Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V10_0/V10_017.TXT

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Date: Mon,  4 Sep 89 00:19:26 -0400 (EDT)
Subject: SPACE Digest V10 #17

SPACE Digest                                      Volume 10 : Issue 17

Today's Topics:
         Re: Voyager Interstellar Trajectory
         Astronaut Eugene Cernan to speak in Acton MA
             Re: Voyager & Star Parallax
        Where the hell are electric-ion thrusters????
           Re: RTG's and nuclear reactors.
              Re: Impossible Space Goals
             Re: Neptune on the Boobtube
             Re: Voyager: Tape recorder?
              Hipparchos Rescue
             Re: How is Voyager powered?
----------------------------------------------------------------------

Date: 29 Aug 89 05:18:41 GMT
From: bfmny0![e-mail]  (Tom Neff)
Subject: Re: Voyager Interstellar Trajectory

Casey and others have it right.  The superficially silly assertion that
FIRST Voyager will make its closest approach to Barnard's Star and THEN
it will pass through the Oort cloud, is trivially true because Voyager
is hardly heading towards Barnard's star at all!  Its technically
closest approach to BS will occur about 1ly out, and the Oort cloud, if
it exists, is about 1.25ly out.  (If you believe current thinking
anyway.)

The only "real" approach on the books is with a little red dwarf called
Ross 248 and that's 40k years away, with a 1.7ly approach.  If humanity
is still around 40k years from now, all space within 100ly of earth will
probably be subdivided 3 dimensionally for condos and Voyager will be
netted and braked and warehoused for the public safety. :-)

Seriously, all of this is moot.  The sole remaining event of interest is
the heliopause encounter, and that is of great interest indeed so it's
vital that power conservation take hold once Neptune dwindles to
impracticable viewing size.  The RTG decay figures appear to make it a
race between crossing the solar wind boundary -- a vital shot at
understanding the nature of the interstellar medium -- and underwattage
fatal to transmission.  I predict that if the funding holds, the
struggle to win the heliopause race will rival all the behind the scenes
Voyager heroics heretofore.

And I renew the call for MEDALS FOR THE VOYAGER TEAM!  Let's have a
goddamn "phone tree alert" about something REAL!
--
"We walked on the moon --    ((    Tom Neff
    you be polite"         ))     [e-mail]

------------------------------

Date: 24 Aug 89 20:14:40 GMT
From: peregrine!ccicpg!zardoz!tgate!ka3ovk!drilex!axiom!linus!alliant!merk!spdcc!ftp!huxley![e-mail]  (Steve Stein)
Subject: Astronaut Eugene Cernan to speak in Acton MA

Astronaut Captain Eugene Cernan, the last man to walk on the moon,
will be speaking at Acton-Boxboro (MA) Regional High School on
Thursday, October 12 from 6:30-8:00 PM.  Tickets are $12 for an
adult, $10 for children.  Proceeds will benefit the Acton
Discovery Museums.

There will be a reception following the talk at the Acton
Science Discovery Museum.  A limited number of tickets are
available for the reception at $35 apiece.

For tickets, please call the Acton Discovery Museums at
[phone].

------------------------------

Date: 29 Aug 89 04:09:30 GMT
From: [e-mail]  (Terry Hancock)
Subject: Re: Voyager & Star Parallax

In article <[e-mail]> [e-mail] (Rick Coates) writes:
>
>Now, just a little while ago someone asked about putting a telescope in
>orbit way out to study star parallax.  Why not use images from Voyager?
>It's not its primary mission, but they should work.  I realize that the
>craft is in no way set up to do whole sky surveys, but even a few images
>might be helpful.  The navigator also said that they could resolve images
>to sub-pixel precision - would this be good enough?
>
    NO.  I was one one of the posters who suggested a project
like you describe -- and in fact, my first impulse was to figure
out how good Voyager could the job.  It turns out that it's
absolutely lousy for this -- you can do a lot better from Earth.

    Voyager's cameras were designed to take relatively close-up
shots of planets.  Even the 1500mm narrow angle camera really can't
be called a telescope, certainly not for parallax purposes.  A single
pixel on a Voyager image is just too big to do it.

    Another poster mentions alignment problems with using
Voyager as an astrometry platform.  I'll just refer you to him on
that subject.

    It's too bad really, I wish somebody could prove me wrong
on this and figure out a way to do it with these instruments.  Perhaps
there's some way to set Voyager slowly rotating and use some part of
the spacecraft to occult stars and watch it with the photopolarimeter.
Is it possible to point the scan platform at one of the magnetometer
booms, for example?

    Of course, you'd have to keep Voyager rotating at a very
constant rate, or the data would be useless.  And, of course, it
could only be one end of the baseline (you could in principle just
wait for it to move, but this causes a lot of problems that limit
you to about what you can do on Earth anyway), although if it
were to work, you could use both Voyagers.  I can think of a lot of
other problems too.  Perhaps they are soluable, though.  Anyone
else have ideas?

