Washington Post article on how astronauts eyesight goes bad in zero-G. In 2005, astronaut John Phillips took a break from his work on the International Space Station and looked out the window at Earth. He was about halfway through a mission that had begun in April and would end in October. When he gazed down at the planet, the Earth was blurry. He couldn’t focus on it clearly. That was strange—he’d always had 20/20 vision. He wondered: was his eyesight getting worse? “I’m not sure if I reported that to the ground,” he said. “I think I didn’t. I thought it would be something that would just go away, and fix itself when I got to Earth.” It didn’t go away. During Phillips’ post-flight physical, NASA found that his vision had gone from 20/20 to 20/100 in six months. It happens to 80% of astronauts and they don't know why. The truth is, we're not getting to Mars in a zero-G spacecraft that doesn't have radiation shielding. The transit vehicle will have to be shielded and spinning and presently we aren't even working on how to build such a craft, or studying what the required G-levels and spin rates will have to be.
It is easy to spin a spacecraft to generate appropriate G levels. Dr. Robert Zubrin has gone over it thoroughly in his Mars Direct scenarios and "The Case For Mars" book. He has also gone over the issue of shielding both in transit and on the surface of Mars.
That's great, but there's nothing even in the works to fill the requirements. I've suggested launching a re-used Falcon 9 with no payload. The current Falcon 9's, with the full thrust Merlins and super-cooled propellants, can achieve SSTO. That means one could go up as a 230 ft x 12 ft cylinder for making a spin test habitat. Things we don't really know: 1) What RPM's humans are comfortable with, and how this tolerance might vary with the G levels imparted. 2) How we react to fractional G's, and how plants and animals react. 3) The maximal fractional G load at which penguins can fly.
The problem with using spin to achieve G levels is that with a vessel 50 feet in radius (100 feet in diameter) to achieve 1G at floor level, would generate about 0.9G at head height for the average male height. Increasing to 100 feet in radius would drop the difference at floor level from 1G at floor to 0.94G at head height. A difference of 0.1G would be noticeable and probably enough to cause discomfort and possible disorientation.
I wouldn't even try it at a 50 foot radius. The RPM's would probably cause disorientation every time you turned your head. We don't know for sure because NASA avoids orbital spin tests like the plague.
Dr. Robert Zubrin calculated that once Trans Mars Injection was achieved the Habitat Module could separate from the burnt out third stage, turn around and unreel a tether 200 meters long then fire thrusters to begin spin of six RPM. This would produce the same gravity as Mars for the six month trip. disorientation with that long a tether would be negligible. You do the same thing on the way back with the Earth Return Vehicle and the burnt out stage that boosts it into orbit and then toward Earth. Problem solved. NASA makes things too complicated.
Tethers have extremely complicated dynamics. They'd have to test, test, and test some more before being confident enough for such a mission. Large hollow struts are much simpler to manage and could allow the crew full freedom of movement throughout the craft.
How are tethers complicated? You make a multistrand tether a few hundred meters long. It isn't rocket science. Well within current technology. And why would you need freedom of movement "throughout the craft"? The entire craft is the Habitat Module which is two levels and its areoshell and the burnt out upper stage of the rocket which will never be used again. And if a tether somehow breaks? Who cares? You can continue on to Mars in zero G if you have to anyway. Its only six months. Many cosmonauts and astronauts have lived for that long in zero G just fine. And once you arrive at Mars they are only subject to one third of a G from then until they begin Earth return. People make manned space exploration overly complicated. As Zubrin points out.
NASA has had very bad experiences with space tethers. You have to keep them under tension or they wander all over the place, which is a deployment issue. If you spin up a bit before deployment you have to manage constant accelerations very carefully because the spin rate of the outer loads of course wants to drop (maintaining constant velocity, not constant RPM's) while the hub wants to maintain its initial RPM. So if you start while spinning the tethers will just want to wrap up the hub. Unless you do everything very smoothly you'll set up standing waves in the tether (like a plucked string) and some nasty oscillation modes, and in space there's nothing to naturally damp those out. Then, whenever a course change is made, all three units must fire simultaneously to keep everything in coordination. A two body tether is simpler, but still presents issues with oscillations, such as will be caused by astronauts moving around. So such systems will have to be tested in orbit, and at present NASA can't even manage to launch a person. All available money is going to the SLS and Orion, with schedules running out to the 2030's and not including any real attempts at a Mars craft or precursors to one.
