# Why No Space Stations with Artificial (centrifugal) Gravity? **URL:** \ **Category:** Factual Questions\ **Created:** [December 22, 2014, 9:51pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956 "2014-12-22T21:51:49Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![Peaton\_of\_peat](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@Peaton\_of\_peat](https://boards.straightdope.com/u/Peaton_of_peat)\ **Post date:** [January 6, 2015, 2:06am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/61 "2015-01-06T02:06:32Z")
There’s also a more prosaic reason than any of this engineering stuff. This sounds trivial, but generally speaking the answer to the question “how come there’s no X in space yet” is “because nobody’s put one up yet.” The world only has so much payload capacity coming from a limited number of spaceports, and governments generally have first dibs on those launches. I don’t know how it works in Russia and China, but in the United States most of NASA’s budget is directly earmarked by Congress for reasons other than scientific value. So in short, the reason we don’t have a centrifugal space station is because it’s not a priority for anybody in Congress. I really wish the answer was more interesting, myself. ☹ ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [January 6, 2015, 2:29am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/62 "2015-01-06T02:29:54Z")
> [@davidm](#): > > Hey, why even bother to reproduce. We can just build robots to run everything in the future. > > Heck, if we had had robots back in the 15th century, Europeans wouldn’t have had to bother coming to the Americas. It was a dangerous trip, after all. And now we get the inevitable hyperbole which is advanced any time the discussion of why remotely operated (robotic) missions are and will likely remain a vastly better choice in terms of science value per dollar for the foreseeable future. There are a number of reasons that uncrewed (or more properly, remotely crewed) missions are preferable aside from essentially eliminating the hazard to personnel, but the biggest is not having to pay for the trip to return a human crew from the destination back to Earth. The cost of this isn’t just extra supplies and the necessary thermal protection for a crew-sized reentry vehicle, or even additional redundancy in order to achieve required reliability over the full mission duration including return; everything about a crewed mission has to be larger, better protected, be more robust, operate with more narrow environmental constraints, capable of landing a much larger payload (or many separate large payloads), and function around the needs of the human crew that are functioning primarily as cargo for most of the trip. The cost of this is not minor; the minimal cost for a quick, flags & footprints opposition class mission with a crew of three or four remaining at Mars for ~30 days (total mission duration ~640 days) is in excess of US$200B. A more useful conjunction-class mission with a crew of six at Mars for ~18 months (total mission duration ~910 days) is in excess of US$500B. These are based on existing costing models that in no way pad the costs, and given the history of inflation of large aerospace projects like the STS and the ISS, multiplying the estimate by a factor of 2 or 3 is probably a reasonable guess to the final costs. By contrast, the Mars Science Laboratory was executed for a budget of ~US$2.5B, expected to operate for at least planned duration of 23 months, and will not have to be returned at end of mission. The scientific value per dollar spent is enormous, and even though the rover is only covering a small area, it is more than we could expect for a opposition-class mission to cover. If we lose the rover to sand damage, or an electrical fault on the main buss, or any other issue before planned mission life the mission would still be considered a resounding success. For the cost of a single conjunction-class mission, we could pepper the surface of Mars with two hundred Curiosity-type rovers (likely more if they were actually built en masse and launched at a high rate due to economies of scale) which are well equipped with a variety of instruments which are more perceptive and capable of a wider variety of measurements than any astronaut. Remote probes can also be sent to explore environments that no human astronaut could withstand or are too remote to realistically visit and return from, including the corrosive high pressure environment of the surface and lower atmosphere of Venus, the Gallelilan moons of Jupiter that are awash with lethal radiation, or the frigid extents of the Kuiper belt and Oort cloud that would take years to reach. Even bodies that we can reach with extant or practicable technologies, e.g. Luna or Mars, may pose a substantial hazard to human habitation, to wit, the fine charged