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Let's talk about this whole Moon vs. Mars thing for human spaceflight

Posted By Jason Davis

26-01-2017 6:01 CST

Topics: Humans to Mars, human spaceflight

In case you haven't heard, the Trump administration may direct NASA to land humans on the moon.

At least, that's according to scattered media reports; officially, NASA remains on course for a Mars landing in the 2030s. Starting with the scheduled inaugural launch of the Space Launch System with Orion late next year, the agency plans to start establishing a human presence in lunar orbit, using it as a proving ground to prepare for deep-space missions to Mars.

Landing on the Moon's surface currently isn't part of that plan. Should that change? 

Moon or Mars?

NASA / GSFC / ISRO / Jason Davis / The Planetary Society

Moon or Mars?
NASA's current human spaceflight goal is Mars. Will the moon eclipse that?

The science

It only takes a glance at a planetary exploration roadmap to see Mars is currently one of humanity's leading targets of interest. There are eight spacecraft operating on or around Mars; five belong to NASA. 

Following the loss of the Mars Observer spacecraft in 1993, NASA established the Mars Exploration Program, a systematic series of missions to determine whether Mars "was, is, or can be, a habitable world," and to pave the way for eventual human exploration. The program has been wildly successful; in 2013, drill samples analyzed by the Curiosity rover confirmed the answer to the "was" question is yes.

Whether or not life actually existed there—or still does—will be harder to answer. Back in late 2014, Ellen Stofan, who served as NASA's chief scientist for three years, told me the question would be best solved by astronauts.

"I have a bias as a field geologist that it's going to take astrobiologists, geologists, and chemists on the surface of Mars, being able to go out and read the landscape, pick up rocks, and take them into a lab, to really resolve the question of whether life arose on Mars," she said.

NASA's Opportunity rover has been exploring the surface for 13 years. Mike Seibert, an Opportunity driver and a lead spacecraft systems engineer at NASA's Jet Propulsion Laboratory, said a big advantage of humans over rovers is their ability to improvise quickly. An example of this, he told me, was a meteorite Opportunity inadvertently discovered while the rover was driving between two waypoints.

"We were taking pictures behind us just to record and document, and we downlinked them several sols (Mars days) later," he said. "A half-kilometer down the road, we saw an image of a meteorite. So we doubled back to go see it."

Astronauts haven't been to the Moon since 1972, and if they go back, they won't be searching for signs of life. But that doesn't mean our celestial neighbor doesn't have other secrets worth uncovering. 

The National Research Council's decadal survey, which is published every 10 years to define priorities for planetary science, lists a lunar south pole sample return as one of five top targets for an upcoming mid-tier science mission. The survey says there are important questions to be answered about the Moon's internal structure, and the composition of its mantle.

"There's good science to be gained on both the Moon and Mars," Seibert said. "We haven't had that many surface assets on the moon—or Mars, for that matter. It's like saying you've visited Earth because you've been to Beijing and Lawrence, Kansas."

I put this question to a NASA scientist who works at a key NASA human spaceflight center. The scientist, who preferred to remain anonymous, agreed, telling me, "There's lots of science to do on the moon. But that's not likely going to be the motivating reason we go there."

If not for science, why?

There's a more practical reason some people prefer the Moon over Mars: it's easier to get there.

NASA's current "Journey to Mars" plan is pyramid-shaped. The base includes the International Space Station, Space Launch System and Orion. The middle layer involves learning to live and work around the Moon. And near the top are Mars transfer vehicles, landers, habitats and the all-important ability to come back to Earth.

It's the top of the pyramid that worries many people; right now, it's fuzzy, and without the funding to work substantially on more than one thing at a time, NASA's fully formed plans won't be ready anytime soon.

"There's a number of things that don't exist yet that need to happen," the NASA scientist told me. "And they may require several miracles."

Moon landings enjoy a moderate amount of international support; namely, from European Space Agency Director General Jan Woerner, who has vigorously promoted the idea of an international "Moon Village."