***********************
Terry Hancock
[e-mail]
***********************
***********************

------------------------------

Date: 29 Aug 89 05:44:51 GMT
From: vsi1!daver!lynx![e-mail]  (Neal Woodall)
Subject: Where the hell are electric-ion thrusters????

In article <[e-mail]> [e-mail] (Terry Hancock) writes:

>1> Depends on what you mean by "current technology" -- Ion
>drives capable of doing this (with the appropriate power source),
>do exist and have undergone vacuum chamber testing.  That they have
>not been used for main thrust on a spaceprobe has more to do with
>senseless conservativism than with any real technical challenge.

Now here is a subject that I have been meaning to bring up lately:
ion thrusters. Where the hell are they? I understand that the US has
had one tested and ready to go for over seven years now. I uses
(mercury, cesium....I cannot remember which) as the reaction mass, and
uses very high voltages to shoot the charged particles out at HIGH
velocities....not much thrust, but an exceedingly high specific
impulse (although I have read that some tested designs will produce
about 50 lbs of thrust!).

These things don't give very high accelerations, but they can run
continuously for *months*.....thrust all the way! Accelerate for half the
trip, decelerate for the other half. How do the thrust forces compare
to the chemical thrusters used on deep-space probes of today? I know
that chemical rockets give more acceleration, but they can only burn
for short periods of time.

Didn't the US conduct a test called SERT a few years ago? (SERT = Space
Electric Rocket Test) What was the outcome of the test??

Also, it seems that an electric-ion thruster is perfect for earth orbit
transfer vehicles....you could move large masses with them, it would just
take awhile.

>(Some people are distrustful of letting a probe use such "new and
>unproven technology" -- of course it wouldn't be such a big risk if we
>had a larger exploratory program than we do, but as it is, losing one
>probe would be catastrophic to the program).

Is this the real reason? Why the hell cannot our country do with two fewer
B2 bombers, and give that money to JPL for probes, including a full
program of electric-ion engine probes to various parts of the solar system?
All of the JPL people on TV the other night were saying that this is the
last Neptune mission in their lifetimes (even some of the younger ones said
this).....if we had an working electric-ion system, we could send probes
to all points in our system! Hell, a manned Mars mission would be fast
with an ion thruster!




Neal

------------------------------

Date: 29 Aug 89 05:01:07 GMT
From: vsi1!daver!lynx![e-mail]  (Neal Woodall)
Subject: Re: RTG's and nuclear reactors.

In article <[e-mail]> [e-mail] (Rikhit Arora) writes:

>In response to my original queries regarding Galileo's RTG's, Henry
>mentioned that one should get rid of RTG's altogether and switch to
>nuclear reactors.  Well, what exactly is an RTG then?  It *is* called
>a Radioisotope Thermoelectric Generator.  How much different is it
>from a genuine nuclear reactor?

I am going to work from old memories here....a nuclear reactor is a device
which uses the neutron emissions of certain radioactive elements to build
up a nuclear chain reaction (you know the old stuff....one neutron causes
multiple neutrons, each of which causes more multiple neutrons, etc). In
the case of a nuclear reactor which generates electricity, the reaction is
moderated by some control rods made of cadmium (which is a "hungry" neutron
absorber), else the reaction can get out of hand and meltdown! These things
are tricky to moderate, and need an active control system to keep them
from going super-critical.

The RTG's used in spaceflight are probably based on nuclear fuel that does
not emit neutrons, and therefore cannot start a nuclear chain reaction. The
decaying nuclear fuel will emit large ammounts of energy for a long time,
but there is no chance of a runaway reaction and its possible consequences.
Because there is no "chain reaction" possible, there is no need for an active
control system, and the thing is both simpler and safer than a real reactor.
Of course, the power output is lower, but you don't get something for nothing.

Any comments from net.land??


>And what technical difficulty
>prevents NASA from using a real nuclear reactor for these deep space
>probes?

Probably that they are too tricky to moderate (needs that active control
system) and there are more failure modes, some of them catastrophic!





Neal

------------------------------

Date: 29 Aug 89 05:16:03 GMT
From: vsi1!daver!lynx![e-mail]  (Neal Woodall)
Subject: Re: Impossible Space Goals

In article <[e-mail]> [e-mail] (Tom Neff) writes:

>The first real answers about Lunar composition (remember to forget Zond)
>came from Surveyor.  It did everything you could have asked -- it was a
>terrific program, my all time favorite until Viking.

How interesting....I too have always had a "soft spot" in my heart for the
Surveyor craft. They were really neat little probes, and looked very
much like what one would imagine a "space probe" (as opposed to a plain-old
earth-orbiting satellite) looked like.

BTW, what ever became of the pieces of a Surveyor that wer brought back by
some of the Apollo astronauts....one of the Apolllo landings just happened
to be about 300 meters from one of the Surveyors.

