Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V16_2/V16NO209.TXT

Internet Message Format | 1993-07-13 | 34KB

Date: Sun, 21 Feb 93 05:47:07
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V16 #209
To: Space Digest Readers
Precedence: bulk


Space Digest                Sun, 21 Feb 93       Volume 16 : Issue 209

Today's Topics:
                   Book Computers/AI in Shuttle-SSF
                        Canadian SSF effort ??
                        Getting people into S
  HOW TO OBTAIN INFAMOUS ARTICLE "DOES AMERICA SAY YES TO JAPAN"***
                      Magellan Update - 02/19/93
                    Mars Rescue Mission, what if!
                   McElwaine disciplined! (2 msgs)
                  Nobody cares about Fred? (4 msgs)
   Reliable Source says Freedom Dead, Freedom II to be dev (2 msgs)
                       SETI and Virtual Reality
                             SIRTF Update

    Welcome to the Space Digest!!  Please send your messages to
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----------------------------------------------------------------------

Date: Sat, 20 Feb 1993 01:22:50 GMT
From: Gary Coffman <ke4zv!gary>
Subject: Book Computers/AI in Shuttle-SSF
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>
>I wonder if the non-directional LED-based IR systems used in some office
>networks and the like would have enough range to be practical for the job.
>That wouldn't be so difficult.

I'd expect them to be blinded by the light of the raw Sun and it's
reflections off the white Shuttle, white suits, and the Earth. It's
far from an indoor office environment. These things work indoors by
bouncing their signals off walls and ceilings. Might work inside the
Shuttle until someone drifted in front of the sensor, but then so would
a thinnet coax.

Gary
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

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

Date: Sat, 20 Feb 1993 01:17:09 GMT
From: Gary Coffman <ke4zv!gary>
Subject: Canadian SSF effort ??
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Rohit Sharma) writes:
>
>Does anyone know what's going to happen regarding the C S A effort towards
>SSF now that it's been cancelled (eh!) ????           ^^^^^
>                                                    Canadian Space Agency

Fred's not dead. It's just going to go through yet another descoping
and redesign. I'm sure the US' "partners" in Canada, Europe, and Japan
are totally fed up at this point. I wouldn't be surprised if they didn't
jump ship and move in with the Russians leaving the US government holding
the bag. This is just the latest in a long string of failures by the US
government to hold up it's end of partnership deals with other countries.
If the truss goes this time, there go the lab modules and the arm, and
the power to run them.

Gary

--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

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

Date: 19 Feb 93 13:33:00 GMT
From: Jerry Laplante <[e-mail]>
Subject: Getting people into S
Newsgroups: sci.space

DR>Newsgroups: sci.space
DR>From: [e-mail] (Dave Rickel)
DR>Subject: Re: Getting people into Space Program!
DR>Message-ID: <[e-mail]>
DR>Date: Thu, 18 Feb 1993 03:49:40 GMT
DR>Keywords:
DR>Followup-To:


DR>In article <[e-mail]>, [e-mail] (Pat) write
DR>|> BUt the X-15 was a massive success, for a very difficult problem
DR>|> and it delivered a functionally operational spacecraft.

DR>Hmm.  Stretching a bit, i think, to call the X-15 an operational spacecraft.
DR>NASA gave the X-15 pilots (well, some of them) astronaut's wings, but the
DR>craft never came close to orbital velocity.  I doubt (but could well be
DR>wrong) that the X-15 could have survived reentry from orbit.

DR>I've heard that there were plans to launch a derivative into orbit.


Could that be the vehicle commonly called the flying Bumble Bee, whose
clai to fame in the ordinary world is the vehicle that opened the series
and each episode of the six million dollar man. If I recall correctly it
was aeronautically considered to be unflyable, but flew anyway and was
for a while touted as a great instrument for flight to and from low
orbit. It finally crashed on descent when it touched down and did a pop
up unfortunately a Helo was in the way and the two collided as the story
goes. I think That its prototypes and other s are still on display at
the yearly aeronautical show at Edwards airforce base.