To what degree ESA could financially back this effort is unclear. The agency's 2017 human spaceflight and robotic exploration budget is just $675 million; by comparison, NASA spends almost $3 billion annually on SLS and Orion alone, and another $1.5 billion on the International Space Station.

China and Russia have also indicated an interest in moon landings, but the U.S. is prohibited from cooperating with the former. As for the latter, Russia's 10-year space budget was recently slashed by 64 percent, its long-term plans are uncertain at best, and it has more pressing problems at the moment: severe quality control issues plaguing its launch fleet.

I asked Dave Belcher, an analysis manager at the Washington, D.C.-based Avascent consulting group, whether he thought it was realistic for NASA to expect international partners to make meaningful contributions to lunar landings.

He framed the answer a different way: "I would say that if lunar surface operations would be a financial stretch for international partners, Mars would be even a larger financial stretch," he said.

Moon village

ESA / Foster + Partners

Moon village
ESA Director General Jan Woerner wants to build an international moon village. But the extent to which it could financially back such an effort is unclear.

The commercial argument

Another reason often given in favor of returning to the lunar surface is that it could spur investment from the private sector. But what exactly does that mean?

In 2010, the Obama administration canceled NASA's Ares I crew transportation rocket, along with the rest of George W. Bush's return-to-the-Moon Constellation program. In the process, NASA went all-in on spaceflight-as-a-service, outsourcing ISS crew and cargo transportation to private companies. Despite a few setbacks, the program has been mostly successful; the space station now regularly receives private cargo shipments, and next year, SpaceX and Boeing are expected to begin long-awaited crew flights. 

In the process, SpaceX—by far the shining example of the NewSpace industry—has become a successful company, while giving America's entrenched aerospace industry a much-needed shakeup.

But NASA is still SpaceX's biggest customer. A recent investigation by the Wall Street Journal showed NASA contracts accounted for 43 percent of the company's projected 2016 revenue. And this is in the flourishing market of low-Earth orbit; how such a business model might work at the Moon—beyond providing services to NASA and other international agencies—is unclear.

In recent years, a smattering of space startups have proclaimed an interest in mining the Moon. Water ice on the surface could be turned into rocket fuel, regolith could be used for building materials, and Helium-3 could be converted into nuclear energy.

The challenges to make these concepts a reality are not trivial. As an example, water ice is found near the Moon's poles, in permanently shadowed craters, where temperatures plunge to -249 degrees Celsius—just 24 degrees above absolute zero. Mining in such an environment would be difficult. The extracted ice also has to be converted to propellant, stored, and shipped somewhere useful, such as to an ascent vehicle and back into lunar orbit. 

Many experts I spoke with expressed skepticism. "Mining the regolith doesn't strike me as a winner," the NASA scientist told me.

Belcher, the aerospace analyst, was slightly more optimistic: "I think those (ambitions) are probably longer-term, but potentially viable," he said.

What about practicing for Mars missions? Could learning how to harvest lunar water come in handy later?

Briony Horgan is a planetary scientist at Purdue University who is working with other scientists to select landing zones for crewed Mars missions. If astronauts land near the Martian equator, they would need to bake water out of hydrated minerals—basically, rocks with water trapped inside. Further north and south, actual ice sheets exist, but they lie below the surface. 

"I think resource extraction on the Moon would inform techniques needed for Mars, but it would not be the same technology," Horgan told me. "It would be pretty different in the end."

The technologies needed to land on Mars are vastly different from those needed for the Moon. Curiosity, bundled up inside its protective descent shell, had a mass of 3.3 metric tons when it hit the Martian atmosphere in 2012. Human landings on Mars could easily tip the scales at more than 20 metric tons, requiring much more powerful EDL systems.

NASA / JPL

Crossover tech: landing

To what extent are the technologies and techniques required to land, live and work on the Moon applicable to Mars? The answers are mixed.