It might be interesting to have everyone post to the net which of the
unmanned probe missions is their favorite.....it could provide some
interesting discussions! Pioneer, Voyager, Viking, Surveyor, Mariner....which
is your personal favorite?



Neal

------------------------------

Date: 29 Aug 89 07:37:50 GMT
From: uhccux!munnari.oz.au!basser![e-mail]  (Peter Merel)
Subject: Re: Neptune on the Boobtube

In article <[e-mail]> [e-mail] (Brian A. Jarvis) writes:
>In article <[e-mail]> [e-mail] (John Sparks) writes:
>>The answer was that in 8,000 years it will fly by Barnard's Star, in 20,000 or
>>so it will pass Proxima Centauri, and then the Oort cloud.
...
>said eventually it would be dragged back into the solar system by the
>sun's gravity and could conceivably fly by Earth again in a few centuries.
...

You yanks don't know scientific illiteracy from Uranus. Australia has three
commercial TV operators, all of whom seem to be in some sort of competition
to see who can make the stupidest statement about Voyager. My favourite so
far was a guy called Brian Henderson, who's been the anchorman on one channel
here for about twenty years and is touted as informed and respectable.
He said: "In a stunning scientific breakthrough the latest pictures from
NASA's jet propulsion labs reveal ice volcanoes on Triton ... This makes
Triton the third body in the solar system to exhibit volcanoes, the other
two being Earth itself and the jovian moon, Ten."
--
 "We got to figure out the right way to go around and make people not understand
  their bullshit" - SRV.                                       [e-mail]
 ([e-mail]){uunet,mcvax,ukc,nttlab}!munnari!basser.oz!pete

------------------------------

Date: 29 Aug 89 05:24:42 GMT
From: vsi1!daver!lynx![e-mail]  (Neal Woodall)
Subject: Re: Voyager: Tape recorder?

In article <[e-mail]> [e-mail] writes:

>I keep hearing of data being spooled to the tape recorder for later
>transmission to earth.  Surely they don't use a conventional mag.
>tape drive.  Wouldn't something solid state like a large array of
>eeprom be more robust?

Remember: the Voyagers were built with 1975 technology....our "bag of
tricks" did not include anything like eeproms in 1975! The Voyager's
recorder is an 8-track mag-tape drive...reel to reel!




Neal

------------------------------

Date: 26 Aug 89 16:44:52 GMT
From: cs.utexas.edu!ut-emx!hutto![e-mail]  (Henry Melton)
Subject: Hipparchos Rescue



Suppose Hip were to adjust its perigee down to 100nm.  Then atmospheric
resistance would begin to decay the orbit more.  This would cause the
apogee to decrease.  By making corrections regularly to maintain the
perigee, the orbit might be circularized at a low enough level so that a
rescue might be able to match orbits with it.

I realize that this will never happen due to non-technical reasons,
however, is there any technical flaw in this suggestion?
--
Henry Melton ...!cs.utexas.edu!ut-emx!hutto!henry
1-512-8463241 Rt.1 Box 274E Hutto,TX 78634

------------------------------

Date: 29 Aug 89 04:41:47 GMT
From: vsi1!daver!lynx![e-mail]  (Neal Woodall)
Subject: Re: How is Voyager powered?

In article <[e-mail]> [e-mail] (Paul Dietz) writes:
>In article <[e-mail]> [e-mail] (Dennis M. O'Connor) writes:

>>I believe Voyager uses one or more RTGs. RTGs contain a sub-critical
>>mass of a fissionable material ( plutonium, I think ? ). The
>>fission occuring causes the fuel to be hot.

>No, the RTGs use Pu-238, which undergoes alpha decay, not fission.

I read recently (in the Wall Street Journal maybe) about a company that
wants to market a commercial RTG in the US! The design was their own, was
based on decay of strontium 90, was the size of a large coffee can, and
supplied 75 watts (no voltage/current given). It can supply power for
about 25 years (even though the article did not say it, I am sure that
the power by the end of the RTG's lifetime must be way down).

And, the company wants to market a larger version in the future! They
envision a larger version that will be about the size of a 5 gallon water
container (like is used on a water cooler) and will produce about 300
watts! Now we are talking about some useful power!

The article said nothing about the cost of such a device, but since it was
in the WSJ, I would assume that the thing has some kind of viable life as
a commercial product, IFF the NRC will allow the sale of them in the US.
"Who would buy them", you ask? Well, how about survivalists, or people who
live in very remote areas, or even people who are pissed off at the electric
utilities (assuming the cost is not prohibitively high). I would buy one of
the large ones if it were less than 2500 $ or so.....it would be a great way
to power a VERY remote mountain retreat.

I will try to find the article an post more details......







Neal

------------------------------

End of SPACE Digest V10 #17
*******************