

Herschel space telescope goes blind, ends mission - jacquesm
http://earthsky.org/space/herschel-telescope-goes-blind-ends-mission

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andyjohnson0
The telescope was positioned at the L2 point about 1,500,000 km from Earth
[1], which presumably precluded replenishing the liquid helium. Thats rather
different to the Hubble telescope's 560km orbit which was (only just)
reachable by the space shuttle.

Does anyone know why the Herschel was so far from earth? Easier stationkeeping
at L2?

[1]
[http://en.wikipedia.org/wiki/Herschel_Space_Observatory#Laun...](http://en.wikipedia.org/wiki/Herschel_Space_Observatory#Launch_and_orbit)

~~~
arethuza
"By orbiting at L2, some 1.5 million kilometres from Earth, Herschel is not
troubled by any atmospheric absorption. In addition, the spacecraft avoids any
problems caused by thermal infrared radiation from the Earth interfering with
observations. The L2 orbit also prevents the occurrence of temperature changes
due to the spacecraft moving in and out of eclipse in an Earth orbit, which
are a particular problem for infrared instruments requiring extreme thermal
stability."

[http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=...](http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=34682)

~~~
mturmon
"Put it over there by itself, in the shade."

~~~
jholman
Actually, if I read various web sources correctly (IANA rocket scientist,
sadly), the Earth-Sun-L2 point is always in the sun. The earth is not big
enough to provide shade from that far away, especially since the Hershel's
orbit is 700Mm across. Like how a (cruising-altitude) 747 cannot shade (sea-
level) me from the sun, no matter where I stand.

So it's always in the sun, which means it has continuous heating, and thus
reaches thermal stability, and doesn't expand/contract/whatever as it passes
in and out of shade.

~~~
mturmon
Yes, I was being facile for the sake of a one-liner. Sorry about that.

It will be in a certain kind of orbit around the L2 point, not right on top of
the L2 point. This means it is not in Earth's shadow, as you note. It's in its
_own_ shadow. But if it were in a conventional orbit centered around Earth, it
would have thermal issues, observational issues, etc., because pointing to
keep the telescope away from the sun would be a big problem.

My friend across the hall uses data from Planck, which is at L2, and I used to
use data from SoHO (before its camera was shut down), which is at L1. We used
to joke around about how each other's instrument was in a too cold/too hot
place.

Here's a nice illustration of Planck's orbit. (Planck was the sister satellite
to Herschel.) <http://planck.cf.ac.uk/mission/orbit>

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smackfu
We get so used to space missions outlasting their planned duration that it
seems surprising when one actually ends on schedule.

~~~
jacquesm
It actually went 11 months and 15 days beyond the planned lifespan, that's
nearly 30% longer.

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ChuckMcM
So here is some interesting math. Estimated cost of the Herschel Space
telescope, lets say 850 million dollars [1]

The cost of a SpaceX launch to LEO, 85 million dollars. So we launch three
Spacex cargos for a tanker, a refueling facility, and a pre-fueled space
craft. And for 250 million dollars we get a fully re-fueled telescope. How
cool would that be? Since its in the L2 point it will wait for us to get to it
without de-orbiting.

[1] [http://esamultimedia.esa.int/docs/herschel/Herschel-
Factshee...](http://esamultimedia.esa.int/docs/herschel/Herschel-
Factsheet.pdf) \- 1100 british pounds for total cost of instrument and
mission, if half is the cost of the instrument that is 550 million pounds or
about 852 million dollars.

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stephengillie
It seems like such a waste to scrap have a functional, responsive electronic
device hanging at the L2 orbit point, just because it can no longer cool its
sensors. Can't it be repurposed?

~~~
ISL
Sometimes, but often not. Maintaining communications links and operations
consumes resources too. If someone can find a solid science case for reusing
the instrument, ideas get traction.

Sometimes experiments get handed off as training tools for new aerospace
students. When Gravity Probe B ran out of helium, control was transferred to
the US Air Force Academy for navigation experiments [1]

It's sad to see good experiments go, but with finite resources, sometimes the
young must take precedence over the old.

Edit: Looks like there are alternative repurposing plans, perhaps [2], but a
bummer of a use for that mirror.

[1]
[http://einstein.stanford.edu/content/faqs/faqs.html#sv_contr...](http://einstein.stanford.edu/content/faqs/faqs.html#sv_control)

[2]
[http://www.spaceflightnow.com/news/n1210/26herschel/#.UX_n7n...](http://www.spaceflightnow.com/news/n1210/26herschel/#.UX_n7nGlf0o)

------
option_greek
Why is the helium exhausted ? is it because of imperfect sealing ?

~~~
jacquesm
Evaporation.

From WP:

"The instruments are cooled with over 2,300 litres liquid helium, boiling away
in a near vacuum at a temperature of approximately 1.4 K (−272 °C). The
2,300-litre supply of helium on board the spacecraft was a fundamental limit
to the operational lifetime of the space observatory; it was originally
expected to be operational for at least three years."

more info:

<http://en.wikipedia.org/wiki/Herschel_Space_Observatory>

~~~
iwwr
They could have used an active refrigeration system, no?

~~~
jacquesm
This is right along the lines of 'why didn't they?'.

When you get this close to absolute zero active refrigeration systems are no
longer effective, but the phase change of liquid helium to gas still manages
to extract a little bit of energy from the system cooling it down just that
much further.

