I would like to be wrong, I will be very happy to eat my words if I am, but I am not convinced by panspermia. The real objection to panspermia is that it solves nothing. The problem that it purports to solve is the very fast speed with which life appears on Earth, about 3.8billion years ago. If life arose elsewhere in the solar system, say planet X, the problem remains, how could life, the most complex system of which we are aware, have arisen so fast anywhere in a young solar system? But worse than this, it creates another problem: life must have arisen even faster on planet X because it had to have time to evolve AND then transfer from X to Earth; two improbable events instead of one. If life comes not from inside the solar system but from a passing star there are another set of problems (leaving aside the survival in space question). The cloud would have to pass at just the right time in Earth’s development and be essentially saturated with life forms. It could happen, but I doubt if the probabilities are that good. And we still haven’t got anywhere near the meat of the problem, which is how does a super-complex system like a cell evolve in the first place.

I can accept a very weak panspermia, organic molecules such as amino acids could have come to Earth on meteorites etc, but this works for only the most simple amino acids, glycine, alanine, aspartic acid, valine etc.. I have not seen any evidence for complex amino acids like tryptophan or histidine, which are necessary for all living organisms coming from space.

You cannot simply deep freeze bacteria and expect them to be OK. If ice forms outside the bacterial cell wall water is drawn out of the cell by osmosis and irreparable damage occurs. Similarly if crystals form inside the cell, they damage the integrity of cell and it dies. Cryopreserving even simple cells like bacteria is delicate, it requires careful control of the freezing conditions and a cryopreservant , usually dimethylsulfoxide or glycerol. To be confident of success many cells should be frozen together, around 10 million. At best only very small number of cells will survive by throwing them randomly into space and allowing them to freeze on the way up. There is a better chance for spores to survive but many bacteria are not sporulating. In the Scientific American article cited Aluminium sheilding of spores – not live cells- in space led to a 20% decrease in viability in 6 years, so within a human lifetime all the spores would be dead. Without shielding the death rate was 99.99 percent in six years. This is not encouraging.

Even moderate heating will kill most bacteria (that’s one reason we cook food). You don’t need to incinerate most bacteria, gentle warmth, say 60C will harm most bacteria.

Arguing that the existence of extremophiles proves that bacteria could survive a trip on a meteorite is nice, but I am not convinced. Extremophiles are highly adapted organisms, they have evolved to survive hostile conditions; no organism has evolved to be blown through space for millions of year. What is more interesting about extremophiles is not whether they could travel through space by accident, but what they are saying about the kinds of conditions here on Earth that can support life, and by extension what conditions might harbour life elsewhere in the universe.