Space, the final frontier

Discuss Marilyn's column in PARADE magazine.

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Space, the final frontier

Postby robert 46 » Thu Mar 15, 2012 9:35 am

Ask Marilyn: Would Marilyn Have Liked to be an Astronaut?
ask marilyn Marilyn vos Savant March 15, 2012
Loree Pons of Menifee, California, writes:


Marilyn: 1) Have you ever wanted to be an astronaut? 2) Does it bother you that no woman has ever landed on the Moon? and 3) Do you think there should be manned space missions to explore places more distant than the Moon?

Marilyn responds:

1) No; 2) no; and 3) no.

Very sensible answers [1]. Although I find scientific value in space exploration, it can be best done by robotic space probes- at less expense and no risk. Everywhere in the solar system away from Earth is guaranteed fatal without life-support equipment brought from Earth. Terraforming Mars would take 10,000 years, so the best option is not to destroy the Earth environment first. All other projects should be subordinated to the task of maintaining a stable environment on Earth- at whatever cost is necessitated.

[1] If not exactly explanatory.
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Postby davar55 » Tue Apr 03, 2012 3:30 pm

Don't we "eventually" want to leave home and visit other places?
Like all those inhabitable earth-like planets we just haven't found yet?

Sure it's early, but I think Marilyn meant "not at this time" or
"not very soon" regarding manned space flight beyond the moon.
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Postby robert 46 » Wed Apr 04, 2012 12:10 pm

davar55 wrote:Don't we "eventually" want to leave home and visit other places?

No. If robotic craft provide data, it is sufficient compared to being there- which is extremely dangerous.
Like all those inhabitable earth-like planets we just haven't found yet?

Most people do not comprehend what a lightyear means as far as it concerns the isolation of humanity from the rest of the universe. It is being in cosmic quarantine.
Sure it's early, but I think Marilyn meant "not at this time" or "not very soon" regarding manned space flight beyond the moon.

I think you are reading more into Marilyn's economical response than is even slightly implied.
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Followup

Postby robert 46 » Sat Jun 09, 2012 3:12 pm

robert 46 wrote:Most people do not comprehend what a lightyear means as far as it concerns the isolation of humanity from the rest of the universe. It is being in cosmic quarantine.

Parade wrote:ask marilyn Marilyn vos Savant June 08, 2012
Ask Marilyn: Will We Ever Colonize Other Planets?
Demitris Hatzis of Springfield, Massachusetts, writes:

Marilyn: Knowing that travel to earth-like planets will take longer than one human life span, will humans ever be able to travel and establish colonies at them?

Marilyn responds:

No way. This is never going to happen.

Consider getting to the nearest star Proxima Centauri 4.243 lightyears away. To get there in a reasonable time requires accelerating to a significant speed. Assume that fusion power is perfected for propulsion, that it is 0.7% efficient at converting mass to energy, and that the fusion fuel is half the embarkation mass.

m0*c^2/(2*sqrt(1-v^2/c^2))-m0*c^2/2=0.007*m0*c^2/2

The left side is the kinetic energy of the payload at speed v, and the right side is the energy available from the fusion fuel. The equation is not actually correct because it doesn't take into account that some of the fusion fuel must also be accelerated to a speed before it is converted into energy and the waste mass ejected, and it is impossible to transfer 100% of the energy to motion of the payload. So an integral is required to get a proper answer; but the above equation tells us the maximum theoretical speed, and that the actual speed must be less.

1/sqrt(1-v^2/c^2)-1=0.007
1/sqrt(1-v^2/c^2)=1.007
sqrt(1-v^2/c^2)=1/1.007
1-v^2/c^2=1.007^-2
v^2/c^2=1-1.007^-2
v/c=sqrt(1-1.007^-2)=0.1177

Thus v=0.1177c or ~12% of the speed of light. So the time to Proxima Centauri is a minimum of 4.243/0.1177=36 years. The time experienced by the passengers is comparable because there is not much time dilation at this marginally relativistic speed. If the acceleration is at 1 g then it would take 354 days to get to 0.1177c. The remainder of the duration of the trip would be coasting.