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

Date: 19 Feb 93 20:29:59 GMT
From: Andre Robotewskyj <[e-mail]>
Subject: HOW TO OBTAIN INFAMOUS ARTICLE "DOES AMERICA SAY YES TO JAPAN"***
Newsgroups: sci.space

Hello,
    Over the past few months, a paper entitled, "Does America Say Yes
To Japan?" (Louis Leclerc 1992) has been circulated over various computer
networks and BBS's. Public response has been overwhelmingly positive.
The paper addresses the issue of U.S.-Japanese trade in the context
of our uncertain economic future. Mr. Leclerc provides the reader with
a wealth of referenced background and historical information concerning
our relations with Japan, as well as important facts concerning
ongoing developments which directly effect every American consumer/worker.
The paper, however, is not published and so is only available in the
form of circulating hard copies or electronic format. Anyone interested
in obtaining a copy of "Does America Say Yes to Japan" please send email
to:
[e-mail]

Andre

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

Date: 20 Feb 1993 00:21 UT
From: Ron Baalke <[e-mail]>
Subject: Magellan Update - 02/19/93
Newsgroups: sci.space,sci.astro,alt.sci.planetary

Forwarded from Doug Griffith, Magellan Project Manager

                         MAGELLAN STATUS REPORT
                           February 19, 1993

1.  The Magellan spacecraft is operating normally under the G3043
command sequence.  Starcals (star calibrations) and desats (desaturation
of the reaction wheels) continue to be very successful.  Spacecraft
temperatures are in the expected range.

2.  A Design Review of the Transition Experiment (TEX) was held in
Denver on Thursday.  TEX will begin May 25, 1993, with an OTM (Orbit
Trim Maneuver) to lower the periapsis from 170 to 146 km.  This will be
followed by a series of small OTMs to "walk in" to the nominal aerobraking
corridor.

3.  The aerobraking corridor has been designed to minimize the time
required to achieve a near-circular orbit while keeping below the
temperature and dynamic pressure limits of the spacecraft.  The
corridor design also has to take into account the uncertainty and
variability of Venus' atmosphere.

4.  Periapsis will be kept within this corridor by enabling OTMs during
the aerobraking period, which is expected to take 70-80 days.  The
preloaded OTM parameters provide eight variations to orbit
corrections, four up and four down.

5.  During TEX the orbit period is continually shrinking, so the
spacecraft is controlled by a looping sequence which contains a series
of mini-blocks separated by pause periods.  The length of the pause
periods are controlled by global variables which can be updated by
ground command.

6.  The preparations for TEX are going well.  Engineers and mission
planners are confident that the analyses and design have a high
probability of success.

7.  A Spacecraft Technical Interchange Meeting was held on Friday in
Denver to review the spacecraft performance since last fall.  All
activities have gone very smoothly.  There was some discussion of the
upcoming high-rate gyro calibration (March 1993) and the analyses of
differences in solar panel output.

8.  Magellan is 95 days from the end of Cycle 4.
     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | [e-mail]
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | If you don't stand for
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | something, you'll fall
|_____|/  |_|/       |_____|/                     | for anything.

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

Date: Fri, 19 Feb 93 17:10:59 PST
From: Brian Stuart Thorn <[e-mail]>
Subject: Mars Rescue Mission, what if!
Newsgroups: sci.space

>Wierd question? What is the mission to Mars fails, and the astronauts are
>stranded, maybe not int he end, but early on. What steps would be done if
>anything to rescue them?

   If we do it right, the Mars Mission will be the first of many. Aldrin's
   concept calls for cycler ships always travelling back and forth with
   supplies, et al, so a stranded crew wouldn't be too far from help.

   There should be backups for all essential systems and provisions to
   ride out an accident or the "stranded" type. If the accident were
   severe enough to knock out the supplies and backups, there would
   likely be no one to rescue anyway.

-------------------------------------------------------------------------
Brian S. Thorn                                    "If ignorance is bliss,
[e-mail]                               this must be heaven."
                                                -Diane Chambers, "Cheers"
-------------------------------------------------------------------------

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

Date: 19 Feb 93 11:54:24 GMT
From: Paul Johnson <[e-mail]-mrc>
Subject: McElwaine disciplined!
Newsgroups: sci.skeptic,sci.physics,sci.med,sci.space

I came across this on comp.society in the Computers & Academic Freedom
abstracts.

