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.