BS: Space program goals

Subject: RE: BS: Space program goals
From: Steve Shaw
Date: 22 Dec 16 - 05:20 PM

Give over, Bruce. I'm a born comedian. Even Teribus thinks so.

Subject: RE: BS: Space program goals
From: Iains
Date: 23 Dec 16 - 05:42 AM

Not levitation Mr Shaw, it was balloon assisted. Maybe hot air was the motive force.

Subject: RE: BS: Space program goals
From: Mr Red
Date: 23 Dec 16 - 08:28 AM

It is all a matter of scale. If you were Syrian living in Allepo, the focus is dodging the Russian bombs. Global warming, however concerning, is a minor consideration for them, it will wait.

I don't yet see what technology will come out of sending men or menesses to Mars will contribute to solving the privations of Global Warming.
Terra-forming is all very well on another planet when the only consequence of mistakes is a few human deaths but who is going to risk a different kind of tinkering with the air we breath? Trump maybe - he is stoopid enough to think of it.

I personally think mining asteroids has a better chance of pay-off, once we figure how to keep a lander glued to an irregular surface in virtually zero G.

It is all about profit. However that is measured.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 08:49 AM

Mr Red,

"It is all about profit. However that is measured. "

As was the exploration and colonization of the Americas, and all the European Empires expansions.




"I personally think mining asteroids has a better chance of pay-off, once we figure how to keep a lander glued to an irregular surface in virtually zero G."

Agreed, in the short term. A new source of resources, not in a gravity well.
Attachment is fairly easy- Magnetic to metallic asteroids, sunken in anchors ( by melting) to the ice and rocky ones.




"I don't yet see what technology will come out of sending men or menesses to Mars will contribute to solving the privations of Global Warming."

Closed loop resource recycling, non-carbon energy production, CO2 utilization for hydroponics... and all the ones we can't see until they are developed.



No need to terra-form Earth- we already have an atmosphere and lots of free water!

But remember, the difference between a self-contained space colony and a star-ship is only propulsion.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 08:51 AM

Actually, I should have said
"...and all the expansions of any human culture."

Just depends on what the leaders decide is "profit"

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 09:21 AM

An interesting take on space is in Blish's "Cities in Flight" series.


https://en.wikipedia.org/wiki/Cities_in_Flight

Written 55-65 years ago.

"In the period in between the first and second parts, the Cold War ended with the peaceful merging of the East and West blocks into a single, planet-wide Soviet-ruled dictatorship, which hardly made any perceptible change, as the West's political system had already become virtually identical with the Soviet one. However, this dictatorial power was broken by the spindizzy drive which works for very large objects, so that dissidents and malcontents have an easy way of escaping and going off into space. First factories, then eventually whole cities migrate from the economically depressed Earth in search of work; these space-wandering cities are called Okies."

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 10:21 AM

Jeri,

I lost a post

"Also, does Mars get more radiation than Earth? "

Mars gets less solar radiation, being farther from the sun. It has some atmosphere, which will reduce cosmic rays, but not as much as here on Earth. We have a noticable difference between sea-level cities and, say, Denver.

Another reason to land on Mars rather than stay in orbit- one can bury the habitat in that available Martian soil, to provide shielding.

Subject: RE: BS: Space program goals
From: Greg F.
Date: 23 Dec 16 - 10:29 AM

It is all about profit.

Its also about weaponization, Mr. Red.

Subject: RE: BS: Space program goals
From: Jeri
Date: 23 Dec 16 - 11:01 AM

Thanks Bruce.
Centrifugal force. I wonder why they didn't do that on the ISS. Probably money, engineering and...logistics. That, and they didn't expect people to stay longer than a year. So the rehab they got after returning was enough.

I accidentally watched part of a show on something like the Discovery channel. "Mars". It was a documentary ("mockumentary"?)about how the colony on Mars came to be. Have you seen it?

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 11:25 AM

"Centrifugal force. I wonder why they didn't do that on the ISS. "

Well, the ISS was designed as an Earth Observation system- much of what it does is tied to looking at the Earth. Harder to do if it was spinning. In addition, the structural requirements of a spinning system would have made the solar arrays a LOT more difficult to install and use.

