Although I’m taking a break from posting, a recent note from Marc Millis suggests something productive that can happen while I’m gone. The founding architect of the Tau Zero Foundation and former head of NASA’s Breakthrough Propulsion Physics project, Marc deals with the issues he asks about below on a daily basis, and so do I. But compiling accurate, non-conflicting information about them can be tricky, which is why we could use your help. Read on.
by Marc Millis
While Paul is away, I have a request for you. We have a number of questions for which we need some help to find reliable answers. For all of you who have been wanting to help, here now is a chance.
When answering these questions, we need to know where you got the information. Please cite the document where you found the information. We are looking for reliable information, so avoid articles that cite values to advocate a particular solution. We have found that some papers skew estimates for such things so that it will support their other assertions (yes, it does happen). Instead, we want information that is without bias. It is even better if that information includes uncertainty ranges to help convey how much the values might vary, and why.
1) What is the minimal sustainable population size for planning world ships?
2) What are the minimum requirements to keep each person alive (volume, biological throughputs [water, food/waste, air], energy)?
3) What is the maximum demonstrated rocket specific impulse (empirical, not theoretical)?
4) What is the minimum energy required to send a detectable signal back to Earth from 4.5 light-years distance?
5) What is the maximum demonstrated duration for reliable operation of contemporary spacecraft hardware?
6) What are the estimates for the maximum duration for reliable operation of space hardware? In other words, what is best warranty period we can anticipate for a deep space probe?
7) What is the best demonstrated pointing accuracy for beamed energy (lasers, microwaves)?
8) Using our own Earth as an example of an extraterrestrial world, how close would our SETI – type detectors need to be in order to detect signals coming from our current emission levels from Earth?
We are looking for the information in this format:
• Value, +/- uncertainty.
• Academic style citation for the document where you got that value (No URLs!).
Example:
• Experimentally measured specific impulse: 17,200 +/- 10% sec
• Byers, D. C. (1969) An Experimental Investigation of a High Voltage Electron Bombardment Ion Thruster, Journal of the Electrochemical Society, Vol. 116, No.1, pp. 9-17.
Post your answers in the comments. Paul tells me he is moderating these daily.
Dual-stage 4-grid ion engines have shown iSPs around 20,000 in the laboratory, and should be able to achieve 30,000. There was a flurry of research in the early 2000s, culminating in a lab prototype in 2006, but not much seems to have been done since then — possibly because it was ESA research, and the ESA doesn’t need an ion engine right now.
http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-IAC2006-DS4G-C4.4.7.pdf
Here’s a mission design paper using a DS4G engine:
http://www.esa.int/gsp/ACT/doc/MAD/pub/ACT-RPR-MAD-2006-InterstellarMissionWithHighSpecificImpulseEngines.pdf
And a paper from 2009 on general uses:
http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2009index/IEPC-2009-157.pdf
HTH.
Doug M.
Hello
For 1) I would suggest looking at at this http://www.newscientist.com/article/dn1936
giving a minimum of 80 to something in the range 160/200
Interestingly enough at 148 the Dunbar number( maximal number of relationships a human could have at once based one the neocortex size ) is in this range .
“What is the maximum demonstrated duration for reliable operation of contemporary spacecraft hardware?”
The longest continuously functioning piece of space hardware was the ATS-3 communications satellite, which was launched in 1967 and functioned continuously for 44 years before finally shutting down for good in 2011. There are a number of other satellites that have gone over 30 years, including Marisat F2 (1976-2008) and GOES-3 (1978-present). You may recall that Landsat 5 created a bit of a flurry when it was recently shut down just short of its 30th birthday.
As for “oldest still functioning satellite”, I’m aware of two candidates. One is Amsat Ocsar 7 (AO-7), which was launched in 1974. Its batteries are long since, dead but it also had solar panels and so still works when in sunlight. The second is LES-1, which was launched in February 1965 (!). It was considered a failure because it did not reach geostationary orbit. However, it is still in orbit — and as of 2011 it was still sending a carrier on 237Mhz.
Once you leave Earth orbit, the champions are of course the two Voyagers. Within the Inner Solar System — arguably a more challenging environment — there are a bunch of candidates, but the champion is probably SOHO, which launched in 1995 and is still active (and doing serious science) today. SOHO is in a halo orbit around the Earth-Sun L1 point, meaning it’s a bit less than 1 au from the Sun at all times. Mars Odyssey is the oldest active orbiter around another planet, having been continuously active for almost 12 years now.
Doug M.
On question 8 there is a paper :
John Billingham, Jill Tarter, 1992. Detection of the Earth with the SETI Microwave Observing System Assumed to Be Operating Out in the Galaxy Acta Astronautica26, 185-188.
(Can’t find a electronic copy on line , tho you can pay a king’s ransom for one!)
Billingham and Tarter showed that Arecibo could pick up the missile early warning system BMEWS at 18 light-years.
If I remember correctly the never built Project Cyclops could have seen BMEWS at a Milk Way Galaxy diameter!
John Billingham, Jim and Greg Benford have addressed SETI beacons in recent years.
Note that for a lot of these guys — most satellites, for instance, both around Earth and other planets — the limiting factor on lifespan is propellant for maneuvers and station keeping. Running out of propellant will probably be the death of Cassini, Mars Odyssey, and the MESSENGER probe at Mercury (if nothing else gets them first).
