8. Target Selections and Probability(From the Journal of the British Interplanetary Society 1997, 50, 93-102. Michael N. Mautner)
Directed panspermia will target extra-solar planetary systems, nine of which are already known to have habitable planets, and many more are expected. We can also reach clusters of newly forming planetary systems in interstellar clouds, where local life would not have yet started. The following sections compare the probability of capture and survival for different types of targets.
The fraction of launched panspermia swarm that will reach the target zone (the interstellar cloud, protostellar condensation etc.,) P(target), was calculated above. We consider here the further term P(capture), the probability that once in the target zone, the payload be eventually captured into the habitable zone of a planet. The overall probability for capture in the target planet is then obtained from equation (5).
P(planet) = P(target) x P(capture) (5)
As noted above, for calculated values of P(target) > 1, we use P(target) = 1. The following sections summarise the considerations to estimate P(capture), and from it, P(planet). The results are summarised in Table 1. The following discussion applies to solar sail missions (v = 5E-4c), but results for advanced missions (v = 0.01 c) are also shown in Table 1.
Sections in this chapter
Equation (2) yields P(target) > 1 for the dense Rho Ophiuchus cloud fragment. In other words, because of the large size of the target cloud, virtually all of the microbial capsules launched at it will arrive to the 3E16m radius, 1E33kg target. The cloud contains four dense cores with a total mass of about 1E31kg, one of which has already formed protostellar condensations, and the others with the potential to form such condensations . In addition, capsules may be also captured into the already formed 78 young stellar objects, which would have 100 au (1E13m) radius dust shells or disks. Assuming that the cloud will eventually form 100 stars of 1E30kg, from the mass ratio of each star to the overall dense cloud fragment, 1E-3 of the launched mass will be captured into each accreting solar system, ie., for each star, P(target) = 1E-3. By the mass ratios of 1E17kg dust captured by a planet during the suitable 1E9 yr prebiotic period to 2E30kg mass of the protostellar condensation, about 1E-13 of the capsules will be captured, giving P(capture) = 1E-13. Altogether, therefore, Pplanet = 1E-16 for each accreting solar system, ie., 1E-16 of the mass launched at the cloud will be captured by a terrestrial planet in each accreting system. In total, 1E-14 of the launched mass will be captured in terrestrial type planets in the 100 accreting stars in this cloud. Note that with this strategy, individual stars are not targeted, and the mass that is launched must provide for seeding the entire cloud.
Targeting individual protostellar condensations. The calculations above yielded P(target) > 1 also for specific protostellar condensations, and therefore such regions can be targeted individually and we can use P(target) = 1. From the mass balance ratios as above, P(capture) = 1E-13, giving also P(planet) = 1E-13. The advantage of targeting individual protostellar condensations, rather than the overall cloud, is the greater chance for reaching a known, already established star-forming zone. This strategy also decreases the exposure time and radiation dose received when the payload would be diffusing through the cloud. A disadvantage is that, although the calculations yielded P(target) > 1 for both the cloud and the individual protostellar condensations within it, the value was 1.4E4 for the cloud and only 1.4 for the condensation region, and realistically, the chances of capture are much greater in the larger cloud. Another disadvantage of targeting existing protostellar condensations is that the missions will miss many new star-forming condensations that form after the launching of the capsule swarm.
Targeting early accretion disks. The 78 young stellar objects observed in Rho Ophiuchus are dust embedded or are in the T Tauri stage, with 100 au radius accretion disks. Because of their small size, P(target) = 3.9E-3 for these objects. On the other hand, the capsules will be distributed only in the circumstellar dust but not in the star mass, avoiding a major source of loss. Assuming that the majority of the dust is accreted into the original 1E13 comets with a total mass of 1E28 kg, of which 1E17 kg is eventually captured by a planet, gives P(capture) = 1E-11.
Targeting late accretion disks. Targeting accretion disks at the late stages of comet formation is advantageous because the capsules will be accreted into the outer cometary shell, which is most readily released subsequently. The theory of cometary accretion is uncertain, and a zone of some tens of au, say 10 - 20 au about the star may be considered for initial comet formation. For this area we obtain P(target) = 1.2E-4. It will be assumed that all the payload reaching the zone will be captured into orbit and eventually accreted into cometary shells. Assuming capture into the 100 m outer shell in 1E13 initial comets of 5,000 m radius, the microbial payload will be embedded in 3.1E26kg dust, of which 1E17kg will be delivered eventually to the planet, yielding P(capture) = 3.2E-10, and P(planet) = 3.8E-14.