As it is, one can "park" something outside the ISS, and it stays in that relative position. If there were spin on the ISS, each approach would have to be to the central axis, and anything placed outside at any other point would go off at the spin velocity.



Not the Nat Geo special. I have seen the fictional "The Martian", which is not a bad film. Not always scientifically accurate, but that's Hollywood for you.

Subject: RE: BS: Space program goals
From: Donuel
Date: 23 Dec 16 - 01:44 PM

Von Braun was heavily invested in centrifugal stations.
The ISS crew is always poised and ready to bug out in the event of an enormous solar event.

Its interesting to point out that women's eyes are a bit more robust than men's eyes in space. There are other differences also.

Evolution in as little as a generation could occur when dormant genes wake up. That may indicate previous exposure to the rarified stress of space travel. Imagine a 50,000 year old adaptation we have no idea we already have. This is all far flung speculation but good filler for sci fi.

What other cool stuff do know Bruce?

Subject: RE: BS: Space program goals
From: Greg F.
Date: 23 Dec 16 - 02:10 PM

Vunce der rokets go up, who cares vere dey come down
Dat's not mein department, sez Verner VonBraun.

What's old is new again.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 02:15 PM

The new Emdrive looks promising, but still developmental- too low a thrust, and no idea how it scales. ion drives are in use now (DAWN)

http://www.space.com/28732-nasa-dawn-spacecraft-ion-propulsion.html
but still low thrust and requires material to eject.

Light sails have the most potential, longer term, IMO. Space based laser systems could provide a decent thrust, or be used for deacceleration.

Linear accelerators /mass drivers are also in development ( one man's railgun is another man's launcher.) Give me a way to impart a speed of 25,000 mph at 120 miles up, in the right direction, and I can put you on the moon.

SNAAP systems ( like the one shown in "The Martian") have been in use, but provide low levels of energy for extremely long periods of time- OK for a coasting spacecraft like Voyager, but not much use for an accelerating platform.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 23 Dec 16 - 03:13 PM

Should have been "SNAP"
System for Nuclear Auxillary Power.

I had the engineering unit of a NIMBUS SNAP power conditioner.

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

Subject: RE: BS: Space program goals
From: Greg F.
Date: 23 Dec 16 - 05:44 PM

Fascinating as Mr. Spock would say;

Post about the weaponazition of space es desaparacido.

And the Mudcat Junta knows....

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 27 Dec 16 - 10:39 AM

Greg,

Weapons in Earth orbit are prohibited, like poison gas is. Did not keep China from testing one on one of their satellites.



On the several SDIO/BMDO programs I have been on, we were only allowed to have sensors in the space segment.


No limits on the moon or beyond, unfortunately. And a dumb rock, in the right trajectory, is a weapon (See Caveman 100).

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 29 Dec 16 - 12:18 PM

But if the US does not work on space exploration, others will...


English is the language of international flight- Will Chinese be the one of interplanetary flight?


https://www.yahoo.com/news/china-wants-mars-2020-190121312.html


http://www.newsmax.com/TheWire/china-moon-mars-white/2016/12/28/id/765822/


https://www.yahoo.com/tech/china-going-beat-united-states-back-moon-010043116.html





https://en.wikipedia.org/wiki/Indian_human_spaceflight_programme


http://www.businessinsider.com/japan-is-launching-an-asteroid-mining-space-program-2014-9



http://motherboard.vice.com/read/irans-space-program-may-be-a-cover-for-developing-better-ballistic-missiles



http://spaceref.ca/space-quarterly/military-space-drives-israel-space-program-for-now.html

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 30 Dec 16 - 08:21 AM

https://www.yahoo.com/news/nasa-neowise-mission-spies-two-072652143.html


Yet still no systematic effort to identify bodies that might impact the Earth.

