Sending people to Mars, and Space more generally ----------Warren D. Smith Jan 2001-------------- President R.Reagan, and later other presidents and candidates, have told us of their plans to do a manned mission to Mars. This is a stupid, extremely expensive, dangerous, and pointless idea, with potential benefits not even close to being worth the expenditure. The manned moon missions, the manned space shuttle, and the manned space station were also bad ideas (probably in increasing order of badness?). For example unmanned moon probes by Russia got the same job done with a lot less cost. (They returned moon rocks to Earth from more places on the moon, for example. They also photographed and mapped the moon's far side and measured moon thermal flux. Etc.) We are perpetually informed of how humans walking on the moon was "a giant step for mankind" and constituted True Greatness. But: how exactly? Where did all this "greatness" take us? Unmanned rockets do a heck of a lot cheaper job of launching satellites (even if they are, as currently, "throwaway" use-once rockets such as the Ariane - this fact of economics is a tremendous indictment of the manned shuttle) although I admit that when it comes to servicing low-orbit satellites the shuttle currently wins (and is the only alternative). I suspect an unmanned version of the shuttle could have outperformed both the shuttle and Ariane - for less cost on most missions too - however. (Not to mention, NASA currently estimates the chances of catastrophic failure as 1/78 per shuttle mission. Risky. Manned space travel has historically had a risk of death of about 5%, e.g. one of the 17 Apollo launches killed its 3 astronauts, as did the 25th shuttle launch.) Another huge indictment of our space program was the very attempt to claim that it was justified by its technological "spin offs" such as nonstick pots! The International Space Station is bullshit supported by bullshit claims like "we are developing better crystals than one can grow on Earth, better drugs, etc." which suffer from a complete reality disconnect. But the Mars idea is truly appalling - probably with a cost-to-benefit ratio 100 times worse? You can get a good start to understanding how the US space program became so insane by reading Robert L. Park's excellent book "Voodoo science, the road from foolishness to fraud," Oxford Univ. Press, ISBN 0195135156. On the other side, you can learn all about the Space Zealot point of view by reading the book "The case for Mars" by Robert Zubrin (with Richard Wagner) Simon&Schuster 1996. I enjoyed both books, and have a lot of respect for both authors, despite their divergent attitudes. Indeed, Zubrin is the co-inventor (with D.Andrews) of the "magnetic sail" space propulsion concept, as well as (with D.Baker) the "Mars direct" approach to visiting (and returning from) Mars, both of which are superb and highly original ideas. (Incidentally, both Zubrin AND Park have a lot of nasty things to say about NASA...) But... nevertheless I cannot support Zubrin's views about Mars exploration. Let me quickly outline Zubrin's book. First, the "Mars direct" plan is as follows. 1. launch unmanned ERV ("Earth Return Vehicle") to Mars on a Hohmann min-energy trajectory (258-day travel time. 3.3 km/sec.) It lands on Mars with the aid of aero-braking and retrorockets. It carries 6.3 tonnes of liquid hydrogen, a 3.5 tonne nuclear reactor, and an empty cabin with life support facilities and stored food and water. After landing on Mars, a robotic truck wheels the nuke a few hundred meters away and turns it on, generating 80 kWatts of electricity. This electric power is used, over a 6-month period, to convert the stored hydrogen and CO2 extracted from the Martian atmosphere (which is 95% CO2, at about .007 Earth atmospheric pressure) to stored liquid methane (CH4) and liquid oxygen, totalling 94 tonnes in all. Plus it makes 12 tonnes of water and also manufactures oxygen (alone, without methane and without consuming hydrogen) from Martian CO2. 2. Launch manned "hab" to Mars on a free-return ellipse trajectory at 5.08 km/sec (180-day travel time to Mars). Carries crew of 4. Generates Mars simulated gravity (.38g) using a tether connected to the final stage of its launcher and centrifugal force. Upon arrival near Mars, the tether is cut and the useless launcher stage is jettisoned. The hab lands near the ERV. 3. Crew places sandbags on the Hab roof for shielding cosmic rays, explores Mars with aid of pressurized "rover" fueled by methane/oxygen. 4. Crew detaches nuke, leaves behind rover, truck, chemical plant, etc., gets in ERV, takes off for Earth using rockets fueled by 82 tonnes of the methane and oxygen. (Transit time 7 months.) 