After propellant, the next limiting factor seems to be power. Nuclear reactors run down steadily; that’s what will likely kill both Voyagers. Solar panels degrade over time (though, as seen in the case of AO-7, even a 50 year old solar panel can still generate some power).
A third limiting factor is gyroscopes. Modern ring laser gyroscopes are awesome, but they tend to degrade after 10 or 15 years — this is a minor problem with Odyssey right now. It’s probably fixable, in the sense that we could likely design a gyroscope that didn’t degrade — we haven’t bothered because Odyssey will run out of propellant and die before it gets old enough to matter.
Now, if you can solve the propellant and power issues, it looks like it’s perfectly possible to design hardware — including complex avionics, sensors and communication electronics — that will last for many decades. Note that MESSENGER has survived 26 months in the hellish environment of Mercury orbit, and is expected (fingers crossed) to last until the summer of 2015 — more than four years in an environment of seven times the heat and radiation of Earth orbit. And those four years included the 2012-2013 Solar Max. So, we’ve gotten pretty good at shielding and hardening.
Doug M.
On question 3 , you might know better than me!….
That the Rover/Nerva test stand measurements were ~ 950 seconds specific impulse.
I don’t know how you count Project Orion, since we have indeed tested many thermonuclear devices , so if that empirical enough , that would be ~ 6000 seconds without a test stand (theoretical improvements could yield 100,000 seconds!)
Marc: Thinking about worldships, part of one was built in Arizona for $150 million and it did not work….to be forwarned to is to be forearmed….large amounts of money can be found for your starship research but it’s got to have good odds of producing at least a partial success….if this research in Arizona was poorly conceived and executed by amateurs we must make that clear to future investors otherwise interested in interstellar flight….taking little baby steps at first is just fine….How about a sub-space radio based on the Bell Theorem…. See article below….JDS
By MICHAEL WINERIP
Published: June 10, 2013
In the fall of 1991, eight men and women marched into a glass and steel complex that covered three acres in the Arizona desert and was known as Biosphere 2. Their mission: to test whether they could be self-sustaining in this sealed-off environment, with hope that the model would someday be replicated to colonize outer space.
They wore “Star Trek”-style jump suits, which, depending on your view of the grand experiment, either made them look very scientific or like inmates at the county jail.
Either way, there was serious intent and money behind the project, $150 million, underwritten by Edward Bass, environmentalist heir to a Texas oil fortune.
This Retro Report video looks back to the ideas behind the ambitious experiment, sorting out what was worthwhile science and what was hucksterism, and what happened once the rest of the world moved on.
The original idea was that the inhabitants would grow all their own food, and that the wilderness areas would naturally recycle their air and water.In Discover magazine the project was called the most exciting science venture since man landed on the moon.
Early on, there were problems. One Biospherian accidentally cut off the tip of her finger and left for medical care. When she returned, she carried in two duffle bags of supplies to the supposedly self-sustaining environment (which presumably would not have been feasible on, say, Mars).
But the most damaging discovery was that a carbon dioxide scrubber had been secretly installed to protect the occupants from dangerous levels of the gas.
By the end, as one of the Biospherians put it, they had been suffocated, starved and gone mad.
To all,
Thanks for the findings so far ! I would appreciate if other volunteers could help extract and distill the information into a suscint form, such as this EXAMPLE (for high values of Specific Impulse Measured in Laboratory (or in flight):
—————————————————–
Value:
17,200 +/- 10% sec Isp
Citation:
Byers, D. C. (1969) An Experimental Investigation of a High Voltage Electron Bombardment Ion Thruster, Journal of the Electochemical Society, Vol. 116, No.1, pp. 9-17.
——————————————————
I realize it takes more than a quick google search to do what we are asking. If it was easy, we would have already done it. If you can take the time do dig out and verify THE key information, and then take the time to present it as shown above, that would be fantastic! This will help us, and all the others, from having to review and extract that information on our own from the links given.
Many thanks for this! With each of us contributing such steps, we will more quickly create progress
Ad astra incrementis,
Marc
Question 4 has been referenced in this very blog – look at this paper:
Space Communications Technologies for Interstellar Missions
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/33239/1/94-1175.pdf
They estimate a 20 watt laser output would be enough for a probe to communicate with Earth from 4.5 LY.
For Q1, let’s not forget that we need a minimum sustainable population of not just humans, but all the livestock, companion animals, fish, wildlife, crops, trees, etc. that will be making the journey. This is something I have been thinking about for a long time but have not found any literature that specifically discusses the issue.
Marc, I don’t think answers to Question (5) as currently framed — “What is the maximum demonstrated duration for reliable operation of contemporary spacecraft hardware?” — can be “extracted and distilled into a succinct frame”, because it’s a really broad question.
Any spacecraft is going to have a lot of different parts: propulsion, radio, antenna, instrument package, attitude control systems, thermal control, power generation, on-board processors, fuel tanks, probably batteries, probably gyroscopes, possibly shielding against heat and radiation, possibly recording equipment, et cetera und so weiter yadda yadda. And some of these, in turn, have different levels of performance. For instance:
Radio (simple carrier wave only)
— LES-1 satellite, 1965 – present
Radio, complex
— Voyagers (around 160 bps), 1977 – present
Radio, advanced
— SOHO (around 60 kbps), 1995-present
But even with this, a host of questions arise. (Like, is bps a really good measurement for radio performance? Probably not, but anything else is going to take up a lot more space.) Similarly, if you ask about solar panels, you’ll get different answers depending on what your question is:
Oldest solar panel still providing power to a spacecraft
— Amsat Oscar-7 (1974)
Oldest solar panel still providing over 50% of nominal power
— TDRS-3 (1988)
So, if you want succinct answers, you’ll probably need to break that one down a bit.
cheers,
Doug M.