Which is not to imply that there are any known planets at Proxima Centauri to visit, so the expected trip to somewhere interesting is much farther-- likely on the order of 100 lightyears.

There is also the problem of slowing down at the destination. So there must be a deceleration from 0.1177c to 0 which requires more energy. Assuming the same fusion process, half the remaining mass would have to be fusion fuel. So the trip takes 36 years and 3/4 of the embarkation mass is fusion fuel. To get to higher speeds requires a higher ratio of fuel:payload. To get back to earth requires refueling at the destination with fusion fuel, but if such fuel is not expected to be available then the fuel for the return trip must also be carried along-- less than 1/16 of the embarkation mass would be payload, the rest fusion fuel. The integral makes the numbers much worse.

Could there be an energy source more efficient than fusion? All that comes to mind is antimatter, but antimatter is extremely energy intensive to create and basically impossible to store, so can be ruled out indefinitely. Any trip requiring generations of travelers to complete is out of the question. A 72 year trip to the nearest star and back is pointless. A one-way trip there to collect data to be sent to earth by radio communication is problematical, and who would go on such a suicide mission for 36 years in the void of space? No mind could handle the psychological strain, so anyone who would choose to go is not psychologically fit for the trip. Clearly a robotic craft would be indicated, but 36-year fail-safe reliability technology doesn't yet exist.

The difficulties are enormous. Using passive sensors on earth, in earth orbit or on the moon, is the only practical way of learning about the rest of the universe outside of the solar system. We're stuck here indefinitely.
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Re: Space, the final frontier

Postby phobos rising » Sun Jul 08, 2012 7:52 pm

[quote=robert46]
Thus v=0.1177c or ~12% of the speed of light. So the time to Proxima Centauri is a minimum of 4.243/0.1177=36 years. The time experienced by the passengers is comparable because there is not much time dilation at this marginally relativistic speed. If the acceleration is at 1 g then it would take 354 days to get to 0.1177c. The remainder of the duration of the trip would be coasting.

[/quote]

You mentioned 354 days to accelerate but not the 354 days to decelerate.
And then it is either unknown or unstated for what duration of time the
craft would be coasting at 0.1177c.
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Re: Space, the final frontier

Postby robert 46 » Thu Jul 12, 2012 9:12 am

phobos rising wrote:You mentioned 354 days to accelerate but not the 354 days to decelerate. And then it is either unknown or unstated for what duration of time the craft would be coasting at 0.1177c.

Yes, however I mentioned coasting before bringing up the matter of decelerating.

v=a*t, d=I(a*t,dt)=a*t^2/2=g*354days^2/2 [I()=integral, a=g]
Let Tc=coasting time=(L-2*d)/0.1177c=(4.243ly-2*d)/0.1177c

I'll leave putting in the relevant conversion factors and calculating a numeric answer as an exercise for you. I suggest converting d to lightyears so that the time is in years because c drops out as 1 ly/y.
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Re: Followup

Postby phobos rising » Thu Jul 12, 2012 10:05 am

robert 46 wrote:... * The remainder of the duration of the trip > > would be < < coasting.

[Certain text between these sentences was removed for emphasis.]

** There is also the problem of slowing down at the destination.




phobos rising wrote:You mentioned 354 days to accelerate but not the
354 days to decelerate.




robert 46 wrote:Yes, however I mentioned coasting before bringing up the matter of decelerating.


I am wrong, because in my mind, I interpreted "would be" above as meaning "is."

So, if the ideas were to be juxtaposed with each other, I might read your point as:



"The remainder of the duration of the trip would be coasting, excepting
that there is also the problem of slowing down at the destination."
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Re: Followup

Postby robert 46 » Thu Jul 12, 2012 3:12 pm

phobos rising wrote:So, if the ideas were to be juxtaposed with each other, I might read your point as:

"The remainder of the duration of the trip would be coasting, excepting that there is also the problem of slowing down at the destination."

One might hypothesize a flyby of Proxima Centauri and a slingshot effect to a farther destination: which makes the coasting duration indefinite. This would make sense for a robotic probe, anyway.
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