Re: First Ark to Alpha Centauri

From: George Dishman (george_at_briar.demon.co.uk)
Date: 11/30/04 

Date: Tue, 30 Nov 2004 21:20:21 -0000

"AA Institute" <abdul.ahad@ntlworld.com> wrote in message
news:adbf5bc1.0411281057.7a40e443@posting.google.com...
> "George Dishman" <george@briar.demon.co.uk> wrote:
>
>> http://www-ssg.sr.unh.edu/tof/Outreach/Interstellar/?Interdepth.html
>>
>> 75% hydrogen, 25% helium, both transparent, at about 1 atom
>> per cc, and a tiny amount of fine, black dust. Although the
>> density is very low, you may still need to consider drag
>> over such a long voyage.
>
> Very interesting.
>
> "Our sun (and solar system) are currently moving through a cloud of
> interstellar gas." - quote from the above article.

That's right but it is very thin. Outside the
cloud it would be even thinner.

> As far as I know, the so-called "Pioneer effect" was never
> satisfactorily resolved and I think both probes were retarded by
> microscopic amounts in their solar system escape trajectories out
> towards interstellar space. Do you suspect the interstellar
> winds/medium could have had a part to play in this?

http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/f23.gif

Pioneer is still inside the termination shock so
subject to the force of the solar wind and radiation
pressure (light) going radially out from the Sun.
The anomalous acceleration is towards the Sun so
in the wrong direction. To put them in context,
the radiation pressure is about a thousand times
(IIRC) larger than the solar wind force (both are
inverse square with distance). The anomalous
acceleration exceeded the radiation pressure when
the craft was at about 14AU.

Many people think the effect is just the pressure
of stray thermal radiation but it's hard to find
enough to account for the size of the effect (about
63W).

George