Directed panspermia

Directed panspermia is a type of panspermia that implies the deliberate transport of microorganisms into space to be used as introduced species on other astronomical objects.
Shklovskii and Sagan and Crick and Orgel hypothesized that life on the Earth may have been seeded deliberately by other civilizations. Conversely, Mautner and Matloff and Mautner proposed that humanity should seed other planetary systems, protoplanetary discs or star-forming clouds with microorganisms. Motivations for directed panspermia often stem from panbiotic ethics and as a last resort existential risk mitigation strategy. However, more recently directed panspermia has also been heavily criticised from the perspectives of contamination and interference with indigenous life, wild animal welfare concerns, and procreative ethics, highlighting in particular, concerns about its irreversibility in the context of its uncertain ethical consequences.
Directed panspermia is becoming possible due to developments in solar sails, precise astrometry, the discovery of extrasolar planets, extremophiles and microbial genetic engineering.

History and motivation

An early example of the idea of directed panspermia dates to the early science fiction work Last and First Men by Olaf Stapledon, first published in 1930. It details the manner in which the last humans, upon discovering that the Solar System will soon be destroyed, send microscopic "seeds of a new humanity" towards potentially habitable areas of the universe.
In 1966, Shklovskii and Sagan speculated that life on Earth may have been seeded through directed panspermia by other civilizations, and, in 1973, Crick and Orgel also discussed the concept. In the controversial 2008 documentary Expelled: No Intelligence Allowed starring Ben Stein, Richard Dawkins mentioned directed panspermia as a possible scenario and that scientists may find evidence of it hidden in human biological chemistry and molecular biology. Conversely, Mautner and Matloff proposed in 1979, and Mautner examined in detail in 1995 and 1997 the technology and motivation to secure and expand organic gene/protein life-form by directed panspermia missions to other planetary systems, protoplanetary discs and star-forming clouds. Technological aspects include propulsion by solar sails, deceleration by radiation pressure or viscous drag at the target, and capture of the colonizing micro-organisms by planets. A possible objection is potential interference with local life at the targets, but targeting young planetary systems where local life, especially advanced life, could not have started yet, avoids this problem.
Directed panspermia may be motivated by the desire to perpetuate the common genetic heritage of all terrestrial life. This motivation was formulated as biotic ethics that value the common gene/protein patterns of self propagation, and as panbiotic ethics that aim to secure and expand life in the universe.

Strategies and targets

Directed panspermia may be aimed at nearby young planetary systems such as Alpha PsA and Beta Pictoris, both of which show accretion discs and signs of comets and planets. More suitable targets may be identified by space telescopes such as the Kepler mission that will identify nearby star systems with habitable astronomical objects. Alternatively, directed panspermia may aim at star-forming interstellar clouds such as Rho Ophiuchi cloud complex, that contains clusters of new stars too young to originate local life. Such clouds contain zones with various densities that could selectively capture panspermia capsules of various sizes.
Habitable astronomical objects or habitable zones about nearby stars may be targeted by large missions where microbial capsules are bundled and shielded. Upon arrival, microbial capsules in the payload may be dispersed in orbit for capture by planets. Alternatively, small microbial capsules may be sent in large swarms to habitable planets, protoplanetary discs, or zones of various density in interstellar clouds. The microbial swarm provides minimal shielding but does not require high precision targeting, especially when aiming at large interstellar clouds.

Propulsion and launch

Panspermia missions should deliver microorganisms that can grow in the new habitats. They may be sent in 10⁻¹⁰ kg, 60 μm diameter capsules that allow intact atmospheric entry at the target planets, each containing 100,000 diverse microorganisms suited to various environments. Both for bundled large mass missions and microbial capsule swarms, solar sails may provide the most simple propulsion for interstellar transit. Spherical sails will avoid orientation control both at launch and at deceleration at the targets.
For bundled shielded missions to nearby star systems, solar sails with thicknesses of 10⁻⁷ m and areal densities of 0.0001 kg/m² seem feasible, and sail/payload mass ratios of 10:1 will allow exit velocities near the maximum possible for such sails. Sails with about 540 m radius and area of 10⁶ m² can impart 10 kg payloads with interstellar cruise velocities of 0.0005 c when launched from 1 au. At this speed, voyage to the Alpha PsA star will last 50,000 y, and to the Rho Opiuchus cloud, 824,000 years.
At the targets, the microbial payload would decompose into 10¹¹ 30 μm capsules to increase the probability of capture. In the swarm strategy to protoplanetary discs and interstellar clouds, 1 mm radius, 4.2 kg microbial capsules are launched from 1 au using sails of 4.2 kg with radius of 0.37 m and area of 0.42 m² to achieve cruising speeds of 0.0005 c. At the target, each capsule decomposes into 4,000 delivery microcapsules of 10⁻¹⁰ kg and of 30 micrometer radius that allow intact entry to planetary atmospheres.
For missions that do not encounter dense gas zones, such as interstellar transit to mature planets or to habitable zones about stars, the microcapsules can be launched directly from 1 au using 10⁻⁹ kg sails of 1.8 mm radius to achieve velocities of 0.0005 c to be decelerated by radiation pressure for capture at the targets.
The 1 mm and 30 micrometer radius vehicles and payloads are needed in large numbers for both the bundled and swarm missions. These capsules and the miniature sails for swarm missions can be mass manufactured readily.

