Remember last August, when like half the freakin' world was following the final nerve-racking hours of the Mars Science Laboratory's journey to the red planet? It was one of the most exciting days in the history of Mars exploration, also one of the most exciting days in the history of me being a person. And, success! The Curiosity rover is still roving in Gale Crater as we speak, studying the planet with its fascinating array of tools, including the Mast Camera and X-ray Spectrometer. We're going to learn a lot about Mars from Curiosity, and hopefully some of the data it sends back will help us prepare for future manned expeditions.

But what if I told you that the MSL has already given us some of the most important information we need, at least in terms of sending humans to Mars while it was on it's way there? And what if I told you that data is not very encouraging? (sigh)

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You might remember that we've talked before about some of the big-picture questions that come up when we start thinking about sending people to Mars. But we love ourselves some Mars exploration, and since then there have been some fascinating developments that may shed light on the feasibility of humans ever getting to the red planet.

There are a number of puzzles we need to solve in order to safely and efficiently get there, and one of the trickiest is radiation. While it will certainly be an issue once humans are on Mars, the larger problem is the huge amounts of radiation that astronauts will be exposed to en route.

We've learned a lot about this in the last year, thanks to Curiosity which is equipped with a radiation assessment detector, or RAD. Engineers build the toaster-sized device and its sensors with the idea of using them on the Martian surface. But some scientists wisely suggested that if they turned on the instrument during the MSL's 253-day, 563 million kilometer voyage to Mars, it would help NASA learn what radiation levels humans might be exposed to on the trip. And given that the shielding used to protect the MSL is similar to what might be used on a future manned spacecraft, the data could not be more relevant. 

Radiation doses are measured in units called sieverts and the results from the RAD published in the journal Science in May of 2013 indicate that astronauts would be exposed to roughly two thirds of a sievert over the course of a round-trip excursion to Mars. That is a lot.

On Earth, humans receive about a thousandth of a sievert per year from outer space and potentially a few more thousandths from x-rays and CT scans and other man-made sources. Sensors on MSL's radiation detector-- which, keep in mind, were protected on the way there-- counted the number of energetic particles like protons that hit the instrument, revealing an average dose of 1.84 millisieverts per day. You would have to get yourself a full-body CT scan at a hospital every five days to expose yourself to that level of radiation on Earth. And according to the Nation Cancer Institute, those 12-16 months spent traveling in space would raise the astronauts' chance of dying from cancer from 21 to 24 percent. That's not insignificant. It also guarantees that a voyage to Mars, including time spent on the planet, would exceed NASA's current guidelines for exposure which ensure that an astronaut's cancer risk will not increase by more than three percent. 

Radiation can do a lot of bad things to a body, both in short- and long-terms. Not only can large doses increase your cancer risk, but it can lessen your resistance to infections, cause short term memory loss, and blindness, and increased risk of heart disease. And these are problems that could start showing up while the mission is still en route.

Even worse would be an unforeseen cosmic event, like a radiation burst from the sun or deep space, that could expose astronauts to much larger doses, resulting in Acute Radiation Syndrome which can quickly result in organ shutdown and an incredibly horrible and painful death. Obviously, pretty much the last thing on the list of things we want happening to astronauts that are 300 million kilometers from Earth. 

So, how do we attack this problem? Because we are going to attack it. Well, there are two types of radiation that we have to address. The first are called solar energetic particles, which are emitted by the sun during huge explosions on its surface. You know these events as solar flares or coronal mass ejections. Episodes like these happen fairly randomly, though they are less frequent at quiet times during the sun's eleven-year cycle. The MSL traveled to Mars during one of these quiet times, but it still recorded five solar-particle events.

More worrisome are galactic cosmic rays, or GCRs, which originate outside the Solar System, usually from exploded stars or the vicinity of black holes. It's actually easier for them to penetrate the solar system when solar activity is low. More unfortunate is the fact that GCRs can also penetrate deep into human tissues and damage our DNA. And shielding a spacecraft from them is not easy. Even a 30 centimeter-thick aluminum hull would do little to affect astronauts' GCR exposure. And scientists have found that piling on additional layers of aluminum, polyethylene, or even containers full of water would reduce exposure to solar radiation, but not GCRs.

On the Martian surface, radiation exposure is somewhat less of a concern. Data from Curiosity have revealed that astronauts would be exposed to about 0.7 millisieverts per day. That's still not ideal, but it's about what crew members of the International Space Station are exposed to each day. So NASA engineers are naturally trying to develop more effective radiation shielding systems for future Mars spacecraft; they pretty much have to.

