
Gaia: The 'impossible space mission' ready to fly - choult
http://www.bbc.co.uk/news/science-environment-23779294
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Gravityloss
The article is very well written. It explains what the motivations are, but
you also get actual technical insight on how it all is done.

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acchow
Could someone with better knowledge of astronomy/aerospace explain (with less
hyperbole) which part is the "impossible" part here? I don't really get it.
Reading the article, I got that they're sending a telescope into space with a
very large camera sensor and will record information about many stars.

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lutusp
> Could someone with better knowledge of astronomy/aerospace explain (with
> less hyperbole) which part is the "impossible" part here?

Hyperbole is the operative term. When first proposed it would have been
impossible with then-current hardware, but since such missions take time to
germinate, sometimes people use the term "impossible" to add a sense of drama
and daring, but with the expectation that the goal will become possible in
time.

Also, according to Arthur C. Clarke, "When a distinguished but elderly
scientist states that something is possible, he is almost certainly right.
When he states that something is impossible, he is very probably wrong."

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Zikes
With the incredible amount of data I'm sure this will produce, I wonder if any
new distributed computing initiatives will be launched to handle it.

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noahdesu
Gaia will produce quite a lot of data, approximately 200 TB over 5 years,
according to this Wikipedia article
([http://en.wikipedia.org/wiki/Gaia_(spacecraft)](http://en.wikipedia.org/wiki/Gaia_\(spacecraft\))),
however that feels a bit hand-wavy. Even if we assume an order of magnitude
error, and its more like 2 PB, it is still far far less than the 15 PB (images
plus metadata) that will be produced over 10 years with the Large Synoptic
Survey Telescope. And yes, the LSST is driving a large number of distributed
computing research projects because of its unique processing requirements.

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celoyd
I’d be interested to know the ratio of the L0 data size to the L2/3/4 data[0]
size – in other words, how many downlinked bits of sensor readout are
processed per bit of useful science data? Of course that won’t be a hard
number; I’m just curious to get a sense of the general scale on which the
pipeline reduces pixels to physical parameters.

0\.
[http://en.wikipedia.org/wiki/Remote_sensing#Data_processing_...](http://en.wikipedia.org/wiki/Remote_sensing#Data_processing_levels)

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astrowilliam
It truly boggles my mind how scientists and engineers keep doing these amazing
feats.

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avmich
"There are also no moving parts anywhere on Gaia. Even the antenna to
communicate with Earth has been designed to point electronically, not
mechanically."

Good, but on the next line...

"And if the satellite does need to make fine adjustments, it has been equipped
with thrusters that can squirt just 1.5 micrograms of nitrogen gas a second."

Is there really no moving parts on Gaia?

Hmmm, they could try to use the Earth magnetic field for orientation; I guess
the microthrusters were better...

P.S. And saying "accuracy of about seven micro-arcseconds for the nearest
stars" is a little bit inkorrect.

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ars
Those micro-thrusters are probably piezoelectric, like the nozzles on ink jet
printers.

They move a bit I guess, but there is no net motion - they transmit no force
or vibration outside of the device. They flex, but have no relative (sliding)
motion.

There are other parts on the satellite that can bend - will you call that a
moving part as well?

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avmich
Piezoelectric actuator still relies on a mechanical motion. It has to be - the
idea of nitrogen nozzle is to have a storage of nitrogen under some pressure
in a closed volume, and to be able to open that volume to let some gas out.
That opening has to be mechanical - unless they use some exotic force fields
to keep nitrogen from escaping, in which case it's hardly a (regular) nitrogen
thruster anymore.

Flexing is still a motion. That, however, can be compensated with an opposite
motion in another gas channel.

If other parts of satellite can bend on demand, they'd need an actuator or,
alternatively, one can use Earth magnetic fields to reorient the spacecraft
and let external forces affect it unevenly, thus providing bending. This,
however, is outside of the scope of the article.

So, to summarize - either secondary subsystems on Gaia are quite
unconventional, or the article isn't quite precise.

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ars
Conventionally a moving part is one which moves relative to another, but
flexing parts are not included in the definition.

The reason for excluding them is that _EVERYTHING_ can flex. So if you
included that in the definition it would make it meaningless.

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rorrr2
I don't get what's so "impossible" about it.

Even if it weren't built, it sounds completely plausible.

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sp332
A single device that observes and catalogs a billion stars sounds like science
fiction. That would be a feat even in a software simulation. Doing it in orbit
is crazy.

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JshWright
The 'single device' will just be doing the observing. The cataloging will be
done by many more devices on the ground.

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AYBABTME
The answer is 42.