> 6.  McElwaine is apparently guilty of posting "canned prefabricated
> materials" to numerous newsgroups and mailing lists, generating so
> many complaints that the school's director of computing services has
> now decreed: "If, after being requested [by a mailing list or
> newsgroup's 'owner'] to stop, he [McElwaine] continues to send his
> materials The University of Wisconsin - Eau Claire will take steps
> revoke his Internet privileges."
>        <[e-mail]>



Quite who the "owner" of the sci.* groups is I do not know, but could
s/he please ask him to stop?  :-)


Followups to sci.skeptic.

Paul.

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

Date: Sat, 20 Feb 1993 03:35:37 GMT
From: [e-mail]
Subject: McElwaine disciplined!
Newsgroups: sci.space,sci.astro,sci.space.shuttle

In article <2517@snap>, [e-mail]-mrc (Paul Johnson) writes:
>
> I came across this on comp.society in the Computers & Academic Freedom
> abstracts.
>
>> 6.  McElwaine is apparently guilty of posting "canned prefabricated
>> materials" to numerous newsgroups and mailing lists, generating so
>> many complaints that the school's director of computing services has
>> now decreed: "If, after being requested [by a mailing list or
>> newsgroup's 'owner'] to stop, he [McElwaine] continues to send his
>> materials The University of Wisconsin - Eau Claire will take steps
>> revoke his Internet privileges."
>>        <[e-mail]>
>
>
>
> Quite who the "owner" of the sci.* groups is I do not know, but could
> s/he please ask him to stop?  :-)

Perhaps the "regulars" of sci.astro, sci.space, and sci.space.shuttle
(the three newsgroups I have seen his postings on) could just send a
request to the UW-EC?  It might be a good idea for the other various
science groups too.

I'll be willing to include my name in such a request.

-Bill Gawne,  Space Telescope Science Institute

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

Date: 20 Feb 93 00:54:39 GMT
From: Gary Coffman <ke4zv!gary>
Subject: Nobody cares about Fred?
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Allen W. Sherzer) writes:
>In article <[e-mail]> [e-mail] writes:
>
>>>>So, NASA adds EVA time whenever it can on future Shuttle missions.
>
>>>Too little too late.
>
>>Merely your opinion Allen.
>
>No, they have found that EVA now poses serious risk to Fred assembly.
>For very very small amounts of money this risk could have been
>examined and accounted for at a time when design modification would have
>been a lot cheaper.

Heh. When *I* say that *your* pet programs are likely to face unforeseen
difficulties and schedule slips, you say, "no way they're fast track."
But when NASA faces an unforeseen problem it's "bad planning." When
NASA decides that on orbit assembly is too costly, you scream "bad
planning", but when it's *your* project it's, "we don't need heavy
lift, we can send it up in little pieces and assemble it in orbit."
Double standard?

Gary
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

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

Date: 20 Feb 93 02:39:38 GMT
From: "Allen W. Sherzer" <[e-mail]>
Subject: Nobody cares about Fred?
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Gary Coffman) writes:

>NASA decides that on orbit assembly is too costly, you scream "bad
>planning", but when it's *your* project it's, "we don't need heavy
>lift, we can send it up in little pieces and assemble it in orbit."
>Double standard?

Huh? Sorry Gary but I have never proposed using DC for station
assembly.

Now if you read my posts on station assembly and logistics you will find
that HLV's (Energia and Zenith Star) play key roles.

Now if DC works, it would make a superb logistics vehicle and ACRV but
that isn't assembly or launch. However, I do hope that we become good
enough at EVA to accomplish large scale assembly ASAP.

Maybe this is the source of our disagreement? Maybe if you read my
posts you would agree with them more.

