Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V16_2/V16NO286.TXT

Internet Message Format | 1993-07-13 | 14KB

Date: Sun,  7 Mar 93 05:24:32
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V16 #286
To: Space Digest Readers
Precedence: bulk


Space Digest                Sun,  7 Mar 93       Volume 16 : Issue 286

Today's Topics:
                   Alternative space station design
                   Alternative space station power
                    Followons to Venus and Jupiter

    Welcome to the Space Digest!!  Please send your messages to
    "[e-mail]", and (un)subscription requests of the form
    "Subscribe Space <your name>" to one of these addresses: listserv@uga
    (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle
    (THENET), or [e-mail] (Internet).
----------------------------------------------------------------------

Date: Sat, 6 Mar 1993 17:47:38 GMT
From: Frank Crary <[e-mail]>
Subject: Alternative space station design
Newsgroups: sci.space

In article <[e-mail]> Dr. Norman J. LaFave <[e-mail]> writes:
>> Another solution would be to have a big array (as big as Freedoms
>> arrays or bigger) attached to your station by a short tether (~100m).
>> The drag and the gravity gradient would probably produce a couple of
>> microgees though.  You would need some way of controlling it, stopping
>> it spinning and getting tangled up.  Perhaps the array could be flown
>> like a kite, deflect one half of the array slightly so that its drag
>> produces a torque in the opposite direction to the rotation you want
>> to control.

>If you're going to use a tether, why not avoid the increased reboost from
>the solar array and generate electricity with the tether itself.

Using a tether to generate power actually increases the need for
reboosts: The current creates an IxB force (essentially an
electromagnetic drag) which slows the station. Essentially it
just transforms the station's orbital kinetic energy into electricity.
You can also reverse the process: Force current through the tether
in the opposite direction and increase the station's orbital
velocity. If you had a station with a teather, that would probably
be the best way to reboost: No reaction mass is required and (I think)
the process is fairly power efficient.

                                                 Frank Crary
                                                 CU Boulder

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

Date: Sat, 6 Mar 1993 17:39:23 GMT
From: Frank Crary <[e-mail]>
Subject: Alternative space station power
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>I doubt it very much.  Have you *looked* at thermocouple efficiencies?
>They are, roughly speaking, terrible... even by photovoltaic standards.
>Why do you think they haven't replaced photovoltaics already?  There
>are plenty of commercial satellite builders who would kill for better
>power systems.

Not necessarily: Thermocouple efficiencies depend on the temperature
at which waste heat is rejected. Ideally, you could lower that down
to 3 deg Kelvin. At that point, I suspect themocouples would do
much better than a photovoltaic. However, even getting close to 3
Kelvin would be unrealistic. In Earth orbit, for example, you would
have to point it away from not only the Sun, but also the Earth and
Moon. You would also need near perfect insulation between the cold
plate and the rest of the craft. I'd say this is another idea that
looks good in theory, but...

                                                        Frank Crary
                                                        CU Boulder


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

Date: 7 Mar 1993 09:01 UT
From: Ron Baalke <[e-mail]>
Subject: Followons to Venus and Jupiter
Newsgroups: sci.space

In article <[e-mail]>, [e-mail] (Henry Spencer) writes...
>In article <[e-mail]> [e-mail] (Ron Baalke) writes:
>>The Grand Plan for exploring the solar system is a simple four step process:
>>
>>    1. Flyby
>>    2. Orbiter
>>    3. Unmanned Landing
>>    4. Manned Landing

>What's step 5?  Or is one manned landing the end of exploration for any
>planet?

It was implied that each step can consist of multiple missions.
Also, some of the steps can be combined in a single mission (ie: Viking with
both a orbiter and lander).

>What are the priorities for the various steps on the various planets?

The main factor that determines this is amount new science can be obtained
within the cost constraints.  This would be particularly true for something
we have never done before, like with Galileo parachuting a probe into
the atmosphere of Jupiter.  In fact, the criteria for the selection
of the Discovery missions followed along these guidelines:  high science data
return, feasibility of the mission, and relatively low risk.

>Is there some rational reason, for example, why we continue to spend
>billions on the outer planets when our own Moon -- closer, easier, and
>far more near-term significance to spaceflight -- remains mostly
>unexplored?

Well, if the emphasis is on new science, and we know less about the
outer planets than the Moon, then the outer planets gets the nod.  I'm
not saying we should ignore the Moon.  The Moon, after all, is the one
object that we've sent the most number of spacecraft to, and is the only place
that we've physically set foot on.  We should consider the entire
solar system (including comets and asteroids) and have a balanced
approach with emphasis on high science data return.

>Major gaps in our knowledge of the Moon could be filled
>for a small fraction of Cassini's budget.  (Note, I'm not claiming that
>we should *ignore* the outer planets in favor of the Moon, but neither
>should we ignore the Moon in favor of the outer planets, which is more
>or less what we are doing.)  For example, surely the Moon should be at
>least as well mapped as Mars, which it's not.

I agree with you completely.  There hasn't been a major mission to the
Moon since Apollo.  JPL has proposed the Lunar Orbiter mission, but it
wasn't funded.  SEI has proposed two low cost lunar mission, but they
weren't funded either.  The intent is there, but the support from the
people who make the budget decisions isn't.

>Where do rovers fit in?  Or geochemical orbiters (a step which has never
>been done for the Moon)?  Or atmosphere probes?  Surely steps 2 and 3
>need to be broken down into far more detail, to say nothing of step 4
>and whatever follows it.


Rovers and atmospheric probes would be categorized as unmanned landers.
Impacters (such as the Ranger series) would be considered flybys since
there only cover a short time span.  Penetrators would be considered
landers since they continue to collect data after impact.  How they are
prioritized depend on their science data return.

