[FPSPACE] planetary orbiting relaystations

[Geert Sassen]

Sometimes you can find little jewels of snaps of information in your 
archives which apparently you overlooked for a long time.

Based on the problems Phoenix is having with the data relay via MGS and 
Odyssee, I checked out the history of other landers which relied 
completely on orbiting relay stations, and so arrived at the Soviet 
landers of '71 and '73. Back in those days two landers (Mars 3 and Mars 
6) seemed to have come quite intact through EDL, only to lose all 
connections on touchdown or shortly afterwards (Mars 3 lost contact a 
few minutes after touch-down, Mars 6 lost contact on touch-down).

In his very interesting booklet 'The difficult road to Mars' ( 
http://klabs.org/richcontent/Reports/mars/difficult_road_to_mars.pdf ) 
V.G. Perminov gives first-hand information about both missions. Quite 
apart from several interesting insights into Mars 3, he describes on 
page 61 how, after the experiences with Mars 3 (which was not designed 
to transmit any science data during EDL) an additional radio system was 
installed on the lander backshell, which would transmit data during the 
parachute descent. This data was relayed to earth by the main bus of the 
spacecraft, which continued on a flyby trajectory. Shortly prior landing 
the backshell was released and the automatic landingstation was 
released. This lander was more or less identical from Mars 3. Then, on 
page 65, Perminov notes: "The additional channel was operative only 
during the descent of the lander. Information from the Martian surface 
had to be transmitted by the main radio channel to the Mars 5 spacecraft".

In other words, apparently the main bus of the craft (Mars 6) only 
functioned as a relay during the descent, after touch-down the relay 
function was taken over by the Mars 5 orbiter, working on a different 
radio channel. However... all contact with Mars 5 was lost on February 
28, while Mars 6 only arrived on March 12. With no functional orbiting 
relay station (and apparently no method to transmit surface data via the 
main Mars 6 craft), all surface activities of Mars 6 and 7 were doomed 
from the very start. The only hope they could have was getting some 
descent information via the main bus (which more or less worked). The 
Mars 6 lander might very well have survived touch-down and run his 
complete set of surface activities, but with no-one listening to its 
signal we will never know...

The MER rovers, as well as Viking, are equipped with a direct-to-earth 
connection to back up the data transmissions relayed from orbit, this 
requires more expenses and more complicated equipment but at least you 
do not run the risk that problems with the orbiting relay spoil your 
day. The Soviet landers, as well as MPL and Phoenix, relied only on the 
orbiting relay and did not have a direct-to-earth link. At least on Mars 
6, but probably also on Mars 3, this resulted in loss of the mission.

-- 
Geert Sassen

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