The most hair-raising part of any Mars mission is entry, descent and landing, known collectively as EDL.

Like Earth, Mars has an atmosphere. When a fast-moving spacecraft plunges into an atmosphere, molecules in the vehicle's path compress and warm up. The result is a fiery plasma plume that engulfs the spacecraft. If you want to survive the process, you need a heat shield.

The Moon has no atmosphere. When astronauts descended to the surface during the Apollo missions, they did so inside their ungainly lunar lander, with all of its unprotected appendages sticking out. 

On Earth, once you make it through the atmosphere, you're pretty safe. Air drag will slow you down enough for a relatively soft landing under parachute or thrusters.

But Mars's atmosphere is too thin to finish the job. When NASA's Curiosity rover deployed its parachute in 2012, it was still hurtling toward the surface at a blistering Mach 2. The Apollo 11 capsule, by comparison, was traveling at a leisurely Mach 0.3 when its parachute deployment sequence started.

Curiosity, bundled up inside its protective descent shell, had a mass of 3.3 metric tons. Human landings on Mars could easily tip the scales at more than 20 metric tons; the EDL system required for such a mission will have to be much more powerful. In 2014 and 2015, NASA's Jet Propulsion Laboratory conducted two inflatable heat shield tests. The results were promising, but revealed a different problem: A parachute double the size of Curiosity's shredded apart both times. 

Crossover tech: living

In total, the crew of Apollo 17 spent more than 22 hours exploring the moon during three separate moonwalks. By the end of the mission, the jagged lunar dust had started to chew through the astronauts' gloves and spacesuits. Had a fourth moonwalk been planned, NASA might have called it off. 

Because the moon has no atmosphere, it is continually bombarded from small grains of dust that would normally burn up as shooting stars in a planetary atmosphere. This bombardment creates a fine layer of dust on the Moon's surface. The micron-sized dust grains are sharp and jagged.

Mars dust is more Earth-like. The fine particles tend to be round and polished, and scientists think they may come from rocks shaped by flowing water, oxidation, and wind erosion. 

"On the moon, I imagine the dust as being kind of like asbestos," said Horgan, the planetary scientist. "What might be irritating on Mars could be deadly on the Moon."

These differences mean there is not necessarily a one-to-one design crossover for things like filters, seals, and garment fabrics. 

Mars is also subject to global dust storms. A dusty day on Mars is similar to a cloudy day on Earth; high temperatures drop, while low temperatures rise. These temperature swings can mean the difference between acceptable working conditions and a scenario where astronauts have to hunker down and wait out a storm.

Temperature ranges are a big deal when it comes to the design of things like batteries (batteries hold less charge in colder temperatures) and moving parts (which may require certain temperature ranges to operate), as well as keeping humans comfortable in a pressurized habitat, rover or spacesuit.

At Gale Crater, near Mars' equator, daytime highs can climb above freezing in summer, but drop below -60 degrees Celsius at night. In winter, daytime highs may stay below -20 degrees, while plummeting below -80 degrees at night.

That makes daytime working temperatures at Gale Crater similar to the ones found at McMurdo Station, Antarctica—chilly, but safe enough to operate a rover or walk around in a spacesuit. 

A dust storm can change that. Seibert, the rover driver, said this could even mean having to skip launch opportunities—which only comes every 26 months—because temperatures at a Martian landing site could be too dangerous for an incoming crew to set up shop.

The Moon has a different set of temperature challenges. 

A day on the Moon lasts 27 Earth days. During daylight, temperatures can top 120 degrees Celsius. At night, they drop to a frigid -150 degrees. 

In general, that means crews on the moon will work during the day and take shelter at night. And during the day, astronauts might face a problem they wouldn't on Mars: overheating equipment.

One final difference between living and working in the two environments is spacesuit design. The cooling systems on most spacesuits, Seibert said, generate ice, which is sublimated into the vacuum of space.

"On Mars," he told me, "there's enough of an atmosphere that the design might not work very well."