6 inches of water rise and slow climactic change over decades- PANIC

2000 foot wave from impact and major climactic change in a few weeks- ho hum, no worries here

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 30 Dec 16 - 08:32 AM

http://www.unz.org/Pub/AnalogSF-1966mar-00061

An excellent non-fiction analysis of the problem.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 01 Jan 17 - 02:54 PM

Sorry, I must challenge a couple of things that beardedbruce has said. First, there most definitely is a systematic effort to identify bodies which might impact earth, and details are given here and here. The primary discovery facility currently is Pan-STARRS. To date 90% of all NEOs bigger than 1km have been discovered, and they are now pushing to smaller sizes. The thing that will clean the statistics up will be the Large Synoptic Survey Telescope which will be operational from 2021.

Second, Bruce says that "the new Emdrive looks promising, but it doesn't because its complete baloney. This is a proposal by a British engineer, Roger Shawyer, for which he once succeeded in getting a government grant. Its been around for years, and has been debunked by both theory and experiment. Basically it is a tapered microwave resonant cavity, and Shawyer's claim is that thrust can be generated by from the electromagnetic field inside. But it can't, except in miniscule quantities. See articles from Greg Egan here, and from John C. Baez, Professor of Mathematical Physics at the University of California, Riverside, here, here and here.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 03 Jan 17 - 08:32 AM

"The new Emdrive looks promising, but still developmental- too low a thrust, and no idea how it scales."

The jury is still out.

90% is not goofd enough- Not that we are doing nothing, but are we doing ENOUGH?

And NEO misses most of the higher velocity objects.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 03 Jan 17 - 10:32 AM

Scales with what Bruce? The reason it has too low a thrust is nothing to do with development, its to do with basic physics as Baez and others explain. You say that the ion drive "requires material to eject", of course it does, anything does. Because of conservation of momentum, anything which generates momentum in a forward direction must also shoot stuff out of the back so that the total momentum is conserved. Stuff can be matter, as it is in any conventional rocket and an ion drive. Or it can be radiation. Photons (including microwaves) carry momentum, and when I heard Shawyer talk about this about a decade ago, photons were carrying the reaction momentum. Photons have a momentum p=E/c, where E is the energy of a photon, and c is the speed of light. But as we know, c is a very big number.

I came away from Shawyer's talk not knowing whether the device wouldn't work at all, or whether it would work, but in order to get an emdrive rocket to take off you would need to put out enough microwaves to cook every living thing for hundreds of miles around.

Fortunately, having subsequently read Egan's and Baez's analyses, its the first.

Don't be fooled by Harold White of Eagleworks either, he has a paper which is claimed to be peer-reviewed, but who peer-reviewed it I don't know because others have subsequently pointed out large holes in the analysis. See here.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 03 Jan 17 - 10:41 AM

And no, the searches don't "miss most of the high velocity objects". These things are in elliptical orbits around the sun, and their velocity is determined by Kepler's laws. If they have really high velocity they are on hyperbolic orbits, and nothing to worry about because they are not bound to the solar system. The 10% which havn't been found are either small, or far away. If they are far away it while be a while before they get any closer. But LSST, which has 16 times the collecting area of a single Pan-STARRS telescope, and a survey of much higher cadence, will be good at detecting these. But if you can build an 8 metre wide field telescope plus associated CCD or other detectors and have it ready before 2021, then go right ahead.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 03 Jan 17 - 11:16 AM

High velocity as in coming in from the Oort cloud- Those would not be caught as NEO until they come in- and the first pass might be a collision. IMO a "standing watch" for such objects should be put in place.

As for Emdrive, the amazing thing is not how much thrust it produces, but that it DOES produce ANY thrust ( according to the recent articles in peer reviewed journals.

ANY analysis is only as good as the validity of the assumptions. IF ( and it IS a big if) it produces thrust that cannot be explained by errors, then it should be looked at even if it violates some "law"- after all, when analyzed as a fixed wing design, the bumblebee cannot fly. Perhaps there is something we are not aware of- quantum effects, tunneling, etc.

What we don't know we don't know.


" but in order to get an emdrive rocket to take off you would need to put out enough microwaves to cook every living thing for hundreds of miles around."

