"I disagree. Look at the purported asteroid that wiped out the dinosaurs and (I think) 80% of life on Earth"

No it didn't. It didn't have hardly any impact except at a macro level of a couple orders of vertebrates. What phlya of plants and animals became extinct ? None. Which of even the vertebrate classes became extinct? Reptiles? birds? Mammals? Fish? Amphibians? None. Which of the invertebrate classes (there are several dozens) became extinct? What micro fauna? What classes in the plant kingdom became extinct? None.

The cause of the dinosaur extinction is still debatable. The fish, amphibians, mammals, other reptiles, etc, that livesd in the same environmnets emerged unscathed. Let alone bacteria and other micro organisms that are difficult to wipe out in controlled conditions even when we target them.

"What about the current theory for the creation of the moon? A Mars-sized body crashing into Earth....sure, it's probably not something that's gonna happen again - ever - but if another collision like that took place I don't think life would be able to survive. "

Where is this 'object'.? It's not 'probable' that it isn't going to happen again but a certainty. Venus or Mars aren't going to crash into the Earth unless the laws of physics as we know them are all wrong. The position of the Earth and inner planets have been quite stable and will remain so until we're fried by the Sun a few billion years from now.

Not quite unscathed. It appears that not much larger than a housecat in the land animal range made it. Just so happened not a lot of dinosaurs were below the housecat range, and most frogs, insects, and so on would be straining to get above that range.

Debatable perhaps, but isn't it generally accepted that the impact caused the extinction? And weren't there massive die-offs for most species? Weren't large animals the most likely to be affected?

It is strongly suspected that Jupiter has acted as a sort of cosmic vacuum cleaner, ejecting many asteroids out of the system. If we didn't have a Jupiter, it is quite possible that a major impact would occur every million years or so, and smaller impacts that could devastate a continent would happen much more often.

"Not quite unscathed. It appears that not much larger than a housecat in the land animal range made it. Just so happened not a lot of dinosaurs were below the housecat range, and most frogs, insects, and so on would be straining to get above that range.'

Sort of true at a macro level. Anything larger than a housecat is about 0001% of life on earth. Even 'land' life forms are a minority of life forms. I collect fossils in the Late Cretaceous/ early Tertiary deposits of Alberta . We collect vertebrate specimens as varied as Champsosaurs, terrestrial crocs and terrestrial turtles that were bigger than housecats post-Cretaceous. These come from the Scollard Formation that spans the C-T line.

Here are some of the fossils from my collection:

http://www.geocities.com/joe2nora/PaleoSociety.html

Feel free to cruise through the 'drawers'. There's about 15,000 specimens in the collection.

Got my own theories on that, based on "footprint" of the animal (energy use plus specialization in foodsource) which favors small generalist animals over large specialists... means technically "larger" true reptiles would survive where a similarly sized warm-blooded mammal (or dinosaur) would not.

Sorry, I accidently used the wrong word in my last post - what I meant to say was 80% of species, not life forms. ops: From a quick google it appears I over-estimated by about 20%, however. #-o

There have other (more dated) 'mass extinction events' over the course of our planet's history, however. One of them caused the disappearance of 90% of species (oops almost wrote 'life forms' again :P) that were apparent at the time.

Sure it is, but the cause is of little concern when we are considering possible sources of extinction totality.

What about rogue planets? We don't know enough at the current time to say anything is 'certain', even if such an occasion is unlikely. However, like I mentioned previously, solar systems with larger asteriod belts could easily have larger bodies ranging up to several thousand kilometres which may cross the orbit of a life-bearing planet.

According to the Sky&Space magazine, the asteroid belt that I mentioned earlier with roughly 25 times the amount of material as our own could cause the appearance of a major impact as soon as every 1,000 years.

Ah, so they did finally get around to naming that dinosaur I discovered...? :P

with regards

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Sign up to sue the Sun

I just watched a PBS program called Guns, Germs, and Steel that made some interesting points about civilization. Human civilization may have started in the Mid-east not just because of plant agriculture but because of animal husbandry. The argument is that some societies, such as that in New Guinea, did not develop higher technology because they had no large domesticated mammals to be beasts of burden. The mid-east had more large domesticated mammals than anywhere else, and after the climate turned dry they spread out. This means that plant agriculture was human labor intensive and thus there was no surplus food or time for inventing higher tech.

Perhaps these ideas should also be plugged into the Drake Equation. A planet not only needs to evolve one smart species, but several moderate intelligence domesticable species that can act as force multipliers in agriculture allowing for spare time to develop higher tech that leads to spacefaring and interstellar communications. Consider earth: North America, Sub-Saharan Africa, Australia, and Oceana have no native large domesticated mammals. South America has one, the Llama. Eurasia and northern Africa are the areas that had 13 of the perported 14 large domesticated mammals.