Gene Cernan after moonwalk

NASA

Gene Cernan after moonwalk
Astronaut Gene Cernan covered in lunar dust on the Moon. Following three moonwalks during Apollo 17, jagged lunar dust had started to chew through the astronauts' gloves and spacesuits.

Independence

Would sending NASA astronauts to the Moon give Mission Control any operational experience that could be applied to Mars trips? 

Aboard the International Space Station, astronaut time is meticulously scripted. Communication with the ground occurs in practically real-time. Out in lunar orbit, that changes—but not drastically. Communications signals, traveling at the speed of light, take about 1.3 seconds to travel one way. That makes for a slight delay, but in an emergency, Mission Control is still available (except when an orbiting crew is on the other side of the moon, though a communications relay satellite could fix that problem).

Out at Mars, the one-way communication time with Earth increases to between three and 22 minutes, depending on the planets' relative orbital positions. At best, that makes one question-and-answer session take more than six minutes—and at worst, 45.

"(Mars astronauts) can check in with NASA and get general guidance," Horgan said. "But their minute-to-minute—and even day-to-day—activities are going to be decided much more independently."

For training purposes, NASA could simulate this time delay aboard the ISS, or during a lunar surface mission, giving astronauts more freedom to practice managing their own activities. The NASA scientist I spoke with told me this is a common topic of speculation: How much independence would Mission Control give astronauts on the Moon?

Picking one

Despite the differences between the two destinations—and the questionable case for crossover technologies and experience—the Moon may ultimately represent a more compelling political destination. 

Whereas a NASA-led Mars landing is at least four or five presidential election cycles away, astronauts could be back on the Moon within eight years, if President Trump and Congress gave NASA the funding to make it happen. And if NASA is directed to use its existing capabilities like SLS and Orion—which are supported by influential members of Congress—a pivot to the Moon might not meet much resistance.

NASA could also continue to plug Mars as its long-term horizon goal. This approach was used in 2004 when President George W. Bush unveiled plans to return to the Moon.

But without a major funding increase, establishing a permanent presence on the Moon would all but rule out NASA-led Mars trips for the foreseeable future. A report issued by The Planetary Society in 2015, containing program projections made by NASA's Jet Propulsion Laboratory and The Aerospace Corporation, concluded the agency's current Journey to Mars plan was sustainable only if the ISS retires by 2028. Building a Moon base with an ISS-level operating cost, then, would likely push Mars off the table.  

If NASA isn't sending humans to Mars, SpaceX might. Last year, CEO Elon Musk unveiled ambitious plans to colonize the planet. One D.C. source I spoke with noted if SpaceX develops advanced Mars EDL technologies at its own cost, NASA's path to Mars could get a whole lot easier.

All in all, most people I talked with don't hold a strong opinion on either destination. They just want to see NASA send humans somewhere—anywhere—beyond Earth orbit.

Seibert, who contributed a location of his own to NASA's Mars landing zone selection work, said he sees space community momentum for sending humans to another world. 

"I think a lot of people are working in the right direction, and a lot of people are working together really well," he said. "We would just like to know which destination we're going to, so we can really focus our efforts to make sure we can send folks there."

Horgan seemed to agree.

"Do you try to take that intermediate step, and risk getting stuck there along the way, or do you keep pushing to make it to your end goal first? I think it's whatever your priorities are," she said. "We should pick a destination and go there."

