Crustal Fragments as Asteroids

Consider a solar system with a planet or dwarf planet somewhat smaller than Mars. If it is large enough, when it forms there will be a great amount of heat generated, meaning there will be separations of elements and compounds, and ore mixtures as well, in the core and crust of the planet. The largest glob will likely be an iron and nickel mixture, which would form the core of the planet. If it stays warm enough for long enough, compounds not very soluble in this metal will separate out, and assuming they are lighter in density, will rise to the crust. The crust will have all the interesting materials in it. The core will be fairly homogeneous, but the crust will have different ores separated out, provided the elements to make these up were there in the beginning.

Thus, small planets deep in the solar system, where dust would collect, would be treasure-troves of minerals, some of which might be very important to an alien civilization which had passed the point of being able to travel throughout their solar system.

As far as mining of inner, metallic planets goes, they do not need to be very small for mining to take place. Having no atmosphere may make it easier or harder to do mining, and that is not clear now. But smaller planets, with no atmosphere, would be vulnerable to the rain of smaller asteroids that happens in the early days of a solar system. Many of these would simply pulverize the surface. Those which came in on a grazing trajectory, if the size and speed were right, might chip off some piece of the crust and transfer enough momentum to it so that it escaped the gravitational pull of the small planet. This is a crustal fragment.

A larger planet could conceivably give rise to a crustal fragment asteroid as well, but via spallation rather than a grazing impact. If a large asteroid were to impact a larger inner planet, the shock wave from the impact would travel through the planet, arriving at the opposite point from the impact point. Perhaps it could have enough energy to blast some material past escape velocity, and possibly some of this material might still be intact, that is, some chunks of crust go flying into solar orbit.

Mining asteroids is thought to be a possibly profitable venture, in the sense that the retrieved material is worth more than the cost of retrieving it. For a crustal fragment asteroid, the materials might be much more valuable than an asteroid which simply has the average material of the solar system at some radius. There could be a hundred times more valuable ore on a crustal fragment asteroid than on a usual one. What would be important if finding which was which. This might require visits by some small robotic ship.

Suppose there was an asteroid, formed from one of the crustal fragmentation processes, which was of the order of a hundred kilometers in size. If it were explored, and there were sources of rich uranium ore in the asteroid, it might be able to form a self-sufficient colony of aliens there. Using the uranium as an energy source, the only energy source, could enough energy be generated to provide a habitable environment, where every other material had to be mined from somewhere on the asteroid and transformed into useful materials? If this is possible, a temporary colony could be established, either independent or part of some multiplanetary organization. How long could alien civilization last on the asteroid? Until the uranium ore was depleted so much that it could no longer supply the energy needed to power the entire asteroid and all its necessary activities, of mining, transporting, refining, manufacturing, and all the multiple activities needed to provide a habitable environment.

How likely is it that there would be one or more of these crustal fragment asteroids in an average solar system? We don't know what average is yet. We don't know what asteroids are in our own solar system, so the data is pretty sparse. At best, we can indicate it might be possible.

What might be the orbital characteristics of a crustal fragment asteroid? Ones which are formed from the grazing impact process would have something less than the orbital radius of the incoming asteroid, the one which hits the small planet. That could have been in orbit similar to the planet which was hit, meaning the resulting asteroid would also. However, in the early days of the formation of a solar system, some asteroids might be shot into orbits out of the planetary plane or even retrograde. This happens because of the interaction of the major planets with the small bodies co-inhabiting the solar system. It should be quite rare, but possible.

The spallation situation might serve to give the spallation fragments a higher speed that the incoming asteroid, if the shock wave was focussed sufficiently. Is it possible that it could be given solar escape velocity, and leave the solar system? Is it possible that later interactions with large planets could slingshot it out of the solar system? The later is certainly possible, and the former, maybe. Either way there is a process by which a crustal fragment asteroid could become an interstellar rogue. Since the crustal fragment asteroid is formed in such as way that its orbital parameters could be unusual, this is not too hard to imagine.

This means that for an alien solar system, there might be rogue crustal fragment asteroids passing through it, laden with massive amounts of uranium for energy and other crustal materials for manufacturing. Could an alien civilization, able to travel around its own solar system and very famiiar with mining asteroids, manage to get to such as asteroid before it passed through their system, and establish either a robotic colony or one comprised of some very brave colonist aliens? Only if they had prepared such spaceships in advance, so they could simply concentrate on getting their ship there and down on the surface in the months that the asteroid was present in their solar system.

They might be able to make small adjustments in its trajectory from solar system to solar system. If there were a sufficient number of this type of rogue asteroid passing through their solar system, it would mean that we should not be looking for some giant saucer-shaped ship for visiting aliens, but instead a large rock.

