
NASA calls off attempts to fix Kepler space telescope - anigbrowl
http://www.reuters.com/article/2013/08/15/us-space-kepler-idUSBRE97E16B20130815
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InclinedPlane
The good news is that there's still a lot of juice left in the Kepler data
set. Only about half the data has been analyzed, and a lot of good stuff will
come out of the rest. Also, it takes 3 observations of a transit for planetary
confirmation, but not all 3 observations need to be made by Kepler. With only
2 transits having been observed it'll be easy to extrapolate future potential
transit dates. Ground based observatories can then do targeted observations of
the most promising candidates and fill in that data, essentially moving a lot
of observations from the "candidate" planet category into confirmed territory
for a comparatively minimal effort.

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jamesaguilar
Whenever I see an article like this and something at nasa has broken, it seems
like they always are soliciting a new project. It makes me wonder, why not
just build the exact same thing again (with the broken parts fixed, of
course)?

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jerrya
Building a new something that was designed, built, tested and launched quite a
while ago is difficult because a lot of knowledge of the processes have been
lost, along with tooling, supplies, etc.

However, I do wonder why they build 1 Kepler space telescope and not 2, or 3,
and then store the already built ones until they are wanted.

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sigstoat
kepler wasn't built that long ago, as these things go. and ball aerospace
which designed and built it is already designing and building an almost
identical spacecraft for the b612 foundation. so of all the spacecraft you
might want to duplicate, kepler would be a really easy one at the moment.

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CamperBob2
Interesting, I'd never heard of B612. So they're sufficiently well-funded to
commission a $500M Kepler-class probe?

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sigstoat
last i'd heard they were still trying to get more silicon valley bigwigs
funding it. ball was treating them as a sufficiently "real" customer that it
was talking to subcontractors about the mission, but i hadn't heard about any
big checks for hardware being cut.

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kfury
The Kepler analysis team has only gone through the first two years worth of
data.

There are still two more years worth of exoplanets for us.

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ChuckMcM
Ok, so trolling around I've not found specs for reaction wheels. Lots of
descriptions of what they are, how they work, etc but no specifics on building
one. Given that this type of product is so critical to space operations (The
ISS has replaced several, so has Hubble, its like the oil-filter of space or
something) Seems like either a more durable item or some sort of cartridge
system where you can have like 20 spares ready to go, eject one and slot in a
new one or something.

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lutusp
I'm a former NASA design engineer. Reaction wheels are a real weak point in
spacecraft design. They basically allow one to accurately point an observing
spacecraft, but without using propellant.

If you use propellant, you eventually run out and the mission is over. With
electric-motor reaction wheels (a minimum of three), you can keep them going
with solar energy. So in principle, they're a big improvement over the
alternatives.

The problem is that they're constantly in motion -- constantly. If you turn a
spacecraft and bring it to a new stable position, that new position must be
maintained by constant reaction wheel motion. So they're essentially running
all the time, there's constant friction, and their parts and bearings
eventually wear out.

> Seems like either a more durable item or some sort of cartridge system where
> you can have like 20 spares ready to go, eject one and slot in a new one or
> something.

You're overlooking something. To turn a spacecraft like Hubble or Kepler in a
reasonable time, you must either have a massive reaction wheel, or you need to
make it spin fast (it's all about momentum). Typical reaction wheels represent
a compromise between mass and rotational speed. If you want to reduce
frictional wear, you make them bigger, more massive, which increases the
mission cost. If you need to reduce the size and mass of the spacecraft, you
make the wheels spin faster, and they wear out sooner.

So solving the problem by adding more reaction wheels is a non-starter --
they're too physically large and massive to just add a lot of spares.

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keenerd
> a minimum of three

Correct me if I am wrong, but don't most systems use six? It is a whole lot
easier to use three pairs of wheels. Pairs can be spun up from a dead stop and
in the event of reduced power can be used to turn in either direction by
braking action alone.

Or does "a reaction wheel" imply two counter-rotating flywheels by definition?

For an alternative, I've heard some good things about laser ablative
thrusters. Still fairly young, but they are powered by electricity and have no
moving parts. They do need reaction mass, but unlike traditional engines the
mass is a solid block and can't boil off, corrode, leak, freeze or whatever
problems affect liquid systems.

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lutusp
> Correct me if I am wrong, but don't most systems use six?

Kepler was equipped with four -- three in use, one spare. The idea is that you
need one reaction wheel per dimension.

> It is a whole lot easier to use three pairs of wheels.

All that six wheels would provide is three spare wheels, one per dimension.

> Pairs can be spun up from a dead stop and in the event of reduced power can
> be used to turn in either direction by braking action alone.

That's not how reaction wheels are normally used. Two counter-rotating wheels
cancel each other out, producing zero net rotational force on the spacecraft.

> Or does "a reaction wheel" imply two counter-rotating flywheels by
> definition?

No, just one. If you have two, and if they counter-rotate, they cancel each
other out.

> For an alternative, I've heard some good things about laser ablative
> thrusters. Still fairly young, but they are powered by electricity and have
> no moving parts.

Not nearly enough momentum to rotate a large spacecraft.

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sigstoat
> Kepler was equipped with four -- three in use, one spare. The idea is that
> you need one reaction wheel per dimension.

are you sure? i thought it was four (in a tetrahedral arrangement), but they
were all constantly in use, and one failure could be tolerated.

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lutusp
> are you sure? i thought it was four (in a tetrahedral arrangement), but they
> were all constantly in use, and one failure could be tolerated.

Source:
[http://kepler.nasa.gov/mission/faq/](http://kepler.nasa.gov/mission/faq/)

Quote: "Only 3 reaction wheels are needed to control the 3 degrees of freedom
of rotation of spacecraft. But Kepler was provided with 4 reaction wheels, one
extra for redundancy in case a wheel fails."

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moocowduckquack
Bye Kepler. You were awesome.
[http://kepler.nasa.gov/Mission/discoveries/](http://kepler.nasa.gov/Mission/discoveries/)

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andrewflnr
In the future, will we be able to fix it if we can get a person up there?

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batbomb
It's wayyyyyyy too high in orbit to get up there, past the moon. Not even
remotely worth it.

They are doing the same thing with the Webb.

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andrewflnr
Oh, wow, wikipedia says its actually in a heliocentric orbit. Yeah, not
happening.

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ISL
A huge thank-you to all the teams involved with Kepler. It's a very fine
experiment that's taught us a ton. Hit the lifetime goal too.

Great job!