You assume that just because NASA ISN'T doing something that means they CAN'T do something. False assumption. And I don't see why it is difficult to fire thrusters to begin spin on the Habitation Module and the upper stage simultaneously. NASAs bad experiences with tethers are in part because they have tried to use them as well to transfer power. Something that Zubrins mission planning avoids specifically.
Been there. Done that. the associated article suggests as Ed and GT did above that it isn't viable with today's technology.
In addition, you need at least three tethers between two vehicles so the floor will stay level as astronauts move around. If you use one tether you've essentially hung a basket from a string. If you put a puppy in the basket it's never going to sit level for long, and will sway and tip all over the place. If there are fluid containers in the basket the liquids will flow to the low side, increasing the tilting problem. A fairly large diameter tube (which can resist applied torque) is so much better. Happily, large diameter tubes happen to be what we use as launch vehicles, and by adding lightweight perforated gratings they can provide a simple way to simultaneously test lots of different G levels at once, along with providing vastly more interior space for the journey, even if it is unshielded.
You're not going to launch a hundred or two hundred meter tube into space just to help with artificial gravity. That is foolish beyond belief. And the tether experiments your link refers to involves very short tethers. Not the 200 meter one that Dr. Robert Zubrin calculated. IIRC with a 200 meter tether the difference in gravity from an astronauts head to their feet would be only about 1%. Basically unnoticeable.
Launching a 70 meter (230' x 12' diameter) tube is easy. You just use a Falcon 9 with an empty upper stage and no payload. In that configuration it can get to orbit with just the first stage burning. And handily, SpaceX has recovered first stages that they're not completely confident in, such as the one they used to launch a payload to GTO. So you have a free rocket and just have to pay for the fuel, which is I think about $800,000 for a Falcon flight. Of course you'd want to modify it to better facilitate it's new role, and for a real piece of deep space hardware you'd launch it as the center core of a Falcon Heavy. But even just launching one with a docking adapter at one end would let you mate it with a Dragon and do some preliminary partial-G experiments. Connect three such rockets end to end and you have your 200 meter boom, infinitely more useful than a tether because it would have 2,200 cubic meters of internal, pressurized volume. For comparison, the ISS only has 931 cubic meters of pressurized volume. So you have a massive storage capacity for a trip to Mars and variable G levels from the axis to the ends, where you can conduct partial-G experiments on a host of plants and animals. Additionally, that internal volume provides about 200 man-days of breathable air even if the environmental control system is not working. That gives the crew plenty of time to make any necessary repairs. And it would give you an orbital platform with artificial gravity, where you could finally study the G requirements and spin limits for deep-space long-duration missions. Since NASA isn't doing anything serious in terms of deep-space manned hardware (the Orion is a non-starter), the ball is really in Elon's court. The question is how he'll proceed past Dragon flights to the ISS.
Way too much trouble for questionable benefit. And once again, if a tether failure occurs you simply continue to Mars in zero-G. Six months of zero-G doesn't exactly cripple astronauts. Especially when they are only going to a one third of a G destination.
But you still have to get back from Mars, so you're looking at years in zero-G if there's a tether failure (depending on the propulsion system, mission architecture, etc). The weight of three empty Falcon 9's, stripped of engines, would be about 33 tonnes. The weight of a Dragon capsule is about 4.3 tonnes, and is of course far too small to support such a long-duration mission, with an internal volume of only 10 cubic meters (as opposed to 2,200 cubic meters in the three Falcon tube. The Orion capsule has a habitable volume of about 9 cubic meters. One of the problems with the tether system is that you have to do your burns to head to Mars before you try to deploy the tether. So you don't know if you'll have gravity for the flight or not. That means you have to design a craft for two completely different type of long-term environments, not knowing for sure which one will apply. It also means you'd want to despin and retract the tether prior to any entry burns to drop into Mars orbit. And of course you'll want to be in zero-G to deploy the landers. With a permanently spinning tube design you can just do all the zero-G work near the axis, which also happens to be the best place for engines, tanks, and solar cells so they don't have to carry an extra structural penalty that being near 1 G would require. Zubrin tries to cut mission to the bone to make them more feasible on a limited budget. Musk drastically lower's launch costs and improves payload capability so he doesn't have to cut to the bone. His idea of a Mars transporter is something that carries a hundred passengers.
