scott <sco### [at] scottcom> wrote:
> But presumably an object with the same mass but higher density would allow 
> you to get a faster slingshot without crashing into it (because it's 
> smaller)?

  In theory yes. OTOH it still depends on the momentum of the larger object.
You could get a stronger slingshot from a star of the same mass if it's
moving faster than the black hole.

  Yet, this is not as simple as it sounds in any case. The slingshot effect
is always relative to something. For example interplanetary gravitational
assist is relative to the Sun. The Sun itself cannot be used for a slingshot
effect inside the solar system (it could be used for a slingshot relative
to the galaxy, but not relative to the solar system).

  With this taken into account, can you just go from Earth to the nearest
black hole, get an enormous speedup and come back at 100x the speed and
slam onto Earth at that speed? From a gravity assist only, I don't think so.
I think it would be against conservation of energy. If you were travelling
from Earth to another star system, then maybe, but I don't think it works
in the closed case.

  There are also some practical things to take into accout:

- If you go so close to a black hole that it will give you a stronger
slingshot effect than a regular star would, the tidal forces would
probably rip you apart. Not very practical.

- Black holes usually have an accretion disc around them, usually much
larger than the size of a normal star of the same mass. The accretion
disc could slow you down or be dangerous.

- The humongous amounts of radiation around a black hole would probably
be enough to fry you to ashes in a fraction of a second, no matter what
kind of shielding you use, especially if you go so close that you would
get a larger speed boost than from a regular star.

-- 
                                                          - Warp

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