3 comments

• Tom Billings

  August 11, 2015 at 10:46 am

  One wonders about jets and pits, and causation versus result.

  That they are not linked is almost impossible, IMHO.

  Which precedes the majority of the other, however? It seems that such pits as are large in relation to the size of the comet would be the results of long-term continual jet material expulsion. However, that they often seem filled with dust and relatively nonvolatile material prevents us from seeing just how deep inside the comet they go. I wonder if that fill material could be the result of ejected material during perihelion staying close enough to the nucleus that, as the comet rises towards aphelion, it is attracted back to the nucleus. That would happen with the less volatile material’s larger pieces, projected by the jets at lower velocities, if at all.

  The pictures tell us little bout the depth of the pits underneath the loose fill material, and nothing of their structure at deeper depths. Yet, I have seen speculation about great depth, resulting from heat near the core of the nucleus. It seems the only heat source is the Sun’s radiation. However, at perihelion, one wonders about the gravity gradient along the long axis of the nucleus. As the nucleus tumbles, that gravity gradient would shift, creating strains all the way to the core of the nucleus. Even if it did not cause all that much internal heat those strains could create cracks, through which gas can move.

  We know that jets change the orbit of the comet through reaction on the nucleus as a whole. But what about localized effects? Are gasses in the pits that are not projected outwards being projected inwards? Gas expansion pressure *should* be isotropic, after all. Could gas, heated near the surface by solar radiation, actually move *into* the core, through those cracks, providing heat that drives out the lowest volatility gasses, enlarging the cracks to passages?

  If this were the case, then what of CC Asteroids? It is often speculated that these are “burnt out comets”. If that were true, as well as shifting gravity gradients causing deep cracking being common among comet nuclei, then we might see these “burnt out comets” retain the passages opened up and expanded before they were “burnt out”. That would allow several things to be done. First, exploration of CC Asteroids could tell us much about whether this is one method of pit and crack formation in cometary nuclei. Second, these passages could be exploited as habitat shelters by asteroid miners accompanying the robots that would do most of the work of asteroid mining, doing things robots cannot quite do yet.

  That would provide ready made shelter from radiation, just as lava tube caves would on larger bodies. There are far more CC Asteroids than the lava tube cave candidates so far seen on the Moon and Mars. They might yet provide shelter for larger human populations than any planets ever will, even before being turned into O’Niel type Space Colonies. Of course, Ceres, given its size and composition may also provide more such living space than all the CC Asteroids from outgassing of its ices during its own perihelion. Still, those asteroids that many disdain as targets for examination could yet be bite-sized settlement locations for any number of different settlement groups.
• Blair Ivey

  August 11, 2015 at 9:55 pm

  Rosetta and New Horizons are adding to human knowledge of the universe by the minute. Two very cool and (so far) successful missions. I sincerely hope that these aren’t the high-water mark of human exploration, but given the increasing drag of unsustainable societal priorities, I’m not especially sanguine.
• Edward

  August 12, 2015 at 3:28 pm

  Don’t forget about Dawn, exploring two asteroids/dwarf planets in one economical mission.