 
See other posts from January 2017

 

Read more blog entries about: Humans to Mars, human spaceflight

Comments:

Red: 01/26/2017 10:10 CST

I favor Mars myself, but like others 'anywhere' beyond Earth orbit is good. Elon Musk's Mars plans are very ambitious, but they appear more achievable than attempts NASA made. The Orion, as is, needs to be reworked as it barely has the power to enter lunar orbit, but the SLS is a perfect workhorse at least until a commercial entity (SpaceX, Blue Origin) can build an equal to it later.

dougforworldsexplr: 01/26/2017 12:21 CST

One thing I agree with some other people, I think you mentioned this in your above article too Jason, is that it would certainly be quicker to get a human mission to the surface of the Moon. If NASA is taking people back to the Moon I hope the mistake of just going there to plant the flag alone isn't made again but work will begin on a long term human base on the surface. However the missing part at least for SLS and Orion is a descent/ascent module like the Constellation Altair or the Apollo Lunar Module. Could this still fit in the top of SLS as currently designed, or if a less extensive propulsion module for Orion was used? Surface life-support, transportation and communication infrastructure would also have to be prepared. I think the Moon is important for science, strategic, economic, inspirational and accessibility reasons although the science associated with any past or present life would of course not be like that associated with Mars. I would also be OK if NASA wanted to use SpaceX and their Falcon 9 Heavy and Crewed Dragon to get astronauts to the Moon to help build a base there but they have to get some of that equipment working yet. I saw Apollo 11 live and hope not too much of the deep space mission will be left to robots that could be done by humans as robots don't give the same inspirational or identification value including hearing their personal experiences. I agree Mars is more interesting but I don't want the US and freer world to forfeit the Moon to others that don't promote as free a political system namely Russia and Communist China. Any word when a decision will be made including about a lunar descent/ascent vehicle and basic lunar infrastructure. However I admit Mars in itself has more inspirational and scientific value but too bad it isn't the closer astronomical object.

Haruki Chou: 01/26/2017 12:26 CST

I don't see the point of landing humans on Moon or on Mars. A waste of money.

Jason : 01/26/2017 12:54 CST

Even before we get to the "landing" part, there's a lot we need to learn about long-duration spaceflight beyond LEO, which we could test out closer to home cheaper, lower risk, and with faster turnaround times. And there's still a lot of basic infrastructure research that could make a future mission to Mars much cheaper and reliable, like propellant depots, advanced robotics, base construction, radiation shielding/mitigation, heavy lift reusable rockets, etc.

Jason Davis: 01/26/2017 01:18 CST

Just wanted to clarify, with all due respect to the Jason above, that's not the author, I am :) When I reply to an article I list my full name to make it clear. Hopefully we can iron that out in our full website redesign, which is on our tech roadmap this year—stay tuned for that!

Hamdragon: 01/26/2017 02:40 CST

The Moon and Mars should be a singular objective, as it were. Getting to the Moon needs to get to a place where we all think of it as "commonplace". Mars should be pursued at the same time. Lets get NASA's budget increased to facilitate both pursuits, including funds for continued robotic missions out into the solar system and beyond, and continue doing wonderful science! Let's make space easy!..... OK, at least a bit easier :-)

Outer Space Colonization: 01/26/2017 04:42 CST

The focus should be on technology for the future. Nasa had 30 years between 1987 and 2017 but they have largely the same or lesser capabilities. SpaceX had 10 years between 2007 and 2017 yet vastly different capabilities. Yet no one has any serious capability to mine things in space. No one has any serious ability to farm in space. No one wants to consider seriously any rocket fuel alternative technologies for fast thrust. Nearly all the key technology fields are laying fallow...Only SpaceX and perhaps Mars One are spending any money on the actual things that matter.

Stephen: 01/26/2017 10:10 CST

Outer Space Colonization: “Nasa had 30 years between 1987 and 2017 but they have largely the same or lesser capabilities.” That statement is outrageously ridiculous. The ISS didn’t exist thirty years ago. Or doesn’t that count as a “greater capability”? Neither had there been unmanned probes to Pluto or Saturn or rovers on Mars or an unmanned orbiter around Mercury. Outer Space Colonization: “SpaceX had 10 years between 2007 and 2017 yet vastly different capabilities.” What does that statement mean? Do you mean “different” as in different to NASA’s capabilities or do you mean SpaceX have vastly INCREASED capabilities compared to NASA? With the exception of landing its rockets vertically, there is nothing SpaceX has done which NASA hadn’t done already; and several times over. Plus there are things NASA has done (e.g. landing people on the Moon and sending unmanned probes to Mars, Mercury, Jupiter, and other planets) which thus far SpaceX have not even attempted.