~~~
weichi
Not sure exactly what you mean by "active", but there are plenty of
cryocoolers that work great at temperature below 1.4 K, some of which are also
qualified for space. So there's no physical reason for Herschel to not use a
cryocooler.

But I believe that Planck was the first satellite to cool to below 1 K using
only cryocoolers. Planck actually uses a dilution fridge to get to 100 mK,
which is kind-of astounding.

I don't know why Hershel used liquid helium instead of a cryocooler, but my
guess is that the technology for cooling with liquid helium in space is very
well understood and reliable, so it's a risk thing. That's not to say that
nothing can go wrong; there was a japanese telescope that lost all its helium
within some very short period of time thanks to an engineering mistake.

This is a nice whitepaper about cryocooling in space:
[http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-
holmes.pd...](http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pdf)

~~~
mturmon
"I don't know why Hershel used liquid helium instead of a cryocooler...it's a
risk thing"

The cryocooler technology (say, to a few kelvins) has proved hard to get ready
for space. For example, out of the 10 or so technologies that were judged most
risky for JWST, the 6 K cryocooler for the MIRI instrument was the last to be
judged ready for space ("at TRL 6" in the jargon)
(<http://www.stsci.edu/jwst/news/2007/jwst-passes-tnar>).

Despite being judged ready, the JWST cryocooler has proved _very_ challenging
to build. The effort now has frequent reviews with the director of JPL (and a
high-level counterpart at NGST), and tens of engineers are now working on the
system.

Part of the problem, as I understand it, is that the heat has to be taken away
and radiated at a site distant from the IR detector. This requires a large
structure, and a deployable radiator. This large structure can't leak much
heat back into the spacecraft bus or instruments, and must not be disturbed by
the vibrations of launch. Additionally, vibrations of the cryocooler must not
affect the telescope optics (2 micron resolution).

You can tell that these requirements are fundamentally opposed to each other
("be large, don't vibrate, don't touch anything else").

~~~
VLM
There's also a lifetime issue. Say you decide, "forget the whole vibration
thing, we'll run the cryocooler half the time and take data half the time
while the cooler is powered off"

Then you either get half the mission data, or extend the mission by a factor
of two, which means twice the operational labor cost, plus every other part of
the craft has to be double lifetime rated, which could get expensive. Hubble
used to burn thru gyros on a regular basis, so now you need them to last twice
as long or launch with twice as many spares. Or maneuvering propellant if any,
now you need twice as much for station keeping.

You can easily get painted into a corner where the cheapest way to run a
mission twice as long is to launch two of them. At that point you're better
off saying "you know the cryocool half the time and take data half the time
idea? Yeah scrap that idea"

This is before we started on energy issues. A heavy helium tank doesn't use
much energy. But cryocoolers on earth take quite a bit indeed, well, at least
compared to a couple watt transmitter and all that. Whats heavier, a tank big
enough to last mission lifetime, or a cooler and a stunning array of solar
panels to run the cooler? Or since it only runs half the time as per above to
prevent vibration issues, you could put a battery in which is heavy and
becomes another exciting point of failure. This makes the existing power
system more complicated and less reliable possibly shortening the theoretical
craft lifetime to less than you'd get if you just launched a big simple tank.

~~~
iwwr
How about just making a detachable LHe tank and send some replacement helium
every few years? Shouldn't be too costly even given the day-to-day cost of
running a space observatory.

~~~
mturmon
Given the weight of the LHe tank, I think they'd rather send up a new
telescope along with the tank.

~~~
VLM
Even worse the helium was the limiting consumable in this particular case, but
for the next scope the last gyro might burn out in 5 years. So now you need a
supply of gyros, more likely an entire nav unit bolted permanently on. And
next time the battery croaks so you need a battery and a way to install it and
isolate the old one such that you can never end up with neither installed. And
next time the transmitter final power amp goes thru too many thermal cycles
and cracks so you need a complete telemetry unit.

Yes I know there are good reasons why they put it at a L point, but
unscheduled unplanned maint is exactly what the ISS could have been perfect
for. Position this dude in the same orbit but at a phase 10 miles away so its
not too close, and send a live astronaut with a new battery or whatever
anytime "something" fails. To say the ISS project is not managed this way
would be an understatement. But someday, an unscheduled repair shop in space
will exist, and probably be quite profitable.

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ISL
Congratulations to the Herschel team, and a huge thank-you to the taxpayers
that funded it! These experiments are anything but easy.

It is good for great experiments to have an end.

