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Impact survival
Impact survival is a theory that life, usually in the form of microbial bacteria, can survive under the extreme conditions they are exposed to during a major impact event, such as a meteorite striking the surface of a planet.〔Melosh, H., 1989, Impact Cratering: A Geologic Process, Oxford Univ. Press, Oxford.〕 This step is a necessity for the possibility of panspermia, for the microbial life must be able to survive both the escape out of the initial planetary atmosphere, likely due to a major impact, as well the reentry and collision with a second planetary body.
== Dangers to life ==
For small organisms to leave a planet's orbit, escape velocity must be reached, which is dependent upon the mass of the planet. To reach these velocities,both the surviving organisms and the pieces of debris that they live on must withstand large amounts of acceleration and jerk.〔Burchell, M. ''Survival of bacteria and spores under extreme shock pressures''. Monthly Notices of the Royal Astronomical Society, 2004, p. 1277.〕 One calculation has determined that for possible organisms to be launched out of an orbit of a planet the size of Mars, jerk would be nearly 6x109 m/s2.〔Mastrapa R., Glanzberg h., 2001, Earth Planet. Sci. Lett. 189,1〕 In addition to this, the velocity of incoming meteorites are projected to be in the range of 20 to 25 km/s.〔Hughes, D. W., and I. P. Williams 2000. The velocity distributions of periodic comets and stream meteoroids. Mon. Not. R. Astron. Soc.315, 629–634〕 Thus any organisms that could survive a major impact event and be sent to outer space must be very small, light, and able to withstand large amounts of acceleration and jerk. While concerns over the heat of ejected rocks and the possible sterilizing affects it would have on the microbial life attached, it has been theorized that for rocks even less than 2 kilograms such as ALH84001, internal parts may never reach temperatures greater than 40 °C.〔Weiss, B. P., J. L. Kirschvink, F. J. Baudenbacher, H. Vali, N. T. Peters, F. A. Macdonald, and J. P. Wikswo 2000. A low temperature transfer of ALH84001 from Mars to Earth. Science 290 , 791 – 795〕
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