Messy: 01/27/2017 07:34 CST

http://www.dailymail.co.uk/sciencetech/article-4162296/The-incredible-lunar-TEMPLE.html Someone some time ago proposed a manned flyby mission to Mars as a proof of concept project. That would be a good idea as we and the ESA do the Moon base thing as proof of concept for interplanetary settlement. As to the ISS not existing 30 years ago. In a way, it DID. It as called Mir, and before that it was Called Salyut and Skylab. 30 years ago we had already sent two probes to Saturn. Voyagers 1 and 2, remember?...actually we had sent THREE (I forgot Pioneer 11). In 1970, I bought a magazine that detailed the proposals for NASA's manned spaceflight program. We should have had a moon base in 1980.

Paul_Wi11iams: 01/27/2017 01:42 CST

That shot of Gene Cernan on the final Apollo mission, must be the most cinematographic astronaut photo ever seen in real life. Straight out of the film "The Martian", It leaves little doubt that space is hard and and also not so clean as one would think. What next ? Media and the public will compare any planetary exploration speech by Trump with the preceding "Moon" speeches by Kennedy: https://youtu.be/ouRbkBAOGEw?t=87 and Bush: https://youtu.be/MH9909OVDHE?t=247 . The Kennedy speech led to rapid success and the Bush one not. It would be hard to follow on with *another* Moon speech and be believed. That alone may suffice to tip the balance to Mars. For the cost aspect, and in agreement with Jason Davis, a private contribution to the Mars effort would be convincing both to Trump and his public. What would become of SLS ? Whatever pressure from Congress, Trump doesn't seem exactly afraid of making enemies. And then, even people inside Nasa seem doubtful of SLS which is the remnants of a scaled-down program just as the Shuttle was in its time. For fuel production by the Sabatier reaction, the martian atmosphere is a big bonus for ISRU resource utilization. Fast-moving and more autonomous robots will soon be far better trail-blazers than the slow Oppy/Curiosity type. Their transport to Mars would also validate the EDL doing several landings no humans on board. Despite this, a present Mars project would be initially ahead of its time and almost as dangerous as Apollo was in the 60's. Either the Moon project or the Moon project will be "sold" to Trump. So maybe the choice will depend on who is the best salesman in the space business ?

spacecase: 01/27/2017 02:35 CST

I suggest use Falcon 9 for Mars and SLS for the Moon. A parallel effort that makes sure the USA stays ahead of the rest of the world in Space. If we don't go back to the Moon, China, Russia, and Europe will. We will be left far behind trying to go to Mars while these countries set up bases on the Moon. Does this make sense??! We blew it 45 years ago when we could have set up bases on the Moon and progressed to Mars in a logical fashion. Trying to leapfrog to Mars without going back to the Moon first makes no sense. I hope Trump thinks big and funds NASA and Commercial Space in a Kennedy like plan. It would energize our country like the Moon Race did.

Paul_Wi11iams: 01/27/2017 03:30 CST

firstly correcting the last sentence of my 01/27/2017 01:42 CST comment: Either the Moon project or the Mars project will be "sold" to Trump. So maybe the choice will depend on who is the best salesman in the space business... secondly commenting on the spacecase: 01/27/2017 02:35 CST: Speaking as a European, I'd be delighted if Europe, Russia etc could be a real challenge to American supremacy, but rather doubt it. As for humans to Mars by SpaceX, it would be the Interplanetary Transport System (ITS) rather than Falcon 9 triple core (FH) which can only do demonstration flights and prospecting. However, the development costs would be largely carried by the American taxpayer, at least partly through R&D ordered by the Nasa by contracts with SpaceX or similar with another company like BlueOrigin. This leads to an "either/or" situation between SLS and ITS anticipated by many observers, if politely denied by SpaceX.

phillipwynn : 01/27/2017 03:42 CST

Maybe this is really ignorant, but why do NASA people and others so blithely talk about mining lunar water? Isn't it true that space as a whole could be considered a desert far drier than any desert on Earth? Think a Sahara with few oases, and the closest one a tiny little pond. What would we think of the courageous "explorers" who would gulp down this precious pond to fill the radiator of their car?

