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Curiosity and Perseverance were landed using the rather complex skycrane system because it would prevent the rockets from contaminating the landing site.
But the video shows a great deal of dust being blown around by the rockets, suggesting that the cables weren't long enough to prevent some pretty considerable plume impingement.
Is that OK? What were they actually worried about?
I think you may be confused about the nature of the problem the skycrane is trying to solve (however I also may be confused!). The rover, being a rover, does not need to stay at the landing site. If the landing site has a lot of dust blown off it, well, they can go somewhere else where the dust has not been blown off the surface. As the other answer says there may even be advantages to having the dust blown off the site as it may reveal interesting underlying geology.
However it is quite desirable that the rover should survive the landing, ideally without significant damage. And it is also desirable that it should be able to leave the landing site.
As is visible from the landing videos, the engines on the skycrane raise a fair bit of dust on the surface. They're doing this from a distance of at least 7 metres (the cables are 7m long). Imagine what they'd be doing from a metre or less above the surface (the exhaust velocity of the engines is significantly supersonic): they'd be digging large holes in the surface and picking up stones and throwing them in all directions. This would almost certainly damage or destroy the rover and surround it, probably, with significant holes in the surface which would make leaving hard or impossible. Curiosity, in fact, was damaged on landing: one of its wind sensors was damaged, they think by stones picked by the skycrane engine plumes.
So the skycrane makes the difference between a landing which is almost certainly survivable and one which is almost certainly not.
As to why the cables aren't longer: well, longer would probably be better in terms of possible damage, but would also involve carrying more cables to Mars which isn't cheap, and I think would also mean the rover would swing about more which would be problematic. There will be engineering tradeoffs here that they have thought hard about.
Here is some information from NASA, from before Curiosity's landing, which helps to explain how they were thinking:
"With a payload this size, the rockets could kick up enough dust to compromise the rover and its instruments," explains Sell. "And the rockets could excavate craters Curiosity would have to avoid as it drives away. Add to that the risk of a big, heavy vehicle driving down off the lander via an exit ramp to reach the surface."
I believe that Steve Sell (who is the person quoted above) leads the team that came up with the skycrane.
because it would prevent the rockets from contaminating the landing site.
There was some possible advantage from that, to be sure. But missions to Mars undergo serious levels of decontamination to ensure contamination from Earth doesn't happen, so the risk was marginal at best. If you meant contamination from hydrazine, that's not a hydrocarbon and isn't interesting in the search for evidence of life. And if you meant contamination from dust, on the surface of Mars that's basically a lost cause because dust gets everywhere.
They were landed by skycrane because it was the most practical way of getting a very heavy vehicle on the ground with least risk. It's as absolutely simple as that. See here for reference (link courtesy of @OrganicMarble - thank you).
The two media articles I have been able to find on this, at Spaceflight Now and Space.com, didn't really talk about the problem of material being blown around by the rockets. According to both articles, putting a rover on a platform, and landing that on legs, was ruled out because the platform would have to be really wide for stability, and hence heavy, and in addition there was concern about rolling the rover safely down a ramp off the platform.
The articles do not explain why the rockets weren't just mounted to the rover. Here I am speculating about what the engineers were thinking, but that would certainly add mass for the rover wheels to carry around for the rest of the mission.
I will say that, for the lighter but still rather massive Viking landers, just using multiple small nozzles brought the scouring problem down to acceptable levels. So that also calls into question the idea that the scouring problem by itself required the skycrane.
I ask a related question here, but I don't think this one's a duplicate.
It seems like the real answer is not a single reason but a set of reasons. Because a single approach has to be selected, that approach is the best compromise of the competing cases. The one that stands out to me is the Martian atmosphere. To land without damage without using the skycrane (or rockets) would require an impossibly large parachute (I think the inflatable ball in the previous mission is another approach to this problem).
Curiosity and Perseverance were landed using the rather complex skycrane system because it would prevent the rockets from contaminating the landing site.
That’s not why the skycrane was used. It was used because it took up less mass and volume than a landing platform, and because Curiosity and Perseverance are too massive for the airbag method used for Pathfinder and the MERs.
Everything had to fit in an Atlas V fairing, so smaller was better.
In a sense you are right, the immediate landing sites are affected, but both rovers weren't placed on Mars to just study dust. Also, both rovers were not designed to be stationary, they're mobile and they will travel far.
Curiosity is far from its landing site exploring uncontaminated regions. Perseverance will do the same.
Blowing away dust can also be beneficial. It can potentially uncover a rock or geological unit of interest which may not have been noticed under a covering of dust.
To see what is below the surface, geologists have to disturb a site by either drilling holes or digging trenches/costeans to see the soil/regolith profile below the surface and to take samples. Blowing away dust from a small area just comes with the territory or exploration.
The Mars rovers are a crew less, semi autonomous equivalent of the Apollo 15 to 17 missions that were conducted on the Moon.
The only reason I can legitimate see for sky crane, was that nasa wanted to be able to impact the surface and penatrate deeply into it. Kind of like how an excavator might use high explosives to blow a hole into the ground and observe the hole.
NASA has done this with the moon. Orbiting probes were allowed to de orbit into the moon. They only observed the debrer from very far.
But it would be my opinion that they wanted some sizable object to crash into the surface, to turn up soil, and also open up lower levels of the ground to see. Keep in mind, we've only sent a few rovers with drills and what not. We don't know everything about mars, and having this crash into the ground, allows us to gain a lot of natural insite about mars. Having a lander which drops off the rover and flies away accomplishes this. And then it may have been simpler to have a crane, with a soft landing. The crane simply extends and going too far doesn't hurt. The lender hovers with some degree of accuracy. It's fairly soft and allows for error in the hover, accuracy of ground contact, etc.
The prior rovers to skycrane used an airbag. But similarly they presumably impacted the ground with some energy. And I presume this was to help excavate covered soil of Mars.
From this theory it is an efficient use that f mars potential energy of gravity, to help removed surface soil to explore lower into the ground. Other means to do so would require drilling, and while may be able to penetrate as deep, can not do so for the same volume.
That's what I think. Just a theory.
