This comment suggests that orbit before descent to Mars' surface allows a mission to delay the landing if the weather conditions are bad. I think that Tianwen-1 will be the first to put a lander rover on Mars from Mars orbit, but as @JohnHoltz points out in a comment below Viking 1 and 2 landers were deployed from Mars orbit.

Launches to Mars happen each Earth-Mars synodic period:

$$T_{syn} = \frac{1}{\frac{1}{T_<} - \frac{1}{T_>}} = \frac{T_< T_>}{T_> - T_<}$$

With $T_>, T_<$ of 1.881 and 1.0 years that's about 2.14 years. There's a short launch window and usually a predetermined arrival time.

From time to time Mars can have huge dust storms, with a good fraction of the planet's surface invisible from space due to the amount of particles blowing in the wind.

Question: Do these global dust storms have a particular "season" for happening, and are years where the launch window would result in landings during dust storm season avoided?

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PIA03170: The 2001 Great Dust Storms - Hellas/Syrtis Major image: NASA

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From Space.com's Mars Dust Storm 2018: How It Grew & Killed the Opportunity Rover image: NASA

Piqueux et al (2015)'s Enumeration of Mars years and seasons since the beginning of telescopic exploration (found in the Planetary Society's Mars' Calendar) shows that Martian Year 33 (MY33) stared on 2015-06-18 on Earth.

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    $\begingroup$ Tianwen-1 is not the first lander to first enter orbit and then land. Vikings 1 and 2 entered orbit before the lander portion descended to the surface. If I remember correctly, Viking 1's landing was delayed from July 4, 1976 (America's bicentennial) to July 20 (also a noteworthy day for landing on celestial bodies) while the orbiter scouted for a safer landing zone. $\endgroup$ – JohnHoltz Feb 11 at 5:09
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    $\begingroup$ @JohnHoltz ah thanks! I've made an edit (the first rover) How does it look now? $\endgroup$ – uhoh Feb 11 at 7:08
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    $\begingroup$ Many craft go into a capture orbit on arrival at Mars rather than landing directly. If it was in issue, the craft could remain in orbit for months and land when the storm season was over. $\endgroup$ – Level River St Feb 11 at 23:27

Are launch windows to Mars avoided if they result in landings during dust storm season?


It's pretty well known that the gusts of wind on Mars are relatively harmless to a person standing on Mars (one of the big things the movie The Martian got wrong). Even though winds on Mars could reach up to 130 km/h, the atmosphere is very thin and thus does not result in a huge amount of force. However, this is mostly a bad thing for descending spacecraft as it can offset the final landing location.

Here is a wonderful source from the ESA that talks specifically about spacecraft descending to the surface during dust storms on Mars. This source specifically talks about the ExoMars Schiaparelli lander in 2016.

Once it was decided to launch ExoMars toward the Red Planet during the 2016 launch window, the laws of physics determined that the spacecraft would arrive at Mars in October, during the northern autumn – usually a period of significant dust storm activity.

The direct answer to your question, Are launch windows to Mars avoided if they result in landings during dust storm season? is ultimately no because the Schiaparelli lander attempted a landing in October even though there was a significant risk of a dust storm during landing. There is also the added benefit of more science and a better understanding of the Martian atmosphere by attempting to land during a dust storm, and gather data about the atmosphere during the descent.

However, the ESA did think of putting the spacecraft into orbit before they released Schiaparelli.

"We considered going into Martian orbit before Schiaparelli was released, but we eventually decided to save fuel and mass by choosing to release it from a hyperbolic arrival trajectory for a ballistic entry,"

Slight modifications had to be done as a result from the dust.

One problem we looked at was erosion of the thermal protection material on the front heat shield, due to high speed impacts by dust particles during the entry phase. Dedicated wind tunnel tests with a dust-loaded flow allowed us to derive a dust erosion model that we used when we designed the heat shield. As a result, we increased the thickness of the ablative thermal protection material by 3 to 4 mm.

Any dust storm would not have seriously damaged any of the other hardware. The parachute was shown to work against the dust, and radio communications were not a concern.

"Studies were made on the parachute materials to ensure that it would not be affected by the dust particles. The simulations verified that the parachute strength is adequate for the predicted changing atmospheric densities. (. . .) After various analyses, the mission team concluded that atmospheric dust would have little effect on Schiaparelli's radio communications, although the altitude and velocity measurements made by the radar during the descent would be slightly affected by a small attenuation of its return signals.

So to summarize: Dust storms are definitely taken into account when planning a Martian landing, but it never cancelled a launch in a once in a two year launch window as seen from the European ExoMars mission.

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    $\begingroup$ Thank you for the wonderful answer! Perseverance will use optical navigation cameras to select a safe landing site (1, 2), so visibility will be a concern for this mission. $\endgroup$ – uhoh Feb 11 at 7:13
  • $\begingroup$ Even if the dust storms were a major problem, any mission could simply park in orbit until the storms cleared up before making a landing insertion. No reason to miss a whole launch window for that. $\endgroup$ – Darrel Hoffman Feb 11 at 18:41
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    $\begingroup$ @DarrelHoffman Most Martian landers don't have the excess fuel on board to brake into a stable orbit around Mars and decide to land later. The ones that did were he ones that brought along an orbiter, such as the Viking missions. $\endgroup$ – notovny Feb 11 at 22:51

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