Rich Clouds, Poor Clouds

It seems that normal stars do not produce the amounts of heavier elements, those higher in atomic number than iron, that are observed in the galaxy. Some other source is indicated. One theory involves a collision between two neutron stars. This might be effected by starting with a binary with two neutron stars. In a binary with only one neutron star, it eats up the atmosphere of the other star. But a neutron star has no atmosphere similar to a normal sequence star, so one cannot strip mass from another. They have little to do but radiate energy and eventually collide, leading to another type of explosion.

This makes sense, as higher elements are formed by neutrons being added to lower elements’ nuclei, and a neutron star is nothing but neutrons. An explosion would lead to the rapid formation of elements, but the spectrum would be quite different from that of a stellar interior, where elements are kept in equilibrium by a very different set of processes.

One question this theory raises is how well a binary can survive a supernova explosion of one of the two stars involved. Perhaps a well-separated binary could survive it, with only a orbital change, perhaps from near circular to an orbit with large eccentricity. Would the first supernova strip off part of the atmosphere of its binary companion, reducing the amount of fuel for it to burn, and thereby hastening the second supernova? This theory of binary neutron stars raises many intriguing questions.

Binary stars do form fairly frequently, so it would make sense that some of them would have two stars which could both evolve into neutron stars. It’s not exactly clear what would happen if one of the stars became a black hole, just barely. Perhaps the same type of collision would also add to the heavier elements.

A fairly obvious question arising from this is: Are clouds uniform in the production of double neutron star binaries? Are clouds which are larger or smaller, more or less dense, more or less turbulent,more or less spherical, hotter or colder, dustier or more gaseous, more likely to produce these special binaries? There are many parameters by which clouds can be described, and it would seem some of these would affect the production of predecessor stars to neutron stars, and binaries to boot. If these factors do play a role in the relative density of these binaries, then around the galaxy there would be, sometime into the lifetime of the galaxy, clouds which are rich in heavier elements and clouds which are poor in heavier elements.

If the technology development of an intelligent species requires the presence, on the planet, of a certain amount of heavier elements, this means that some clouds in the Milky Way are more prone to civilizations which can eventually travel to other stars, and other clouds are too deficient to allow any intelligent species to climb high enough in technology to do this.

Clouds are much larger than solar systems or intersolar distances, so this means the galaxy might be like a large continent, part of which was habitable with rain and rivers and vegetation, while other large areas were barren deserts. Similarly, it would mean that travel within one’s original cloud might be much easier than from one heavy-element-rich cloud to another. Huge distances would have to be traveled, in comparison to the typical interstellar ones. Just to provide some food for thought, suppose the distance between good planets was 100 light years in a rich cloud, and the distance to another rich cloud might be 10,000 light years. While it might be reasonable to travel the first, the second might be simply too far. Thus, the galaxy would be necessarily divided into pieces which cannot communicate between one another.

The heavier element distribution is an additional galactic distribution factor on top of the diversity that already is known to exist, with different major components such as the bulge and the disk, and the variations known to exist in the disk with the rotating spiral density waves and the gulfs between them. Cloud density variations are huge to begin with, and with this latest theory on the peculiar ways in which heavier elements are formed, there is yet another factor contributing to the geography of the galaxy.

It should be possible for an advanced alien civilization to map out the distribution of heavier elements in the galaxy, using large wide spectrum photon collectors. They would therefore know, before they made any decision as to seeding other planets or doing anything else interstellar, just how much territory they could operate in. They might find out that they are in an extreme situation themselves.

If the density distribution of heavier elements is very peaked, in other words, the processes that make heavier elements, such as the proposed neutron star collisions, are quite rare, and there are only a few pockets within the galaxy where there are higher densities of these elements, then they might find that there is no hope to seed the galaxy. Basically they might find they were in one pocket, that there were no other similar solar systems within that pocket, and the nearest other pocket was on the opposite side of the galaxy, much too far to travel to under any circumstances at all.

This distribution may be yet another surprise awaiting Earth scientists as they explore the galaxy. Right now in our history we are just beginning to understand the galactic environment that we live in, and the question that has caught our fancy is the possibility of life originating on other planets. For this we search for some attributes which might be signatures of life. But it could very well turn out in a few years or decades that we realize that we are indeed located in a very unique corner of the galaxy and are the only ones alive and civilized at this time. The galaxy is too harsh a place for life to evolve and develop a technological civilization except in only a tiny fraction of the existing solar systems.

Another implication of this is that heavier elements might take billions of years to accumulate, so that if we had come into existence five billion years later, the galaxy might have many more alien civilizations, traveling from one star system to another or at least communicating between one another. It is too bad we can’t wait around for all the excitement to begin.