Partial answer to #2: The average water consumption for personal use (not watering lawns) in the U.S. is 70 gallons per day per person. Most is for showers and flushing toilets. If we’re talking world ships with full recycling and more efficient water use, less than 20 gallons per day per person may be a comfortable amount. The actual amount of water needed per day for consumption to maintain adequate hydration is 0.8 gallons per day for most men and 0.6 gallons per day for most women.
But let me turn the question around. It sounds like you’re really asking what’s the maximum plausible lifespan of a spacecraft given roughly current technology. Yes?
Well, that one is actually easy-ish. If you can solve the power and propellant issues, then we could start building a 100-year spacecraft tomorrow. And by “100 year spacecraft” I mean something that would have a 95% or better chance of lasting 100 years in space.
If you’re sending it to interstellar space, so much the better — that’s actually a pretty benign environment compared to Earth orbit; less radiation, less thermal stress, no solar wind, fewer micrometeorites. (More cosmic rays, but you’re still coming out ahead.) The current generation of comsats have nominal lifespans in the 15 year range, but are expected to last for double that; we currently have no trouble building radios, solar panels, gyroscopes and electronics — including some very advanced and high performance systems — to last 30 years in near-Earth space. The only reason we’re not building comsats to last even longer is that they run up against the limiting factor of propellant; they need it for station keeping, and inevitably they run out. (Also, by federal law, US satellites have to keep a last gasp of propellant to kick themselves upstairs into a graveyard orbit when they decommission. That’s a good and wise regulation, but it peels a couple of years off the satellite’s lifespan.)
Now, solving the power issue is nontrivial. If you’re talking interstellar space, then you need an RTG. Nobody has ever built an RTG that could last 100 years, so that’s a nontrivial design challenge. Current space RTGs use plutonium, but its 88 year half life means that if you want to go much beyond a century you need to swap in something else. Americium-241 has a half life of over 400 years, but its power output per gram is much lower, so you’d need a much heavier RTG to get the same amount of power. And there’d be some very interesting design challenges WRT the thermocouple and the long-term effects of RTG radiation on power plant components.
That said, it doesn’t look like a deal breaker. So if the question is “spacecraft lifespan given current technologies?”, the answer is “a hundred years, almost certainly.” Beyond that, well, that gets pretty speculative.
Doug M.
Please folks, in addition to stating the values, we’ll need to cite the document where that information came from – and that future audiences can check.
Please list citations.
Re questions (1) and (2), I don’t see that answers yet exist. Any worldship will of necessity, I believe, have a long design heritage from Salyut, thru the ISS, thru a long series of manned space stations, hotels and colonies which are increasingly remote from Earth and increasingly more self-sufficient, including colonies in the Main Asteroid Belt and beyond into the Jupiter Trojans and the Centaurs.
While the ISS obviously gives hard data on keeping professional astronauts alive in space for short periods, this may not be very relevant to a worldship, in which psychological, social and political factors come into play. The people will not be professional astronauts, and will live in an environment very different from ours due to ongoing progress in information technology, and to the cultural changes of living in a space-based rather than a planetary civilisation.
A reasonable worldship precedent in terms of small, isolated societies is the remote Pacific Island of Tikopia, which has sustained a long-term population of about 1200 for 900 years or so, tho at a very different technological level from the one we are thinking of. This is discussed, with further references given in the bibliography, by Jared Diamond in his recent book “Collapse: How Societies Choose to Fail or Succeed”.
http://en.wikipedia.org/wiki/Tikopia
http://en.wikipedia.org/wiki/Collapse:_How_Societies_Choose_to_Fail_or_Succeed
Stephen
Oxford, UK
It’s still all going to be scattered about in a series of posts. Maybe a shared google doc spreadsheet is the key to a succinct list of information.
If your goal is to design a world ship, look no further than here:
http://settlement.arc.nasa.gov/spaceres/toc.html
ad 8;
60 to 80 lj
Limits on interstellar messages
may be more :)
Now, if you want to build electronic devices that will last for multiple centuries, you run into some interesting design problems. Solvable, probably, but novel and challenging and tricky.
Solid-state electronic devices are not immortal! They’re prone to entropy just like everything else. And over many cycles of use, they develop all kinds of failure modes: electromigration, hydrogen poisoning, growth of copper whiskers, growth of silicon nodules, electrostatic discharge, you name it. To a certain extent, modern high-performance electronics are more vulnerable to some of these modes; IANA electrical engineer, but my understanding is that as you pack ever more circuits into an ever smaller area, things like electromigration become ever more problematic. (If there are some engineers on this thread, please jump in.) Thus, modern flash memories and EEPROMs have pretty strictly limited lifespans; they’ll degrade after a certain number of cycles, and even if they’re only used lightly they’ll degrade over decades anyway. Whatever you have on that thumb drive on your keychain will be gibberish in fifty years or so, whether you retrieve it regularly or not.
So while I’m pretty confident about that first century, once you get much beyond that you’re climbing an ever steeper design curve. I don’t see any reason we couldn’t build thousand-year electronics — but not with off-the-shelf technology. It would require a significant research effort over at least several years.
Doug M.
“1) What is the minimal sustainable population size for planning world ships?”
