On Thu, 30 Sep 1999, Robin Hanson wrote:
> At 04:26 PM 9/29/1999 -0400, you wrote:
> >Apparently, some believe this is caused by a planetoid's gravity:
> >
> >
> > http://news.bbc.co.uk/hi/english/sci/tech/newsid_460000/460095.stm
> >
> >Don't know whether there is a consensus about this.
>
> That doesn't make sense to me at all. The paper cited,
> http://xxx.lanl.gov/ps/gr-qc/9808081 , describes the
> acceleration as constant in time and directed toward the
> sun, for three different spacecraft (Pioneer 10,11 & Ulysses)
> in very different directions. Ulysses was at 1-5 Au
> and the Pioneers at 20-40 Au. Even if the Ulysses data
> is a mistake, didn't the Pioneers go off in substantially
> different directions? And wouldn't a planet's attraction
> vector vary as a craft moved from 20 to 40 Au?
>
I agree. The BBC post might explain the problem with Pioneer 10. But I believe the probes did go off in different directions and so you would have to suggest that they both have encountered such objects. But that would seem to imply a really high abundance of such objects!?! I think there are also problems if the acceleration vector is towards the sun, since if you went flying past an object with enough mass to pull you back towards it (with the sun behind it in a line-of-sight), then that object is likely to significantly alter your trajectory as well.
Between this report and the Mars mission probable crash one could suspect that long-range interplanetary/stellar navigation is difficult.
I've thought in the past that nanoseed interstellar colonization would be feasible, but perhaps it isn't due to the difficulties of aiming non-powered/non-sighted seeds someplace useful. If your chance of hitting a planet light years away is so small that you have to send out 10^12-10^20 of them, then you haven't gained much on the old problem of having to send a World Ship (which has to travel very slowly due to the energy requirements). If the cost of sending seeds at relativistic speeds and the cost of sending World Ships at 1/10 c speeds are comparable, then I suspect you could begin to ask whether it makes more sense to take the entire star system (at even slower speeds)?
Robert