Targeting planets. The most direct approach is to target planets in already accreted planetary systems. As noted above, this may be better applied to planets at least 0.5 Gyr after accretion, as the initial conditions may be sterilising. Targeting planets directly may be appropriate if older accreted planets are identified, or if further research suggests that young planets are survivable. We consider capture of the payload within <3.5 au from the star, which yields P(target) = 4.9E-6. From the Zodiacal dust and meteorite capture statistics, P(capture) =1E-5, and therefore P(planet) = 4.9E-11.
The amount of material that needs to be launched is calculated from the Pplanet values, allowing for the delivery of 100 capsules. The factor of 100 also corrects for other uncertainties in the mission. The mass required for the delivery of 100 capsules of 1.1E-10 kg each is then given by m = 1.1E-8 per Pplanet. The results are shown in Table 1.
For targeting the entire dense star-forming region, a very massive program of 1E8 kg per accreting star in the cloud is required, which can be only accomplished using space resources (see below). If targeted at individual protostellar condensations or accretion shells or disks, requirements on the order of 1E5 kg for a sail mission, and especially 1E3 kg for an advanced mission, are realisable. Finally, if already accreted planetary systems in the cloud or closer are identified and targeted, the mass requirements on the <1 kg to 100 kg scale are easily met. Such panspermia programs should be affordable to small motivated groups or even individuals, which increases that likelihood that the program will be actually enacted.
Swarm missions to nearby stars. It is of interest to evaluate the swarm method also for closer planetary systems. For alpha PsA (Fomalhout), d = 22.6 ly, Ptarget was found as 1.2, and for beta Pictoris, 0.25, for capture into orbit in the habitable zone. For Pcapture we use 1E-5, although of course it may be different in different solar systems. With this assumption, Pplanet = 1E-5 and 2.5E-6, respectively, is obtained for the two targets. These stars are in the local low-density interstellar medium, and the sail method described in the previous papers [4 - 6] may be used, miniaturised for launching 30 Tm radius, 1E-10kg capsules by small, 1.8 mm radius sails. These sails may be, for example, envelopes of thin reflective film that enclose the payload, mass-produced using industrial microencapsulation technologies. As few as 1E7 or 5E7 capsules, ie., 1 or 5 g of microbial payload launched toward these stars in a swarm, respectively, could then deliver 100 capsules to a planet. Remarkably, with current launch costs of $10,000/kg, a panspermia swarm with a reasonable probability of success can then be launched to these stars, nominally, at the cost of $10. Of course, it should be easy to scale up such missions by a factor of 1,000 to kilogram quantities for increasing the probability of success or for allowing much less accurate, easier methods to launch the capsules, still within a very low-cost program of $10,000. Therefore,directed panspermia swarms to nearby planetary systems can be easy and inexpensive.
The missions to star-forming regions can arrive into solar systems at stars in various stages of star formation, that may coexist in a target cloud. Stars that are at the dust-embedded or T Tauri stages when the missions are launched will last in these stages 1E5 - 1E6 years, similar to the transit time. When the missions arrive, these stars will have formed accretion rings. The subsequent planetary accretion lasts for 1E8 years, and high temperatures, intense solar UV flux, and frequent major impacts may make the new planets habitable only after 5E8 yr. However, capsules arriving at this stage can be preserved frozen if captured in asteroids and comets at r > 2.3 au at temperatures of T < 150 K, as calculated from the temperature function T = 250r-0.6 (r distance in au). Furthermore, capsules accreted into a depth of several hundred g per cm2 in the comet will receive a radiation dose reduced by a factor of 100 from those on the cometary surface, which can assure survival on the Gyr time-scale.
Optimally, a fraction of the capsules may be embedded into the protected layers of the outer cometary crusts. These loose porous icy aggregates and embedded dust evaporate losing several hundred gm per cm2 in the first perihelion passage , and further inner layers evaporate gradually during further transits, releasing dust that is later captured into planets from the zodiacal cloud. Capsules that are more deeply embedded in cometary nuclei or asteroids may also arrive on planets with impacts , and within the meteorite rock can survive atmospheric transit.
Of the original 1E13 comets formed, 99% are ejected to interstellar space , but where Jupiter-sized planets fail to form, the cometary populations that remain bound to the solar system are greater, and barriers to penetration to crossing Earth-like planetary orbits are smaller. Jupiter-family comets can then remain in these orbits for 1E7 - 1E8 yr, instead of the present 1E5 yr, and the frequency of major cometary impacts increases from 1E-8 per yr to 1E-5 per yr . In such planetary systems, the amount of cometary material and embedded microbial capsules that is delivered to the planets can increase by a factor of 1,000. In addition to comets, microorganism capsules may also become embedded in asteroids, and in the meteorites fragmented from them. Compared with the 1E26 kg total cometary mass, the total asteroid mass of 1E21 - 1E22kg is much smaller, but it can provide a favorable nutrient microenvironment, see below.
Please note: numbers in square brackets refer to the references that you will find under "resources"