Re meteors, sorry, I used the wrong number for Venus atm. pressure, it should have been 90 atm, not 600 atm. Anyhow, same conclusion: I think it is going to be extremely difficult to eject anything from Venus.

Warren>Here is the situation: A. meteor falls in at somewhat more than escape velocity, perhaps as much as 1.5X escape.

This was Barringer's delusion. Stuff is falling in toward the *Sun*, 42.1km/s =rt2*Earth orbital speed, vs 11.2km/s Earth escape speed.

So his impactor was (42.1/11.2)^2 ~ 14 times less massive than he thought, and was largely vaporized on impact. Then he had the great misfortune to discover that angled impacts make round craters, and drilled fruitlessly on.

--Aha. OK, good, that corrects another error by me. So a large body hitting earth (1000 tons or more) could so so at sqrt(42.1^2 + 11.2^2) = 43.6 km/sec if it came in from cometary regions. If interstellar even faster speed possible under pessimal circumstances, as large as 73km/sec. I However, I think most things that hit us are co-orbiting the sun in orbits not hugely dissimilar to the Earth's orbit, hence would hit at more like 15 km/sec. --Another error (or rather omission) I made -- which might rescue Melosh to some extent, is this. I argued that if large stuff (8-1000 tons) was unable to be ejected without first being fractured into smaller stuff, then it would be unable to be ejected, period. However, an "escape clause" could be this. Suppose something huge like a mountain is accelerated roughly upward at 100000gs to 150% escape velocity and also at the same time fractured into gravel. Then the atmosphere might not NOTICE that it was gravel until it got 20 km up since not separated enough. This "swarm effect" might be enough, anyhow it is not obvious to me how much help it provides. (The whole slowdown effect is exponential and whence will exhibit a fairly sharp size threshhold...)

The Earth's surface pressure is about 30 inches of mercury, so a skybound 4" thick steel plate will be facing quite a headwind.

--exactly.

On a mountain, lower atmospheric pressure.

--true, not a very big help though.

I think the most interesting thing about this is that a good fraction of stuff that escapes into interplanetary space will eventually be ejected into interstellar space. So we're broadcasting a wake of bacterial DNA, and should expect some amount of incoming material. I may be out of date, but my impression is that no confirmed extra-solar meteorite has been found.

--here is a 1996 paper http://articles.adsabs.harvard.edu/full/1997JRASC..91...68H which says interstellars are rare and maybe nonexistent. To eject stuff from not just earth, but in fact whole solar system, you'd probably need help from "slingshot" effects around planets like Jupiter. Is it really true system ejection is common? How do you know it is, if it is? There are observation-based claims that there are a ton of "rogue planets" (more than the number of stars) wandering round the galaxy, which were ejected from their systems by such effects. So it seems plausible 10% of stuff inserted into the solar system in a "random orbit" is ultimately ejected. But trying to justify such an estimate mathematically or computationally might be very difficult, since the ejection process could take 100M years.