The seeding organisms need to survive and multiply in the target environments and establish a viable biosphere. Some of the new branches of life may develop intelligent beings who will further expand life in the galaxy. The messenger microorganisms may find diverse environments, requiring extremophile microorganisms with a range of tolerances, including thermophile (high temperature), psychrophile (low temperature), acidophile (high acidity), halophile (high salinity), oligotroph (low nutrient concentration), xerophile (dry environments) and radioresistant (high radiation tolerance) microorganisms. Genetic engineering may produce polyextremophile microorganisms with several tolerances. The target atmospheres will probably lack oxygen, so the colonizers should include anaerobic microorganisms. Colonizing anaerobic cyanobacteria may later establish atmospheric oxygen that is needed for higher evolution, as it happened on Earth. Aerobic organisms in the biological payload may be delivered to the planets later when the conditions are right, by comets that captured and preserved the capsules.

The development of eukaryote microorganisms was a major bottleneck to higher evolution on Earth. Including eukaryote microrganisms in the payload can bypass this barrier. Multicellular organisms are even more desirable, but being much heavier than bacteria, fewer can be sent. Hardy tardigrades (water-bears) may be suitable but they are similar to arthropods and would lead to insects. The body-plan of rotifers could lead to higher animals, if the rotifers can be hardened to survive interstellar transit.

Microorganisms or capsules captured in the accretion disc can be captured along with the dust into asteroids. During aqueous alteration the asteroids contain water, inorganic salts and organics, and astroecology experiments with meteorites showed that algae, bacteria, fungi and plant cultures can grow in the asteroids in these media. Microorganisms can then spread in the accreting solar nebula, and will be delivered to planets in comets and in asteroids. The microorganisms can grow on nutrients in the carrier comets and asteroids in the aqueous planetary environments, until they adapt to the local environments and nutrients on the planets.

Advanced missions

Significantly, panspermia missions can be launched by present or near-future technologies. However, more advanced technologies may be also used when these become available. The biological aspects of directed panspermia may be improved by genetic engineering to produce hardy polyextremophile microorganisms and multicellular organisms, suitable to diverse planetary environments. Hardy polyextremophile anaerobic multicellular eukaryots with high radiation resistance, that can form a self-sustaining ecosystem with cyanobacteria, would combine ideally the features needed for survival and higher evolution. For advanced missions, solar sails can use beam-powered propulsion accelerated by Earth-based lasers or ion thrusters propulsion to achieve speeds up to 0.01 c (3 x 106 m/s), or by ion drives. Robots may provide in-course navigation, may control the reviving of the frozen microbes periodically during transit to repair radiation damage, and may also choose suitable targets. These propulsion methods and robotics are under development. Safeguards are needed against robot takeover, to assure that control remain in human control with a vested interest to continue our organic gene/protein life-form.

Microbial payloads may be also planted on hyperbolic comets bound for interstellar space. This strategy follows the mechanisms of natural panspermia by comets, as suggested by Hoyle and Wikramasinghe. The microorganisms would be frozen in the comets at interstellar temperatures of a few degrees Kelvin and protected from radiation for eons. It is unlikely that an ejected comet will be captured in another planetary system, but the probability can be increased by allowing the microbes to multiply during warm perihelion approach to the Sun, then fragmenting the comet. A 1 km radius comet would yield 4.2 x 1012 one-kg seeded fragments, and rotating the comet would eject these shielded icy objects in random directions into the galaxy. This increases a trilion-fold the probability of capture in another planetary system, compared with transport by a single comet. Such manipulation of comets is a speculative long-term prospect.