30 September 2010
The 4th MSL Landing Site Workshop: Day 3 – Engineering and Safety
Posted by Ryan Anderson
With the details of all four landing sites on the table, we started day 3 of the meeting by hearing from the engineers and several scientists about the properties of the ellipses, the risks for landing and the capabilities of the landing system. First on the schedule was Mike Watkins, who explained why MSL is so unique in terms of assessing the risk for the landing site because the landing safety is essentially the same for the sites, so the tradeoff becomes more science-oriented and requires a lot more knowledge of the possible targets and traverse distances.
After Watkins, Ashwin Vasavada – the deputy project scientist – told us about the atmospheric simulations that his team has been doing to make sure the weather at the sites won’t mess up the landing system. He pointed out that we are one mars-year away from landing: “The next time Mars goes around the sun, we’ll be there to meet it.” That means that measurements being made right now will be really important in predicting the conditions when MSL arrives. It turns out that unlike previous missions, MSL will actually fly for about 100 km at a pretty constant altitude. This guided flight is what shrinks the ellipses down to nearly circular, but it means that we need to understand the weather for that whole distance. It sounds like the weather at the sites shouldn’t be a problem: Vasavada said that MSL should be able to land even if there is a dust storm occurring at the landing site.
Next up, Ken Herkenhoff gave a summary of all the processing that goes into making the high-resolution elevation maps based on HiRISE stereo images. It is incredibly complicated to make these things, but they’re extremely important for planning the landing and traverses. Luckily, the folks at USGS have a lot of really clever techniques to make the products possible. The DTMs are available at the HiRISE website.
Matt Golombeck then gave his first of two presentations. This one was about counting rocks in the landing ellipses to make sure they’re safe. For the purposes of his talk, he defined a “rock” as anything that we don’t want to land on or get in the way. Rock counting has been done for all the previous landing sites on Mars, so we have some good “ground truth” to compare with the rock counts from orbit with HiRISE. They use an automated algorithm to count the rocks and then fit the size distribution to a model based on previous sites to predict the number of dangerous rocks that are too small to count. In the past it has been very successful, and they’re confident that all the MSL sites are safe enough. Golombeck said that Holden is especially safe in terms of rocks: “I think there’s maybe one rock in Holden.” There were a couple of very good questions after this talk. Rob Sullivan asked whether a small softball-sized rock would be a danger if we landed right on it with a wheel, and the answer was basically no. The engineers said that the biggest risk was getting a rock in the belly that would damage the rover and/or prevent it from moving. A second question from Steve Ruff was more generally, “how concerned should we be about landing with a hobbled rover?” The engineers said, again, don’t worry about it. They’ve been investigating some scenarios where the landing system might get stressed beyond the point where the materials have a linear behavior, but they aren’t worried about breaking the rover.
Next up, Robin Fergason presented about the thermal inertia of the sites. Thermal inertia is a measurement of how resistant a surface is to heating. Bedrock takes a long time to heat up and cool down so it has a high thermal inertia. Dust has a low thermal inertia. Fergason said that all the sites look safe in terms of thermal inertia and then discussed some of the details of each site. One thing that sort of bothered me was that she kept saying that lower thermal inertia might suggest alteration. It’s true, it might, but I would never bet a $2.3 billion rover on the fact that altered rocks have slightly lower thermal inertia. The values seen could just as well be
unaltered bedrock with a dusting of sand or dust on top. Of course, we see other evidence that the rocks at the landing sites are altered, but for the rocks with no clear spectral signatures, I don’t think it’s safe to assume they’re altered based on thermal inertia.
After the thermal inertia discussion, Golombeck got up and gave a presentation with 83 slide in 30 minutes, flashing rapid-fire views of lots of different data-sets used to characterize the ellipses. He also told us that the orientation of the ellipses has changed a little bit so that now their long axis is due east-west. Taking a look at the 5 meter slopes he said that Mawrth and Eberswalde were significantly rougher than Gale and Holden (which makes sense, since Gale and Holden both land on top of nice flat alluvial fans). The topography at Mawrth and Eberswalde would be comparable to that seen in the Columbia hills, where Spirit has been exploring. Golombeck also showed that there are very few “inescapable hazards” such as craters in which you could land but then couldn’t escape. At this point he jokingly made fun of engineer Gentry Lee who had been worried about a big crater in the Mawrth ellipse, saying that the crater was not a target rather than an obstacle! There was also the amusing mention of a small mesa in Eberswalde where MSL would land but would have to go down some pretty steep slopes to escape. This possibility was later refferred to as the “Lion King” scenario by Rob Sullivan. It’s incredibly unlikely, but still hilarious to picture the rover greeting the sunrise from atop this viewpoint, which would, of course, be called Pride Rock.