From what I presently see, this is correct. but if we want to launch
nanotechnology to another star, the total mass may be reasonable (drive plus payload)

I prefer the idea of a Bussard Ramjet (collect interstellar dust and heat it through a fusion reaction)- but the radiation problem is still there, for a living payload And it seems to work only with LARGE masses- the functional fusion units we have now are called "stars". Makes a nice idea for a trans-galactic trip, though.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 03 Jan 17 - 12:49 PM

As for the Emdrive, the amazing thing is that anybody takes it seriously at all. The thrust produced is entirely consistent with a number of sources of systematic error, such as radiation from some component of the experimental apparatus. See here.

It has been true in the past that new physics has resulted from an unexpected experimental result, notably the Michelson-Morley experiment. But far, far more often experimental error is the explanation. As with faster than light neutrinos recently.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 04 Jan 17 - 03:24 PM

But brute force works, too...



http://www.bizjournals.com/losangeles/news/2016/12/30/spacex-unveils-photo-of-falcon-heavy-rocket.html?ana=yahoo&yptr=yahoo

"The rocket's first stage is composed of three Falcon 9 nine-engine cores whose 27 Merlin engines together generate more than 5 million pounds of thrust at liftoff, equal to about 18 747 aircraft. Only the Saturn V moon rocket, last flown in 1973, delivered more payload to orbit.

SpaceX founder Elon Musk has been aiming to build a rocket that will eventually carry people to Mars to form a colony. In September, the billionaire/business mogul said he plans to design a rocket that will take up to 100 people per launch to Mars, with the first flights happening as early as 2023.

"Falcon Heavy was designed from the outset to carry humans into space and restores the possibility of flying missions with crew to the Moon or Mars," SpaceX said on its website."

Subject: RE: BS: Space program goals
From: Mr Red
Date: 09 Jan 17 - 06:00 AM

Here's another thing to factor in: space debris.

BBC TV prog "the Trouble with Space Junk" last night (available for 30 days).

Yea Yea, they are cataloging billions (literally) of things down to quite small pieces. & the probability of strikes will be low. But when you are talking human life, the repercussions are such that the stakes are sooooo much higher. Your Marsonauts will pass through this debris field.

And it was clearly stated that the logistics are way beyond current computers to predict all of the relevant bits. (It took 12 months to process 14 months data for the bigger items on several supercomputers around the world). then there is all of the bit on bit strikes that will occur over the years, generating ever smaller, more numerous debris that is still significant. Clean-up is not a viable option yet, by a long chalk.

We better get to Mars within a decade, because by 2100 the debris field will be denser. Smaller bits maybe, but kinetic energy rules!

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 09 Jan 17 - 07:24 AM

There are plans in place to send up garbage sweepers to clear out debris from NEO.


http://www.space.com/24895-space-junk-wild-clean-up-concepts.html

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 09 Jan 17 - 07:29 AM

and , until recently ( but still under Obama, so no Trump comments please):


https://www.nasa.gov/directorates/spacetech/niac/gregory_space_debris_elimination.html

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 09 Jan 17 - 07:32 AM

Space debris is not ignored, but again, are we doing enough?



https://nasasearch.nasa.gov/search?query=space+debris&affiliate=nasa&utf8=%E2%9C%93

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 09 Jan 17 - 09:41 AM

ESA have a programme on Space Debris, you can read about it here. Their programme includes Analysis and prediction, Scanning & observing, Re-entry and collision avoidance, Mitigating space debris generation, Debris removal, Hypervelocity impacts and protecting spacecraft and International cooperation. And international cooperation certainly includes with NASA. So although it is reasonable to ask "are we doing enough", the professionals at the space agencies who are best placed to answer this are certainly doing a lot of things.

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 09 Jan 17 - 02:37 PM

http://www.businessinsider.com/asteroid-flyby-nearer-than-moon-2017-1

"The asteroid, dubbed 2017 AG13, was discovered only Saturday by the University of Arizona's Catalina Sky Survey, according to an email from Slooh, a company that broadcasts live views of space.

It's between 50 and 111 feet (15 to 34 meters) long, and when it swung by Earth, 2017 AG3 was moving at 9.9 miles per second (16 kilometers per second). The near-Earth object, or NEO, came within about half the distance that the moon is from Earth, according to Slooh.

