At any stage of this random walk through the solar system, the fragment may actually hit one of the planets, ending its journey. This is presumably how the SNC meteorites reached Earth. Any microorganism contained in these rocks would thus have an opportunity to colonize the new planet, if it was able to survive both the journey and the fall to its destination. This question is of paramount importance for the transfer of viable microorganisms from one planet to another, since even dormant organisms might not be able to survive a long trip. Furthermore, cosmic rays, ultraviolet light or even radiation from the enclosing rocks might kill the organisms along the way.
Many microorganisms stand up surprisingly well to the space environment.
Subjected to high vacuum, some bacteria quickly dehydrate and enter a state of suspended animation from which they are readily revived by contact with water and nutrients. Medical laboratories routinely use high vacuums for preservation of bacteria. Viable microorganisms were recovered from parts of the Surveyor 3 camera system after three years of exposure to the lunar environment. However, these instances of preservation have only been tested over times approaching decades, not over the tens to hundreds of millions of years necessary for interplanetary travel. Nature, however, has been kind enough to give us several instances of long-term preservation of viable microorganisms.