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About every two years, Earth and Mars are on opposite sides of the Sun (a Mars-solar conjunction). Plasma expelled from the Sun can interfere with radio transmissions between Mars spacecraft and the Earth1.

The last time this happened (2019), NASA issued a "command moratorium". Commands are no longer sent from Earth, as they may be corrupted and thus misinterpreted. Vehicles are put to rest. Data collection is reduced to low-bandwidth measurements, and transmissions to Earth are reduced:

Solar conjunction occurs every two years. This time, the hold on issuing commands — called a "command moratorium" — will run from Aug. 28 to Sept. 7, 2019.

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On the surface of Mars, the Curiosity rover will stop driving, while the InSight lander won't move its robotic arm. Above Mars, both the Odyssey orbiter and the Mars Reconnaissance Orbiter will continue collecting data from Curiosity and InSight for return to Earth. However, only Odyssey will attempt to relay that data to Earth before conjunction ends. Meantime, another orbiter, MAVEN, will continue to collect its own science data but won't support any relay operations during this time.

Will there be another command moratorium in 2021? If so, when?

(This time, there are more spacecraft, including those from other nations besides the U.S. China will make their own decision, but the others rely on NASA's Deep Space Network. On the other hand, the various solar probes will have plenty of radio time available!)

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Yes!

Using the previous moratorium for reference, I found the separation angles (using JPL's Solar System Dynamics HORIZONS Web-Interface ,DE-441 ephemeris): 2019 Moratorium

This seems to suggest the cutoff limit is ~1.3° of Sun-Mars-Earth angle (the angular separation of Earth and the Sun in Mars' sky).

The next time this occurs is in October 2021 (~2-Oct-21 to ~15-Oct-21): 2021 Moratorium

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    $\begingroup$ Nice work! Eventually NASA will make a decision; I'm curious to see how well it matches your answer. $\endgroup$
    – DrSheldon
    Jun 25, 2021 at 15:08
  • $\begingroup$ @DrSheldon not too bad a guess: "The solar conjunction moratorium on commanding all Mars spacecraft is between Oct. 2 and Oct. 14, 2021, when Mars is within 2 degrees of the Sun." from NASA news bulletin $\endgroup$
    – BrendanLuke15
    Oct 3, 2021 at 16:05

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