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Comment by fsiefken | Hacker News Reader

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?

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> What are terrestrial planets?

Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.

1. https://en.wikipedia.org/wiki/Terrestrial_planet

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.

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.

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/

It's the mass ~ size cubed that's deceptive.

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 :)

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.

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.

Terrestrial planets usually means inner 4 in our solar system, and "earthlike" elsewhere

Re. dynamic lifetime: you are right that there are many rocks. You underestimate the vastness of space though. Collisions are extremely rare and the Tesla is pretty small.

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