  Allen

--
+---------------------------------------------------------------------------+
| Allen W. Sherzer | "A great man is one who does nothing but leaves        |
|  [e-mail]     |  nothing undone"                                       |
+----------------------116 DAYS TO FIRST FLIGHT OF DCX----------------------+

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

Date: 20 Feb 93 02:42:54 GMT
From: "Allen W. Sherzer" <[e-mail]>
Subject: Nobody cares about Fred?
Newsgroups: sci.space

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

>actually the biggest paradox here is McDac.  AS Prime contractor on DC-X,Y.
>THey seem to be doing a hell of a job on building a cheap,sturdy, reliable
>LEO transfer vehicle. Oh, I forgot to mention flexible.

>AS a major sub to Freedom, they are participating in a technological
>boondoggle...

I don't think it is a paradox. McDac is a large company and each
project is for different managers with different expectations and styles.

The lesson is to run projects like DC and not like Freedom.

  Allen
--
+---------------------------------------------------------------------------+
| Allen W. Sherzer | "A great man is one who does nothing but leaves        |
|  [e-mail]     |  nothing undone"                                       |
+----------------------116 DAYS TO FIRST FLIGHT OF DCX----------------------+

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

Date: 20 Feb 93 02:59:25 GMT
From: "Allen W. Sherzer" <[e-mail]>
Subject: Nobody cares about Fred?
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Brad Whitehurst) writes:

>    Welll, Allen, I'm just a lowly mechanical engineer (hate those
>li'l electrons!!),

Some of my best friends are mechanical engineers :-)

>but I would propose that there are some qualitative
>differences in forging a large, mechanical assembly out of steel,
>aluminum, and composites, compared to primarily software
>constructions.

I think they may be closer than you think but reasonable people can
disagree.

>The laws of physics tend to thwart many of my best ideas! :-)

Me too. Two weeks ago I spent the whole weekend at work because the
speed of light is too slow. The processor couldn't process the signals
on the sensor I am building fast enough so I had to find an alternative
algorithm before the demo.

>Natural laws, particularly with respect to materials'
>behaviours, are far from perfectly understood, especially outside
>research labs.

Exactly why building prototypes and EVA research are so important.
Exactly why more effort should have been spent building prototypes
rather than paper.

>I can well understand how difficulties ("design error"
>assumes a goof...sometimes there's no predicting somethings) may
>appear in a new project far into the construction.

Sure. That doesn't absolve the engineer of the job of working
to identify those problems ASAP. I don't expect them all to be
cought but I do expect reasonable effort.

Let me give an example from everybody's favorite project: DCX. They
had a serious problem with one of the tanks. The builder had major
problems with welds on one of the tanks. This resulted in a 30 to
60 day schedule slip. Now had DCX not been built and they just took
a stab at DC-1 (the way NASA did with Freedom) that problem wouldn't
have been found for years and would have resulted in major schedule
slips and a big overrun.

I don't expect perfection but I do expect reasonable efforts at risk
reduction.

    Allen
--
+---------------------------------------------------------------------------+
| Allen W. Sherzer | "A great man is one who does nothing but leaves        |
|  [e-mail]     |  nothing undone"                                       |
+----------------------116 DAYS TO FIRST FLIGHT OF DCX----------------------+

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

Date: 20 Feb 93 01:10:27 GMT
From: Brian Stuart Thorn <[e-mail]>
Subject: Reliable Source says Freedom Dead, Freedom II to be dev
Newsgroups: sci.space

>Agreed. It's too early to say but this may result in a real station. I
>look forward to being able to support this.
>
>  Allen

    Actually, this sounds like we're abandoning a Space Station in
    favor of an Industrial Space Facility variant. While I'm not
    complaining, I can't see calling this thing a "Space Station".

    If the cutback is as massive as the original message in this
    thread indicates, then why don't we just get the Mir 2 specs
    from Russian and build a module or two for it?

    What on Earth are Canada, ESA, and Japan going to do with all
    their Freedom hardware? After ISPM and Freedom, international
    cooperation in space is likely to be dead, dead, dead.

    Oh well, I guess our youth can look forward to building nice
    highways in the future, not much else.


