I recently consumed little piece of hard holiday candy on The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth.
It's a fun little paper about interstellar colonization, vonNeumann self-replicating machines, and why the heck aren't we knee deep in inquisitive and curious alien life forms? At one very brief point, the idea of panspermia is broached, in the sense of we-have-met-the-aliens-and-they-is-us solution to the Fermi Paradox, but the treatment last about as long as this sentence. And why should it? I mean, the paper itself is informed speculation, but for hard answers? Why, the standard excuse, into the foreseeable future is, Insufficient Data.
But what the heck, let's think about it a little bit. And for my object lesson, I choose the movie Starship Troopers. Now, most people would say "Oh my goodness why? That's a horrible movie. Why would you choose that for your speculative background?" And then, of course, there is the objection from the humorless little prig known as the Heinlein fan, who views the movie as an awful perversion of the book. Well, director Paul Verhoeven knew what he was doing. He presented a perfect fascist society of the future and showed that the only thing a perfect fascist society of the future was good for was killing alien Bugs. Not to mention, Verhoeven directed Showgirls, Robocop, and Total Recall, so the charge of taking things over the top is, at best, to identify yourself as a cluelessly humorless little prig. Besides, Heinlein book does serve a useful purpose. For those with a healthy and mature mental metabolism, it serves as a vaccine to Ayn Rand's wretched works.
Anyway, I'd argue that the alien Bugs in that movie provide a very good example of panspermia. I mean, we actually told in that movie that they have the ability to colonize other planets by "hurling their spore into space". To those geeks that object to this method as haphazard and inefficient, I'd suggest a scuba dive during the full moon when the coral reefs are spawning. The water is thick with eggs and sperm, which should freak most geeks out, and the chances of all that milky organic stuff turning into a new coral reef are frighteningly small.
Alright, you know what? Clearly this essay is not going to be a serious attempt at discussing panspermia. I mean, your'e lucky it's only now that I snigger at the "sperm" part of it.
But I'll tell you what. I'll take a brief crack at it. Starship Troopers was a movie. So, let's get "real" for a sec. Is panspermia possible without violating the laws of physics? Well, clearly if we play by the rules, the speed of light cannot be violated. So, let's give the Bugs a moment of respite, and talk about bugs. As in bacteria.
If there is any life on Earth that might survive a voyage through space without technological assistance, it would be bacteria. Recall the brief period of excitement back in 1996, when it was believed microbial fossils were found in a piece of meteorite? It was dubbed ALH84001, a chunk of Mars that had landed in Antarctica 16 million years ago.
Let's send a bacterium from the Earth to the nearest star. Assume bacteria are well insulated within a rock from an event catastrophic enough to propel them into orbit. That's 7 mi/sec or 25,000 miles per hour minimum. That's a formidable event, itself undoubtedly an asteroid or comet impact upon the surface of the Earth.
Further assume that the bacteria are hardy enough to survive the journey by being flash frozen into cryogenic suspension, and that the rock protects them from the ravages of hard radiation (the number one space hazard for organic life). Given that some bacterial spores have been revived from a dessicated state in salt deposits after 250 million years, it's not an unreasonable assumption.
Further assume that a serendipitous series of celestial mechanical jugglery speeds up our spore enough to break free of our solar system. That's 26 mi/sec, or 93,600 mph. And now, we wait. Alpha Centauri, the nearest candidate star, is 4.3 light years away, or 25.8 trillion miles. Dividing our distance by speed gives us a travel time of ~275,600,000 hours, or 31,461 years. That's not that bad.
Figure on a latency of a few hundred thousand to million years between chances for further "voyages", and the majority of the galaxy could be colonized by bacterial life in as little as 3 to 4 billion years. Given that the life on Earth has been around some 3.9 billion years, there's a slim chance Earth life has colonized the galaxy and we will never know.