regolith of Earth’s Moon and the abrasive silicate scale of Mars that has caused erosion problems with Curiosity’s wheels. We are so used to this world, which seems so perfectly adapted to us (although it is really the other way around) that it is almost impossible to conceive the almost immediate and pervasive hazard of extraterrestrial environments. Neither Mars, nor any other celestial body in the solar system, is comparable to the New World, which far from being challenging offered an ease and wealth of resources for ship crews exhausted by the transit to replenish themselves, especially after the natives fell to disease or were slaughtered. Mars is likely the most hospitable body in our solar system, and it is (as far as we have been able to tell) a lifeless body with no nitrates to support Earth flora, and just enough atmosphere to be problematic to land upon and carry dust storms without providing sufficient protection against radiation, containing a useful amount of oxygen, or allowing low speed flight. With all that being said, robots and automatons are not going to replace humans any time soon except in science fiction movies. While machine intelligence is improving incrementally in being able to operate heuristically with less rigorously defined rules and guidelines, nobody working in the machine intelligence field expects computers to become sentient or display the kind of creative problem solving and holistically integrative insight that human scientists and engineers can. In other words, the robots still require people to command them, and exist as tools to serve those people who interpret the data that is returned. It may be more effective–once habitation, propulsion, and in-situ resource utilization technologies improve–to send a human crew to accompany robot probes and landers in order to have near-real time command of operations and be able to inspect samples in greater detail than the equipment carried by a rover. I expect future space exploration to combine robotic and human elements almost seamlessly, to the point that the probes are just considered another tool like a rock pick or a microscope. But we won’t get to that point by closing eyes to the inherent hazards of space or engaging on showy and outrageously expensive destination-oriented programs which develop technologies only just to the point of barely being able to plant a flag and do some minimal research. We need to be able to use resources in space rather than pulling them up, laboriously and with great risk, from Earth’s surface, and be able to maintain a self-sustaining human presence in space using those resources before we can seriously engage in crewed interplanetary exploration. > [@Mangetout](#): > > Your citation says _a little bit_ about being a staging base for interplanetary missions, and _a whole heaping lot_ about research in a microgravity environment (and not just research about human survival in that environment. The ISS is a terrible platform as a “staging base of interplanetary missions”; the high inclination of the ISS orbit (necessary to allow access by Russian Proton and Soyuz launch vehicles from Baikonur Cosmodrome. (The Vostochny Cosmodrome, scheduled to start operations in the next few years, doesn’t improve this at all, and may even reduce payload capability to the ISS orbit.) The ISS also has no facilities to support interplanetary missions, and will be well beyond end of life by the time any nation or interest is ready to initiate an crewed interplanetary mission to Mars. There is a certain value to having a staging point for a multi-element mission to Mars or other destinations (which will almost certainly be required for a crewed mission) but there is no strong need for a permanent orbiting platform versus integrating around elements of the eventual spacecraft unless there are plans for regular crew or resupply missions, e.g. the Mars cycler trajectories. In short, don’t uncritically accept claims that the ISS is designed to or will directly support interplanetary missions as any kind of staging platform or depot. Stranger ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [January 6, 2015, 2:52am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/63 "2015-01-06T02:52:00Z")
> [@Stranger\_On\_A\_Train](#): > > These are based on existing costing models that in no way pad the costs NASA’s NAFCOM model [predicted Falcon 9 1.0 development costs](http://www.nasa.gov/pdf/586023main_8-3-11_NAFCOM.pdf) of $3.6 B using a standard cost-plus “NASA approach”. Actual development costs (again, F9 1.0 only, not 1.1 or Dragon, though including Falcon 1) [were $390 M](http://www.spaceref.com/news/viewsr.html?pid=38858). That’s not to say that SpaceX or anybody else can magically cut costs by an order of magnitude on any old project. Or that NASA itself would be capable of running such a low-cost program given their political exposure. Just trying to illustrate the hazards of depending solely on NASA cost models. ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [January 6, 2015, 3:50am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/64 "2015-01-06T03:50:57Z")