-------------------------------------------------------------------------
Brian S. Thorn                                    "If ignorance is bliss,
[e-mail]                               this must be heaven."
                                                -Diane Chambers, "Cheers"
-------------------------------------------------------------------------

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

Date: Sat, 20 Feb 1993 02:19:20 GMT
From: Jeff Bytof <[e-mail]>
Subject: Reliable Source says Freedom Dead, Freedom II to be dev
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Brian Stuart Thorn) writes:
>From: [e-mail] (Brian Stuart Thorn)

>    Oh well, I guess our youth can look forward to building nice
>    highways in the future, not much else.
>Brian S. Thorn                                    "If ignorance is bliss,

Or filling potholes, pickup up trash and sweeping steets under
Clinton's new "National Service Plan".

-Jeff "Falling Down" Bytof

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

Date: 19 Feb 93 13:33:00 GMT
From: Jerry Laplante <[e-mail]>
Subject: SETI and Virtual Reality
Newsgroups: sci.space

TT>From: [e-mail] (Tim Thompson)
TT>Newsgroups: sci.space
TT>Subject: SETI and Virtual Reality
TT>Date: 17 Feb 1993 17:49:44 GMT
TT>Message-ID: <[e-mail]>
TT>Reply-To: [e-mail]

TT>   In response to an earlier query, I am quite confident that the NASA SETI
TT>program has nothing to do with virtual reality. Remember, communication with
TT>any possible aliens is definitely NOT part of the NASA SETI project. It is
TT>strictly a radio search for extraterrestrial signals of an intelligent natur
TT>Detection of such a signal is the project's sole purpose, responding to any
TT>such signal has never been considered as part of the NASA SETI project. Of c
TT>e,
TT>I suspect that if such a signal is detected, someone will become interested
TT>the communication aspects.


TT>---
TT>ALL OPINIONS EXPRESSED ARE MINE ALONE. BELIEVE THEM AT YOUR OWN RISK.
TT>I AM NOT AN OFFICIAL OR UNOFFICIAL SPOKESMAN FOR THE SETI/HRMS PROJECT.
TT>------------------------------------------------------------
TT>Timothy J. Thompson, Earth and Space Sciences Division, JPL.
TT>Assistant Administrator, Division Science Computing Network.
TT>Secretary, Los Angeles Astronomical Society.
TT>Member, BOD, Mount Wilson Observatory Association.

TT>INTERnet/BITnet:    [e-mail]
TT>NSI/DECnet:         jplsc8::tim
TT>SCREAMnet:          YO!! TIM!!
TT>GPSnet:             118:10:22.85 W by 34:11:58.27 N
The only possible use of VR that I can imagine is training astronauts to
do things in some particular setting. Actually constructing that setting
in an artificial environment  might be too unweildly to work with . VR
could allow others to see what a space walk was like or walking on the
moon was like or to thow it out further...What a walk on mars would be
like without actually having been there.I'm sure I've missed some more
promissing aspects of VR but ...it's late and the suns rising over the
curbstones...it's time for bed..


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

Date: 19 Feb 93 23:20:24 GMT
From: Steve Willner <[e-mail]>
Subject: SIRTF Update
Newsgroups: sci.space,sci.astro

I've been meaning to post a progress report on SIRTF, the fourth (and
greatest, if it ever flies) of the Great Observatories.  As many of
you know, SIRTF was the highest priority recommendation of the decade
survey (Bahcall Committee) report, but the last year has seen a
reexamination and redesign of the project.

The highlights are that the mission has been rescoped to reduce the
cost, but the key performance determinants (mirror size and lifetime)
have been retained.  This is still an observatory-class mission, not
an Explorer.  Meanwhile, the detectors that have been developed for
the program are making their way onto ground-based telescopes.  I'll
give an overview below and answer specific questions at the end.

The major effort in the rescoping was to get the mass down so as to
launch on an Atlas instead of a Titan.  The biggest change was to
plan a Solar orbit instead of high Earth orbit.  Essentially, SIRTF
will slowly drift away from the Earth and will reach a distance of
about 0.5 AU after five years.  This dramatically cuts the heat load
from the Earth at the price of reducing the data transmission rate.
The second change was to simplify the instruments to reduce the mass
that must be cooled.  This means that the straps holding the
instruments in place can be reduced in cross-section, which reduces
their heat conductance.  A third change was to recognize that the
newer detectors will generate less power than the old ones, so there
will be less heat input, further reducing strap size.  Finally, the
lifetime requirement was reduced from five years plus margin to three
years plus margin.  The net effect of these changes was to reduce the
amount of liquid helium required from 3800 liters to 920 liters.
(Obviously the reduction in helium mass and tankage further reduces
the cold mass and the strap size.)  For comparison, IRAS flew 520
liters and ISO will have 2140 liters.  The new design looks like a
telescope with a modest dewar wrapped around it.  The old design was
a much larger dewar with a telescope buried down in the middle.

