms and tiny particles of space dust; over a million of years
that would likely accumulate to a major change. But it's not likely to hit a
rock of "dent-making" size before it gets sucked in by some major planet.

~~~
kijin
It's also worth noting that Earth is more massive than Mercury, Venus, Mars,
the Moon, and the asteroid belt _combined_.

Since the Tesla doesn't go anywhere near Jupiter's orbit, if it gets sucked in
by anything at all, it's likely to be Earth.

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dfee
I thought there was no way this could be true - I was at the Griffith
Observatory yesterday and saw that Venus was nearly 80% the mass of Earth.

But indeed, Mars has only about 1/10 the mass of Earth! And Mercury is about
half of that.

[https://nssdc.gsfc.nasa.gov/planetary/factsheet/](https://nssdc.gsfc.nasa.gov/planetary/factsheet/)

~~~
kijin
Yeah, Earth is huge!

It also has the highest density of all the planets, thanks to the iron and
nickel in the core. Earth is so dense that its surface gravity is higher than
that of Saturn and Uranus (for a suitable definition of "surface" for the gas
giants) despite the fact that the latter are much more massive as a whole.

We should make note of these facts and use them to burn the aliens if we ever
come to an interplanetary rap battle :)

~~~
stephengillie
It's as though Earth were a gas giant, but with the outer layers of hydrogen
and helium blown away. The magnetosphere is closer in magnitude to Uranus's or
Neptune's than Venus's or Mars's. We think it's why we still have water.

~~~
zentiggr
That makes me wonder - given that Jupiter and Saturn are so massive as to have
solid cores, how do their core sizes compare to Earth's size? never compared
those numbers, not sure where to find core size estimates for our four giants.

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snakeboy
> In our numerical model, we do not integrate the orbit of the Moon and
> instead use a single particle with the combined mass of the Earth and the
> Moon.

Is that a standard assumption for this kind of model? For such a chaotic
simulation, would it not be very important to approximate the effects of the
Earth and moon very precisely? Especially since these bodies would have such
dominating effects early on?

~~~
sudhirj
Only the initial launch has the car near the earth and moon, but the
calculations are already factoring in a range of starting points - I’d assume
the moon’s effects are contained in that range.

After it moves away from the earth the distance ought to be negligible,
although even if they do reduce the earth moon to a single point I don’t know
if they model a wobble.

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manaskarekar
Very cool, looks like this partially answers my curiosity
[https://news.ycombinator.com/item?id=16324894](https://news.ycombinator.com/item?id=16324894).

It'll be fun to read papers about other projected fates of the various aspects
of the Tesla.

~~~
mirimir
I'm wondering about the fate of the paint, plastic, rubber and leather.

[https://www.autoblog.com/2018/02/07/tesla-roadster-
starman-s...](https://www.autoblog.com/2018/02/07/tesla-roadster-starman-
space-radiation-elon-musk/)

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xixixao
It’s pretty incredible to imagine that after most of the monuments man has
built for man have vanished, Elon’s Tesla might still be orbiting the Sun.

Feels implausible.

~~~
taneq
Within a couple of hundred years (maybe even much less) someone's going to go
and collect it for bragging rights and/or as a trophy. See if they don't.

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noonespecial
It will be designated a solar system historical site and be strictly off
limits. You can't exactly sneak up on something in space...

~~~
pronoiac
Aw, that reminds me of a story about the future Mars rover:

[https://www.wired.com/2016/12/jon-rogers-a-martian-goes-
hunt...](https://www.wired.com/2016/12/jon-rogers-a-martian-goes-hunting-for-
the-past/)

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cyberferret
As high as 6%? In the total vastness of space, 6 in 100 seems like an
incredibly high chance that the rocket payload will hit either Earth or
Venus... It's a bit like me pulling out a rifle and firing into the air,
hoping to hit a specific dinner plate in a back yard two suburbs away on a
windy day, isn't it?

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SeoxyS
Plates a few suburbs away don't exert massive gravitational pulls on their
surrounding.

~~~
cyberferret
But doesn't gravitational pull mainly deflect objects travelling at high
velocity? Hence their use to accelerate spacecraft into different
trajectories? If we looked at the 'indentations on a rubber mat' model of
gravitational influence, the object would still have to be aimed almost
directly at the planet in order to hit it rather than bend around it?

But I guess that comes down to the velocity of the object in question, and
after repeated trajectory adjustments due to gravitational pull, it could
quite conceivably end up aiming directly at a massive body in space that
exerted the pull in the first place...

~~~
kijin
Currently, the Tesla is aimed almost directly _away from_ Earth. After all, it
was launched from Earth!

On the other side of the orbit, that becomes almost directly _at_ Earth.

Go round and round a few million times, and Earth might actually be at a point
in the orbit where it can exert a significant gravitational pull on the
passing Tesla.

~~~
cyberferret
I hope that other members more in the know can clarify, but I thought that the
launch into deeper space was done as a 2 stage thing - first, they got the car
up into earth orbit, THEN they did a second burn to break orbit. i.e. the
second burn was done while the vehicle was moving tangential to the Earth, and
not radially away from it?

~~~
kijin
That just means that when the Tesla returns to the same point in its orbit, it
will pass a few hundred miles above Earth's orbit (where the final burn was
performed) instead of directly intersecting Earth's orbit.

The difference is virtually meaningless compared to orbital perturbations from
other sources. Either way, it's close enough for Earth to affect the Tesla's
orbit if it were at the right place at the right time.

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dsacco
Hah. This is cool.

I’ve become so used to reading papers with a footnote that states something
about an NSF grant being used to support the research that I was initially
surprised when I didn’t see it here. Then I realized what I was reading and
chuckled :)

~~~
TeMPOraL
There is one in ACKNOWLEDGEMENTS section:

> _This research has been supported by the NSERC Discovery Grant
> RGPIN-2014-04553._

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jkh1
As if there was not enough junk in space already.

~~~
ars
There's junk in low earth orbit. But this object is in solar orbit. There's
very little junk there. There's very little anything there.

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TheRealPomax
Very little junk, but _loads of stuff_ in solar orbit in the form of the
asteroid belt, the jupiter greeks/trojans and the hildas, the kuyper belt, and
the oort cloud.

There is so much there.

~~~
Sharlin
For extremely low values of ”so much”.

~~~
kybernetikos
Maybe, but on the other hand, I expect you've hardly ever used 'so much' to
refer to more - since a small fraction of that 'so much' includes Earth and
everything on it.

As with everything, it depends on what your comparisons are to.

