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Current Mars rovers collect large amounts of data and image sequences on Mars and store them in memory, and usually they then transfer the data to a spacecraft in Mars orbit for rebroadcast to Earth at scheduled times.

But is it possible for a rover like Curiosity, to transmit 'live' HD video from the surface of Mars to an orbiter which, at the same time, will rebroadcast the images to Earth ?

From Wikipedia:

Curiosity is equipped with significant telecommunication redundancy by several means – an X band transmitter and receiver that can communicate directly with Earth, and a UHF Electra-Lite software-defined radio for communicating with Mars orbiters.

[...]Curiosity can communicate with Earth directly at speeds up to 32 kbit/s, but the bulk of the data transfer should be relayed through the Mars Reconnaissance Orbiter and Odyssey orbiter. Data transfer speeds between Curiosity and each orbiter may reach 2000 kbit/s and 256 kbit/s, respectively, but each orbiter is able to communicate with Curiosity for only about eight minutes per day (0.56% of the time).

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    $\begingroup$ "Is it possible?" questions are problematic because the answer is very often "It depends how much time and money you have" is the answer, and so not much is learned. I've adjusted the wording of your question to a "What are the technical challenges...?" format that tends to match the Stack Exchange format better. You are welcome to adjust it further if I've missed something, or just click edited below the question and then rollback if you are not comfortable with it. $\endgroup$ – uhoh Mar 18 '18 at 23:22
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    $\begingroup$ With live HD color TV from the rover it is not possible to steer the rover from ground control real time at walker's speed. The transmission delay time in both directions is too long for steering at about 1 m/s. $\endgroup$ – Uwe Mar 19 '18 at 16:14
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    $\begingroup$ @uhoh Thank you. Looks more professional now. $\endgroup$ – Conelisinspace Mar 19 '18 at 20:28
  • $\begingroup$ @Uwe Yes, that's a problem. But cooperation between ground control and an intelligent automatic pilot could be a solution. $\endgroup$ – Conelisinspace Mar 19 '18 at 20:31
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According to this answer, the Mars Reconnaissance Orbiter has up to 5Mbps of bandwidth back to Earth in good conditions (favorable orbits, close range), which could support HD video in terms of bandwidth needs.

It is unlikely that MRO itself has the ability to generate and send HD video though - it has high bandwidth to send back high resolution satellite imagery (of which it's generated several terabytes.)

I believe this proves HD video is possible, but there isn't currently any equipment in the field to send such video back to Earth. It also demonstrates that the requirements to deliver HD would vary over the years, depending on the orbital alignments of Earth and Mars. If you want HD in the worst of times you'll need to improve the bandwidth at range - more transmit power, more gain, both?

There's also some work being done into laser communication. The Mars 2022 orbiter (NeMO, Next Mars Orbiter) is proposed to carry a broadband laser communication package specifically capable of relaying high definition video from the surface of Mars.

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    $\begingroup$ "up to 5.22 Mbps" is not the same as "about 5-6Mbps". If 5.22 Mbps are possible over the minimum distance Mars to Earth, the data rate will be much smaller at maximum distance. $\endgroup$ – Uwe Mar 17 '18 at 20:33
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    $\begingroup$ True, but it seems the theoretical maximum is 6Mbps and they have experienced 5Mbps as a typical close range high value. The low end ranges as low as 500Kbps at maximum distance. $\endgroup$ – Saiboogu Mar 17 '18 at 22:25
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    $\begingroup$ @Uwe I think that this proof-of-principle answer is sufficient to answer the OP's question that the biggest problem is just distance, there's no other major barrier to Mars-Earth HD. Distance varies from 0.5 AU to 2.5 AU, so some combination of transmit power and gain is needed to get that factor of 25. Factor of 5 in transmit power plus a $\sqrt{5}$ increase in transmit antenna diameter of the relay satellite would do it for example. Since the one-way light time varies from 4 to 20 minutes, there's a philosophical issue of what "live" from deep space would mean, but that's a separate question $\endgroup$ – uhoh Mar 18 '18 at 2:38
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    $\begingroup$ fwiw according to this answer from 2015, the Mars 2022 orbiter is supposed to have optical communications capability, and I'm assuming this means from Mars orbit to Earth (rather than to the surface of Mars). $\endgroup$ – uhoh Mar 18 '18 at 14:28
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    $\begingroup$ @uhoh Found a recent NPR article that didn't have much fresh info, but confirmed the plans. $\endgroup$ – Saiboogu Mar 18 '18 at 14:38
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It's just whether you'll spend the money. To speed up a link you need bigger dishes and/or more power for your transmitter. Both are extremely expensive when you're talking deep space.

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According to this article, the left and right MastCams can take true color 1600x1200 images, or 720p video at 10 frames per second.
So HD video is possible with these cameras !

According to this article, MastCam hardware and internal processing permit a wide range of operational flexibility, but it is not clear if these allow for taking images and transmitting them at the same time.

Finally, according to this article, throughput efficiencies at either the Mars Reconnaisance Orbiter or the Mars Science Laboratory end of the link limit the effective maximum rate to approximately 1.35 Mbps.
If well understood, this means that only about 1 frame per second of HD video can be transmitted from Curiosity to the relaying orbiter.
But, since most of the time, no fast moving objects occur on Mars and the rover's speed is limited, a 'live' HD picture of sufficient quality could be obtained and transmitted during the 8 minutes the orbiter passes over the rover.

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