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--I'm skeptical on some of your/Melosh's claims, but agree re others.

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(big atmospheres). Earth has an intermediate size atmosphere and I think it would be difficult to eject earth rocks.

Melosh specifically talks about how stuff can be ejected from the Earth -- in particular, the meteor event of 65 million years ago.

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To get a meteor thru an atmosphere, it needs to be large, otherwise it vaporizes.

Which direction are you talking about? Launching or re-entry? According to Melosh, re-entry is relatively easy -- it happens all the time.

--To correct what I said before, my formula for the "threshold diameter" really pertains not to melting/vaporizing, but to possibility to traverse atmosphere in either direction without a huge slowdown. I.e. if you launch something from ground in some upward direction at escape velocity plus some extra safety margin, you are NOT going to be able to eject it from the earth, UNLESS it is large enough that the air-drag slowdown is small fraction of launch speed. In particular Melosh said some bullshit about a small iron object he thought a nuke explosion had ejected from Earth based on a high speed film frame. I claim that is FALSE -- it was not ejected from Earth. This in turn suggests that Melosh never thought about this whole issue, which if so was a major omission in his thinking. My formula for threshold size of the object, roughly delineates how big it has to be so that atmosphere traversal can happen. Melting and vaporizing are a bit harder to think about because a lot of the heat energy does not go into melting or vaporizing but instead is radiated or convected away. But my argument goes through using the corrected statement just about slowdown (for that purpose, convecting or radiating is fine, that is still an energy-loss mechnaism); that suffices.

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However, one might conjecture that an impact big enough to blast huge rocks to well above escape velocity would shatter those rocks, i.e. not giving us big intact ones.

Melosh specifically addresses this issue. Rocks on the surface near by to the collision, but not directly impacted by the collision, will see a huge acceleration _upwards_. His example is if a meteor crashed through the lecture hall and impacted beside him (or more likely some of his bacteria) would cause his body to accelerate upwards at perhaps 100,000 G's. Although he wouldn't survive, some of his bacteria would certainly survive.

--I'm ok with *that*. My problem is, the fast flying resulting stuff, will then get slowed down, melted, and/or vaporized by the atmosphere hence will not escape Earth. It will only be able to escape if object being ejected is large enough. But if all large objects are shattered that will not happen. Will they be shattered by 100,000 Gs? I suspect "yes" for objects of the calculated threshold size, or larger. A mere 20G or so is enough to total your car.

Melosh also suggests that the upward-expanding fireball would actually help propel the object upwards. I suspect that this effect may be similar to those Soviet-era "supersonic" torpedoes that had small rocket motors facing _forwards_ which pushed the water ahead of the torpedo out of the way.

--??huh? That kind of idea definitely sounds bogus at supersonic speeds.

I don't know whether anyone has directly addressed the issue of Venus.

--you had said any planet to any other was possible. I dispute that. Sounds like you now also dispute it.

I get the impression that this has been a relatively active area of research over the past 10 years, so there may be later results than this 2009 video. Re bacteria surviving: Melosh discusses one of the instruments dropped onto the Moon before one of the Apollo missions brought it back. This instrument had _not_ been sterilized prior to being placed onto the Moon, and was kept sealed when brought back from the Moon. When this instrument was cultured, it showed the presence of live microbes.