....
Had a porous rock asteroid of 111 feet (34 meters) long hit Earth at a 45-degree angle, the simulator found, it would have exploded as an air burst. The blast would have released about 700 kilotons' worth of energy — dozens of times more powerful than the atomic bomb detonated over Hiroshima."


BTW, the Hiroshima bomb had a yield of 12-18 KT...

Subject: RE: BS: Space program goals
From: Steve Shaw
Date: 09 Jan 17 - 02:49 PM

Maybe it would have exploded much higher up. Fireworks rather than firestorm.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 09 Jan 17 - 02:54 PM

700 kilotons is a factor 20 less than Castle Bravo though.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 09 Jan 17 - 02:58 PM

Plus what Steve said. Tunguska was approximately the same yield equivalent as Castle Bravo, and a third that of Tsar Bomba.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 09 Jan 17 - 03:01 PM

And 700 kilotons is marginally more than Chelyabinsk, which was estimated to be 20 metres in diameter. So that is pretty much what we would be looking at if that had hit.

Subject: RE: BS: Space program goals
From: Steve Shaw
Date: 09 Jan 17 - 03:37 PM

Not a nuke either. We worry far too much.

Subject: RE: BS: Space program goals
From: Iains
Date: 09 Jan 17 - 04:07 PM

The evidence for previous impacts is strong, even if there is a little uncertainty as to exact chronology. These previous impacts occurred in a far less populated and urbanised world. Today increasingly mega cities are situated in the littoral zone and the resulting tsunamis from oceanic impacts would be globally catastrophic. To ignore the very real risk, even if it is low, would be the height of stupidity.

http://elib.sfu-kras.ru/bitstream/handle/2311/1636/03_.pdf;jsessionid=CBA11409F1A8B01C8ADE7E38FDE6754E?sequence=1

Subject: RE: BS: Space program goals
From: Steve Shaw
Date: 09 Jan 17 - 04:49 PM

Well thank God my house is half a mile inland and a hundred feet above sea level. I promise to dash down and help the dudes in Widemouth Bay should the Big One arrive. I'll start at the Bay View Inn and progress to the Widemouth Manor. Priorities, Stevieboy!

Subject: RE: BS: Space program goals
From: beardedbruce
Date: 09 Jan 17 - 05:05 PM

The wave expected from a major sea impact is many hundreds of feet, and will go tens to hundreds of miles inland.


At 1300 ft above sea level and many hundreds of miles inland, I'll be hunkering down and hoping my shelter will survive. ( underground basement)

Subject: RE: BS: Space program goals
From: Iains
Date: 09 Jan 17 - 05:48 PM

The link is quite an old paper and there is, I believe, still dispute as to the height of tsunamis generated by an oceanic impact, and the degree of continental penetration. However the most pessimistic calculations give rise to huge waves so complacency really has no place in the consideration.


http://users.tpg.com.au/users/tps-seti/spacegd7.html#tsunamiimpact

Subject: RE: BS: Space program goals
From: Steve Shaw
Date: 09 Jan 17 - 06:28 PM

Yebbut the anatomy of tsunamis does not fit your predictions. The water piles up in narrow confines, not along open coastlines. My house at Widemouth will be fine. I wouldn't be so confident about places in the narrow bits further up the Bristol Channel though. Anyway, I shall continue to enjoy life and worry about far more likely events, such as being run over by a Blackpool tram on my next visit to the Golden Mile, which could be some years off. I think a sea view a hundred feet above sea level in a wide bit is a pretty good compromise.

Subject: RE: BS: Space program goals
From: Joe Offer
Date: 09 Jan 17 - 07:10 PM

My understanding about the threat Trump poses to NASA, is that he threatens NASA's authority to study earth science. Earth science has always been a major part of NASA's research, and it has provided much of what we know about greenhouse gases and global warming and other matters that are anathema to some conservatives. The Conquest of Space is one of those "make America great again" things that are so popular with Trump's supporters, but Earth science isn't sexy to them. So, there may be some trouble on that front.

http://www.motherjones.com/environment/2016/12/grinspoon-trump-nasa-inquiring-minds

Subject: RE: BS: Space program goals
From: SPB-Cooperator
Date: 10 Jan 17 - 03:28 AM

Pity the goals do not say anything about protecting the well being of the planet we actually live on, its biosphere and humanity? I would not be surprised if Trump already has a business plan to be first in the queue to profit from the commercial opportunities of his proposals.