> [@Mangetout](#): > > Your citation says _a little bit_ about being a staging base for interplanetary missions, and _a whole heaping lot_ about research in a microgravity environment (and not just research about human survival in that environment. I didn’t notice anything about it being a staging base for much of anything but occasional space walks or releasing nano satellites. I was thinking more of things like “studying the Earth’s climate.” Does that require weightlessness? I don’t know. Maybe some of the instruments do, but it seems like being in orbit and having that vantage point is more important than experiencing microgravity. Telecommunications satellites don’t really need to experience microgravity, but being in orbit, obviously they have to function in microgravity, free fall, weightlessness or whatever we want to call it. They also have to deal with cold and heat and radiation. We don’t go to space to experience those things. But sure, if we figure out how to produce really valuable pharmaceuticals and advanced materials in zero G then yeah, that pretty much requires zero G. ---
**Author:** ![davidm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidm/32/225_2.png) [@davidm](https://boards.straightdope.com/u/davidm)\ **Post date:** [January 6, 2015, 11:44am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/65 "2015-01-06T11:44:05Z")
Damn **Stranger** , I was just having a little fun following in the theme of the previous two posts by **eburacum45** and **EdwardLost** that humans are basically their own reason for existence and that _eventually_ we’ll want to move out beyond the Earth as more than just a robotic presence. I don’t think it deserved the “inevitable hyperbole” remark followed by that TLDR wall of text. I understand that it’s dangerous and that we need to move slowly. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [January 7, 2015, 7:12am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/66 "2015-01-07T07:12:54Z")
> [@Stranger\_On\_A\_Train](#): > > and just enough atmosphere to be problematic to land upon I’ve heard this sentiment from you and other writers on the subject, and I’m calling bullshit. There’s no question that EDL (Entry, Descent, and Landing for the peanut gallery) is harder for Mars than Earth, given our rich, creamy atmosphere. But that’s only half the claim you’re making–the other half is that Mars’ tenuous atmosphere is actually worse than _no atmosphere at all_. It’s clearly wrong. From a Mars transfer orbit to Martian soil is a delta V of about 6.4 km/s. That’s 2.3 km/s to Low Mars Orbit and another 4.1 km/s to the surface. If there were no atmosphere at all, that means no aerocapture and no aerobraking. Every single m/s of that has to come from a rocket, and 6.4 km/s is not small potatoes. If your engines are 300 s Isp with storable propellants, you need a mass ratio of 9. That does not leave a lot of room for payload, and that’s without any extra margin. In contrast, the MSL spacecraft that delivered _Curiosity_ weighed 3,900 kg and landed a 900 kg rover. Nearly 25% of the original mass was delivered to the surface. And there’s no reason to believe that’s anywhere close to optimal–being a science lab, the _Curiosity_ descent operated under a number of constraints (such as limiting the surface contamination from the rocket exhaust) that probably would not apply to cargo drops to Mars. The _Red Dragon_ proposal demonstrated that you can deliver a capsule with a relatively high ballistic coefficient (i.e., no parachutes or inflatables or the like) to the surface with a modest delta V budget for landing rockets–on the order of 600 m/s. Larger systems with even higher ballistic coefficients require more delta V, but in no case would you come close to the 6.4 km/s required if Mars were simply a vacuum. The entry profile is hair-raising but so is pretty much everything about EDL. We can also contrast with the Descent Propulsion System used on the Apollo landings. It had 2.5 km/s of delta V despite the relatively piddly gravity of the Moon. The lack of atmosphere _allowed_ the LM to be a tin can, but it also _required_ it. They had no choice but to spend that mass on fuel. Mars is undoubtedly difficult. But the implication that its atmosphere is worse than useless is clearly not the case. We would have little hope of any significant exploration of the surface, robotic or not, if it didn’t exist. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 7, 2015, 3:15pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/67 "2015-01-07T15:15:45Z")
