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A New Way to Prepare Samples of Mars for Return to the Earth

Mars 2020’s strategy to cache samples is evolving as the mission matures

Posted by Casey Dreier

23-07-2015 14:18 CDT

Topics: Future Mission Concepts, Mars

Mars 2020, NASA’s next and yet-to-be-named Mars rover, will be the first mission to collect and prepare samples of the martian surface for return to Earth. This process is known as caching, and it is the crucial first step of a fully-born sample return campaign that could define the next two decades of robotic Mars exploration. Recently, the Mars 2020 engineering team proposed a new caching strategy that differs from previous concepts in some interesting ways.

JPL calls this adaptive caching, but I like to think of it more as the cache depot strategy. This means that after coring samples and placing them into hermetically-sealed tubes (the same process for any sort of caching), the rover will then deposit groups of samples on the ground throughout its drive. A future rover would retrieve some or all of these samples, place them in a rocket, and launch them into Mars orbit.

The adaptive caching cycle proposed for the Mars 2020 rover

Casey Dreier / JPL-Caltech

The adaptive caching cycle proposed for the Mars 2020 rover
After landing, the rover would drill and store soil samples in sealed tubes. At the team’s discretion, groups of these sample tubes would be placed at strategic ‘cache depots’ for a future retrieval mission. This can repeat as needed until the Mars 2020 is out of sampling tubes.

Now, this wasn’t always the idea. For years, caching concepts focused on a single repository for the samples, essentially a high-tech bucket that would hold 30 or so tubes in a honeycomb pattern. This would then be dropped off at a single location for future retrieval.

Here’s an example of this “cache bucket” concept from the European Space Agency:

A concept design for a Mars sample cache container


A concept design for a Mars sample cache container
This would hold all of the samples in one container, which would be loaded into a rocket and launched into Mars orbit to await a return to Earth.

It sounds straightforward, right? Drill some rock samples, seal them in a sample tube, and stick the tubes in a caching bucket, and set it on the ground for a future mission to grab and launch into space. Honeybee Robotics has a video depicting this basic concept for a Curiosity-like rover:

But it wasn’t until teams of engineers and scientists started to think about the scientific goals and the risk factors of a single caching storage system that people began to realize this may not be the best way to go.

Think of the following scenario: a hypothetical Mars 2020 rover is on the surface. It has 31 empty sample tubes that it can fill, but “mission success” is defined as collecting 20 different samples (this comes right from the actual science definition team’s report for the mission, by the way).

With the old-style cache bucket, you hit an interesting problem once you reach your minimum success goal of 20 samples. By definition: the mission is now a “success.” You have a treasure trove of samples that you could set on the ground for future collection, but you could also fill it with 11 more samples of indeterminate importance. You don’t know what or how important these samples could be, since you’d have to continue looking for interesting rocks to drill. But every meter you rove carries with it risk: risk of breaking down, risk of getting stuck in sand, risk of a computer malfunction, you name it. Despite JPL making it look easy to operate a rover on Mars, it’s not.

So there would be an enormous amount of pressure to drop the cache container once minimum success is reached. Do you cash out your winnings or let it ride? Even if you kept going, the levels of risk that the team would accept for roving to interesting places would be very low. This risk-aversion would increase with every sample collected beyond the minimum success point.

With the adaptive cache (or cache depot) strategy, you don’t have the same problem. You just tell the rover to drop the 20 samples on the surface, effectively creating a cache depot at that spot on Mars (the rover can carry a certain number of sample tubes with it while it drives). The rover then continues on to other interesting places where it can drill additional samples, safe in the knowledge that it has already deposited a successful mission’s worth for future retrieval.

There are additional practical benefits for the cache depot strategy. Since Mars 2020 would no longer be limited by the size of a cache bucket designed to return every sample to Earth, the rover could bring along more sample tubes to use throughout the mission. And since the samples are deposited frequently, which frees the rover to go on in search of more science, the team would have more time in which to debate which samples to return. Without this, the team would have to decide far more quickly about where to drill with less contextual information about the overall geology of the landing site.

If there will be a retrieval rover to grab these samples, it will face a similar problem of minimum success criteria for sample retrieval. But the risks it would face are different. Unlike Mars 2020, which would simultaneously explore, sample, and cache, the retrieval rover would travel over known terrain. It would likely travel to a single cache depot—or just a few of the most promising cache depots—to gather the previously-collected samples and place them in the return rocket.

The evolution of caching from its original single-container concept—a concept, I should mention, that’s been around for years—to the adaptive cache strategy illustrates the natural process of designing a mission. It’s only when you really get down and dirty with a problem that these types of subtle complexities arise.