Since the instruments now dominate the heat load (fully 75%), the
lifetime could go back up if the instrument heat generation can be
reduced.  (In the old orbit, the Earth contributed most of the heat
load.)

The mirror diameter remains at 85 cm.  (Sky and Telescope said it was
down to 70 cm, but they were wrong.)  Shrinking the mirror does not
save much money, since the observatory mass is determined by the
spacecraft, payload, helium, and structure and not by the telescope
itself.  The estimated mass is 1839 kg; launch capability of the
Atlas (I think it's a II AS, but I can't find the notes right now.)
is 2459 kg, so there's reasonable margin for this stage of the
project.

The capabilities retained amount to imaging, mapping, and
spectroscopy at all wavelengths from 3 to 200 microns.  Major
scientific goals include studying the Universe at large redshift and
detailed study of star formation.  In fact, almost every field of
astronomy from the Solar System outward will be affected in some way.

The instruments have been simplified considerably, but the core
capabilities have been retained.  The biggest losses are: 1) all
capability beyond 200 microns is gone.  This is a real loss, but it
is also a real simplification in such areas as baffling and
temperature stability requirements.  It also eliminates the adiabatic
demagnetization refrigerator, considered by some a technical risk.
2) The requirement for diffraction-limited spatial resolution at
wavelengths as short as 2.5 microns has been dropped.  This relaxes
requirements on the fine guidance system and on mirror quality.  The
wavelength at which diffraction-limited resolution will be provided
is subject to future technology developments but will be at worst
10 microns.  (I'd be willing to bet it will end up no worse than
5 microns and possibly as low as 3.5 microns.)  This is more of a
loss than it seems at first, though, since 3.5 microns is the ideal
wavelength for seeing the most distant objects.  3) Capability to
survey large areas quickly (as opposed to making long exposures on
small areas) has been compromised.  4) There are additional deletions
that mean some kinds of observations will take longer but will still
be possible.

The detectors for the 3 to 27 micron range have met (or nearly met)
all performance goals.  For example, the indium antimonide detectors
for 3 to 5.3 microns are being built in 256x256 format and are
delivering 90% quantum efficiency (with an anti-reflection coating),
10 electrons read noise, and less than 1 electron per second dark
current.  Some of these arrays are already in use on ground based
telescopes.  At wavelengths longer than 27 microns, individual
detectors have met the performance requirements, but there are still
issues of how to package them in the required array formats for
flight.  Nobody sees any show-stoppers, though.

The cost of the rescoped SIRTF will be considerably (35-40%) less
than the $1.3G estimated by the Bahcall committee.  (The Bahcall
committee did not include launch, as was customary when the report
was written.  In round numbers, we are talking something like $1G
_including_ the Atlas launch but _not_ corrected for inflation.)

The schedule is highly uncertain.  As far as I can tell, there's not
anybody in Washington who can say "yes" to any mission at the moment.
One reasonable schedule would be a two-year phase B starting in 1995
and a four year development/construction phase leading to launch in
2001.  Keep your fingers crossed.


In article <[e-mail]>,
[e-mail] (Dave Rickel) writes:
> There was a proposal in SPACEFLIGHT a couple months back to build a
> large passively cooled IR telescope.  I seem to remember that they
> proposed sticking it in the L-2 spot (presumably the sun-earth L-2
> spot).  Is this SIRTF, or something else?

This is not SIRTF; I think it's Edison.  This is a scaled-down
version of Large Deployable Reflector (LDR).  Compared to SIRTF, it
gives up sensitivity by running warmer in order to achieve better
spatial resolution with a larger mirror.