Mainly because centrifugal force is just that, it isn’t gravity. If you were not permanently attached to the CF, I.E. try running… you will become detached from the CF and become weightless. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 7, 2015, 3:29pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/68 "2015-01-07T15:29:27Z")
> [@vomit\_comet](#): > > Mainly because centrifugal force is just that, it isn’t gravity. > > If you were not permanently attached to the CF, I.E. try running… you will become detached from the CF and become weightless. Not true. Your body would try to keep moving along the last direction it had when you push off the “ground” and the effect would be the same as if gravity were present. This is why astronaut David Bowman was shown jogging in the centrifuge aboard the Jupiter spacecraft in _2001: A space Odyssey_. If what you are saying were true, he couldn’t do that. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [January 7, 2015, 3:30pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/69 "2015-01-07T15:30:39Z")
Before you start casually tossing around your “bullshits” and making accusations of ineptitude you may want to educate yourself further. First of all, what I said was, “[Mars has] just enough atmosphere to be problematic to land upon and carry dust storms without providing sufficient protection against radiation, containing a useful amount of oxygen, or allowing low speed flight,” all of which are true. Yes, that Mars has an atmosphere that can be used for aerocapture, and that has figured into the EDL profile for every Mars Design Reference Mission from 1.0 onward. That doesn’t negate the fact that during descent the spacecraft will have a very high Q (dynamic pressure) but low drag forces for a given profile, resulting in a longer entry duration and considerably more aeroheating than a comparable craft would experience in Earth reentry. This means that either the beta must be really high, necessitating the use of very large deployable ringsail conical decelerators (the more recent proposals have two or even three stages of decelerators), supersonic retropropulsion, or both, and really beefy thermal protection systems to protect a 40+ ton capsule, not to mention protection from ablation of suspended dust if the capsule is forced to enter through one of the months long Martian dust storms. The MSL is the largest payload delivered to Mars and is at the mass limit of what conventional EDL systems can deliver to Mars. SpaceX hasn’t “demonstrated” anything; they’ve developed a conceptual design for a propulsive reentry system which has no basis in prior art and for which even scale models have not been demonstrated, so to argue that such a design can go from the drawing board to reality without substantial development and testing (in Earth’s upper atmosphere and likely on Mars as well before it would be sufficiently mature for a crewed landing) is the height of absurdity. Demonstration means that someone has actually built and operated the technology in an operational-like manner. Supersonic retropropulsion technology is nascent to date it has been demonstrated only using cold gas jets in supersonic wind tunnels, and this largely to provide baselines for validating CFD models. The AIAA _Journal of Spacecraft and Rockets_ had a “Special Section” on supersonic retropropulsion in the May-June 2014 issue (Vol 51, No 3) which goes into extensive detail on the challenges, including the difficulty predicting stability conditions because of the interaction between the rocket plumes disrupting the supersonic boundary layer. This is never a problem for forward propulsion or subsonic retropropulsion so we have almost no actual experience in flying vehicles in this fashion. The NASA Technical Reports Server has literally hundreds of papers on the difficulties of Mars EDL modes with capsules large enough for crewed use. I’ve personally worked on two different crewed Mars mission proposals and can attest to how what are initially seemingly small difficulties (in the EDL profiles and elsewhere) can become magnified when attempting to demonstrate the reliability numbers necessary for a realistic crewed mission. Next time you want to spout “bullshit”, please do more than uncritically regurgitate press releases and unvalidated claims. Stranger ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [January 7, 2015, 3:39pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/70 "2015-01-07T15:39:16Z")
> [@vomit\_comet](#): > > Mainly because centrifugal force is just that, it isn’t gravity. > > If you were not permanently attached to the CF, I.E. try running… you will become detached from the CF and become weightless. Ermm… Is this true? Your body still has mass, momentum and inertia, as well as the atmosphere surrounding you. So even though jumping from the deck might land you in a weird spot if compared to the same jump on earth (since it’s angular momentum forcing you to the floor), I don’t think jumping would cause you to go pinwheeling uncontrollably about the station. ETA: ahh, CalMeacham’s post wasn’t there when I hit reply. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 7, 2015, 4:00pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/71 "2015-01-07T16:00:58Z")
