[TomMckenny]

Slightly related, I wonder if anyone has figured out the density of interstellar comet or asteroid like objects.

I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

[wahern]

I've wondered the same thing. Also consider that

  [t]he energy released by a cosmic collision increases as the 
  square of the incoming object's speed, so a comet could pack
  nine times more destructive power than an asteroid of the
  same mass. (https://www.space.com/26264-asteroids-comets-earth-impact-risks.html)

ʻOumuamua reached a barycentric speed of 87.71 km/s. The tables on Wikipedia's Impact event article (https://en.wikipedia.org/wiki/Impact_event) assume a speed of 17 km/s relative to Earth. The energy of objects local to our solar system is limited in a way that interstellar objects are not.

With a single observation we can't deduce much of anything concrete except to floor the incidence of these interstellar objects at greater than 0. I'm no astronomer, but I assume models of interstellar objects as they reflect actual risk to Earth wouldn't be very useful without more observations. Whatever the average density in galactic space, I'm betting they're not uniformly distributed. Our solar system is speeding through space that could be littered with clouds of objects.[1] Are we entering a cloud? Leaving a cloud? We can't know without more observations.

[1] There are theories that posit that the ~30- and ~225-million year cycles we see in extinction events are a function of our solar system's orbit in the galaxy, which takes about 200-250 million years. Shorter cycles could relate to the inclination of our orbit (and other stars' orbits) relative to the galactic plane.

[ChuckMcM]

One of the interesting things about the galactic orbit is that our system is not exactly in the galactic plane, so as it orbits it passes through the plane to the other side and then back again.

The kinetic energy of objects is proportional to the square of the velocity (Ke = (mv^2)/2), so an object going 4 times faster than a solar system object has 16 times the energy for the same mass. This makes it possible to have extinction level events from rocks that are 1/4 the size of planet killing asteroids.

[mhandley]

Ok, here's an extremely rough back-of-the-envelope calculation. As you'll see, these numbers can be out by orders of magnitude, and it doesn't greatly change the conclusion.

Oumuamua interstellar asteroid. 230x35x35m, ~= 280000 m^3

Density assumption: 2 x water. => mass is ~500,000 metric tonnes.

Spotted only after passing the Sun. Assume we'd spot such objects only if they came within the orbit of mercury so are well illuminated. Assume one such object every 10 years (we've not been searching very long with automated telescopes), and we spot all of them.

Mean mercury orbit radius ~ 60,000,000 km

Area of mercury's orbit: 1.1 x 10^16 km^2

Mercury's orbital area x path length in 10 years = volume swept by one visible object in 10 years.

Asteroid velocity ~100,000 km/h

Path length in 10 years = 100,000 x 10 x 24x365. Swept volume ~ 10 x 10^25 km^3

Distance to Alpha Centauri: 4.37 light years = 4.37 x 9.5 x 10^12 km = 4.15 x 10^13km

Sol's "cube of influence" ~= 7 x 10^40 km^3

Cube of influence / swept volume = rough estimate of number of asteroids in cube of influence. Number of asteroids: 7 x 10^14

Mass of asteroids: 3.5 x 10^20 tonnes. Mass of sun: 2 x 10^27 tonnes.

Conclusion: dark interstellar asteroids like Oumuamua are a tiny fraction of the visible mass of the galaxy.

[coldtea]

>I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?

[Baeocystin]

Practically instantaneous on the lengthy time scales the universe operates in, don't you think? And our observations have far from complete coverage of the sky.

[azernik]

That's a pretty small volume and a pretty short timespan, all things considered.

[Firerouge]

I'm not qualified to answer your question, but the observation of Oumuamua alone doesn't give a large enough sample size to estimate how often large interstellar comets or asteroids pass through our solar system.

[privong]

> I'm not qualified to answer your question, but the observation of Oumuamua alone doesn't give a large enough sample size to estimate how often large interstellar comets or asteroids pass through our solar system.

It's an observation, so it sets some level of constraints on the rate. Though it's true that an estimate of that rate would have large uncertainties.