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From Wiki:

The Viking program ended on May 21, 1983. To prevent an imminent impact with Mars the orbit of Viking 1 orbiter was raised on August 7, 1980 before it was shut down 10 days later. Impact and potential contamination on the planet's surface is possible from 2019 onwards.[4]

Does this mean Viking 1 is set to impact Mars at any time?

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    $\begingroup$ I found a paper from 2009 whose summary suggested that (at that time) nobody knew whether V-1 had already de-orbited or not. $\endgroup$ Sep 17 '20 at 16:00
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First of all, the standards in 1970 were quite a bit higher for planetary protection of Mars than they are today. The Viking Landers were sterilized in a giant oven, a step which is not done today. We can build a lander to the same standards as an orbiter back in the day, which basically involve constructing in a clean room. In the 1970s things were sterilized completely that landed.

It is entirely possible that Viking 1 is still orbiting Mars, or it may have impacted. We just don't know. The way to figure this out would be to somehow search the orbit of Mars for a small object, which is theoretically possible using the spacecraft orbiting Mars now. They would be able to find a tiny asteroid orbiting, or a dead spacecraft. This hasn't been done, it just isn't considered enough of a priority to make it happen.

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  • $\begingroup$ I was surprised to see that JPL's Horizons has nothing for the Viking-1 mission trajectory., so I've asked Why aren't there any trajectories for the Viking missions in JPL Horizons' database? $\endgroup$
    – uhoh
    Sep 22 '20 at 0:40
  • $\begingroup$ " the standards in 1970 were quite a bit higher for planetary protection of Mars than they are today" Interesting, can you provide some more information on that? How were they higher? Do you have any sources for this? $\endgroup$
    – Polygnome
    Sep 23 '20 at 20:59
  • $\begingroup$ I'll see if I can find something better, but the Viking landers were placed in a huge oven to heat them high enough to sterilize them before going to Mars, a step which has not been done since. I've added a source that talks about the sterilization process for the Viking landers, but not how they have loosened that requirement today. $\endgroup$
    – PearsonArtPhoto
    Sep 24 '20 at 11:20

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