QUESTION:
Are the Viking spacecraft that landed on Mars some 20 years ago still
sending any relevant data today?  If not, when did they stop functioning?

ANSWER from Smart Filter on August 25, 1997:
The following web site will answer most of your questions:
http://nssdc.gsfc.nasa.gov/planetary/viking.html

Viking 1 was launched on August 20, 1975 and arrived at Mars on June 19,
1976. The first month of orbit was devoted to imaging the surface to find
appropriate landing sites for the Viking Landers. On July 20, 1976 Viking
Lander 1 separated from the Orbiter and touched down at Chryse Planitia
(22.27 degrees N, 49.97 degrees W, 2 km below the datum elevation). Viking 2
was launched September 9, 1975 and entered Mars orbit on August 7, 1976.
Viking Lander 2 touched down at Utopia Planitia (47.57 degrees N, 225.74
degrees W, 3 km below the datum elevation) on September 3, 1976.

Viking Lander 2 ended communications on April 11, 1980, and Viking Lander 1
on November 13, 1982, after transmitting over 1400 images of the two sites.