50 unrelated, breeding adults (i.e., effective population number Ne = 50)
For long term (>150 generations) viability, Ne = 500.
Franklin, Ian Robert. “Evolutionary change in small populations.” Conservation biology: an evolutionary-ecological perspective. Sinauer Associates, Sunderland, Massachusetts (1980): 135-149.
The total (“census”) population is 2-3 times higher, depending on your demographic assumptions.
PLEASE…. We NEED THE INFO IN THIS FORMAT:
• Value, +/- uncertainty.
• Academic style citation for the document where you got that value (No URLs!).
Example:
• Experimentally measured specific impulse: 17,200 +/- 10% sec
• Byers, D. C. (1969) An Experimental Investigation of a High Voltage Electron Bombardment Ion Thruster, Journal of the Electrochemical Society, Vol. 116, No.1, pp. 9-17.
This is not about off-the-cuff estimates. This is not about pontificating. There is not just one answer for each question. At this stage these questions cannot be answered perfectly, but estimates have been published in reliable documents. Please cite any reliable estimate you’ve encountered – ALONG WITH citing its source as if you were writing a term paper. It is okay to list more than one answer from more than one source. There is value in seeing the span of values and from seeing where those estimates came from.
We are collecting this info for scholarly purposes, not just for pondering. That is why we need the source cited in an academic style (such as style set by the American Society of Civil Engineers). We do not have staff to read through all the suggested URLs, dig out the key value, and then present the citation in the correct form. We are asking your help to do that – with the expectation that many of you have run across that information already and can find it more easily than the rest of us. PLEASE present your findings in the manner requested.
If you’ve already listed here, please restate your findings in the requested format as a service to the thousands of other readers here.
EXAMPLE: Using a prior post, reformatted into the form we need:
– Detectable range of early missile warning system BMEWS = 18 light-years.
– John Billingham, Jill Tarter, 1992. Detection of the Earth with the SETI Microwave Observing System Assumed to Be Operating Out in the Galaxy Acta Astronautica26, 185-188.
As to question #6 — “What are the estimates for the maximum duration for reliable operation of space hardware? In other words, what is best warranty period we can anticipate for a deep space probe?” — I’m not aware of any. You might think the Long Now folks would have done something along these lines, but no, or anyway, not yet.
There’s plenty of stuff out there on keeping space hardware alive out to — can you guess? — 25 or 30 years or so. Yup, the current upper limit for commercial satellites. After that, nothing.
Part of the problem is that nobody’s going to pay for such a study. Comsats die from propellant loss (and obsolescence) sometime in their third decade, so why study how to keep them alive longer? Lots of space missions hit 15-20 years, but past that, not so much. Also, the mission structure imposed by NASA and ESA strongly discourages that kind of long term planning — you’re supposed to plan for the nominal mission, and maybe a first extension. You’re not supposed to waste money thinking about what might happen 20 years down the line; not only might the spacecraft not live that long, but it’s /not budgeted/.
The sole and partial exception is New Horizons, which was designed with a 12 year primary mission and freely allowed to contemplate a much longer Voyager-style extended mission. Unfortunately, New Horizons got kinda screwed over on its RTG. To make a long story short, it launched with a smaller RTG and less Pu than it could have (or, some say, should have). So instead of the ~45 year lifespan anticipated for the Voyagers, New Horizons will die a slow cold death sometime in the 2030s, just ~30 years after its 2004 launch.
So, in terms of formal, rigorous estimates done by scientific, academic, or commercial entities, AFAIK the answer to your question is “none”. But if anyone knows better, I’d be very interested to hear.
Doug M.
On question 4, David Messerschmitt (http://www.eecs.berkeley.edu/~messer/) has a recent paper “End-to-end interstellar communication system design for power efficiency” (http://arxiv.org/abs/1305.4684). He addresses in a general way the fundamental tradeoffs between power and data rate, for example.
@Marc, might I suggest that if you have a desired format, you do an edit and add it to the main body of the post? Because not everyone is going to read the whole thread and spot it 22 comments down.
Also, can I ask what you’re planning to do with this information? Right now it looks like you’re crowdsourcing some sort of academic article. Not that there’s anything wrong with that, but if that’s what we’re doing we should know up front.
Doug M.
This information, once scrubbed, will be posted at the new “CHALLENGES” page of the Tau Zero website.
This invites a discussion on the wisdom or practicality of “world ships”.
But I greatly respect Mr. Millis and he is making serious inquiries and asking scientific/engineering questions.
Question 8 is a bit open ended, but the SKA phase one will be able to detect a simple airport search radar from 50-60 lyr. The EIRP of BMEWS radars like Cobra Dane and Sea-based X-band are orders of magnitude more powerful as will the detection capability of the completed SKA. If right now empirical is what you want, then Tarter’s 18 lyr for BMEWS. If you want what’s in development, then thousands of lyr. My Starship Congress talk is on this very subject.
SETI on the SKA, Astrobiology Magazine, 25 June 2012.
From Centauri-Dreams July 13, 2010:
Benford, James, Gregory, and Dominic. “Messaging with cost-optimized interstellar beacons” Astrobiology 10 (5) 475-490.
Benford, James, Gregory, and Dominic. “Searching for cost-optimized interstellar beacons” Astrobiology 10 (5) 491-498.
As well as the reference from A.A. Jackson, Tarter, et.al., Acta Astronautica.
Sorry about not including links for water use.
