8.1 Dark Cloud Fragment
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 3E16 m
radius, 1E33 kg target. The cloud contains four dense cores with a total mass of about
1E31 kg, one of which has already formed protostellar condensations, and the others with
the potential to form such condensations [8]. In addition, capsules may be also captured
into the already formed 78 young stellar objects, which would have 100 au (1E13 m) radius
dust shells or disks. Assuming that the cloud will eventually form 100 stars of 1E30 kg,
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 1E17 kg dust captured by a planet during the suitable 1E9 yr
prebiotic period to 2E30 kg 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.
8.2 Protostellar
Condensations
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.
8.3 Accretion
Disks and Planets
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,
and P(planet) = 3.9E-14.
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 3.1E26 kg dust, of which 1E17 kg 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.
8.4 Biomass
Requirements
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/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.
8.5 Missions
to Nearby Stars
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 T m radius, 1E-10 kg 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.
8.6 Survival and Growth in Comets and
Asteroids
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 cm-2 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 cm-2 in the first perihelion passage
[11], 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 [21], and
within the meteorite rock can survive atmospheric transit.
Of the original 1E13 comets formed, 99% are ejected
to interstellar space [12], 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 yr-1 to 1E-5 yr-1 [22]. 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 -
1E22 kg is much smaller, but it can provide a favorable nutrient microenvironment, see
below.
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