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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?

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  • $\begingroup$ I think this is a great point. Skycrane really is not a great solution for a lander. But you may not nasa has tried a few hairball schemes. $\endgroup$ – marshal craft Mar 1 at 11:45
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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.

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    $\begingroup$ You also have the problem of getting the rover off the lander once it's all on the surface. $\endgroup$ – jamesqf Feb 28 at 18:11
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    $\begingroup$ The Apollo LM engines were significantly less than a meter above the surface at touchdown, and none of them dug a hole (not even Apollo 11, which didn't shut the engine off until after touchdown). $\endgroup$ – Mark Feb 28 at 19:27
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    $\begingroup$ @Mark Presumably the presence of an atmosphere makes a difference. Also, one would think they are two completely different engines with different output characteristics. But don't quote me on that. $\endgroup$ – Arthur Feb 28 at 20:25
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    $\begingroup$ @ikrase: Viking &c didn't have rovers. And the Apollo landers had to have the descent stage as a launch platform for the ascent stage. $\endgroup$ – jamesqf Mar 1 at 3:29
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    $\begingroup$ @ikrase: Well: "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.", from here. Perhaps Steve Sell & all the other people at JPL are just wrong, but I don't think so. $\endgroup$ – user21103 Mar 1 at 10:04
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because it would prevent the rockets from contaminating the landing site.

No they weren't

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).

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    $\begingroup$ I agree with what you say - dust reduction was not the goal-, but can you provide a reference? For example, space.com/16889-mars-rover-curiosity-sky-crane-landing.html $\endgroup$ – Organic Marble Feb 28 at 20:42
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    $\begingroup$ @ikrase The skycrane is a lander, just as trimmed down as you can get. Some kind of large capsule with an exit ramp would be much much heavier and still have the same problems needing to be solved for landing. $\endgroup$ – spacetyper Mar 1 at 4:02
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    $\begingroup$ @ikrase It is, but despite that it works. Remember that the "skycrane" idea wasn't invented by NASA - it's a standard way for heavy-lift helicopters to carry equipment including trucks, and the name "skycrane" is actually the name of the Sikorsky helicopter that first did it. A truck-size helicopter would be a heavy structure - much better to just attach cables to the truck chassis. And so too with the Mars rovers and the lander. $\endgroup$ – Graham Mar 1 at 8:39
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    $\begingroup$ @ikrase As for complexity, the ability to come to a hover 7m up before starting the winch is exactly the same as the ability to come to a hover 0.5m up for a soft landing. The only new elements are three winches and three cable-cutting mechanisms, and these are very simple and reliable. $\endgroup$ – Graham Mar 1 at 9:01
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    $\begingroup$ @OrganicMarble Thanks - I was on my phone so it was a little harder, and TBH I thought this was so trivially googlable as to not need it. But you're right that I should have it. Thanks for a good source. $\endgroup$ – Graham Mar 1 at 11:03
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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.

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    $\begingroup$ Thinking this way, you can see how the skycrane would evolve: 1) mount braking rockets on lander. 2) Have those rockets detach and fly away once down, so you don't need to carry them around. 3) Move the rockets onto a separate platform and detach the whole thing. 4) Lower the lander on a cable from this platform. $\endgroup$ – Oscar Bravo Mar 1 at 9:54
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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.

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  • $\begingroup$ That makes it sound like the skycrane was less focused on plume contamination than commonly implied? $\endgroup$ – ikrase Feb 28 at 19:02
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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).

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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.

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.

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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.

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  • $\begingroup$ As to why its not talked about, I find often some aspects pertain to a higher level of design, aren't talked about in typical interviews and questions. It can be because of the complex management and design strucuture which probably uses a great deal of compartmentalized technologies. So one expert really doesn't know some other areas. Also computers may be used as well in the process. And the entire process of designing such broad components of the system could of been highly complex and more so then I've described here. But I see they seem to be employing Landers, which impact the ground. $\endgroup$ – marshal craft Mar 1 at 12:07
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    $\begingroup$ This is wrong. The sky crane crashed into the ground with maybe 100km/h, not with orbital velocities like some lunar probes and spent stages. Making a crater was neither possible nor a significant goal. $\endgroup$ – Jens Mar 1 at 21:10
  • $\begingroup$ I never claimed it would have orbital velocity. Key point we see with nana is using vehicle mass coming from or it to penetrate some level into a planet or moon. 100km/h is in no way insignificant, in ability to remove surface material to understand Martian geology. I really see no evidence to suggest otherwise. But of course especially on space se, I can see the upset over my theory. That's fine and perhaps I'm wrong. But I make strong attempt to solve the question. And MY answer is the closest thing I've seen yet. Skycrane doesn't seem the best way to land a rover to me. I haven't seen ... $\endgroup$ – marshal craft Mar 1 at 23:51
  • $\begingroup$ Strong evidence to suggest it is, which relies on math, physics, and engineering. A common theme for a while now had been recycling vehicle to study further into planets and and moons. When is see that, now skycrane does make more sense. It even can seem like the most natural adaptation to Mars and its atmosphere, that strategy of using the lander as a secondary purpose. $\endgroup$ – marshal craft Mar 1 at 23:54
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    $\begingroup$ This is baseless speculation, not backed up by any fact. As far as I know, the rover will not investigate the crash site of the sky crane, and the images from orbiters do not have sufficient resolution to see any terrain features uncovered by the impact. $\endgroup$ – Polygnome Mar 2 at 9:39

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