Adequate water consumption (0.8 gallons per day for most men and 0.6 gallons per day for most women) is provided at the following path:
http://www.mayoclinic.com/health/water/NU00283
Water use per person per day in the U.S. is estimated at 70 gallons per day for indoor use and the basis is the following:
http://www.epa.gov/watersense/our_water/water_use_today.html
The EPA link says roughly 70 percent of 300 gallons per day per family is for indoor use.
The following link indicate 2.58 members per household in the the U.S.:
http://www.census.gov/prod/cen2010/briefs/c2010br-14.pdf
Converting the above data provides an estimate of 81 gallons per person per day for indoor use. However, I work for a municipality that provides water and wastewater services. Our community gets a lot of rainfall, so there is very little outdoor use. Our internal data and an inflow and infiltration study conducted a few years ago supports the above conclusions, but the water use in our community is estimated at closer to 70 gallons per day per person for indoor use.
Question 8, more specifically there is a nice article at
http://mostlymissiledefense.com/2012/04/12/cobra-dane-radar-april-12-2012/
Gene Stansbery, “Preliminary Results from the U.S. Participation in the 2000 Beam Park Experiment,” Proceedings of the 3rd European Conference on Space Debris, Darmstadt, Germany, March 19-21, 2001, pp. 49-52.
It goes into great technical detail about Cobra Dane (ref above). This radar is typical of the BMEWS PAVE/PAWS radars the US uses on a continual basis to scan the entire sky visible from US, Canadian, and UK territory. It has a peak radiated power of 15.4mw with a 0.6 degree beam giving it a gain that could be 30-60bd depending on the length of each pulse (duty cycle) and losses within the system. 30db is a 1000 fold jump in what is called Effective Isotropic Radiated Power because of the focusing provided by the phased array system. So we are looking at more than a gigawatt of highly directed rf energy that would blip on and off very rapidly, then slowly die away as the planet rotated. Bottomline, if SKA phase one can detect a simple airport radar at 50 lyr, then it can detect something with 1000 times (approx. 2^10) the EIRP at ten times the distance which would be about 500lyr.
5D ‘Superman memory crystal’ heralds unlimited lifetime data storage
Data written to a glass “memory crystal” could remain intact for a million years, according to scientists from the UK and the Netherlands who have demonstrated the technology for the first time. The data-storage technique uses a laser to alter the optical properties of fused quartz at the nanoscale. The researchers say it has the potential to store a staggering 360 terabytes of data (equivalent to 75,000 DVDs) on a standard-sized disc.
http://physicsworld.com/cws/article/news/2013/jul/17/5d-superman-memory-crystal-heralds-unlimited-lifetime-data-storage
2) What are the minimum requirements to keep each person alive (volume, biological throughputs [water, food/waste, air], energy)?
Value: 100 m^3 per person
Citation: Harry Jones (NASA Ames Research Center), “Starship Life Support,” SAE Technical Paper 2009-01-2466, doi:10.4271/2009-01-2466 (2009). See also: “Living Large” by R. Lovett, Starship Century, J. & G. Benford, eds, 2013.
4) What is the minimum energy required to send a detectable signal back to Earth from 4.5 light-years distance?
Value: 100 kJ (This is not the right question, so this is an approximation. It’s not just energy. Several other things must be specified. See the citation.)
Citation: “Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, to be published, JBIS (2013). For an earlier version, see http://arxiv.org/abs/1102.1938.
7) What is the best demonstrated pointing accuracy for beamed energy (lasers, microwaves)?
Value: Microradians
Citation: .” Starship Sails Propelled by Cost-Optimized Directed Energy”, JBIS 66, pg. 85, 2013. See also: “Sailships’ by J. Benford, Starship Century, J. & G. Benford, eds, 2013.
8) Using our own Earth as an example of an extraterrestrial world, how close would our SETI – type detectors need to be in order to detect signals coming from our current emission levels from Earth?
Value: Leakage cannot be detected in reality. Video carriers cannot be detected beyond Pluto. BEMS radars, which do not radiate anymore, have a probability of intercept of about one part per billion.
Citation: “Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, to be published, JBIS (2013). For an earlier version, see http://arxiv.org/abs/1102.1938.
BMEWS PAVE/PAWS operate on a continual basis to this day to track everything from ICBMs to wayward wrenches dropped by shuttle astronauts in the 1980s. I am not sure what James Benford alludes to. Also, we can detect the feeble signal of Voyager which broadcasts at far less than a dim light bulb far beyond the orbit of Pluto. A phased array antenna like Cobra Dane scans a sector many times a second. We would see it as a continual pulse just like we see TV which blinks on and off 60 times a second, just like I did in my EP-3E in the late 80s. That EIRP gigawatt signal would be visible for hundreds or possibly thousands of light years by any alien civilization that happened by chance to pass through it or the six other radars beams who are as I write this doing the same thing…the METI arguments are moot.
The Apollo and Lunakhod Lunar Laser Ranging Retroreflectors continue to work well; the oldest (the Apollo 11 array) is 44 years old. Of course, these have neither moving parts or electronics.
Instead of giving a reference, I will give NASA ADS links, from which you can get a bibtex file and make your own reference list in your own format. For the above,
http://adsabs.harvard.edu/abs/2004PhRvL..93z1101W
http://adsabs.harvard.edu/abs/2009PASP..121…29B
are fairly recent and up to date.
“8) Using our own Earth as an example of an extraterrestrial world, how close would our SETI – type detectors need to be in order to detect signals coming from our current emission levels from Earth?”
