James Webb Space Telescope is a boondoggle

James Webb Space Telescope (JWST) is going to
take mostly infrared pictures of the universe.
To reduce unwanted infrared light coming from
the Sun, JWST will be launched into the Sun-Earth
L2 point, which is 1.5 million kilometers away
from the Earth. This far out location was
justified by the shade made by the Earth. A quick
calculation proves that the L2 point is *not*
in the Earth's shade! The complete shade, called
umbra, extends only to a distance of 1.39 million
kilometers beyond the Earth, i.e., 110,000 km
short. The L2 point is in partial shade called
penumbra.

If JWST is launched into the L2 point, it will
sizzle in the sunlight almost as much as the
Hubble Space Telescope. If something goes wrong
with the JWST, the telescope will be difficult to
repair because the L2 point is far away from the
Earth. Worse yet, JWST has a monolithic design
not suitable for telerobotic repair or upgrade.
My conclusion: James Webb Space Telescope
should be redesigned to improve its thermal
insulation and compatibility with telerobots,
and then launched into low Earth orbit.

"Andrew Nowicki" wrote in message
...

My conclusion: James Webb Space Telescope
should be redesigned to improve its thermal
insulation and compatibility with telerobots,
and then launched into low Earth orbit.

Ain't gonna happen.

Andrew Nowicki wrote in news:4216D559.B6CA6011
@nospam.com:


If JWST is launched into the L2 point, it will
sizzle in the sunlight almost as much as the
Hubble Space Telescope.

Which is why it has its own sunshade. Also, it's
away from Earth's thermal radiation, which is the real
reason for that orbit. What's the problem with this?

If something goes wrong
with the JWST, the telescope will be difficult to
repair because the L2 point is far away from the
Earth. Worse yet, JWST has a monolithic design
not suitable for telerobotic repair or upgrade.
My conclusion: James Webb Space Telescope
should be redesigned to improve its thermal
insulation and compatibility with telerobots,
and then launched into low Earth orbit.

What telerobots?

You may have noticed they're not going to repair
Hubble, either. If JWST breaks, well, that's too
bad...and no upgrades.

We aren't going to have a Shuttle much longer and
it's not clear that CEV will be able to provide similar
support at even slightly lower mission cost.

--Damon

"Damon Hill" wrote in message
31...
Andrew Nowicki wrote in news:4216D559.B6CA6011
@nospam.com:


If JWST is launched into the L2 point, it will
sizzle in the sunlight almost as much as the
Hubble Space Telescope.

Which is why it has its own sunshade. Also, it's
away from Earth's thermal radiation, which is the real
reason for that orbit. What's the problem with this?


Absolutely right. 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. The problem is general heating of the
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.

If something goes wrong
with the JWST, the telescope will be difficult to
repair because the L2 point is far away from the
Earth. Worse yet, JWST has a monolithic design
not suitable for telerobotic repair or upgrade.
My conclusion: James Webb Space Telescope
should be redesigned to improve its thermal
insulation and compatibility with telerobots,
and then launched into low Earth orbit.

What telerobots?

You may have noticed they're not going to repair
Hubble, either. If JWST breaks, well, that's too
bad...and no upgrades.

We aren't going to have a Shuttle much longer and
it's not clear that CEV will be able to provide similar
support at even slightly lower mission cost.

--Damon

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

Mike Dworetsky wrote:

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. The problem is general heating of the
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.

Clean, polished gold and silver have *infrared*
emissivity in the range of 0.01 to 0.03. They
are good enough to reach the microkelvin
temperature range in cryogenic equipment.
A passive tube-shaped insulation surrounding
the telescope in low Earth orbit could reduce
its temperature to 70 K or so. It seems that
NASA is going to send JWST to L2 because they
cannot design decent thermal insulation.

Andrew Nowicki wrote:

A passive tube-shaped insulation surrounding
the telescope in low Earth orbit could reduce
its temperature to 70 K or so. It seems that
NASA is going to send JWST to L2 because they
cannot design decent thermal insulation.

You're projecting your own cluelessness onto
the engineers who designed JWST, who actually know
what they are doing.

Paul

In article ,
Andrew Nowicki wrote:
A passive tube-shaped insulation surrounding
the telescope in low Earth orbit could reduce
its temperature to 70 K or so. It seems that
NASA is going to send JWST to L2 because they
cannot design decent thermal insulation.

Hardly. The insulation they designed kept COBE's telescope under 50K in
LEO after its LHe supply ran out.

Trouble is, that requires that the telescope point pretty much directly
away from the Earth. Which is a problem, since as the telescope goes
around Earth, that direction changes constantly. This is workable, pretty
much, for a sky-survey instrument like IRAS or COBE. It's grossly
unsuited to a telescope that wants to point at arbitrary targets, and
preferably stare at them for considerable lengths of time.

Note that when ESA built an infrared observatory, they went to the extra
trouble of putting ISO into a highly elliptical orbit, so it was well away
from Earth most of the time.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:
[ker-snip]
Note that when ESA built an infrared observatory, they went to the extra
trouble of putting ISO into a highly elliptical orbit, so it was well away
from Earth most of the time.

Note also the more recent infrared and longer wavelength
observatories and their designated operating locations:

Launched missions:
WMAP: Earth-Sun L2 Lissajous orbit
Spitzer: Earth-trailing heliocentric orbit

Planned missions:
Herschel: Earth-Sun L2 Lissajous orbit
Planck: Earth-Sun L2 Lissajous orbit
SIM: Earth-trailing heliocentric orbit
SPICA: Earth-Sun L2
JWST: Earth-Sun L2
Darwin: Earth-Sun L2


There are pretty strong trends here.

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
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.