>As a case in point, if you are serious about step 4 being part of the
>process of planetary exploration, it is clear that we need better data
>on the medical effects of long-term free fall and low gravity, because
>manned missions even in the inner solar system will be lengthy.  Since
>"long term" means years, and we're going to need more than one test
>run to get detailed information, a biomedical space station is clearly
>a vital long-lead item for planetary exploration, and should have a
>fairly high priority, because we need to start *now* if we're going to
>have solid data in time for manned missions several decades hence.
>This idea is, shall we say, *unpopular* in the planetary-exploration
>community... which indicates just how seriously this "plan" is taken,
>vague and indefinite though it is.

Well, first let me say that I support the Space Station.  A continued
manned presence in space is very important.  I think
Clinton's plan to redesign the Space Station is in order.  This gives
NASA the opportunity to get the design right. A NASA-designed
Space Station is better than a Congress-designed Space Station.
I've seen debates in this newsgroup about "manned vs unmanned" missions,
and to me it is a non-issue.  You need to support both manned and
unmanned missions.  They complement each other and provide for a
balanced space program.  Selecting one exclusively over the other
would be very detrimental to the space program.

The manned missions are under a lot of uncertainty.  Its main stumbling
block is cost.  Presently, manned missions are limited to the Space Shuttle
and the Space Station.  If we are going to send men back to the Moon or Mars
or anywhere else, we have to get the cost down.  Otherwise, it'll never happen.

>>If you look at this closely, you'll realize that the Galileo and
>>Cassini missions are the second step in the Grand Plan and are the follow ons
>>to Voyager...

>So when do we see unmanned landers for Jupiter and Saturn?

Galileo drops off its atmospheric probe in Jupiter in December 1995.
Cassini drops off its probe on Titan in 2004.  Saturn would of been
targeted for Cassini's probe had it not been determined that Titan
is a more interesting place to drop in on.  Also, the moons of
both planets (particularly the Galilean moons) provide additional
landing opportunities.

>Which is more important?

Well, I would say a Jupiter probe would be equally important as a
Saturn probe, but since Jupiter is closer and cheaper, it gets the nod.

>How do their priorities compare to, say, Mars sample return?

This is a tough one.  I would very much like to see a Mars Sample Return
mission, and the science return would be very high, but unfortunately such
a mission doesn't fit within today's 'cheaper, faster, better' mentality.

>Will these two missions, one per planet, exhaust what can be done with
>orbiters?  Do we fly a geochemical orbiter around Callisto before or after
>a manned Mars mission?

Ask yourself which one gives the higher science return within the cost
constraints.

>At the current stage of things, a plan cannot be expected to give detailed
>and final answers to such questions... but it should be able to give rough
>and tentative ones, based on an idea of the desired sequence of events in
>the absence of major surprises.  No such tentative answers can be found,
>because there is no plan.

>>With Venus, we sent the flyby missions with Mariner 2 and
>>Mariner 10; we've done the orbiters with Pioneer Venus and Magellan, so
>>the next logical step would be to send a lander (the Soviets have sent
>>landers, but the US hasn't).

>The other logical possibility would be to decide that Venus is not worth
>major further effort at this point, which is the sort of thing that a plan
>might do.  Or it might decide that Venus deserves to stay at the orbiter
>stage for a while, since there are a number of questions that can still
>be answered from orbit, and building practical Venus landers is hard.
>(As a case in point of such questions, there were a number of atmosphere
>experiments and such slated for VOIR which were scrapped to save money
>on Magellan.)

Good point. This fits very well within the plan.  You can have multiple
orbiters mission before doing a lander mission.  In fact, a number of
Venus orbiters were proposed as Discovery missions.  There were also
a couple of long term Venus landers proposed, but they did not make
the final cut, probably because they were considered high risks.

>>The Fast Pluto Flyby mission is the start of the first step for Pluto.

>What is the relative priority of the advanced-propulsion work that will
>be needed to go beyond step 1 for Pluto?  Given how much even PFF is
>going to have to spend on launchers -- millions of dollars per kilogram
>of probe weight, assuming they use Titan -- surely this should have a
>high priority as a crucial long-lead item for outer-planets exploration.
>There is good reason to mount PFF now with off-the-shelf technology,
>since Pluto's atmosphere won't wait, but there seems to be a notable
>lack of strategic planning here for other missions.

The need for better propulsion to the outer planets has been recognized, and
research into this is already underway at JPL.  A couple propulsion
systems (ion propulsion I believe) were obtained from the Soviets last
year.  They are being carefully studied and will be improved upon
for use in exploring the outer planets.

>>The proposed MESUR mission is step 3
>>for Mars and will eventually help pave the way to a manned landing.

>Will that be before or after the first manned asteroid expedition?
>(If you pick an Earth-approaching asteroid, that mission is easier and
>has a higher payoff in some ways.)

There does seem to be more support for a manned mission to Mars
than an asteroid.  A lot of data will be collected from the Mars
Observer, Mars '94 & '96 and MESUR missions.

>>It may not always be obvious but there is plan, and it has been in effect
>>for over 30 years.

>This plan is so un-obvious that it's awfully close to invisible.

If you don't see it, then so be it.  The Discovery missions will go on
as planned regardless.  There is a good selection from the Discovery
missions along with the MESUR, NEAR and Fast Pluto Flyby missions to provide a
good balance to the unmanned portion of the space program.  The Space Station
will give us a long term manned presence in space.  Once we get the costs down,
then we can proceed to send men to the Moon or Mars.
     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | [e-mail]
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | It's kind of fun to do
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | the impossible.
|_____|/  |_|/       |_____|/                     | Walt Disney

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

End of Space Digest Volume 16 : Issue 286
------------------------------