Paul_Wi11iams: 01/27/2017 04:42 CST

replying to phillipwynn 01/27/2017 03:42 CST "why does NASA... talk about mining lunar water? Isn't it true that space as a whole could be considered a desert far drier than any desert on Earth?" We shouldn't be using the comments section as a forum but... the nebula then protoplanetary disk from which Earth and Mars condensed was full of hydrogen and oxygen that constitutes the water on both planets. In fact the whole solar system is awash with water, mostly as ice but some as liquid. The problem is that the remaining water on Mars is ice nearer to polar latitudes. Read about the Mars phoenix mission and also about martian brine which is briefly liquid.

Paul_Wi11iams: 01/27/2017 05:03 CST

In fact I misread you, phillipwynn, but what is true for Mars may be true of the Moon. Polar hydrogen has been detected, indirectly, suggesting water ice has deposited from gas in dark craters, but the presence of available water is less certain on the moon than on Mars where the detection is direct via landers. For any more questions, better qualified people than me could reply on http://www.astronomyforum.net or similar.

Stephen: 01/29/2017 02:20 CST

@Messy: “As to the ISS not existing 30 years ago. In a way, it DID. It as called Mir, and before that it was Called Salyut and Skylab. ” FYI, Mir & Salyut were Soviet space stations, not NASA ones. As for Skylab, it preceded 1987. (It was launched in 1973 and crashed back to Earth in 1979. BTW, I was responding to @Outer Space Colonization’s claim that “Nasa had 30 years between 1987 and 2017 but they have largely the same or lesser capabilities.” What happened prior to 1987 wasn’t part of that debate. In making that statement @Outer Space Colonization either knew nothing about what NASA had done over that period or didn’t much care. @Messy: “30 years ago we had already sent two probes to Saturn. Voyagers 1 and 2, remember?...actually we had sent THREE (I forgot Pioneer 11). ” What’s your point? The Voyagers and Pioneer 11 were flybys. The Cassini probe is an orbiter. There’s a difference, even if only in the capability sense I was referencing in my response to @Outer Space Colonization.. Again what NASA did prior to 1987 was irrelevant to that response.

Karen: 01/29/2017 06:21 CST

"There's a more practical reason some people prefer the Moon over Mars: it's easier to get there." So, in short: "We choose to go to the Moon, rather than Mars, not because it is hard, but because it is easy"? Next up: "Don't ask what you can do for your country; ask what your country can do for you."

Rob: 01/29/2017 12:17 CST

"Helium-3 could be converted into nuclear energy." This is unfortunately a pipe dream. As of yet there is no realistic way to use Helium-3 for commercial use to produce a reliable energy supply and there is no real estimate on how long it will be for this to happen, if ever.

Karen: 01/29/2017 12:30 CST

Indeed, @Rob. Lunar helium-3 is a solution looking for a problem that we would consider ourselves lucky to have actually exist. Whenever I hear anyone advocate for the moon for "helium mining", it really comes across as them being desperate for some reason, any reason, to set up a base there. There may well be good reasons to go back to the moon. That is not one. And it does a disservice to the side of lunar advocacy to bring it up. 1) Lunar helium 3 is still rare there, just not as rare as on Earth. And it's diluted into orders of magnitude more helium 4. All in turn embedded into rock at low quantities. 2) We don't even have D-T fusion, which is *vastly* easier. Until D-T fusion is common and economically competitive, 3He-p or 3He-3He fusion isn't even worth talking about. 3) If you wanted aneutronic fusion, 11B-p is even better. Abundant fuels, less side reactions, and if you're going crazy-high barrier anyway.... 4) We can, and do, make 3He on Earth; it's a product of tritium decay, tritium coming from 6 or 7Li + n. For a while it was such a non-in-demand isotope that it was actively being converted to tritium. Now, if you go totally aneutronic, and you don't have any neutron sources *anywhere*, then sure, you're not making tritium. But that's a world that's never going to happen. And even in that case, it still makes up a about 1,4 ppm of the helium in the atmosphere.