Rogue Asteroids

In current news, it was reported that Earth astronomers have detected their first interstellar asteroid within the solar system. Temporarily named A/2017 U1, its size has not been determined, simply its trajectory. It traveled in from the north of the ecliptic, passed by the sun within Mercury’s orbit where the orbit was bent back toward the north, and on its way out of the solar system it passed within 25 million kilometers of Earth. This latter fact allowed it to be detected by our sky survey instruments, which are looking for near-Earth asteroids.

The size was bounded by maximum 400 m diameter, as otherwise it would be less faint. There is no albedo measurement, so the true size will stay unknown. Let’s throw caution to the wind, and try and understand the implications of this detection. If we say an asteroid with diameter between 300 and 400 meters would have been detected, can this be used to figure out the number which pass through the solar system? The sky survey telescopes can see this object out past 25 million kilometers, but perhaps not detect it initially. Let’s simply suppose that this is the only one of this size which passed through the sphere of detectability of this radius during the last twelve months. Neptune’s orbit is about 180 times this distance, so by looking at cross-sections, we might say that of 32 thousand penetrations of a sphere of this radius, only one would go through the Earth detectability sphere. This means that of the order of 32 thousand asteroids in this size range come through the solar system each year.

If we assume that the size distribution of interstellar asteroids is the same as the asteroids in our solar system, this size range represents about one thirtieth of the total asteroid population with diameters greater than 100 m. So, a little multiplication tells us something like a million asteroids bigger than 100 m shoot through the solar system every year. We’ve seen one.

This number could be off by an order of magnitude or even two. If the sky survey astronomers were really lucky, and this was the only asteroid to come through the detection sphere in a century, then everything would be 100 times too high. But the simplest guess is that this is not the one year when it happens, just that there was not much interest in such objects before, and the detection rate was affected by the attention given to them. Now things are different, and the sky eyes will be looking for the next one.

This asteroid could have been formed similarly to a orphan planet, just condensing out in interstellar space from a small cloud that congealed. Probably it was instead formed in a solar system, and then chucked out in the early days of orbital interaction. There could even be some late time interactions which propel an asteroid from a solar system. We don’t live in any unusual part of the galaxy, just a normal section of a spiral arm, and so it would be reasonable to assume that other solar systems have similar amounts of interstellar object penetration. What would an advanced alien civilization make of this?

One thing they could do would be to use the asteroids as free shipping objects to other solar systems. Put some memorial on an interstellar asteroid, and a million years later it might pass through another solar system. Stars move around a lot, so it might be hard to write something that would be meaningful as to where the memorial was inscribed, but perhaps that problem would be solvable if some dating were possible. Is there anything in the galaxy that tells time?  We can date supernovas and nebulae formed by them by determining the relative speed of the nebula gas, and backtracking the trajectory to find out the date when the supernova exploded. This might be accurate enough to enable some announcement in the memorial as to when the alien civilization inscribed it.

To get the memorial out to an interstellar asteroid requires some high-power propulsion. This asteroid we see is going at about 25 km/sec relative to the sun. For comparison, LEO velocity is 8 km/sec and it is still within Earth’s gravitational well. To comprehend better what launcher requirements are, think of putting a multistage rocket into space outside of the moon’s orbit. The payload of the rocket would have to include a lander, plus control systems able to bring it into orbit near the interstellar asteroid. This would have to be done within a period of a couple of months, between detection and departure of the asteroid. It exceeds our capability significantly, but we haven’t even been launching extraterrestrial rockets for a century yet. It should certainly be within our capability within another century, probably much less.

Digging into an asteroid would provide a radiation shield for anything that the alien civilization wanted to send to another solar system. Digging machines would mean a much larger payload however. It would be good, for such a massive mission, to have as much lead time as possible. However, doing a sky survey requires a telescope that can be oriented and scanned over large sky areas. Using a kilometer sized telescope rules out rapid scans. Thus, the task of landing on an interstellar asteroid and creating something there within the allotted time is certainly technologically challenging.

Could something more significant be done with these opportunistic travelers? Perhaps if there was a rogue planet nearby. If we assume the ratio of planets to asteroids is the same in those early solar systems that were launching asteroids as in our present day solar system, perhaps one millionth as many planets would get launched on interstellar trajectories as asteroids. So, there is some possibility that one will come by. It is also quite possible that the dynamics of planets is such that there is a much lower probability of launching a planet on an interstellar path than an asteroid, so the number might be a billionth instead of a millionth. If this is the situation, we shouldn’t expect a planet anytime soon.

If there was one, and it had an energy source such as large amounts of uranium ore, it might be possible to put a robotic colony on it that would be self-sustaining. It is barely conceivable that such a rogue planet could be used on a seeding mission, especially as there is no way to choose the target solar system or the arrival time. More likely, memorials will be the only thing possible for these star-traipsing asteroids and planets.