James Webb Space Telescope is a boondoggle

February 21, 2005

The Mighty Krell wrote:

Dude,

there is a distinct difference between him and you, he posts content,
you post ****.

I would rather read his thought provoking content, no matter how wrong,
than your ****.

plonk

Thomas Lee Elifritz
http://elifiitz.members.atlantic.net

In article ,
Jim Oberg wrote:
re SE L2, that is also the staging area of choice
in the recent IAA (Huntress) study for stepping stones to Mars...

Earlier work suggested that SE L1 was a better choice, with no major
difference in the dynamics, and the advantage of reliably staying out of
Earth's magnetotail. Did the Huntress study have reasons for picking L2
in particular?
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Andrew Nowicki :

The Planck spacecraft is going to be launched into the Earth-Sun
L2 orbit where it will be cooled by refrigerators to 0.1 K!
source: http://www.rssd.esa.int/SA/PLANCK/in...ayl/node8.html

This idea looks like madness to me because these extreme
refrigerators are not reliable. They have mechanical parts
that may be glued by contaminants.

SPICA is almost as ambitious. Its refrigerators are going to
cool it to 1.7 K.

So many vulnerable satellites... so few telerobots to repair them...

Why can't you admit you were wrong? Are you turning into a kook?

Earl Colby Pottinger

--
I make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos,
SerialTransfer 3.0, RAMDISK, BoatBuilding, DIY TabletPC. What happened to
the time? http://webhome.idirect.com/~earlcp

In article ,
"Mike Dworetsky" writes:
It's thermal radiation from the Earth, not from the Sun,
that causes problems for infrared telescopes in low orbits, where the 300K
Earth fills nearly half the sky.

Can't be right, as Mike will see once he thinks about it a bit more.
1.4 kW/m^2 from the Sun, about 400 W/m^2 from the Earth. Actually
less than that from Earth because of the atmosphere.

The real difficulty, of course, is that in LEO telescopes have to be
shielded against both heat sources, Sun and Earth, which are in
general in different directions. Shielding is really tough if the
spacecraft orbit takes it between the two bodies. Whereas from L2,
both bodies are always in more or less the same direction, so one
shield takes care of both. Also, from L2 the Earth is a lot farther
away and contributes very little heat anyway.

spacecraft, which makes it use up helium coolant much faster. It is very
hard to reflect away this infrared radiation, which is mostly at wavelengths
of about 0.01mm. Most of the Sun's radiation is visible and near-IR light,
which is easily reflected by a sunshade.

As I think someone else has noted, it is easier to make a shield
against IR than visible light. Most metals are more reflective at
longer wavelengths.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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In article ,
Andrew Nowicki writes:
This far out location was
justified by the shade made by the Earth.

Not true, as others have pointed out. "Shade" would make electrical
power rather problematic, don't you think?

If something goes wrong
with the JWST, the telescope will be difficult to
repair because the L2 point is far away from the
Earth.

If telerobots are as capable as you claim in other messages, what is
to prevent sending them to L2?

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote:
In article ,
"Mike Dworetsky" writes:

It's thermal radiation from the Earth, not from the Sun,
that causes problems for infrared telescopes in low orbits, where the 300K
Earth fills nearly half the sky.

Can't be right, as Mike will see once he thinks about it a bit more.
1.4 kW/m^2 from the Sun, about 400 W/m^2 from the Earth. Actually
less than that from Earth because of the atmosphere.

The Sun tends to lie in the same direction relative to the
sky for a fairly long time, so you can easily use a sunshade
and point your telescope in the other direction. But in LEO
the Earth tends to zoom around the local sky every few hours,
and subtends a very large angle while doing so. This is a
lot harder to block out and when you do block it out you end
up with a very small region of the sky that you can look at
safely.


The real difficulty, of course, is that in LEO telescopes have to be
shielded against both heat sources, Sun and Earth, which are in
general in different directions. Shielding is really tough if the
spacecraft orbit takes it between the two bodies. Whereas from L2,
both bodies are always in more or less the same direction, so one
shield takes care of both. Also, from L2 the Earth is a lot farther
away and contributes very little heat anyway.

Precisely. But for LEO it matters more that the Earth is
all over the sky, even a location like L1 where the Earth
and Sun are on opposite sides would be preferable,
regardless of the magnitudes of the heat inputs, because
you could safely view a much larger portion of the sky.
For LEO that safe portion looks like two sets of circles
(one "up" one "down") on the sky, for L1 that looks like a
big, thick vertical strip all the way around the sky (from
up to "left" to down to right back to up), for L2 that
looks like about half the sky, give or take.

Steve Willner wrote:
In article ,
Andrew Nowicki writes:
This far out location was
justified by the shade made by the Earth.

Not true, as others have pointed out. "Shade" would make electrical
power rather problematic, don't you think?

Yup, JWST will actually be in almost continuous sunlight, as
it will be in a halo orbit around L2 that will take it far
from Earth's penumbra. This is actually advantageous, because
it provides a steady source of power and avoids longevity
wrecking thermal cycling. JWST actually has a pretty neat
design that creates almost two separate spacecraft on opposite
sides of the sun-shade. On the sunny side of the shade will
be the spacecraft bus, where the PV arrays, power processing
systems, radiators, propulsion system and such-like will be
isolated, on the shaded side will be the telescope itself and
the instrument package (ISIM).


If something goes wrong
with the JWST, the telescope will be difficult to
repair because the L2 point is far away from the
Earth.

If telerobots are as capable as you claim in other messages, what is
to prevent sending them to L2?

Please be humane, don't hurt his fragile brain with facts.

Andrew Nowicki wrote:

This far out location was
justified by the shade made by the Earth.

Steve Willner wrote:

Not true, as others have pointed out. "Shade" would make electrical
power rather problematic, don't you think?

You mean solar power. This is good point
and argument against the L2 location.

Andrew Nowicki wrote:

If something goes wrong with the JWST, the
telescope will be difficult to repair because
the L2 point is far away from the Earth.

Steve Willner wrote:

If telerobots are as capable as you claim in other messages, what is
to prevent sending them to L2?

Dextre telerobot is inexpensive (US$154 million) and
flight-qualified, but there is one big problem: Dextre
was made by a canadian firm called MDA. If MDA moves
to Texas, the sky will be filled with Dextre telerobots :-)