This question is about the orbital parameters of ESA's ExoMars Trace Gas Orbiter (TGO), so I am optimisitic that it belongs on astronomy SE. More specifically, it is about the orbital geometry of TGO and how it is Helping Perseverance phone home. Here, Preserverance is the brand new Mars rover which landed on February 18th, 2021.

I know that TGO is on an areocentric orbit, which is the Martian equivalent of a geocentric orbit. I assume that TGO is not areostationary, i.e. not always looking at the same spot on Mars, like many weather satellites (see e.g. Meteosat). Is that assumption correct?

What confuses me is the following paragraph which talks about 24/7 coverage on the one hand, but 14 hours of possible relay-coverage on the other hand. The highlighting in the following quote is from me:

To ensure that this information gets to the engineers on Earth as quickly as possible, TGO and NASA’s Mars orbiters will be able to communicate with deep space ground stations on Earth almost twenty four hours a day, seven days a week for the first two weeks after landing. ESA’s ground station network will provide roughly 14 hours a day of this ‘low-latency’ coverage.

Then, there is a infographics on the ESA news page from which I cropped the part about the "relay slots": How TGO helps NASA's Mars lander phone home

Here, there are time stamps given for "relay slots", and I am unsure how that relates to the above mentioned quote. Any clarification would be appreciated.

  • $\begingroup$ There's a difference between 'deep space ground stations' (=from all space faring nations) and ESA's ground stations. That explains the difference. As to Mars orbit: if TGO is not aero-stationary it will be behind the horizon for the Rover, thus cannot faciliate relay function. And aero-stationary, Mars might still be in line-of-sight between it and Earth $\endgroup$ Commented Feb 19, 2021 at 10:13
  • $\begingroup$ @planetmaker I found ESA's groundstations to be to be distributed enough around the globe, but I guess you are right that NASA only relies on ESA once it has to. $\endgroup$
    – B--rian
    Commented Feb 19, 2021 at 10:17
  • 4
    $\begingroup$ "Areocentric orbit" just means "It's in an orbit around Mars." Given that TGO's orbital period around Mars is only about 2 hours, it can't maintain 24.65-hours-a-sol line-of-sight to any designated point on Mars $\endgroup$
    – notovny
    Commented Feb 19, 2021 at 18:05


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