When we talk about intelligent spacefaring civilizations we should keep in mind that we are distinguishing them from non-spacefaring intelligent civilizations. I suspect there may be hundreds if not thousands of intelligent civilizations or societies in the galaxy, but it is just a guess. Most of the posts about the Drake Equations focus on indirect observations of habitability and prospects for development. We should remember that we might also include direct observation on our own. This is why I include Bright Radio detection in order to reinforce the Drake-esque calculations. We should also remember the difference between an estimate and a guess. Based on my calculations I estimate that there may be on the order of 10 ISCs, meaning at least 1 and not more than 100. On the other hand, my guess is that there are exactly 6 ISCs in the Milky Way galaxy.

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J Pax

"When we talk about intelligent spacefaring civilizations we should keep in mind that we are distinguishing them from non-spacefaring intelligent civilizations"

Excellent point. We don't even know if a 'spacefaring' civilization is possible. Because of the great distances between stars, I'm inclined to believe we're stuck in our own solar systems (hope i'm wrong).

Mythozdog: yes, truly remarkable. I participate in a few other science forums and Badastronomy is refreshing and is unique in sticking to science. Maybe we are all just superior intellects (and modest). :^o

Why wouldnt space-fairing be not possible? I mean in a hundred years or less we probably will be space-fairing.

"Based on my calculations I estimate that there may be on the order of 10 ISCs, meaning at least 1 and not more than 100. On the other hand, my guess is that there are exactly 6 ISCs in the Milky Way galaxy"...why would your guess be different from your calculation?

I think a simple definition of "spacefaring" should include interplanetary travel as a lower bound. Modifying the term with Interstellar would be useful but I don't think it should be a lower limit. Interplanetary travel is only separated from interstellar travel by duration, not technology.

Platinum, the estimate is a range of possibilities, my guess is a specific number that, while related to my estimate, is precise if not accurate.

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"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

Interplanetary travel is only separated from interstellar travel by duration, not technology."

I politely disagree. The difference in the technology needed (if at all possible) to travel between stars is greater than the difference between crossing a river on a raft and zipping around the solar system.

The fact is that once you have a ship that can handle vacuum and can attain or surpass solar escape velocity it is capable of interstellar transit.

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"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

We already have that--but we're not very close to interstellar transit.

Even the ISS, which is capable of long term habitation, was built piece by piece, and has to be resupplied every so often.

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Well, what I wrote was in reference to what most people would consider to be interplanetary capable craft. I don't consider probes or the ISS to be interplanetary capable passenger or cargo craft as they are not purpose built to the purpose, although I suppose they could make interplanetary or interstellar transits if modified. I would arbitrarily limit the definition of interplanetary-to-interstellar mods to less than a 10% modification in habitable volume or speed or technology but not to reaction mass (if necessary) or duration of thrust. This does not mean that we might not construct larger or faster purpose built craft for subluminal interstellar transit. Descriptions of some interplanetary craft designs were listed in another topic, and that is my reference design. However, we would probably operate under the assumption of purpose built interplanetary craft designed with the state-of-theart known or developing technologies without relying on hypothetical propulsion systems. This would include Orion, fission or fusion nuclear thermal rockets, Nuclear Salt Water Rockets, solar and magnetic sails but not space warps, hyperspace, wormholes, vacuum propellers, or antimatter. So, once we have interplanetary craft like this we could probably modify them for interstellar use.

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"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

In no way speaking of myself, here... but I do feel that... you, "as a group", are some of the sharper tacks in this box. Not just throwing out a comment here and there, then walking out cold... but latching on to this subject matter, like a bunch of Bulldogs, tearing it to shreds and examining it from every conceivable angle. And, The fact that the subject matter might be more than a little controversial doesn’t seem to phase you all much!
My hats off to all of you!

Couple things.

I also feel there is a fundamental difference between interstellar and interplanetary space travel. If all you want to do is blow a metal box across space, then OK they are the same. But interplanetary travel is like a cruise ship, it comes back to port and can be resupplied. But interstellar travel requires a completely self sustaining system. It cannot be resupplied, it cannot turn back to port. And then there is the whole length of time it will take.

The original premise of this thread was involving our detecting other intelligent races in the galaxy. For us to meet, someone has to be able to do interstellar flight. The fact that they may or may not be flying between their own planets is irrelevant. SO we must consider who out there is an interstellar SFC.

It may not be possible to travel between stars. The distances are huge. Those distances are the problem, not the ability to make propulsion systems, or airtight hulls.