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Horizons has calculated trajectories for deep space spacecraft, including six objects related to Apollo 8 through 12.

See an (old) list in the currently unanswered question What was Chang'e-2's 3D orbit in space? (since it's not in Horizons) ...yet ;-)

But there's nothing there that I can see for the Viking 1 and 2 missions which came later and were certainly well-tracked.

Why not?

My guess is that there are raw tracking data and that due to time and resource constraints and probably not much need, this is simply lower priority than moving forward.

However, there might be a more interesting story, so I thought I'd ask.

It would come in handy for addressing (in Stack Exchange, not necessarily in reality) Is the 1970's Viking Orbiter due to contaminate Mars?

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The real short version is that Horizons is maintained by a single person who has other assignments at NASA, and only adds missions as they are requested by the public, or newly launched. Older missions that were still active ~20 years ago. There are archives where this older data can be pulled, just not web based.

There are some other reasons for this. One of the more interesting is that each mission has a number associated with it called the NAIF id, some of which were reused, for reasons I don't understand. These NAIF ids are used in the Horizons files, because Horizons is built on top of SPICE, and they are used in the SPICE files. Just to give one example, Viking 2 and Deep Space 1 share the code of -30.

The single biggest exception to this is the Apollo missions, which I assume are in there because of the huge interest in those missions. Viking just isn't that interesting these days, because we have much better data of Mars than anything that came from Viking, except for maybe the landers, which don't move and don't require SPICE files.

You can find the data for many of these older missions, including the Viking Orbiters, in the form of the SPICE kernels, at NASA's NAIF archives.

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  • $\begingroup$ The hardest part of space missions is obviously coming up with an unique identifier for each of them. $\endgroup$
    – SE - stop firing the good guys
    Sep 22, 2020 at 20:24
  • $\begingroup$ LOL. I added a bit more about that, but it's strange they reuse the codes for missions that aren't even related... $\endgroup$
    – PearsonArtPhoto ♦
    Sep 22, 2020 at 20:40
  • $\begingroup$ Good answer, thank you! Pioneer 11 last contact September 30, 1995 which is 25 years ago, not 20. $\endgroup$
    – uhoh
    Sep 23, 2020 at 2:28
  • $\begingroup$ I was looking at 10, which last contacted in 2003, but... $\endgroup$
    – PearsonArtPhoto ♦
    Sep 23, 2020 at 2:35
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    $\begingroup$ That one I will give you is strange... It really is just what people have requested, so... $\endgroup$
    – PearsonArtPhoto ♦
    Sep 23, 2020 at 2:38

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