5. Zubrin did not explain what they do when they reach earth in the ERV. Aerobraking in Earth atmosphere followed by parachuting to splashdown? (But: no parachute was listed in Zubrin's inventory.) Aerobraking capture into Earth orbit followed by pickup by the Space Shuttle (requiring a 3rd launch?) It is not clear what Zubrin intended. The brilliant element of this plan is the idea of synthesizing the fuel needed for the return trip (as well as the water and oxygen used by the crew) ON MARS primarily from Martian ingredients, thus vastly reducing the mass we need to launch from Earth. The main chemical reactions 4 H2 + CO2 --------------------> 2 H2O + CH4 (exothermic) hot Ruthenium catalyst and the electrolysis of water 2 H2O ---> 2 H2 + O2 have both been tested in a pilot chemical plant (built by Zubrin and L.Clark) which achieved 96% hydrogen conversion efficiency, could run 10 hours unattended, could run with CO2 input at Mars pressure, and was "scaled to produce 400 kg of products" although I do not think it actually did so. The other main reaction needed: 2 CO2 ------> O2 + 2 CO with the O2 being separated from the CO by passage through hot zirconia ceramic membrane, has, however, not been comparably tested, as far as I can tell. Zubrin claims the ERV will weigh 28.6 tonnes and the Hab 25.2 (both fully loaded). These are capable of being launched by launchers comparable to the Saturn V or by something he calls an "Ares launcher" cobbled out of space shuttle components. Because liquid CH4 density = 0.453 gram/cc liquid H2 density = 0.071 (could also use colder and denser hydrogen "slush"), liquid O2 density = 1.142, we see that (1) there are .071375 mols of oxygen atoms/cc (2) there are .071 H mols per cc (which would use up .0355 mols of oxygen atoms, i.e. 2.0105 cc's of LOX, on combustion), (3) meanwhile there are .0283 mols of methane per cc which uses up 4*.0283 = .11325 mols of oxygen atoms on combustion, i.e. 0.630 cc of LOX. (4) Note that the HYDROGEN density of liquid methane is actually, at .11325 gramsH/cc, 1.595 times higher than the hydrogen density of liquid hydrogen! Thus if both the O2 and methane tanks are initially filled with liquid H2, we will have just enough hydrogen... i.e. if we begin with two tanks in about 1.587:1 volume ratios both full of liquid H2, then later we will be able to fill the larger one with methane and the smaller one with oxygen. The numbers almost match perfectly. Unfortunately some of this numerical joy must be sacrificed when we realize that there will also need to be a third and fourth tank since mixing any two among methane, water, hydrogen, and oxygen, is forbidden. The presence of these large empty tanks permits an additional possibility Zubrin did not mention. Start with all 4 of the ERV fuel tanks full of H2 and O2 and burn all of the O2 and 40% of the H2 during launch from Earth. (This would allow launching a heavier ERV.) Do I believe all this? Well, I point out that the US "Skylab" space station carried a crew of 3, for a maximum space stay of 84 days (endurance record), and it weighed 89 tonnes. The USSR "Salyut" space stations carried a crew of 2, for a maximum human-in-space stay of 139 days (endurance record), and it weighed 19 tonnes. The US "Apollo capsules" carried 3 men for 5 days, and weighed 45 tonnes. In contrast, Zubrin wants to fly to Mars in a Hab weighing only 25.2 tonnes and carrying 4 people for 180 days. This would be a new endurance record for all 4, and it also would cram more people into a smaller spacecraft (going by spacecraft weight per person - which is approximately half that of the nearest competitor above - Zubrin conveniently fails to point this out). So, I think the conditions would be pretty horrible or the low total weight Zubrin thinks is achieveable, would actually not be achieveable. Furthermore, all long stay astronauts so far have suffered bone loss due to the zero-gravity conditions, with recovery taking 1-2 years once back on Earth. It is unknown whether a broken bone will heal at all in zero-G. Zubrin makes much of his idea of avoiding problems by generating artificial Mars-level gravity on the trip out - but he conveniently avoids noting that the entire return trip will be in zero-G for a record duration. Also, Zubrin has some interesting estimates of radiation exposure in his book, which unfortunately seem considerably more optimistic than the corresponding estimates in his cite [JR Letaw, R Silberberg, CH Tsao: Radiation hazards on space missions, Nature 330,24 (31 Dec 1987) 709-710]. Also Zubrin may have understated radiation risks - if we trust the Encyclopedia Brittanica article on biologic effects of radiation, which claims each additional Sievert (1 Sievert=100 rem) of radiation will lead to .0125-.1 extra cancers for the recipient. In spite of all these quibbles, it seems plausible everything could be done if the 4 astronauts were sufficiently crazy and enough money were spent (Zubrin estimates mission cost at $50 billion in 1996 dollars). Good. But: it seems obvious that this whole mission cost could be cut by a factor of AT LEAST 3, only ONE launch, not two or three, would be required, and zero health risks due to explosions, zero-G, radiation, etc, would be entailed, if the whole mission were run UNMANNED! Duh! Not to mention, I think a considerably smaller spaceship would then suffice, too, so the cost savings ought to be a lot larger than 3. Zubrin's few arguments for why the trip "must" be manned seem unconvincing to me. (Zubrin: the light-time delay of 10-40 minutes between Earth and Mars will make Earth-based control unfeasible. My response: [A] who cares if it takes a long time?, [B] run autonomously WITHOUT Earth control of every move. If computers are not good enough to do that, wait until they get good enough. Zubrin: "We need rock hounds" to explore Mars to seek scientifically interesting things like signs of fossil water, signs of extinct Mars life, etc; robots cannot do this. [Incidentally, see 30 June 2000 issue of Science for exciting cover story showing Mars probably