Study looms and blogging is eating into my assignment writing time, so expect very sporadic writings for a few months.
One thing before I go is a topic Paul Gilster from Centauri Dreams mentioned – the hazards for an interstellar high-speed fly-by from interplanetary dust. Daedalus, for example, was to fly through Barnard’s Star’s system at 12.2 % of lightspeed, which gives a massive energy punch to any dust impacts. So I quantified the hazard – in the Sun’s Inner System interplanetary dust masses about 10^16 kg. If every bit was 1 mm cubes, density ~ 2, then the dust masses 2 x 10^-6 kg each, making for about 5 x 10^21 specks of dust. Sounds like a lot, but the volume of the inner 2 AU of the Solar System is 4/3*pi*(3.0E+11 m)^3 = 1.13 x 10^35 cubic metres – 22.6 trillion cubic metres per speck. That’s a cube 28.3 km on a side per speck.
If Daedalus is 50 metres wide (that’s its dust-shield) then it punches a 2 AU long, 50 m wide hole through the dust and encounters about 26 dust specks – each one packing a punch of 1.33 gigajoules (a few kilos of TNT) which isn’t fun, but blown into plasma by the precursor shielding cloud it’s a survivable dose of energy, heating the beryllium shield to about 1000 K. The Daedalus sub-probes are much smaller and probably have a 50/50 chance of an encounter, but they have precursor shield systems too.
For interest sake I also looked at some figures on mass in the Oort Cloud which Robert Zubrin uses in “Entering Space” – his results are seriously in error. He describes the Oort as containing 100 km cometoids about 10 AU apart – sounds spacious, but the Oort is 100,000 AU in radius, thus he’s describing 10^12 cometoids at 100 km in size. Then 1000 times more (i.e. 10^15) 10 km cometoids 1 AU apart each, then 1000 times that number of 1 km comets just 0.1 AU apart… and so on down to 10 cm chunks just 1,500 km apart. 7 orders of magnitude, giving a total mass of 7 x 10^30 kg (3.5 solar masses!) for comets of 500 kg/m^3 density.
That’s ridiculous! Long period comet orbits wouldn’t be anything like stable, or long period with that much mass out there. In actual fact the total mass is usually quoted as 40 – 30 Earth masses – about 30,000 times less than Zubrin’s wild figures. Fortunately for us, or else we’d have a sky full of comets all the time and mass extinctions every ~ 3,000 years. The real Oort Cloud is very wide open spaces…
Ok. Bye for now!