Subject: RE: BS: Space program goals
From: Mr Red
Date: 10 Jan 17 - 04:36 AM

"are we doing enough"

NO

but then the logistics of removing space junk are obvious to paint in broad brush strokes, even if hazy. Big items - you can get at. Not easy but imaginable. Paper thin shards of aluminium and their number cannot be measured currently, and finding them needs a satellite, which itself would become junk if it got it wrong. The concepts are easy, the logistics are not.

That's GPS in the garbage some day. But think laterally, and Electronic Engineers have had this vision for 40 years (trust me I am one, I read the publications): LED street lights - their light can carry data. Adverts & location data. It would pay for itself in urban areas. Rural - always lose out IME. Maybe phone masts will cover that, approximately!

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 10 Jan 17 - 04:44 AM

Regarding the first link given by iains, what on earth are a group of people at Australian, Welsh and German universities doing publishing in the journal of the Siberian Federal University? My guess is that they couldn't get it published anywhere that we have heard of. That paper proposes that there was an impact of a 1.6km diameter object in 300m of water, in AD1500, just south of New Zealand. Sorry but I think that there would be other evidence for that. A later paper by James Goff et al. dismisses that hypothesis. Unfortunately you can't read any more than the abstract of that paper without paying Elsevier loads of money.

Don't get me wrong, impacts are a threat, and NASA, ESA and others are right to take the measures they are to catalogue the threats, predict possible impacts, and develop mitigation strategies. But in my view the geological hazards are greater, and if I lived on the east coast of Australia it would be Taupo and related volcanos which I would worry about more. Not to mention Toba on Sumatra.

Subject: RE: BS: Space program goals
From: Iains
Date: 10 Jan 17 - 05:18 AM

David Carter. What you state is entirely correct. The group do not carry the support of their peers, but that is a lonely furrow ploughed by many others in the past, who have subsequently had their theories accepted. I would not necessarily accept all they say but would certainly keep an open mind and not discount the possibilities outright.
There is a major problem trying to establish any sort of accurate chronology or frequency because oceanic impacts are hard to find and evidence of terrestrial impacts tend to be weathered out in time, also making them hard to find. Just because we have not managed to construct an accurate data base does not give us the confidence to be complacent and say it does not happen. Shoemaker levy's multiple impacts on Jupiter demonstrated very clearly that impacts do occur.
Our problem today is that we have no idea as to frequency, or intensity of possible impacts and as a result of this many do not accept we may have a problem.

Subject: RE: BS: Space program goals
From: David Carter (UK)
Date: 10 Jan 17 - 09:11 AM

Its really the frequency of future impacts we need to worry about, not past ones. These may be different for a few reasons, including depletion of the stock of NEOs, the changing character of the atmosphere, and also maybe the increasing distance between earth and moon. The last really large impact (with potential global consequences) appears to have been the Eltanin impact about 2.5 million years ago. So the efforts of NASA, ESA and others to calatlogue potential threats is in my view more important than determining the frequency of past events. Which is as you say hard, because even when you find a circular structure, you are hard pushed to pin down the cause.

Subject: RE: BS: Space program goals
From: Iains
Date: 10 Jan 17 - 10:43 AM

The silverpit structure in the north sea is a prime example. Pull apart structure or impact crater? I would agree that the future possibility of impacts is the major consideration - past impacts are merely an academic consideration, although may give some understanding of future events. It would appear that until recent times the possibility of impacts was largely discounted. That is no longer the case, but our knowledge base leaves much to be desired.

Subject: RE: BS: Space program goals
From: Iains
Date: 10 Jan 17 - 11:48 AM

Steve you are getting confused between tidal bores and tsunamis. Very different beasts.


https://en.wikipedia.org/wiki/1755_Lisbon_earthquake

The tsunami was widespread and did not restrict itself to narrow channels.