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Questions for Planetary Radio, answers too!
elakdawalla
post Aug 29 2007, 05:15 PM
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Hi all, I thought I'd start a thread for people to dump questions that they might think are good for Planetary Radio. Planetary Radio is The Planetary Society's weekly half-hour podcast. It features an interview by producer Mat Kaplan with a space scientist, engineer, advocate, or well-wisher, and three regular segments: news headlines, "What's up" with Bruce Betts, and a Questions and Answers segment that I read. Each week, Q and A addresses one question, with half of the answer given at the beginning of the show, and half toward the end, each half lasting between 1 and 2 minutes. I get lots of questions submitted but the majority of them can't easily be answered in this rather strict format, so I am always looking for more questions.

So, please post your questions! They don't have to be questions that you don't actually know the answers to -- also please consider submitting questions your friends and family ask you, as those are often some of the best. And, if you see a question here that you can answer, please don't hesitate to answer it and save me a little research work! biggrin.gif

--Emily


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djellison
post Aug 29 2007, 05:20 PM
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How do they do TCM's with spacecraft that are spin stabilized like MER?

That could be the part two, to a more general look after spacecraft on their way to Mars -does Phoenix have only one fuel supply which is used for cruise and landing?

Both questions for which I have NO idea to the answer!

Doug
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Guest_AlexBlackwell_*
post Aug 29 2007, 05:36 PM
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QUOTE (djellison @ Aug 29 2007, 07:20 AM) *
How do they do TCM's with spacecraft that are spin stabilized like MER?

Typically via pulsed thruster firings timed to the spacecraft's rotation, which allow the spin axis to precess to the desired attitude. If a velocity change is desired as well, then a coupled thruster firing is performed once the spin axis is set.
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Guest_AlexBlackwell_*
post Aug 29 2007, 09:05 PM
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Hey, maybe we should start a separate thread to come up with some harder space trivia questions for Mat and Bruce.

In fact, why don't we turn it around, i.e., we ask the questions and see if the two of them can answer them. If they can't, the originator of the question gets a free TPS T-shirt biggrin.gif
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nprev
post Aug 30 2007, 12:25 AM
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That'd be cool! smile.gif

Here's one I always wondered about: how exactly do Cassini's hemispheric resonating gyros (HRGs) used in the inertial navigation system work?

A-10s use something called a vyro for their stability augmentation system, which is basically a strip of a niobium alloy that's vibrated @ a high freq by piezoelectric drivers. Very similar transducers then measure the amount of force generated 90 deg from the drive plane to determine rate of motion. I suspect that the HRGs work much the same way, but from the name the geometry sounds a little weird...


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Guest_AlexBlackwell_*
post Aug 30 2007, 12:49 AM
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QUOTE (nprev @ Aug 29 2007, 02:25 PM) *
Here's one I always wondered about: how exactly do Cassini's hemispheric resonating gyros (HRGs) used in the inertial navigation system work?

In principle, here's an excerpt from this paper:

QUOTE
One of the most well known examples of the oscillatory gyroscope with continuous vibrating media is a hemispherical resonating gyroscope (HRG). HRG sensitive element design usually is based on the resonating shell that has a hemispheric or so-called “wine-glass” shape. Primary oscillations are provided by standing wave excited in the rim of the shell. In case of no external angular rate, nodes of the wave do not move. If the sensitive element rotates around sensitive axis, which is orthogonal to the plane of the wave, the secondary oscillations can be detected at the nodes. Despite HRG itself has never been referred to as a micromechanical gyroscope, its operation principle has been widely used in the number of micromechanical designs. In particular, the hemispherical shape of the shell has been replaced with a thin cylinder or a ring.

Note also an excerpt from a Northrup Grumman press release:

QUOTE
The hemispherical resonating gyro utilizes a thin-walled quartz shell that is energized by an electrical field to produce an imperceptible vibration pattern within itself. This pattern is electrically sensed and used to determine the gyro's output parameters. The vibration is so minute that it creates virtually no internal stress and fatigue effects, leading to its unmatched reliability. Northrop Grumman is the exclusive producer of the gyros, which to date have accumulated more than 4.5 million hours of operation in more than 50 systems in space without a mission failure.