Mars 2020 is maturing into a real mission. There are teams of people working every day to solve the practical, irritating problems of exploring Mars and storing samples for eventual return to Earth. Adaptive caching is the most visible example of one of the many thousands of clever solutions, compromises, and workarounds that every mission must face to succeed in exploring our solar system. The very fact that we face (and solve!) these problems is, in itself, remarkable.

A discussion of the pros and cons of adaptive caching can be found on the final slides of a recent presentation made by the Mars 2020 Deputy Project Scientist to the Mars Exploration Program Analysis Group in March of 2015.

See other posts from July 2015


Or read more blog entries about: Future Mission Concepts, Mars


Jonathan Ursin: 07/23/2015 07:23 CDT

Great idea! The retrieval robot could carry different instruments, inspired by the first visit.

Arbitrary: 07/23/2015 09:02 CDT

Another rover to the same location? With the ascent vehicle (stationary lander) that makes 3 missions to the same location on Mars, for one sample return. Is that what they are proposing? I expect them to be more clever than that. We could have 3 new MSL missions to three new locations instead.

Mewo: 07/24/2015 02:37 CDT

Let me see if I understand this. They propose a rover to go to a bunch of locations near the landing site, collect samples, put them in metal canisters to be strewn all over the place. Some time later, a second rover will arrive to collect the canisters. It will go to the exact same places as the first rover. There are several problems with this plan. Firstly, it hinges on being able to land the second rover near enough to the deposition site, and that is by no means certain. If it has to do a multi-kilometer trek to get there, the benefit of having scouted out the area is gone. Secondly, why not just have the sample return rover collect dirt and rocks directly instead of canisters? There is actually no need for the first rover. This is a stupid idea IMO.

Rob: 07/24/2015 05:10 CDT

Another important problem with this scenario is the incredible difficulty the fetch rover will have in globally localizing itself and then locating these very small sample containers. This is an open research problem, and will be very difficult for the sample fetch rover to achieve multiple times within a relatively short time frame.

Gregk: 07/24/2015 10:13 CDT

I wouldn't worry folks. I would be very surprised if the plan as described here actually happens, much for the reasons you're pointing out. I would be even more surprised if the second rover actually happens. No use in getting worked up over this, it's not going to happen.

Atom: 07/24/2015 10:38 CDT

Since the first "caching" rover is likely to be much more capable than the second "retrieval" rover, it makes no sense to ever drop these samples, instead the plan should be for the caching rover to deliver the samples to the lander carrying the ascent rocket. In the sample collection effort numerous more samples should be collected. Since the lander wouldn't be carrying a rover, it may be able to carry a larger and more capable ascent rocket.

Bored of silly people: 07/24/2015 10:40 CDT

If it sounds stupid to you, its only because you're reacting and not thinking about it clearly. Keep in mind that these people have actually done quite a few more missions than you have. There are reasons for doing sample return this way that have been laid out in other places.

not bored of silly people: 07/24/2015 02:58 CDT

I understand where B-of-SP is coming from but I also appreciate the thoughtfulness of those commenting on the somewhat bizarre intricacies (and subsequent risks) of the missions identified here This all strikes me as an example of how you have to get your boss to understand the impracticalness (or even foolishness) of his idea by getting OTHER people to tell him what you'd REALLY like to say. That or this is just a trial balloon. Either way, I tend to side with those that think this is a nag right out the gate. Then again, the idea of the “Sky Crane” approach for Curiosity was met with the same “incredulousness” as this approach so what do any of us know? ;)

Ryan_T : 07/25/2015 05:18 CDT

In theory, it's a promising method of mitigating the aforementioned risk of total cache loss, but it's not without it's own difficulties. There will almost certainly be a mass penalty as individual samples are now required to become 'life rafts' in order to survive long duration exposure to the less-than-friendly surround. Furthermore, there's an added risk of never being able to re-ascertain these individual samples as they will be deposited in multiple locations. Whilst this means that, in an ideal situation, a collection vehicle should be able to collect some of the samples, you are left with a situation where a decision must be made as to if those samples deemed as having the highest scientific reward are kept locally at the rover, thus statistically more likely to be recollected, or abandoned en-route. Ultimately, there's a lot riding on precision landing of the collection vehicle to ensure any kind of collection of samples can be established, but it seems this may be addressed by the terrain-dependent landing system that is currently under development at JPL. Food for thought, perhaps. Nothing comes without compromise in space exploration!

