"Mars Science Laboratory: Entry, Descent, and Landing System Performance"
There's another paper floating around (that I can't seem to find) that evaluates and compares all of the alternatives to show why they decided to go with this particular configuration.
I can find several papers in IEEE Aerospace Conference proceedings, but they are not available in free form online. This is a shame, because the government owns the copyright.
Unless something has changed, the US government doesn't own the copyright - works they produce are automatically in the public domain. Most journal put wording on the paper to that effect.
So if you have access to the papers, you should be able to post them for free somewhere without violating copyright.
Not quite. They were almost certainly approved by JPL in its role as a FFRDC. The copyright in such cases belongs with the government. What you write would be correct if they went through copyright release at another NASA center like JSC in which the workers are federal employees (I work at JPL.)
It's very difficult to recreate mars on earth sufficiently (without bonkers expense) to thoroughly test at full scale, but there are some interesting ways you can get close.
For example, the parachutes that will be used on Mars deploy at a certain dynamic pressure, ambient pressure, mach number and mass ratio (the ratio of the payload mass to the mass of the air entrained by the parachute). All of these factors quite significantly affect the magnitude and the shape of force vs time profile of the parachute as it opens.
You care about this because the maximum force that a parachute generates is when it opens, higher than the equilibrium force it would generate under the same conditions, so that's the number you need to do the stress calculations on the entire landing system (and every gram is costing the tax payer a lot, so you can't really afford to just over engineer it to save some analysis).
You also care about this because at one ambient pressure a parachute might oscillate and corkscrew, at another ambient pressure it might form a stable glide (this has a lot to do with where the centre of gravity is of the payload and the air entrained by the payload). If you have a computer vision system calculating landing sites on your lander, you really don't a wobbly 'chute.
So, deploying in a wind tunnel isn't much use as that's at sea level pressure. You also can't really have more than a single-digit percent blockage of a wind tunnel by the test article (in cross sectional terms) otherwise the results are off because the air velocity round the parachute has to increase to maintain the mass flow through a smaller area. Big windtunnels are rare and expensive, big ones that'll do transonic and supersonic basically don't exist, big wind tunnels that will do transonic and supersonic at low pressures (Mars's atmosphere being about 1% the density of Earth's) just don't exist.
But we can do useful things on earth. Our atmosphere density decreases as you get higher to almost nothing. So at some point between sea level and space we can find at altitude that has the right ambient pressure. You can get a model of your lander, tow it above that altitude, to an altitude that you have calculated so that when you release it it will reach (in free-fall) the right mach number at the pressure-matched altitude.
This is done at full scale but it's still very expensive, requiring huge helium balloons over big ranges.
I was involved in a project which did exactly this, but on the cheap at sub-scale, when I was a student. If you're interested you can read the paper here: http://www.cusf.co.uk/CUSF_AIAA_2011.pdf
Of course, that's just the parachute!
As for the sky crane, there is no way to frame it that doesn't make it look extremely ambitious, it is. However, recall that the Mars Phoenix lander of 2008 used steered retrorockets to land successfully: http://en.wikipedia.org/wiki/Phoenix_(spacecraft)
Recall secondly that the Spirit and Opportunity landers used a deploying tether to lower the airbag system away from the retrorockets. Step 8 on this page: http://marsrover.nasa.gov/mission/tl_entry1.html
So, you can argue that the bits have been tested before on Mars, but of course putting them all together into a single system is a challenge in itself.
It will be fascinating to watch, a magnificent achievement if it works, admirably bold either way.
from Wikipedia... "The landing sequence alone requires six vehicle configurations, 76 pyrotechnic devices, the largest supersonic parachute ever built, and more than 500,000 lines of code."
What will they show in the live feed? It makes me sad we can't have a (near) live feed from the delivery case or even the sky crane. Except for the understandable 19 (or so?) minutes or lag due to speed of light, the bandwidth is not nearly enough. It would have been cool of another level to be able to see the rover landing live, 19 light minutes away.
It'll most likely be a panel of people explaining what's happening as they get data coming in, shots of the team as they cheer/look nervous, and eventually a bit after the landing there might even be a photo from the martian surface taken by the rover.
At least that's what it's been like on other similar live coverages.
The moon is relatively (very) close (238,900 miles vs depending on where they are in their respective orbits, mars can be anywhere from 36 million miles to over 250 million miles away from earth), there are good relay satellites for moon and sun never comes in between to disturb incoming signal.
It certainly looks like overly complex. The simplest system, that is, airbags, are unusable because of the size (it's almost like the difference between a big toy car and a regular car)
You probably can't brake it only with retrorockets, and as this mission is aiming to be the most precise landing on Mars ever (and I'm not even sorry for the pun) they need a way to get the lander to an exact location (by deploying it from the crane)
People seem to forget/not realize that the EDL (entry, descent and landing) of Spirit and Opportunity had pretty much all the elements that the Curiosity EDL does PLUS the airbags and the airbag cocoon: http://www.youtube.com/watch?v=Ij33yhdGn_g
Leaving out the sky crane means that the comparison doesn't involve "pretty much all the same elements." The MSL sky crane is insanely complicated. Not much can go wrong with an airbag, relatively speaking.
I'd compare it to Cassini-Huygens, more than Spirit and Opportunity.
"not much can go wrong with an airbag" -- not true. The mission designers for MER (Spirit/Opportunity) admitted that a wrong bounce or an initial collision with a sharp rock could spell doom.
I think the airbag relies considerably more on good luck. You can't really model collisions the way you can a descent on a cable. The latter is 100% engineered.
But as the video shows, the MERs did have a similar system - after the parachute phase, the rover in its cocoon was lowered from the aeroshell at the end of a tether. Then, the retrorockets in the aeroshell fired, bringing the whole thing to a full stop some tens of meters above the ground. After that, the airbags inflated, the tether was severed, and the rover dropped the rest of the way to the surface.
One of the reasons for the skycrane is that it minimizes the area of the Martian surface that will be disturbed by the landing rockets. The skycrane approach moves less surface soil around the landing site (decreasing travel distance to pristine subjects for geological study) and deposits less rocket exhaust (which complicates chemical and biological studies).
NASA has an excellent and free ebook, _When Biospheres Collide_ [1], which goes into great detail about the problems posed by biological contamination (both forward and back). The book dedicates a long chapter to the great lengths taken with the Viking landers to avoid contamination problems. Having read it, I'm not at all surprised NASA is giving alternatives a try.
The Viking program was resigned to regarding the surface soil as untrustworthy for at least some (if not all) of their experiments. To get around this, the landers' robotic arms dug a few inches into the Martian surface to reach uncontaminated soil.
As for driving, the other commenters have it right: Curiosity isn't particularly fast and it's nice to start the scientific experiments sooner, rather than later. Moreover, driving around consumes limited electrical power which has to be shared with all the other systems on board, including transmitters and scientific instruments. Every minute spent moving from place to place is a minute of reduced data collection.
They drive at mm/s speeds. Moving over a few meters because the soil is stirred up is a huge waste of energy and time, not to mention the risk of failure.
Actually, 30 m/hour is 8 mm/sec. But that's a bad choice of units, because seconds is not a good mission time scale.
The point is that, within a couple of hours, you can drive beyond the contamination radius. And, over the span of the mission, you most certainly will.
Sorry, I wasn't able to figure out where I first heard about the contamination concern with MSL. I think I may have heard it in a video interview, which makes it nigh unsearchable. That said, a search of nasa.gov reveals several mentions of the use of a skycrane to reduce rocket exhaust contamination in relation to other missions, particularly ExoMars.