But the most effective way to limit radiation exposure on the way to Mars would be to build a vehicle that can get there faster. Which leads us to a second challenge: propulsion. Even if we stick with the conventional methods we've used since the Apollo program--chemical rocket engines propelled by liquid hydrogen and oxygen-- some enormous hurdles remain. Mainly, how do you store that much fuel? In all, about 90% of the initial weight of a conventional spacecraft would be fuel, and fuel is heavy and therefore expensive in terms of both money and energy to move.

Making matters even less efficient, mission planners expect to lose a lot of the fuel along the way. The extreme temperature changes that spacecraft experience --particularly when in orbit around a planet-- cause some of the rocket fuel to vaporize. Plus, hydrogen is know for its propensity to leak, often at rate up to 4% per month.

So what about unconventional methods? Particularly ones that don't involve words like "warp" and "drive." Can I possibly interest you in some solar-electric or nuclear-electric propulsion? The first isn't even theoretical. We have spacecraft currently exploring the solar system with solar-powered electric engines.

Solar-electric propulsion, or SEP, generates electric power from solar arrays, which is used to convert neutral atoms of fuel --usually the heavy noble gas xenon-- into charged atoms, or ions. These xenon ions are then accelerated by magnets to give the engine thrust. NASA's Dawn spacecraft --which launched in 2007 on a mission to to study Ceres and Vesta, the two largest objects in the asteroid belt-- uses an ion propulsion system just like this powered by large solar panels. SEP spacecraft engines offer long-lasting thrust, but the downside of that efficiency is a lack of power. Using current SEP technology, it would probably take several years to reach Mars in a manned spacecraft; and, as we just talked about, we don't have that kind of time to be floating around up there.

Nuclear powered electric engines are an intriguing option, but the technology isn't really ready yet. Conducting nuclear fission requires a lot of heavy equipment; it also requires hydrogen fuel, which --as we've learned-- is not easy to store in the long-term.

So what about a hybrid of some kind? Using elements of traditional chemical propulsion and SEP is an option, though it remains unclear just how much it would speed up a trip to Mars. The idea would be to use chemical rockets to boost the craft out of low Earth orbit where such a craft would have to be built, and then use an electric propulsion system for the remainder of the trip. If we go that route, we're still going to need larger and more powerful solar arrays than currently exist in order to speed up the trip and lessen all that exposure to radiation. 

All this, and we haven't even gotten to actually exploring Mars yet. I could do an entire episode on the Martian dust situation, which is going to be really bad for engines, tools, and humans.

So, it's hard for me to be very optimistic about our near-term prospects for a Mars mission, but there are always private groups that have their eyes on the red planet, though their plans are a little far-fetched. We talked about the Mars Society just last year, but others keep popping up. One is called the Inspiration Mars Foundation whose goal is to send a two-person American crew to fly within 100 miles of Mars and then return to Earth. Their planned launch date is January 5, 2018, which is when the closest alignment of the planets will allow for a round-trip duration of just 501 days. The non-profit plans to use conventional technologies for the trip, though there are few details about how this will all be funded and carried out in less than five years.

Even more optimistic is another non-profit called Mars One which hopes to establish a four-person Mars Colony in 2023 with a budget of six billion dollars. You may have heard of these guys because A) their plan does not involve humans ever returning to Earth, and B) they recently began taking applications for potential crew members, and more than 76 thousand people have applied. Their plan includes launching a series of unmanned supply missions beginning in 2016 to reach Mars before the people who need them will be there. It also entails funding a lot of the mission by selling broadcast rights for different milestones along the way. 

I'll be honest; I don't think we're going to have humans on Mars in less than a decade. A fly-by mission is far more likely, though maybe not in five years. And while I still wonder if even NASA's stated goal of sending people to the red planet in the 2030's is realistic, I appreciate the vision and enthusiasm of these non-profits. We want people to remain excited about Mars exploration, and it says something about the passion we have as a species to explore the universe that we've finally begun to talk about astronauts --possibly citizen astronauts-- making the ultimate sacrifice and never coming home just for a chance to touch the surface of another world. 

So, what do you think? Would it be worth it? Would you even wanna do it yourself maybe? Can you think of another way? Tell us about it on Facebook or Twitter or in the comments below. And if you want to keep getting smarter with us here at SciShow, go to youtube.com/SciShow, and subscribe.