Ok this question has a fundamental flaw in. If you are assuming ability to detect Earth by other civilizations it’s easier to search for traces of industrial activity in atmosphere by normal telescope detection that we are starting to work on with the new generation of telescopes(and perhaps with hypertelescopes in the future). Radio signals are not that strong. As to radio signals:
Someone here cited following works to calculate detection levels
http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html
Radio Astronomy, John D. Kraus, 2nd edition, Cygnus-Quasar
Books, 1986, P.O. Box 85, Powell, Ohio, 43065.
Radio Astronomy, J. L. Steinberg, J. Lequeux, McGraw-Hill
Electronic Science Series, McGraw-Hill Book Company, Inc,
1963.
Project Cyclops, ISBN 0-9650707-0-0, Reprinted 1996, by the
SETI League and SETI Institute.
Extraterrestrial Civilizations, Problems of Interstellar
Communication, S. A. Kaplan, editor, 1971, NASA TT F-631
(TT 70-50081), page 88.
-Table 1 Detection ranges of various EM emissions from Earth and the
Pioneer spacecraft assuming a 305 meter diameter circular
aperture receive antenna, similar to the Arecibo radio
telescope. Assuming snr = 25, twp = Br * Tr = 1, r =
0.5, and dr = 305 meters.
————-+————–+———–+——–+——–+———–+
Source | Frequency | Bandwidth | Tsys | EIRP | Detection |
| Range | (Br) |(Kelvin)| | Range (R) |
————-+————–+———–+——–+——–+———–+
AM Radio | 530-1605 kHz | 10 kHz | 68E6 | 100 KW | 0.007 AU |
————-+————–+———–+——–+——–+———–+
FM Radio | 88-108 MHz | 150 kHz | 430 | 5 MW | 5.4 AU |
————-+————–+———–+——–+——–+———–+
UHF TV | 470-806 MHz | 6 MHz | 50 ? | 5 MW | 2.5 AU |
Picture | | | | | |
————-+————–+———–+——–+——–+———–+
UHF TV | 470-806 MHz | 0.1 Hz | 50 ? | 5 MW | 0.3 LY |
Carrier | | | | | |
————-+————–+———–+——–+——–+———–+
WSR-88D | 2.8 GHz | 0.63 MHz | 40 | 32 GW | 0.01 LY |
Weather Radar| | | | | |
————-+————–+———–+——–+——–+———–+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 22 TW | 720 LY |
S-Band (CW) | | | | | |
————-+————–+———–+——–+——–+———–+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 1 TW | 150 LY |
S-Band (CW) | | | | | |
————-+————–+———–+——–+——–+———–+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 1 GW | 5 LY |
S-Band (CW) | | | | | |
————-+————–+———–+——–+——–+———–+
Pioneer 10 | 2.295 GHz | 1.0 Hz | 40 | 1.6 kW | 120 AU |
Carrier | | | | | |
————-+————–+———–+——–+——–+———–+
Ok, this paper has all the answers to your question number 8 and it’s a scientific publication:
http://arxiv.org/abs/1207.5540
http://arxiv.org/pdf/1207.5540.pdf
The Benefits and Harms of Transmitting Into Space
Jacob Haqq-Misra, Michael Busch, Sanjoy Som, Seth Baum
Space Policy (2013) 29:40-48
It has a table with relative detectability of sources of radio leakage from Earth on page 4.
The answer to number 7 for lasers (visible and near IR) is simply the pointing accuracy of the telescope used to direct the beam. The best modern telescopes, from amateur to large professional, can usually be made to point, over reasonably long slews, to a precision of around 1 arc-sec or 4.85×10^-6 radians, as James Benford said. Differential pointing could easily be 10 times better, which is roughing the active guiding precision, which is just as critical. This number is happily about equal to the diffraction limit for a ground based telescope with adaptive optics in the visible or near IR or a space based telescope with a good mirror etc (these are not actually the same thing, since you can use many sources to determine where you’re pointing). This means you can actually hit your target with that beam if you know it’s position to that precision (which is the more important but slightly different question). Single microwave dishes on the ground probably point a little worse than this, simply due to the mass, and thus the flexure involved (ask a radio astronomer who’s used the GBT for a better answer). Their differential pointing won’t be any better unless there happens to be a strong, documented source in the FOV. Large, precision pointing, microwave dishes have not yet flown in space. Astrometric surveys in the visible are such now that there are always multiple, high precision sources in a reasonable FOV. If one actually wants references for this you’d have to access the technical manuals for various observatories or telescope manufacturers, which I’m not going to do. As has been shown for optical SETI, you’re almost certainly better off using lasers for communication, mostly bc of the smaller apertures needed since the diffraction limit scales linearly with the wavelength and inversely with the diameter of the telescope.
Well Marc, sounds like your going to be the ‘Christopher Hitchens’ of Deep Interstellar flight advocacy? Giving all the nth degree details to thwart debate opponents. You;ll do well, I saw you on a TED talk… funny as well as informative. thanks for all the hard work.
Marc: don’t give up. I know what you are looking for but many haven’t had the benefit of materials and methods in grad school. It’s a good exercise and with time this motivated audience will figure it out.
I’d add that that this is a very interesting set of responses in any case, reflecting the interests of the readers. The discussions are passionate and informed and reflect wide-thinking and reading.