Astrometry and targeting

The panspermia vehicles would be aimed at moving targets whose locations at the time of arrival must be predicted. This can be calculated using their measured proper motions, their distances, and the cruising speeds of the vehicles. The positional uncertainty and size of the target object then allow estimating the probability that the panspermia vehicles will arrive at their targets.
The positional uncertainty of the target at arrival time is given by the following equation, where is the resolution of proper motion of the target object, d is the distance from the Earth and is the velocity of the vehicle.
Given the positional uncertainty, the vehicles may be launched with a scatter in a circle about the predicted position of the target. The probability for a capsule to hit the target area with radius is given by the ratio of the targeting scatter and the target area.
To apply these equations, the precision of astrometry of star proper motion of 0.00001 arcsec/year, and the solar sail vehicle velocity of 0.0005 c may be expected within a few decades. For a chosen planetary system, the area may be the width of the habitable zone, while for interstellar clouds, it may be the sizes of the various density zones of the cloud.

Deceleration and capture

Solar sail missions to Sun-like stars can decelerate by radiation pressure in reverse dynamics of the launch. The sails must be properly oriented at arrival, but orientation control may be avoided using spherical sails. The vehicles must approach the target Sun-like stars at radial distances similar to the launch, about 1 au. After the vehicles are captured in orbit, the microbial capsules may be dispersed in a ring orbiting the star, some within the gravitational capture zone of planets.
Missions to accretion discs of planets and to star-forming clouds will decelerate by viscous drag at the rate as determined by the following equation, where is the velocity, the radius of the spherical capsule, is density of the capsule and is the density of the medium.
A vehicle entering the cloud with a velocity of 0.0005 c will be captured when decelerated to 2,000 m s⁻¹, the typical speed of grains in the cloud.
The size of the capsules can be designed to stop at zones with various densities in the interstellar cloud. Simulations show that a 35 μm radius capsule will be captured in a dense core, and a 1 mm radius capsule in a protostellar condensation in the cloud. As for approach to accretion discs about stars, a millimetre size capsule entering the 1000 km thick disc face at 0.0005 c will be captured at 100 km into the disc. Therefore, 1 mm sized objects may be the best for seeding protoplanetary discs about new stars and protostellar condensations in interstellar clouds.
The captured panspermia capsules will mix with dust. A fraction of the dust and a proportional fraction of the captured capsules will be delivered to astronomical objects. Dispersing the payload into delivery microcapsules will increase the chance that some will be delivered to habitable objects. Particles of 0.6 – 60 μm radius can remain cold enough to preserve organic matter during atmospheric entry to planets or moons. Accordingly, each 1 mm, 4.2 × 10⁻⁶ kg capsule captured in the viscous medium can be dispersed into 42,000 delivery microcapsules of 30 μm radius, each weighing 10⁻¹⁰ kg and containing 100,000 microbes. These objects will not be ejected from the dust cloud by radiation pressure from the star, and will remain mixed with the dust.
A fraction of the dust, containing the captured microbial capsules, will be captured by planets or moons, or captured in comets and delivered by them later to planets. The probability of capture,, can be estimated from similar processes, such as the capture of interplanetary dust particles by planets and moons in the Solar System, where 10⁻⁵ of the Zodiacal cloud maintained by comet ablation, and also a similar fraction of asteroid fragments, is collected by the Earth.
The probability of capture of an initially launched capsule by a planet is given by the equation below, where is the probability that the capsule reaches the target accretion disc or cloud zone, and is the probability of capture from this zone by a planet.
The probability depends on the mixing ratio of the capsules with the dust and on the fraction of the dust delivered to planets. These variables can be estimated for capture in planetary accretion discs or in various zones in the interstellar cloud.