Aucbvax.6294
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Thu Feb 25 03:57:12 1982
SPACE Digest V2 #115
>From OTA@S1-A Thu Feb 25 03:47:02 1982
SPACE Digest Volume 2 : Issue 115
Today's Topics:
re: giant mass driver to colonize other galazies by accelerating to c/2
re: giant mass driver to colonize other galazies by accelerating to c/2
Re: Quasars
re: giant mass driver to colonize other galazies by accelerating to c/2
Laser & Hydrogen
What a million Gs?!?
Re: Quasars
----------------------------------------------------------------------
Date: Wednesday, 24 February 1982 08:09-PST
From: KING at KESTREL
Subject: re: giant mass driver to colonize other galazies by accelerating to c/2
To: dietz at usc-ecl, space at mit-mc
cc: King at KESTREL
How do you decelerate when you get there?
------------------------------
Date: 24 Feb 1982 1027-PST
From: Paul Dietz <DIETZ AT USC-ECL>
Subject: re: giant mass driver to colonize other galazies by accelerating to c/2
To: KING at KESTREL, space at MIT-MC
cc: DIETZ at USC-ECL
Deceleration: Presumably with rockets, stellar sails, pressure of the
interstellar gas against magnetic fields (you'd have to ionize it first).
Steering when you get to the other galaxy is no problem: just put a
large electric charge on the ship and let the galactic magnetic field
move you around. You could throw away most of the ship, or use it for
fuel.
Length of the Mass Driver: c = 3.0E8 m/sec (m=meters), and g = 10 m/sec^2,
so it takes 1.5E8 / 1.0E4 = 1.5E4 seconds to reach .5c at 1000g's
(ignoring relativity). During that time you travel .5at^2 =
(.5)(1.0E4)(2.25E8) = 1.125E12 meters, or 1.125 billion kilometers.
(darn arithmetic errors!)
One millon g's: Lest this figure seem ridiculous, let me note that
railguns have already achieved accelerations of over 1E6g's. So it's
just engineering to design a railgun 1.1E6 km long.
The big advantage of linear accelerators is that energy is used much
more efficiently than in a rocket, where you have to use exponentially
increasing amount of energy to accelerate a payload to higher and
higher velocities. The energy used in a mass driver grows more slowly.
Also, the mass driver can be used repeatedly.
------------------------------
Date: 24 Feb 1982 11:22 PST
From: Lynn.ES at PARC-MAXC
Subject: Re: Quasars
In-reply-to: Reed.ES's message of 23-Feb-82 8:52:52 PST
To: Space-Enthusiasts at MIT-MC
cc: Reed.ES, Lynn.es
Halton Arp was the astronomer mentioned in the article. Everyone agrees that all
quasars show huge redshifts. Essentially all normal galaxies are receding from
us, causing a red-direction Doppler shift (redshift) in their spectra. The bigger
the redshift, the faster the recessional velocity. The fact that the universe (on
the galactic scale) is expanding requires that distance and speed of recession be
directly related; that is, the faster the recession, the farther the galaxy is from
us.
Now we have two choices in explaining quasars: 1) they follow the rules of
galaxies, and the huge redshifts mean huge recession velocities and huge
distances, or 2) there is some physical means (what means is not clear) that
redshifts quasar light, and they are not so distant nor receding.
Arp is apparently the only astronomer of international note that believes the
second choice. His contention is that some quasars show physical links with
normal galaxies that have small redshifts and are therefore nearby. Most other
astronomers believe that the apparent physical connections have to be
coincidence of wispy objects nearby happening to lie in a line of sight with the
distant quasars and some nearby galaxy. That seems easier to believe since no
one, after years of trying, has come up with a decent explanation of a
non-Doppler way to create huge redshifts in quasars. Arp wants to continue
looking for physical connections to quasars to show that there are too many for it
to be line-of-sight coincidences, while other astronomers want a physical
explanation of the redshifts before spending much on the connections search.
A few years ago, several astronomers were listening to Arp with interest, but
apparently from the Times article and a few others elsewhere on quasars, few
now give Arp much chance of being right.
/Don Lynn
------------------------------
Date: Wednesday, 24 February 1982 11:26-PST
From: KING at KESTREL
To: Paul Dietz <DIETZ AT USC-ECL>
cc: space at MIT-MC, King at KESTREL
Subject: re: giant mass driver to colonize other galazies by accelerating to c/2
The ship has a momentum of 1.5e17 newton-seconds. I suspect
it would be necessary to shoot a dummy load through the mass driver
backwards to avoid losing the mass driver after a couple of shots.
I would design it as a series of separate modules "attached"
by station-keeping hardware. Each module would have its own energy
storage system (and probably its own power plant).
However, one wonders whether something analogous to a
traveling wave tube could be used. Send your energy pulse down a
non-uniform helical superconducting cable wound in such a manner that
the spiraling electrical pulse's velocity matches the ship. The
mechanism is simple, but now the energy has to be supplied all at once.
I'd rather use a laser and have frozen Hydrogen on the tail of
the ship, to be heated within an inch of its life and sent back at
99c. Anyone have any thoughts on this?
Sorry about the lack of detail - I'm pressed for time right
now.
Dick
------------------------------
Date: 24 Feb 1982 1200-PST
From: Paul Dietz <DIETZ AT USC-ECL>
Subject: Laser & Hydrogen
To: king at KESTREL
cc: space at MIT-MC
The problem with this scheme is that you have to accelerate your fuel,
along with the ship. So, the amount of fuel increases exponentially
with the final velocity of the payload. The mass driver avoids this
problem. Also, I imagine that heating the hydrogen to such high temperatures
will roast the ship in a bath of x-rays and gamma-rays.
Recoil can be minimized by making the launcher very massive. If it
weighs 1.0E12 tons the velocity increment is 150 m/sec. By launching
when the launcher is on opposite sides of the sun the delta-v's will
cancel out. Thumbnail calculations indicate that a trillion tons is
bout the mass of an asteroid 5-10 km in diameter, so materials are no
problem.
The biggest problem is the energy source. It can be massive, though, because
you don't have to accelerate it.
------------------------------
Date: 24 Feb 1982 1502-PST
From: Ted Anderson <OTA AT S1-A>
Subject: What a million Gs?!?
To: space at MIT-MC, Dietz at USC-ECL
In support of Paul's claim that a million Gs is "reasonable", consider
this: I understand that experiencing an acceleration of 100,000 Gs is not
uncommon for an artillary shell. Now I don't know that this is the same
type of artillary pieces but I know of at least two fairly sophisticated
devices that live in artillary shells. On is a nuclear warhead, the other
is a frob which actually looks for tanks in some fashion. I don't know
any of the details, however. At any rate, I'd be surprised if something
interesting like an intergalactic probe couldn't be built to withstand
1 million Gs.
Ted Anderson
------------------------------
Date: 24-Feb-82 15:17:52 PST (Wednesday)
From: Reed.ES at PARC-MAXC
Subject: Re: Quasars
In-reply-to: Lynn's message of 24 Feb 1982 11:22 PST
To: Lynn
cc: Space-Enthusiasts at MIT-MC, Reed
If I remember correctly, a large gravitational potential can also cause a
redshift. Is it not possible that a quasar's redshift could be at least
partially gravitational in nature, thus reducing the recession velocity
(and therefore distance) for a given redshift?
On a scifi tack, what would a spaceship travelling away from us at an
appreciable fraction of the speed of light (say .5c) look like to an
observer on earth? Would we see a light source with a redshift? Could a
quasar be such a light source?
-- Larry --
------------------------------
End of SPACE Digest
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