How long will it take before they reaches the 1 lightyear limit? And how is the Sun looks like at that distance? Is the sun shining bright there or is it very dark then. How difficult will it be to make eventual contact with them?

Voyager is the fastest one, covering more than half billion Km each year... deceleration is almost negligible, so it will take something like 18000 years to reach 1 light year distance from the Sun!
At this distance, Sun appear like a -3 magnitude star, so still the most luminous object in the sky (slightly less bright than Venus at its maximum).
Any contact would be impossible because, apart distance, onboard RTG generators will be completely exhaust.

The ships will reach this distance in hundred of thousand years. At that time, even if the equipment is still working, their radioisotopic energy source will be extinct for long. Already today it is wondered if they can still receive a command and reply to it (last test failed). The Pu238 has 80 years half life, which makes than in a thousand years only it will be extinct.

...of course the RTG power output lifetime is far less constrained by the termal output of the radioisotope than it is by the dopant migration and degradation of thermoelectric semiconductor junctions which actually produce the electricity, and thus they will be producing far less than 50% of the initial power at the 80 year Pu239 half life point.

It will be really dark then at 1 lightyear distance with only a bright star (the Sun) in the sky. I think it looks like the same when Jupiter is shining at its brightest here on earth ,is that true?

Don't know if this was already posted:
http://www.southgatearc.org/news/april2006/voyager1.htm

Cannot believe, an "amateur" 20m antenna detected Voyager signal from almost 100 au! ...are we sure isn't another 1st april joke?

'Signal', perhaps - but not data!

Put a bundle of such guys together, though, on five continents...

Bob Shaw

Well - 20m isnt that much less than 34 which is the smaller ( but much more common ) DSN asset - and they DO get Voyager data pulled out of the signal on an almost daily basis.

Granted - the later is about 3x the area Lot's of amaeturs getting in on the amateur DSN scene -it's quite remarkable.

Doug

Except that the RTGs don't use Pu-239, they most commonly use Pu-238 in the form of plutonium dioxide. IIRC, Pu238 is harder to acquire than Pu239 and that's one of the reasons these things cost so much.
But you're right on the half life, it is some 88 years.

Just read this in Spaceflight magazine (May 2006 page 167 ):

PIONEER 10 lost in Deep Space
NASA DSN has failed to contact the venerable spacecraft an it seems that its 'active' mission is now effectively over. ... It is now heading towards a 3.3 light year encounter with the star Ross 248 in 33000 years time.

Well I guess the spacecraft lasted 35 years

AAAArrrrgghhh stupid mistake. I meaned Pu 238 of course, so I corrected in the initial post. Pu 239 has a 24000 years half life, but it would not provide enogh power for a RTG. Tons of it would be required, but it would be "a bit" unstable...

34 to be precise; launched on march 2nd 1972. Would you believed this : it was the very first day I took a car driving lesson!
Anyway, there is one point where we're not consistant : Pioneer 10 will be 3.3 light year from Ross 248, which is 10.3 light year from here (I mean, France ) in 33.000 years, while Voyager is traveling one light year in 18.000 years. Something wrong, eh ?

More information on the final attempt to contact Pioneer 10 here:

http://www.unmannedspaceflight.com/index.p...indpost&p=44298