Hiatus

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!

3 thoughts on “Hiatus

  1. That’s remarkable. I wonder where Zubrin got those figures? On the other hand, it’s probably true that our estimates of the Oort have a great deal of play in them. And what’s your take, Adam. Is that Daedalus beryllium shield really up to the job?

  2. Hi Paul

    As near as I can figure he firstly over-sized Oort comets – 100 km instead of 10 km, which immediately increases the mass ~ 1000 fold. Next he assumed that the numbers of smaller objects scaled directly with decreasing mass – for objects ten times smaller (i.e. 1000 times lighter) there was 1000 times more of them.

    This kind of mass-frequency slope isn’t seen in either the Main Belt or the Kuiper Belt – typically numbers rise a lot slower as size decreases mainly because the Sun’s radiation either causes dust to spiral into the Sun or be blown away. Not such a quick process in the Oort, but dust production is much lower too. Also the intermolecular energy required to fragment an object increases with decreasing size of the pieces – tiny grains are basically indestructible unless vaporised or ionised, which sucks up a lot of energy per unit mass. If you imagine fragmentation energy is proportional to the area of the pieces exposed, and area goes up faster than volume as size decreases, then you can see where the trend is heading.

    As for Daedalus… well I think ejectable precursor shields would be needed to thermalise a colliding particle’s explosion – at 0.122 c the initial impact debris will spray like a jet aimed into the guts of Daedalus’s payload housing. A few Whipple shields ejected ahead would spread out the energy and all Daedalus has to worry about is the heat dissipation. No problem then – the beryllium engine bell is designed to run at 1600 K, so 1000 K shouldn’t be a drama, just a cooling problem.

    Of course the probe could be really unlucky and crash into a cm sized bit of space-trash. Personally I’d rather active defenses with LIDAR scanners and high-energy lasers to vaporise potential impactors above ~ 50 microns. The sub-probes can take their chances with just Whipple shields.

  3. Final note, according to papers I’ve read on Zodiacal dust the size/number distribution goes to power-law indexes between ~ 1.6 – 1.8, which means there’s 40-60 times the number for every 10-fold reduction in size. It also means that most of the mass is locked up in bigger pieces, which will be further apart on an average basis. I think the size peaks around 100 microns, about 1000 times lighter than the coarse sand-grains I assumed. That would make the number of collisions higher, but the energy is lower, more spread out per impact.

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