> [@CalMeacham](#): > > Not true. Your body would try to keep moving along the last direction it had when you push off the “ground” and the effect would be the same as if gravity were present. This is why astronaut David Bowman was shown jogging in the centrifuge aboard the Jupiter spacecraft in _2001: A space Odyssey_. If what you are saying were true, he couldn’t do that. Don’t believe everything you see in movies. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 7, 2015, 4:08pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/72 "2015-01-07T16:08:11Z")
There is no effect from gravity because gravity does not exist. We are talking about inertia, plain and simple. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [January 7, 2015, 4:11pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/73 "2015-01-07T16:11:37Z")
Are you ok with NASA footage of Pete Conrad running inside Skylab? [![](https://img.youtube.com/vi/Awe6vOXURpY/hqdefault.jpg "Skylab") ](https://www.youtube.com/watch?v=Awe6vOXURpY) ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [January 7, 2015, 4:18pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/74 "2015-01-07T16:18:20Z")
Actually - let me retract that. Skylab didn’t rotate but good lord was it huge inside. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 7, 2015, 4:25pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/75 "2015-01-07T16:25:28Z")
[quote=“Grey, post:73, topic:707956”] Are you ok with NASA footage of Pete Conrad running inside Skylab? [/QUOTE] Doesn’t change the fact that gravity does not exist. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 7, 2015, 4:27pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/76 "2015-01-07T16:27:08Z")
If you run counter to the rotation of the spacecraft, and if you can run fast enough, you could become weightless. From the inertial point of view, you’d just be floating there motionless. Now, eventually, a wall is going to come along and smack into you and bring you back into the reference frame of the station, or lacking that, eventually air resistance will do the job. Alternately, you could run the other way, and increase your weight. Inconvenient if you’re trying to get somewhere else in the station in a hurry, but it might make for good exercise. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [January 7, 2015, 4:39pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/77 "2015-01-07T16:39:46Z")
> [@vomit\_comet](#): > > Mainly because centrifugal force is just that, it isn’t gravity. > > If you were not permanently attached to the CF, I.E. try running… you will become detached from the CF and become weightless. This is false – with every step, you will re-“engage”, so to speak, with the spinning surface, and re-acquire the outward force that serves as pseudo-gravity. Just like we’re all spinning with Earth. Earth’s gravity holds us down, but the reason we can jump and come down in approximately the same place is that we have the same rotational speed as the Earth, and “re-acquire” it each time we come into contact with the surface (or something on the surface). ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 7, 2015, 6:27pm UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/78 "2015-01-07T18:27:28Z")
> [@vomit\_comet](#): > > Don’t believe everything you see in movies. Yeah, but they verified it before they filmed it. The effect of being in that rotating centrifuge is, to lowest order, like being in a gravitational field. If you don’t think so, try getting into one of those rapidly rotating drum rides at the amusement parks. If you lift something away from the surface, it doesn’t act as if it can simply fall down – it “falls” to the rotating wall. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [January 8, 2015, 12:23am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/79 "2015-01-08T00:23:38Z")
