4. Propulsion and Launch
(From the Journal of the British Interplanetary Society 1997, 50, 93-102. Michael N. Mautner)Micron-thick solar sails can launch interstellar spaceships, or swarms of capsules, filled with microbes toward new solar systems at interstellar speeds over 0.0001 times the speed of light. These sails, given the minute size of the payloads, do not need to be large - a radius of 0.35m will suffice.
The biological requirements were considered in relation to missions to nearby solar systems [4,5]. Some key points are as follows.
Our previous papers considered technologies for sending large microbial payloads on the order of 10 kg to nearby solar systems [4-6]. We considered relatively simple technology, using solar sail vehicles with areal densities 1E-4 kg/m2 with thin sails of thickness 1E-7 m (0.1 microns), and of sizes on the order of 1E6 m2, which can reach velocities of 5E-4 c when launched from 1 au. The sails must remain stable during transit times of 2E5 years to targets up to 100 ly away, so that they can provide braking by radiation pressure after arrival.
In comparison with the 10 kg payloads of directed missions, the swarm approach launches large numbers of small payloads. The considerations below suggest launching 1 mm radius, 4.2E-6 kg microbial packets. Therefore, the swarm method miniaturises the mass of each launched payload by about a factor 2E6, which further reduces the technological requirements and may allow new propulsion approaches. Once in the target region, the packets can further decompose into 4E4 capsules of 30 mm radius containing 1.14E-10 kg microbial mass, that is appropriate for eventual non-destructive atmospheric entry. The large numbers can also increase the probability of capture.
Even for the milligram payloads, the most imminent technology appears to be solar sailing. For effective devices, the sail/payload ratio should be about 10:1, requiring sails of 4.2E-5 kg. With an areal density of 1E-4 kg per m2, this will require sails of 0.42 m2, ie., sails with a radius of 0.35 m. Such small sails can be mass manufactured easily, which is important since very large numbers are required. For planetary targets in the dilute medium within 100 ly, the 30 T m, 1.1E-10 kg capsules can be launched individually, using 1E-9 kg sails of 0.18 cm radius. These miniature objects can be mass manufactured and launched even more easily.
The thin sail devices with a density of 1E-4 kg per m3 could transit the local low-density medium about the Sun with little drag. However, the sail devices cannot penetrate even a diffuse interstellar cloud with r m = 1E-19 kg per m3, where they will stop rapidly, for example, slow down to 15 m per second in the first 0.4 ly. For this reason, and to minimise scattering during transit, a useful strategy would be for the sails to eject the capsules once they obtained the final velocity of 1.5E5 m per second, possibly with an impulsive ejection using the sail as countermass, to impart the payload further acceleration. Alternatively, the sails may be manufactured of biopolymers that would fold over the payload after exit from the solar system. They can then provide additional shielding in transit, and be used as a nutrient shell once the capsules land on the host planet.
The transit time for a sail-launched capsule to a cloud 100 ly away is 2E5 years, during which the payload will be subject to 2E6 rad of ionizing radiation. This can be lethal, or at least strongly damaging to most microorganisms. It may be desirable therefore to use alternative propulsion methods to achieve greater velocities and shorter transit times. However, at high speeds, ablation and heating of the capsules can be significant, especially in the dense cloud area, requiring velocities <0.01 c. At such high entry velocities, even sub-millimeter size, sub-milligram capsules may penetrate the clouds sufficiently, so further miniaturisation of the microbial packets down to microgram levels may be possible.
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