Artist's concept showing the Mars Sample Return lander and its return capsule. Image courtesy NASA Science Mission Directorate/Planetary Science DivisionNASA has captivated our imagination with it's concept of returning regolith samples back to Earth from the surface of Mars. A technologically-feasible Mars Sample Return mission has been ruminating in the minds of NASA scientists for some time. The Decadal Survey of Planetary Science figures a Mars Sample Return mission as the next decade's most important NASA scientific mission. NASA's Commercial Crew and Cargo Program (CC&CP) invests in the private sector to develop space transportation capabilities that foster government-private sector partnerships. The agency's Commercial Orbital Transportation Services (COTS) division helps American industry develop privately-operated space transportation systems that enabled, for example, the study of a low-cost approach for a prospective Mars Sample Return mission. Previous studies have favorably highlighted just such a mission, a cost-effective mission using a high mass capacity lander. The craft proposed is the spacecraft that is designed and built by NASA - COTS partner SpaceX, the Dragon capsule, whose task is to land the launch vehicle system that returns Martian regolith samples to Earth.
If the MSL Curiosity Rover (whose one-year-on-Mars anniversary is Aug. 6) was NASA's most ambitious mission to Mars to date, the Mars Sample Return Mission will be like finding and bringing back the Holy Grail, on an inhospitable, cold, desert-like planet, no less. I was very excited to speak with NASA and pose some tough questions about the Mars Sample Return mission's MAV, the Mars Ascent Vehicle. I spoke via telephone with Charles Whetsel, manager of the Mars Program Formulation Office at NASA's Jet Propulsion Laboratory, about what the Mars Sample Return mission would look like, what engineering challenges stand in the way, the MSR mission architecture and the future space technology that is budding in NASA mission laboratories.
Michael Venables: Describe a theoretical mission of a Mars Sample Return mission in terms of it's format and mission objectives.
Charles Whetsel: The idea of returning samples from the surface of Mars has been studied a lot over the past several decades. And what it really boils down to is that, as we've learned more and more about the planet from the missions we've sent there, the remaining questions are becoming more and more challenging to answer with the kind of science instruments and the kind of spacecraft that can conduct their investigations in-space on Mars.
The idea is to use the instruments that we can send to Mars to select scientific samples of the history of Mars. How did it form? What was it like in its distant past when it was warmer and wetter? Were there opportunities for life to ever develop and grow to fruition on the surface of Mars back in it's ancient history. All of those are questions that we can answer to a certain level of precision with our missions at Mars, but if we brought samples back we'd have much more laboratory capability, much more sensitive instruments and much more opportunities to really make those measurements, understand the history of Mars and it's ability to support life in its past much more precisely. So that's kind of, for planetary exploration - that's been the Holy Grail for quite some time.
To do that, from a mission perspective, you have to think of it in several different stages, or several different steps, and they could all be done by one big mission, or you could break those steps up across several different missions. The first step is finding the good stuff, finding a good place to land on Mars. And even once you're at a good landing site, moving around, roving around and looking at the different rocks and rock formations - the soil and the regolith, to pick the best samples to answer the questions that the scientists are interested in. So, that's sort of step one, collect a really outstanding set of samples. Then, once you've done that you need to get them back to Earth. And we've thought of that as a two part relay.
You can imagine blasting off the surface of Mars with a rocket, with a launch vehicle that was powerful enough to throw those samples all the way back to Earth. That's a big rocket, a big launch vehicle if you're going to do that in one step. But if you break it into two phases where there's a smaller rocket that lifts the samples just off the surface of Mars and parks them in Mars orbit, leaves them in a stable orbit, where they can just be kind of like an artificial satellite around Mars. Then that package of orbiting samples could be collected by another mission, scooped up if you will with the Rendezvous Orbiter, that then, that mission could grab the samples from Mars orbit and fly them back to the Earth and then land some place safe in a desert somewhere, where there's a lot of open space. We've done that before with other samples from asteroids and samples from solar wind. The Japanese have recently done that from a small asteroid as well. So that part of flying back to the Earth and sending a capsule down with a heat shield, that part's been done before. But never from the surface of Mars. And really the tricky part here is getting off of the surface of Mars. That would be the new, big step here.
Venables: How will completion of the development of the ASRG satisfy the large problem of yielding more efficient power for future missions?
Whetsel: The missions that we've sent to Mars that have used radioisotope power in the past have used a different conversion technology. The radioisotope thermoelectric generators have relatively low efficiency of turning heat into electric power. Those were used on the Viking landers that were sent in the 70s, and they are also currently being used by the Curiosity mission. And there's a desire and an expectation to talk about using those again in 2020. But the Sterling cycle engine is in development as well, and could be a viable contender for near-term missions also. Because it is a moving piston cycle, it converts the energy a lot more efficiently, so if you have a fixed electrical demand for the spacecraft, one of the main benefits is then you can get away with launching a smaller amount of radioactive material, a smaller amount of plutonium. This is not only beneficial from having to not produce as much and handle as much of the material, but also not having to put as much on the rocket when you launch it up, which is something we always try to be very careful about, and only do that whenever it need to be done.
Venables: Describe how Solar Electric Propulsion (SEP) works.
Whetsel: We probably wouldn't use electric propulsion in an ascent vehicle. But in the mission I was talking about where you scoop the samples out of Mars orbit and fly back to the Earth with it - that's also a very propulsion-intensive mission. So to be able to use electric propulsion for that kind of mission would really enable that to either be launched on a much smaller launch vehicle than would otherwise be the case, or one of the other things we've talked about is if you're going to send that orbiter there while it's waiting for the samples to come up from the surface of Mars or if there's time to do other things. It can carry other scientific investigations or other payloads. So, by having electric propulsion instead of the standard, chemical propulsion that orbiter would be a lot more capable and would be able to do its mission, its round-trip mission - flying all the way out to Mars and flying back from Mars, actually flying all the way out to Mars, down into a low-Mars orbit, where it rendezvous with the samples and then out of that orbit back to the Earth. It would do that a lot more efficiently if we use electric propulsion than if we had to use either purely chemical or a combination of chemical and aeroassist technologies like aerocapture, or aerobreaking, which we've studied in the past.
Venables: What is the projected date to deploy full solar electric propulsion in current missions?
Whetsel: SEP has been used on a couple of planetary missions already. The Deep Space 1 was the first technology demonstration of the mission, just as a proof-of-concept showed that you can use it for a long durations, can use it to put a lot of the propellant through the system and use that to fly around the inner solar system. And then the Dawn Mission, which is going out to a pair of asteroids, with the first planetary mission to use electric propulsion in an operational capacity, in a mission where it wasn't just being flown as a technology. So we're relatively comfortable with that as a proven technology for that class of mission. If we were to use it for a Mars Sample Return mission, there are a few additional nuances. It's a very long mission. You have to fly out to Mars and fly back. So there is some additional lifetime qualification or testing and so on. But generally, I think people are of the opinion that the road map for the technologies ahead could be done in a small number of years.
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