> [@Stranger\_On\_A\_Train](#): > > making accusations of ineptitude I have never done this and I never will, especially not in GQ. I dispute claims, not make ad hominem attacks. > [@Stranger\_On\_A\_Train](#): > > First of all, what I said was, “[Mars has] just enough atmosphere to be problematic to land upon and carry dust storms without providing sufficient protection against radiation, containing a useful amount of oxygen, or allowing low speed flight,” I cropped out the rest of the sentence because I do not dispute the claims past the first one. And for what it’s worth, I also don’t think the remainder of your post is wholly dependent on this particular claim, though I do think it slightly weakens it. Mars has enough atmosphere to make it _possible_ to land heavy equipment on. No EDL is easy, whether on Earth, Mars, or the moon, and the Martian atmosphere is “problematic” only in the sense that one has to design with it in mind. > [@Stranger\_On\_A\_Train](#): > > resulting in a longer entry duration and considerably more aeroheating than a comparable craft would experience in Earth reentry. Perhaps you could read my original post again, where I acknowledged that Mars is harder than Earth. But what you said was “[Mars has] just enough atmosphere to be problematic to land upon”, implying not just that the Martian atmosphere is harder to deal with Earth’s, but more difficult than if there were no atmosphere at all. It is this latter implication that I strongly dispute. > [@Stranger\_On\_A\_Train](#): > > SpaceX hasn’t “demonstrated” anything; they’ve developed a conceptual design for a propulsive reentry system which has no basis in prior art and for which even scale models have not been demonstrated It was not a SpaceX proposal that I was referring to but a NASA study for a sample-return mission using a Dragon-like capsule. Yes, it was a paper study (with computer simulations). The heart of the study revolved around the aerobraking and use of the capsule as a lifting body (using an offset center of mass to change angle of attack). Although there’s always more to be learned, this stuff has been studied for a long time. > [@Stranger\_On\_A\_Train](#): > > Supersonic retropropulsion technology is nascent to date it has been demonstrated only using cold gas jets in supersonic wind tunnels, and this largely to provide baselines for validating CFD models. This is false. Supersonic retropropulsion has been demonstrated at least three times so far in flight. SpaceX’s first-stage reusability scheme uses three burns, the second one of which was at mach 4 or so, and each one was successful. NASA sent one of their planes with an infrared camera to see how it went. There’s a [pretty video](https://www.youtube.com/watch?v=_UFjK_CFKgA) if you’d like to watch. > [@Stranger\_On\_A\_Train](#): > > including the difficulty predicting stability conditions because of the interaction between the rocket plumes disrupting the supersonic boundary layer. The _Red Dragon_ presentation talked about this. It is absolutely the case that at low thrust levels, interaction of the plume with the boundary layer is very complicated. This presents a challenge for weak maneuvering thrusters and if you wish to do anything really clever, like using the plume to “inflate” the boundary layer to present a larger effective aerodynamic surface. But this complication pretty much goes away at high thrust levels–the kind that you would have in a scheme relying on landing rockets. The layman way of putting it is that the plume “punches through” the boundary layer. Reality is obviously more complicated but the basic idea is true. And SpaceX already demonstrated that they can get it right both on the first try and three times in a row. Finally, I’d like to reiterate the crux of my argument in the previous post. These advanced proposals are interesting but even if they prove to be unfeasible, they do not invalidate the point that we’d be immeasurably worse off if that 6.4 km/s had to be rid via rocket thrust alone, as it would be if Mars were a vacuum. The laws of physics dictate that the useful payload to the surface would be vastly worse than what NASA is _already achieving_ today. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 8, 2015, 2:16am UTC](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956/80 "2015-01-08T02:16:16Z")
> [@cmyk](#): > > Ermm… Is this true? > > Your body still has mass, momentum and inertia, as well as the atmosphere surrounding you. So even though jumping from the deck might land you in a weird spot if compared to the same jump on earth (since it’s angular momentum forcing you to the floor), I don’t think jumping would cause you to go pinwheeling uncontrollably about the station. > > ETA: ahh, CalMeacham’s post wasn’t there when I hit reply. The direction of angular momentum is at right angle to the surface of the rotating circular ‘sidewalk’ going outward from the centre of the rotating mass. When you run, one foot is in the air, you bound off ‘sidewalk’ with the other foot, in an opposite direction of the angular momentum of the rotating mass; with both feet not attached to the sidewalk that is inducing the angular momentum on your physical body, you would go off into space toward the centre of the rotating mass. [Previous page](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956.md?page=3) [Next page](https://boards.straightdope.com/t/why-no-space-stations-with-artificial-centrifugal-gravity/707956.md?page=5)