Will: 01/29/2017 07:43 CST

I really hope we can go to the moon (again) and mars and at least establish a permanent base on both surfaces. I want to be able to explore farther than Mars and see what else is out there. I even hope that someday I can visit Mars. We've got a lot of work to do!

Lenore: 02/06/2017 02:14 CST

Might it not make more sense to focus money, technology on unmanned exploration of say Saturn and Jupiter moons, and Mars? Technology, research instruments must have come a long way since Curiosity was planned and launched. Might we not get more answers without humans expensive complexities in a more timely way?

Zane: 02/06/2017 09:18 CST

I think that NASA should concertrate on the lunar facility's, and allow SpaceX to spearhead Mars. If we are going to get anywhere, there needs to be delegation and team work. Lunar operations are ideal for budget restricted agency's and cuts down on all aspects of costs. Once NASA is back on the moon, and SpaceX has a good operation on MARS then we can concentrate on what's after Mars, and how both NASA and SpaceX can get us there.

John S.: 02/07/2017 01:51 CST

Largely overlooked in this discussion are human missions to asteroids. Some Near-Earth asteroids have been identified which are energetically easier to reach than Mars - or even the Moon's surface. RT travel times are on the order of hundreds of days, not the 2+ year durations of Mars missions. And they present none of the surface-landing requirements that either the Moon or Mars represent; it's more like you dock with them. NASA's ARM mission is a waste. It was originally to be such a mission, then it was descoped to an asteroid captured into lunar orbit, and descoped again to a boulder returned from an asteroid placed in lunar orbit. Missions to NEO's may have broad public support, because they have never been done, and people are aware of the dangers of asteroid impacts.

Ian Miller: 02/07/2017 03:32 CST

While I would really love to see a mission to Mars, if only because I have an alternative theory of planetary formation and I make specific predictions,including what drove the rivers, I don't think it makes sense until we get away from chemical rockets. The Moon does have mining possibilities, such as rare earths from KREEP, so until a better motor, maybe the Moon.

Zane: 02/07/2017 07:41 CST

@John S. That's because this discussion was about the advantages and disadvantages of the Moon VS Mars.

DougSpace: 02/13/2017 02:17 CST

> how such a business model might work at the Moon—beyond providing services to NASA and other international agencies—is unclear. The LCROSS mission unequivocally demonstrated lunar polar ice in concentrations of 5.6% (one part per 18). Lunar propellant triples the amount of payload delivered to the lunar surface for each launch. So that turns the capability of Falcon Heavy to greater than the SLS Block 2 (for the Moon). This means lunar missions would be in the low hundreds of millions with each seat in the mid tens of millions. Plenty of nations could afford seats at that price and they would love to watch their national astronauts conducting their own "Apollo Program" by doing suborbital hops around the Moon using propellant that companies harvest on the Moon. So what we have here is a business case for lunar propellant about 15 times more than the demand for propellant in support of missions to Mars. So, the Moon is a legitimate destination in its own right an not just a stepping stone to Mars.

eltodesukane: 02/16/2017 10:11 CST

Is there any need or use for men on the Moon or men on Mars? This should be the first question to ask. Consider that such endeavors will be very very costly. Would it be worth it? Costs vs benefits? What else will not be funded because of such missions? What else was lost because of the costly Space Shuttle program? What else was lost because of the costly International Space Station?

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