Expert: Tom Whiting - 1/5/2011

Question
QUESTION: Hi,
 I was wondering(and this has nothing to do with any 2012 doomsday scenario :)), if a rogue black hole came upon our solar system, how close would it have to get to our sun to start drawing material away from it and/or pulling it towards it or at the very least effecting its rotation or orbit? If it passed lets say just outside Pluto's orbit, would the sun be affected? Granted kuiper belt and oort cloud objects and maybe the planets would or at least some but I am strictly speaking about the Sun. For this question, the planets don't matter.
Secondly, lets say for arguments sake the sun is affected but only a small amount. It is nudged slightly out of its current location. Then the black hole passes through and leaves the system. Assuming it wasn't ripped away entirely from its position, would the star eventually work back into its original orbit/location?
Thank You

ANSWER: Part one - Depends entirely on the mass of the "rogue" black hole; 50 solar masses or 3 billion
solar masses?????  Assuming the former, there would be no effect, as the gravitational force drops off as the square of the distance, so 4 billion miles (6.4 billion kilometers) out to Pluto's orbit, SQUARED... would be in the denominator of the equation F = G1G2/d squared, making a 50 solar mass black hole's effect barely measurable at that distance, let alone catastrophic. So no, the sun would be unaffected.
Part 2 - If the sun were to be affected, orbital change around the Milky Way Galaxy (or even thrown out of it, like in a collision with the Andromeda Galaxy like 4 billion years from now), the sun would not return to it's previous orbit (and no one would care anyway, including the sun and Earth...) the sun (and Solar System) would simply assume a 'new' orbit around the Milky Way Galaxy. AND all the planets would be unaffected by the orbital change, and could care less if we're in a galaxy, or not in a galaxy.  The captured planets will continue to follow along with the sun no matter what orbital path the sun takes, in or out... of the Galaxy.
The sun (and captured planets) could care less about whether we're in a galaxy, or not. We don't "need" to be in a galaxy at all.  And a slight orbital change around the galaxy would be... who cares? As long as the deviation isn't great enough to throw us into the center of the galaxy, then we're in deep do-do eventually... as the radiation levels are quite high close to the center of the Milky Way. But even then, it would take millions of years to even reach the center of the Galaxy... time enough to get us off (with future starships) this skimpy 8000 mile diameter rock we live on.
So no, the sun would not return to it's previous orbit (Newton's First Law -would take another 'action' force to return back to original orbit... ) but no one would care anyway. With the Milky Way and the Local Group traveling at about 600 miles per second toward Leo, orbiting the Virgo Supercluster, we're always "cutting into" new territory... new space as it were... all the time anyway, so what's the difference where that "new space" is?  No one would care... humans, Earth,
or the sun. Even our Solar System's orbit around the Galaxy means that the Earth isn't really back at the same "spot" after one year, as the Solar System is traveling something like 180 miles per second around the Milky Way Galaxy. You NEVER return to where you were at, one year ago, in space. You only return, relative to the sun, back to the same place, but the sun has moved a great distance around the Galaxy during that year. So you're always cutting into... "New territory" all the time.  (There is no such thing as absolute rest in the Universe... everything moves relative to everything else.)

When Captain Kirk is tooling along in the starship Enterprise with the starship Potemkin far behind, and he states, "Let's stop here and wait for the Potemkin to catch up"... what the heck does he mean by that statement?  Here? Where is here, relative to what? (That alien solar system just off the starboard bow?)  And what is "Stop?" Relative to what?  That same alien Solar system? (Which planet in that system?)  If so, then you're NOT stopped relative to all the other bodies in the Universe. (So you ain't maintaining a "HERE" either!)  There is no "STOP" in space, and there is no "HERE" except relative to... some other body for some amount of time duration.  So his statement; "Let's stop here..." is meaningless, and just for... your enjoyment and entertainment.
Clear Skies,
Tom Whiting
Erie, PA  


---------- FOLLOW-UP ----------

QUESTION: Thanks that answered my questions very well. One last thing. Again I understand there are many factors and that it depends on the black hole but in general how fast do they move? Would it cross the solar system in a day, week, month, year, etc?
Thanks again

Answer
How fast do they move?  Relative to what?  I thought I made it clear that all places, times, events, velocities and accelerations... are always relative to something else. (So I assume that you mean relative to the sun)... since it contains 99.86% of the mass of the Solar System. (Jupiter has 0.1% so that just leaves 0..04% for all the rest of us).
Plus, what do you mean by... "crossing the Solar System?"  The odds on any body (think incoming comets) coming in on the exact plane of the Solar System are slim and none. Most comets from deep space come into the solar system at some angle to the plane of the solar system, ALL angles in fact, rarely zero on the plane. Remember, space is 3-dimensional. So it would be more proper to say "pass through" as opposed to 'cross over'.
And the velocity would vary on approach and departure, just like a rogue comet. It would  accelerate inbound, and decelerate outbound... there is no such thing as a constant velocity out there.
If the orbit is elliptical, parabolic, or hyperbolic (relative to the sun) then the velocity would not be constant. (The only constant velocity would be a perfectly exact circular orbit, and that doesn't exist in nature). So there is no way to know ahead of time (just like an incoming comet) what the angle of approach is relative to the plane of the solar system, the velocity, or even where would the intercept point (ascending or descending node) would be.  You have to assign all those values first, from your own mind, before you could even attempt to assign a relative speed and relative gravitational effect. If Jupiter's orbit helps, our largest planet, it takes Jupiter just under 12 years to orbit the sun in an elliptical orbit at that distance of average 484 million miles from the sun. Rogue comets, like Hale-Bopp... well, we followed it for nearly two years with our telescopes, and it missed the sun by about 100 million miles. On the other hand, Comet Hyakutake missed the Earth by only 9 million miles, but was only visible for about
2 weeks, if I remember correctly. So, bottom line, there are just too many variables to provide you a definitive answer... the answers are limitless. (For both new comets, or any other incoming body).
Clear Skies,
Tom Whiting
Erie, PA