contains frozen undergound water which, on geologically rare occasions, can get released to the surface in temporarily liquid form. Photos suggest this has happened fairly recently, by which I mean, within the last 10 million years, at maybe 200 places on Mars. Also Mars is known to have once been warm and had extensive surface water including an ocean and water-formed canyons, zig-zaggy rivers, deltas, gullies, etc., but that all was billions of years, not millions, ago, before most of its atmosphere vanished.] We need human adaptability to fix bugs as they arise. My response: Is the benefit worth the cost and risks? Historically, human adaptability has indeed saved some space missions from bugs, but there have been a comparable or larger number of cases where the humans did not adapt because they were killed. And unmanned missions can stay on Mars for an unlimited duration, they do not care about air and water and food and cold and living space, do not need recreation, and do not care about zero G and radiation. And "rock hounds" back on earth might still be able to have considerable decision making input.) Zubrin then continues his book into much more speculative and crazy territory, describing the future history of Mars bases, Mars colonization, Earth-Mars trade, and "terraforming" of Mars. I think all that is nuts. It now costs $10000/kg to launch stuff into low Earth orbit, so it does not seem like many great space trading opportunities are out there. Conceivably there could be rare substances on Earth, worth much more than $10000/kg, which are cheap to mine on Mars. But, I doubt it. I think the cost of doing anything on Mars will be so high that even materials a lot more common on Mars will still not be cheaper to mine. (Zubrin also figures/hopes the cost of space travel will soon decrease by 2 or 3 orders of magnitude...) Zubrin then has the idea that we can construct giant orbiting mirrors to heat up Mars's polar icecaps by 4 centigrade, vaporizing the CO2 in them, thus increasing atmospheric pressure by about 20-30%, and hopefully starting a runaway greenhouse effect. I.e. this will cause the soil to release trapped CO2 gas, increasing the greenhouse effect further, to where the whole of Mars is warmed 20-30 degrees C and the atmospheric pressure increases to .3 Earth atmospheres. Sounds great. But I think this is ridiculous. This is the equivalent of claiming that if Earth's poles were heated 4C, then Earth's atmospheric pressure would increase by a factor of 50 (if we go by multiplicative factors) or by a factor of 2 (if we go by mass of gas released per surface area) thanks to release of gas currently trapped in Soil or Oceans. That is plainly false (and experimentally false). So it seems likely to be a wrong assumption about Mars too. TAKE THIS SIMPLE TEST If you think space trade and Mars colonization are economically attractive, ask yourself this simple question: Why are you not, right now, colonizing Antartica? Or, for that matter, colonizing the bottoms of the oceans? Let's compare. Antartica (South pole) Ocean bottom Mars (at equator) avg temperature -49C? -55C 0C -63C -20C? temp variability +-70C small +-80C air yes ---no (but can synthesize)--- water plenty plenty some frozen underground? cost to go there --at least 1000 times cheaper-- expensive travel time needed 1 day few hours 180 days gravity 1 G 1 G .38 G radiation .1-.2 rem/year tiny 9 rem/year (30yr exposure to 9 rem/yr ==> chance you die of cancer 30% instead of 20%) Looks to me like Antartica and Ocean floors both beat Mars by a lot. (And yes, Antartica and the Ocean bottom presumably have quite a few resources, as presumably does Mars.) The fact is, despite much smaller obstacles, there is no great clamor to colonize Antartica or Ocean bottoms. The economy is not interested. This is all plainly silly. Case closed. (You might also want to ask yourself: suppose it took 180 days to travel to the Ocean bottom. Would we want to do exploration down there by means of manned subs, or by means of robots? Which would be economically more attractive? Related question: do we need people in diving suits prowling around the ocean bottom to find resources, or can we use unmanned robots, sonar, drill coring, etc? Which is more likely to find the resources more efficiently? Still believe in the need for "rock hounds" on Mars - even after answering that question? Yet another question: which will have more useful spin-offs on Earth: [a] researching robotics technology for unmanned Mars rovers, etc, or [b] manned Mars mission?) As far as I can see, Zubrin and his ilk are motivated not by logic, but instead by some kind of science-fiction adolescent psychological fantasies. It makes them feel good to dream about this stuff. I'm touched but don't think this worth funding. Besides, I think there is plenty of room (maybe more room) for huge fantasy satisfaction jollies on a worldwide basis purely by using UNMANNED space missions. Consider: did you get more of a kick from seeing Viking lander photos of Mars, or from canned and scripted space "photo opportunities" such as the Moon landing complete with "planting the flag," and/or from the Soviet-US "handshake in orbit"? Which made you feel like more progress had been made, and which made you feel like a bunch of Public Relations Assholes had just blown some propaganda at you and made you pay for it? I know my answer.