Utter nonsense. Complete tether failure BOTH ways means a total of one year in zero G. With only six months consecutive each way. And you don't reuse the tether. The vehicle for the return from Mars is entirely separate from the one that takes you to Mars. Have you even read any of Dr. Zubrin's work? And Musks idea of a Mars transporter carrying 100 people is pure fantasy for an initial exploration of Mars.
I'm familiar with some of Zubrin's work. He's very excitable and has no tolerance for anything he doesn't think furthers the mission of putting a person on Mars. And yet he includes the tether which he doesn't deem "mission critical" because he can continue to Mars without it. If you don't need the tether, why use the tether? He even allows that it may have uncorrectable dynamic modes that will require the crew to jettison it. That's just throwing a big unknown into the whole mission plan. But it seems that long-term zero-G really does cause problems. Astronauts' vision is going from 20/20 to 20/100 and the changes start happening in a few weeks. NASA is already sending up super-focus adjustable glasses to the ISS. And of course we have no data on the long term response to 0.38 G's. NASA has never bothered building an orbital facility to see how plants and animals will react to partial G. We only have two data points, 0.0 G and 1 full G. We also don't know what spin rates are tolerable. Zubrin picked 1 RPM because most doctors think we could tolerate that. But we really have no data. So we need a long-term spin lab that can impart a variety of spin rates, along with offering a variety of G levels. Someone needs to launch such a lab, and if something like my 200 meter/3 Falcon system could work, why not send it on to Mars once we're ready to go? After several years in Earth orbit for tons of scientific studies, it would no doubt be packed with all kinds of equipment that would be quite useful for supporting Mars missions, things that Zubrin wouldn't include in his mini-van approach.
Why? As Zubrin points out (in regards to study of radiation effects) we don't train bomber crews by having them fly through real flak! If we're going to endanger astronauts health with zero G or high radiation exposures why not do so while on our way to Mars anyway. 20/100 vision is far, far, far from a deal breaker when it comes to travel to Mars. Neither is a 5% increase in lifetime cancer rates. If you have qualified astronauts willing to go then simply send them and be done with it. Their experience becomes your medical research and we get astronauts on Mars in the bargain. It is well worth it.
The deal breaker on radiation exposure is that particle accelerator experiments are showing that after a few months of cosmic radiation, the astronauts' brains would start to get erased, as if they had Alzheimer's. By the time they get to Mars they might not know all the complex procedures required. They might not even know they're on a space ship. What they found in mouse studies is that the high-energy particle collisions rip dendrites apart, literally unwiring the brain. By simply doing some animal experiments outside the Van Allen belt we can find out how fast this problem develops and how severe it is. Or we could watch as a crew of heroic but foolish retards smashes into a planet.
Without some form of artificial gravity, the astronauts will be as helpless as kittens when they get to Mars, until they adjust to gravity. Much better that they are able to move about under their own power as soon as they touchdown on Mars.
Complete BS. We've had astronauts stay in space for months at a time who could play tennis the day after getting back. And on Mars they only have to adapt to one third of the gravity.
Who were those astronauts? How long were they in space? And what happens if there's an emergency within a few hours of them landing on Mars? So, it'd only take them 1/3rd as long to recover as it would on Earth. So, if it took them 3 days to recover on Earth, it'd only take them one on Mars. That's not bad, provided there's no "glitch" on their first day back that they have to solve.
Who cares? No mission to Mars is going to be risk free. Risk is part of the excitement anyway. If astronauts are willing to take the risks, send them and be done with it.
IIRC, most of the muscle weakness caused by zero G occurs in the first few months. It is bone loss that continues steadily over time. But bone loss due to zero G has never been considered a deal breaker.
Chances are good that even a bare bones Zubrin type mission to Mars and back would not result in the death or serious injury of a crew member. Not one of the Apollo astronauts suffered serious injury or death during the Apollo program and that was with mid 1960s era technology.