For question 2, Wikipedia cites “F.M. Sulzman & A.M. Genin (1994), Space, Biology, and Medicine, vol. II: Life Support and Habitability, American Institute of Aeronautics and Astronautics” as the source for: “approximately 5 kg (total) of food, water, and oxygen per day to perform the standard activities on a space mission” per crewmember as inputs, broken down as: “0.84 kg of oxygen, 0.62 kg of food, and 3.52 kg of water”. The O2 figure roughly matches my memory of 2 lb O2/day from Apollo days. The water figure (~3.5 L) is close to the usually recommended 1 gal/person/day for emergencies, and the food number is in the ballpark for a 2500 Calorie diet. Of course this assumes open loop, no recycling, but you’d probably want at least a couple of days’ worth of buffer even with a very good recycling system.
I’m pretty sure I have a copy of that reference in my personal library, I will double check that and see what other answers I can find later. (I’m not where my books are right now.)
Regarding question 1, we can get some upper bound for this number based on population bottlenecks in our own history. Nuclear DNA evidence suggests the population has been as low as some tens of thousands, but the smallest minimum size for such bottleneck seems to be about one thousand:
The following is from “Population Bottlenecks and Pleistocene Human Evolution” in Molecular Biology and Evotion at http://mbe.oxfordjournals.org/content/17/1/2.full
“As it turns out, retention of a large number of ancestral HLA alleles precludes effective population sizes of much less than 1,000 at any particular point in time during human prehistory (Ayala, F. J. 1995. The myth of Eve: molecular biology and human origins. Science 270:1930–1936. ; Ayala, F. J., and A. Escalante. 1996. The evolution of human populations: a molecular perspective. Mol. Phylogenet. Evol. 5:188–201. ; Takahata, N., and Y. Satta. 1998. Footprints of intragenic recombination at HLA locus. Immunogenetics 47:430–441. ). This minimum bottleneck number, 1,000, also seems to be the minimum effective population size compatible with the maintenance of species viability and adaptability (Lande, R. 1995. Mutation and conservation. Conserv. Biol. 9:782–791. ). ” [formatting added, reflinks expanded]
On the other hand, a look at the history of the Pitcairn Islands suggests that the minimum sustainable population could be as low as 27, or even smaller. The unfortunate history of that group leaves it open to question. (A number of men, mostly Tahitian, were killed not long after the original settlement.) Visits by other ships after the first 20-ish years could have, um, added to the genetic diversity of the population, and raise questions as to whether the colony would have been sustainable in the long term without outside contact.
The real problem with Biosphere 2 (the Michael Winerip article quoted is somewhat sensationalist) is that the chemistry wasn’t completely understood. Specifically, the curing concrete (concrete takes a long time to completely cure) absorbed CO2 (to form CaCO3) as it did so. You might think this a good thing, particular in light of the higher-than-expected soil bacteria metabolism producing excess CO2, but the net effect of these two processes was to reduce the O2 available in the atomsphere. With the produced CO2 locked up in the concrete, the plants couldn’t photosynthesize it back to breathable oxygen. The crew didn’t install a CO2 scrubber, that wasn’t the problem. Lack of oxygen was.
The Soviet Union conducted a series of “closed” biosphere experiments before Biosphere 2 (there was some contact between the two groups) in their BIOS 3 facility in Krasnoyarsk. They grew most of their own food and recycled their air and water (algae and wheat were the plants, somewhere I have a sample of the wheat from when I visited.) Under Soviet laws on human experimentation, however, the inhabitants were required to include externally-supplied meat in their diet, which ended up increasing salt levels within the habitat. Also, it was all indoors, so had artificial lighting for the plants and required a significant water flow over the walls of the chamber to keep it cool.
Mostly what BIOS 3 and Biosphere 2 taught us is that you need fairly large buffers to keep the system going wild; the bigger the buffers the more tolerant it is of short-cycle variations (and it will vary, no feedback cycle is perfect in the short run).
I recommend that Thomas Hair and J. Wojciech read the Billington & Benford paper carefully before asserting that all sorts of things from Earth are observable. This is simply not the case; high EIRP is a necessary, but not sufficient, condition. Detection at great range depends on many factors as shown by equations 4 and 7 in our paper. These factors include the bandwidths of transmitter and receiver, signal-to-noise ratio that can be achieved and the receiving systems’ effective antenna temperature. Also coming into it are the length of time the signal is observed and the bit rate of the message. A careful analysis has to be done.
Simple assertions that bright sources must be observable at enormous ranges are simply unfounded. One of the problems of discussions about leakage radiation is that these calculations are not understood by many of the participants. The discussion would be enhanced if these calculations were taken seriously and documented. Without such quantitative arguments, conclusions cannot be made; we must do the math. My recommendations for doing such documentation are in section 3.4 in our paper.
One argument I make in that paper is about the extremely low probability of intercept of any such radars. Though called by the same system name, the radars of today are different from those for which Sullivan and others made their calculations about leakage radiations 35 years ago. The statements about the observability of leakage radiation, radars and transmissions from Arecibo are frequently based on unrealistic transmission or observing assumptions: that the transmission will last and be visible for months even though the earth is rotating, that the listener will be able to listen for corresponding times, or that vast, expensive arrays much larger than any on Earth would be focused on a very tiny part of the sky where we are for a long time.
There’s also the factor that planetary radar beams are not directed to stars at all but typically these days to asteroids. Even transmission advocates, such as Zaitsev, agree, stating 5 years ago “the probability of our radar getting into inhabited zones is insignificantly small”.