Paul McCarthy: 07/25/2015 08:49 CDT

Surely it would be better if the "caching" rover retains half of every (drilled) sample in tubes on board, and caches the other half in tubes on the ground at the several "cache depots". That way, if the rover is still mobile once it is "full" (which is more likely than a breakdown) it can directly transfer a full set of samples directly to the return rocket itself -- even if that rocket only arrives later. If it is clear that the caching rover still retains this functionality, then no retrieval rover need be sent -- saving one whole mission and transforming the economics! In this approach, if a malfunction prevents the caching rover from delivering its on-board samples, only then would a simple "retrieval" rover mission be launched. (Even then, the most secure strategy would be a series of separate "sorties" out to each "cache depot" in turn, i.e.: return one "cache depot" from each single sortie, thereby increasing the chance that at least some gets back to the return rocket before rover malfunction.) Are these not more economical and secure alternatives?

Shane: 07/25/2015 08:04 CDT

Bored of silly people, could you point me to an article that explains why caching and retrieval in two missions is better? I agree, I'm not qualified to judge at this point, but I would be really interested in understanding the economics and risk factors involved. Without that technical explanation, I am left thinking that they are doing one thing at a time because NASA does not have the funding to do both in one mission's time frame. If an objective will cost two decades worth of funding, then it has to be spread out over two decades.

Squirreltape: 07/26/2015 06:11 CDT

Don't concern yourselves over needing a pinpoint-landing... you can get exquisite accuracy if you attain orbit first à la 'Viking'.

Nirgal: 07/26/2015 01:14 CDT

In the first place, nobody has ever done a Mars sample return mission, so nobody has experience with it. And there is no heritage or legacy to help with. Every time a Mars sample return mission has been proposed, the mission has been cancelled because of technological challenges and cost overrun. Second, this scenario is not clever, because the second "retrieval" rover will face exactly the same issue than the first : as soon as he will grab a certain amount of tubes, he will become very valuable, and then the team will have to make a choice between going back immediately to the lander or continuing with the painful process of retrieving the damm caches from the ground. This is clearly a very political scenario, aimed to please the politics, who are very sensitive to "mission success". And of course, having to build more rover means getting more money and more jobs ... The silliness of this mission is so huge that she will clearly not happen, not in this configuration.

Torbjörn Larsson: 07/27/2015 10:12 CDT

Quite an odd discussion, seeing how the provided presentation answers a lot of the concerns raised here. (For example, it looks like the MAV return vehicle, which will place the samples in orbit for later retrieval, is brought on board the sample pick-up rover. And the overall strategy of both missions will minimize cache risk, not eliminate it. Finally, the sample pick-up rover do not need a smaller landing ellipse, so it will likely do some science on its own.) The most salient point besides reduced cache risk seems to be the simplified interfaces in and between missions: "reduces engineering complexity". @Nirgal: The "silliness" of science missions are always there, at least in the mind of armchair observers. Yet they happen, precisely because it is hard to predict - especially about the future. The provided presentation safeguards the potential follow up sample pick-up mission with "possible return mission". I assume you can back up your "clear" scenario with references?

ScienceNotFiction: 07/29/2015 02:20 CDT

Lets face it, we already have plenty of cheap flying technologies on Earth - autonomous flying drones. Why not embed a GPS-guided helicopter drone into the cache-container. As soon as it is filled, the container will fly back to the rocket launch site. A collecting droid (ASIMO type robot) will place the samples on board the rocket near the vicinity. With Mar's low gravity, this drone should have a greater flying range (one way only). This can avoid any rover-stuck situation resulting mission failure. The prerequisite of this idea is the GPS part. You need to launch a set of micro satellites orbiting Mars beforehand. If we are planning to send human to Mars, we really need to have a working GPS system there on the first place.

Rich: 07/30/2015 05:01 CDT

With MRO or equivalent assets in place, geolocation of the cache is trivial. MRO can already see our rovers on the ground, so it would know exactly where the cache is located and where the retrieval rover landed. I think it will be harder to reliably pick up and carry the cache, if it gets a layer of dust on it or falls over. I hope the caching rover is designed so that the sample tubes can be removed by the retrieval rover in case of a system failure. Imagine how sad everyone would be if it failed at sample 19! I think it makes more sense for the return vehicle to land first, and then a collection rover land and start filling it with samples, through slots or whatever on the side. The RV could have a simple backup collection hole that sampled the air dust, or an arm to grab a contingency sample. This way we get at least 1 low quality sample, with a high likelyhood of more high quality samples.

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