At the moment, this is just a concept floating around, not a mission
that has been studied in any detail.  In spite of certain over-
enthusiastic claims (now retracted, I understand), Edison is no
replacement for SIRTF.

> Anyway, the article mentioned that there is enough thermal noise that it
> really wasn't worthwhile cooling the mirror all the way down to liquid
> helium temperatures; if you can passively cool the mirror (they mentioned
> temperatures like 40-60 K) you are no longer limited by the amount of He
> you carry along (larger mirrors, longer liftimes).

It depends on what wavelengths you want to work at.  For example, if
your wavelength cutoff is 5 microns, 100 K is adequate.  This can be
achieved by passive cooling (i.e. clever design and a bunch of
radiators on the cold side of the spacecraft) even in low Earth
orbit.  Getting colder is hard, though, even if you get away from the
Earth, because the power radiated goes as the fourth power of
temperature.  Thus to reach 50 K with a given design, the heat load
has to drop a factor of 16.  Getting much below 50 K with passive
cooling seems very difficult to me.

Warm telescopes also give less penalty at longer wavelengths (beyond
200 microns).  This is because even a cold telescope will still have
residual radiation, but the brightness goes more nearly linearly than
exponentially with temperature.  So it may pay to take the hit in
telescope brightness if you can fly a larger mirror, which gives both
more collecting area and better spatial resolution.

For the prime SIRTF range of 3 to 200 microns, though, if you want
sensitivity, there is no substitute for cooling the whole telescope.
The SIRTF focal plane will run at 2-3 K, the optics not much warmer.
(I can't find it right now, but I think the maximum baffle
temperature is 10 K.)

Somebody else wrote:
> Last I looked we cooled the detectors, not the mirror.  A mirror at 40 K
> might do strange things.

No, it has to be the whole thing if you want good sensitivity.  There
is some worry that cooling the mirror could change its figure, but in
practice, ISO has found it not to be a problem.  (The tolerances are
not as strict as optical telescopes, much less HST.)  The flight
telescope will have to be tested, of course.

> using four stage thermoelectric cooling to get down to temperatures around
> 180 K.  Might be somebody's looking at a 5 or 6 stage cooling scheme.

TE cooling doesn't seem to be practical for low temperatures.  (I
don't know why.  Thermodynamics says it should work great, but when I
investigated, it turned out that suitable coolers didn't exist.)
Flight qualified mechanical coolers are available to get to 30 K or
perhaps below, and the temperature limit could no doubt be decreased
if anybody wanted to spend the money.

> TE cooling would solve a lot of servicing problems.  But with the advances
> made over the last 10 or so years in closed cycle cryostatic refrigerators
> I'd think you could fly a He cooled detector array and keep it cooled for
> many years.

For the combination of reliability, mass, and cooling power, the
engineers say you still can't beat liquid helium.  Servicing is not
planned for any of the cryogenic telescopes.  The advantage of
getting away from the Earth is bigger than the advantage of being
able to service.

If there are more questions about SIRTF, I'll try to answer them.
--
Steve Willner            Phone [phone]         Bitnet:   willner@cfa
Cambridge, MA 02138 USA                 Internet: [e-mail]
  member, League for Programming Freedom; contact [e-mail]

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


From: Dave Ratcliffe <frackit!dave>
Newsgroups: news.admin.policy,alt.privacy,comp.org.eff.talk,sci.space,sci.astro
Subject: Re: David Sternlight and wasted bandwidth
Message-Id: <[e-mail]>
Date: 19 Feb 93 01:41:09 GMT
References: <[e-mail]>
Followup-To: news.admin
Organization: Data Factory Services, Harrisburg, Pa.
Lines: 9
Sender: [e-mail]
Source-Info:  Sender is really [e-mail]

In article <[e-mail]>, [e-mail] (8 February 1993) writes:
> Need I say more?

Unless you want to reveal who you really are, No, you said quite enough.

--
 vogon1!compnect!frackit![e-mail]            |  Dave Ratcliffe  |
 - or - ..uunet!wa3wbu!frackit!dave                      |  Sys. <*> Admin. |
                                                         | Harrisburg,  Pa. |

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End of Space Digest Volume 16 : Issue 209
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