At the end of this talk, Sullivan asked whether there is any concern that dust kicked up by the rockets could confuse the landing radar. The engineeres said that yes, there is some concern, but they are doing tests with the radar on helicopters to learn more. Basically, the engineer responding said “We could probably fly through the blowing stuff. We might not like it, but we could do it.”
Next, Devin Kipp, one of the engineers on the entry, descent and landing teams gave a presentation that took a look at all the things that could go wrong and basically said “we don’t think these things are going to happen”. He said that the chances of landing success in all of the sites are about 98-99%. Kipp also made the biggest understatement and the best example of NASA engineering-speak all day. When discussing the transition from wheel touchdown to actual roving he said: “hopefully the rover-surface interaction perpetuates for the rest of the mission.” I certainly agree with that!
Finally, Paolo Belutta, a rover driver, gave a presentation detailing how the traverse times for each site are going to be predicted. I suspect that these traverse times are going to be the deciding factor for which site is selected. Since landing on the sites is equally safe, the risk gets pushed into the traverse portion of the mission. Belutta is making detailed maps of the sites using HiRISE images and terrain models to estimate how long it will take to drive in any given location. Then, with input from the scientists, he will compute the traverse durations for several options in each of the landing sites. At that point we’ll come to the tough part where we decide whether the cost of a long traverse is worth the possible payoff.
Overall, it was an extremely positive morning and I think it laid to rest a lot of concerns that the science community had, based on rumors and partial information that had spread around the science teams. That said, I think it was also presented with full knowledge that this was a public meeting, and that word might get out if they admitted to any major problems. NASA has this mentality where it is afraid to acknowledge the difficulty of what it does until it has done it successfully. Personally, I think NASA should play up the risks and the difficulties ahead of time: they’re really interesting, and they make the final successes that much more exciting, and prepare people for the worst if it happens.
In any case, it was a fascinating morning. I’ll post about the big afternoon discussion of all the sites tomorrow. Stay tuned!
It would be interesting to hear more about the failure modes on landing as this seems to be the biggest “new” thing that NASA is doing for MSL. Rovers have been used and “proven”, but this Sky Crane just worries the hell out of me – seems like you have too many non-redundant points of failure and a critical path that is crazy filled with risk.
They mentioned all sorts of failure modes: sliding down a slope, blasting the rover with the rockets, failing to separate the skycrane, etc. The message was that none of those are really likely. They seemed surprisingly confident. But as I said, they may have been acting extra confident so that the scientists don’t go running around telling people about how this thing is probably going to crash… 🙂
“failing to separate from skycrane”
Really? They’re that confident? Man, that’s the part that scares me the most. You have to maintain altitude, reel the thing down evenly, release it correctly…
What sort of redundancy do they have in that? What if the reels don’t let down evenly enough? What if the altitude is off? They had problems with their atmospheric readings before, ala Spirit’s landing – how’ve they managed that risk with MSL?
I’m not expecting you to answer all those, BTW, just the stuff that rattles my head about this whole deal. I know they had to come up with something other than the airbags but it seems like the tried and true retrojets lander would be the safest option. This thing just gives me the whillies.
I liked the photoshop job of Curiosity atop the promentaory at dawn. Can you do another one with you riding it like a bronco?
Did Matt mention whether they have published a discussion of the algorithm/code they used to count rocks? I’ve been debating whether to use Weitz et al.’s [2006 para. 8 – 9] software with Meridiani soil, but maybe Matt’s algorithm is easier to implement.
He didn’t mention whether it was published. I noticed that his presentations are not available on the public site, which makes me suspect that it’s not published yet. He did point to the relevant chapter in Jim’s Cambridge book, so that may be a good place to start looking for references.
Both MER landers were lowered on tethers which had to be cut at the right moment. In both cases, all worked fine.
No, the MERs used an airbag system to bounce along the surface until they rolled to a stop.
I think bjm was referring the the fact that the rover dropped on a bridle from the backshell, used retro rockets to bring the descent to a halt, and then separated from the bridle. Both operations seem like non-redundant points of risk.
Obviously what MSL will do is different in many ways, but I think bjm’s point was that JPL’s success with the unreeling of the bridle and subsequent separation for MER should give us some comfort that it knows what it’s doing with the analogous (though different) operations for MSL.
Er, I mean all three operations (the Spanish Inquisition strikes again!).
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