[QUOTE=cheesehead;352660]I'm shocked that NASA decided four were enough for Kepler.[/QUOTE]

Indeed.

Six would have been far smarter.

[QUOTE=chalsall;352662]Six would have been far smarter.[/QUOTE]

Please correct me if I'm wrong.

Using only two flywheels, it is possible to orient the attitude of space-craft (in three degrees of freedom) so long as the said two flywheels don't share an axis.

It should be possible to design the positions of the flywheels (effectively nothing much more complicated than hard-drives) on a space-craft for optimal resilience, much like we design servers with RAID 6.

I was about to ask "roughly how large/bulky are these things?" but decided to peruse the wikipage on [url=http://en.wikipedia.org/wiki/Reaction_wheel]reaction wheels[/url] first, and noted this:
[quote][u]Since the reaction wheel is a small fraction of the spacecraft's total mass[/u], easily measurable changes in its speed provide very precise changes in angle. It therefore permits very precise changes in a spacecraft's attitude. For this reason, reaction wheels are often used to aim spacecraft with cameras or telescopes.

Over time reaction wheels may build up stored momentum that needs to be cancelled. [i][followed by details on why getting rid of the excess angular momentum is surprisingly difficult][/i][/quote]
So perhaps - in fact certainly - there are details here which I don't fully grasp, but it seems having spares onboard could serve 2 highly useful purposes:

1. If one breaks, shut it down or jettison it and switch to a spare;

2. If one builds up a dangerously high amount of momentum, jettison it (and its accumulated angular momentum) and switch to a spare.


[sarc]Perhaps the thinking was "hey, if one primary mirror is enough for Hubble, 4 reaction wheels should be enough for Kepler ... oh, wait..."[/sarc]

[url]http://news.yahoo.com/lhc-celebrates-five-years-not-destroying-world-194300737.html[/url]

[QUOTE=cheesehead;352669][url]http://news.yahoo.com/lhc-celebrates-five-years-not-destroying-world-194300737.html[/url][/QUOTE]

Namaste.

In the modern nomenclature, ROFLMFAO...

So long and thanks for all the data.
[URL="http://www.space.com/22729-voyager-1-spacecraft-interstellar-space.html"]Voyager is out of this solar system.[/URL]

[QUOTE=Uncwilly;352855]So long and thanks for all the data.
[URL="http://www.space.com/22729-voyager-1-spacecraft-interstellar-space.html"]Voyager is out of this solar system.[/URL][/QUOTE]

It's not gone in the "no longer sending data" sense - it's simply entered the final phase of its long and productive life. As the article notes, there should be enough juice to power at least part of the key systems package for another decade or so.

----------------------------

And speaking of age-related funsies, we all tend to get shorter in our dotage, right?

[url=www.reuters.com/article/2013/09/12/us-usa-denali-height-idUSBRE98B04Y20130912?feedType=RSS&feedName=domesticNews]North America's highest peak not as tall as once thought[/url]

Compare that to young-whippersnapper peaks like Everest which are still growing quite rapidly.

[QUOTE=Uncwilly;352855]So long and thanks for all the data.
[URL="http://www.space.com/22729-voyager-1-spacecraft-interstellar-space.html"]Voyager is out of this solar system.[/URL][/QUOTE]

[url]http://xkcd.com/1189/[/url]

[QUOTE=chappy;352870][URL]http://xkcd.com/1189/[/URL][/QUOTE]

You are SO MEAN!

[QUOTE=kladner;352875]You are SO MEAN![/QUOTE]

But, of course, funny....

Did anyone else catch the exoplanetary, erm, I mean explanatory, hovertext?