The paper of Jacob Haqq-Misra and other workers ‘Benefits and Harms of Transmitting Into Space’ should be viewed in this light: this group is promoting an antique Apollo era transmitter, near Carmel CA, in a commercial venture to sell transmissions to the stars to gullible members of the public.
I don’t take their table of leakage signals seriously because they do not state their parameters fully. They do not specify the bandwidth of the receiver, nor the S/N ratio, nor the observing time. They protect their estimates by writing equations 2 & 3 as a proportionality, rather than an equality. Yet provide a table with firm numbers for range with this caveat: “the values given in Table 1 are upper bounds”. I note that the excellent detailed quantative work of Sullivan in 1978 gives ranges roughly 100 to 1000 times shorter. “Upper bound” indeed!
Jim Benford
“I recommend that Thomas Hair and J. Wojciech read the Billington & Benford paper carefully before asserting that all sorts of things from Earth are observable. ”
Sorry if I didn’t make myself clear. This was not my assertion, quite the opposite, for the reasons mentioned by you and others I believe radio SETI to be mostly pointless endeavour.
1) What is the minimal sustainable population size for planning world ships?
• 1, +/- 0. (One female with a container of “a few thousand” frozen embryos).
• Minimum viable population size: A meta-analysis of 30 years of published estimates, Biological Conservation, Volume 139, Issues 1–2, September 2007, Pages 159-166
My point: If you ask the wrong question, you’ll get an answer which will lead you astray. If the goal of a worldship is interstellar settlement, it can be achieved at far less cost and sooner by sending people in frozen form.
1) What is the minimal sustainable population size for planning world ships?
The spaceship would carry large samples of a number of microorganisms. A payload of 1000kg might be made up of 10 samples each containing 10^16 microorganisms, or 100 samples each of 10^15 microorganisms.
Crick & Orgel 1972 Directed Panspermia, Published in Icarus 19, 1973, P341-346
2) What are the minimum requirements to keep each person alive (volume, biological throughputs [water, food/waste, air], energy)?
The calculations also show that even after 25 million years in space, a substantial fraction of a spore population would survive the exposure to cosmic radiation if shielded by 2 to 3 m of meteorite material.
Nicholson et al. 2000 Resistance of Bacillus Endospores to Extreme Terrestrial and Extraterrestrial Environments, Microbiology and Molecular Biology Reviews, Sept. 2000, p. 548–572
3) What is the maximum demonstrated rocket specific impulse (empirical, not theoretical)?
A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft’s payload. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, while the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.
Forward, R.L. 1984 Roundtrip Interstellar Travel Using Laser-Pushed Lightsails, J Spacecraft 21 (2): 187–195
4) What is the minimum energy required to send a detectable signal back to Earth from 4.5 light-years distance?
The propulsion beam can also double as communication link, therefore no additional power is required. Communication equipment is not mandatory for mission success.
George & Beach 2013 Beamed-Energy Propulsion (BEP) Study, NASA/TM—2012-217014
5) What is the maximum demonstrated duration for reliable operation of contemporary spacecraft hardware?
35 years, Voyager 2
6) What are the estimates for the maximum duration for reliable operation of space hardware? In other words, what is best warranty period we can anticipate for a deep space probe?
250 million years+
Veerland et al. 2000 Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal, Nature vol 407
7) What is the best demonstrated pointing accuracy for beamed energy (lasers, microwaves)?
The beam doesn’t target the craft, instead it targets the destination point. The craft rides the beam. Obvious problems would be beam power and coherence, not tracking.
George & Beach 2013 Beamed-Energy Propulsion (BEP) Study, NASA/TM—2012-217014
8) Using our own Earth as an example of an extraterrestrial world, how close would our SETI – type detectors need to be in order to detect signals coming from our current emission levels from Earth?
A BMEWS-type radar could be detected as far away as 15 light years.
Sullivan et al. 1978 Eavesdropping: The Radio Signature of the Earth, Science Vol. 199 no. 4327 pp. 377-388
A propulsion beam for a interstellar sailcraft operating in the can be expected causing direct effects at the target system, such as color controverisity or the star of origin emiting an unusual ammount of radiation unrelated to stellar evolution models.
JohnHunt says:
The beam doesn’t target the craft, instead it targets the destination point. The craft rides the beam. Obvious problems would be beam power and coherence, not tracking.
But the star is moving. Alpha Centauri will move a tenth of a light year while a 0.1 c craft goes there, an angular error of 3 10-3 radian.
Minimum viable population……………………………..
Moore John, H. “Kin-Based Crews for Interstellar Multi-Generational Space Travel.” Kondo Yoji, Frederick Bruhweiler, John Moore, Charles Sheffield (a cura di), Interstellar Travel and Multi-Generation Space Ships, Collectors Guide Publishing, Burlington, Ontario, Canada (2003). *****
Newscientist.com. “”Magic number” for space pioneers calculated – 15 February 2002 – New Scientist.” 2002. Web. 27 Jul 2013. .
Smith, Cameron McPherson, and Evan Tyler Suliëman Davies. Emigrating Beyond Earth: Human Adaptation and Space Colonization. Springer, 2012.
****** I remember in 2002 that this was a AIAA conference in Boston
@ Gregory Benford
Aim where the Star is moving to. If the “packets” are scattered early enough the error margin should can be covered. Also multiple seedships could be launched to cover a large area. These are not very complex vehicles.