Or check out the Northrup Grumman HRG web page.

This post has been edited by AlexBlackwell: Aug 30 2007, 01:01 AM
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nprev
post Aug 30 2007, 02:03 AM
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Answered by our ever-knowledgeable Reference Librarian! smile.gif Thanks, Alex. Pretty cool technology; maybe it's Planetary Radio-worthy if explained with analogies.


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nprev
post Aug 30 2007, 03:09 AM
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Here's a quickie: Why does Mars have such huge volcanoes? Ans: No plate tectonics, with maybe a discussion of the Hawaiian volcanic chain by way of comparison.

Wait a minute...why the hell doesn't Io have huge stratovolcanoes? Ans (?): low crustal tensile strength (and/or the crust is very thin indeed), different materials?...


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MaxSt
post Aug 30 2007, 06:49 PM
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I heard that Phoenix in going to land inside Mars' Arctic Circle.

Does it mean it will certainly stop working as soon as the "very long night" comes, and when exactly it's going to happen?
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elakdawalla
post Aug 30 2007, 07:07 PM
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This is a question that I hear (in various forms) a lot, so I think it's an excellent one for Planetary Radio. I've got some answers on the mission facts page and in the last paragraph of this article.

I like the TCM question but will need a little help on answering that one; thanks for the beginning, Alex.

Big volcanoes is a good one too...

The HRGs one is probably a bit too technical for the format.

And Alex, I like your idea; send it to Mat and see what he says! It'll get a chuckle, if nothing else.

--Emily


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volcanopele
post Aug 30 2007, 07:07 PM
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QUOTE (nprev @ Aug 29 2007, 08:09 PM) *
Wait a minute...why the hell doesn't Io have huge stratovolcanoes? Ans (?): low crustal tensile strength (and/or the crust is very thin indeed), different materials?...

A combination of lava with low viscosity and a short life-span for individual volcanoes compared to those on Mars.


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Guest_AlexBlackwell_*
post Aug 30 2007, 07:28 PM
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QUOTE (elakdawalla @ Aug 30 2007, 09:07 AM) *
The HRGs one is probably a bit too technical for the format.

Just out of curiosity, what do you mean by format? Is there a base level for "difficult" questions beyond which you don't wish to go (i.e., least common denominator theory for audiences)? Or is it simply due to the time constraints for your Q&A segment? Or both? Or more? Etc.
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Paolo Amoroso
post Aug 30 2007, 07:39 PM
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When can we expect to observe features on Eris with clarity comparable to HST's images of Pluto or Ceres? Is it more likely to get such views from a telescope or a space probe?


Paolo Amoroso


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nprev
post Aug 30 2007, 11:11 PM
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QUOTE (AlexBlackwell @ Aug 30 2007, 12:28 PM) *
Just out of curiosity, what do you mean by format?


After posting the HRG question, I realized that PR is a mass broadcast intended for a general audience. Don't think it's a question of dumbing down, but more about engaging an intelligent, interested audience who probably aren't all that familiar with the wonders of the Solar System and UMSF to anywhere near the degree that we regulars are (if I may place myself in your distinguished company!) smile.gif


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brellis
post Aug 30 2007, 11:52 PM
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Emily, thanks for setting this thread up!

I've had a question about Cassini-Huygens, and the release of the Huygens probe. Why did it release the Huygens probe so early in the mission?

After launch, the riskiest moment of the voyage occurred when C-H first arrived at Saturn, when it flew through the ring plane. Once it got into orbit, the danger of a catastrophic accident diminished greatly.

The release was delayed an extra orbit to allow Cassini to get into a better trajectory for receiving the Huygens signal, but no one knew if the probe would land in a sea of methane or on solid ground. The probe was still transmitting when Cassini flew out of signal range, prompting me to wonder why they didn't wait much longer, until the primary mission was further along or even mostly over, to release the probe when they could put Cassini in an even better trajectory.

A few months after the probe was released, Cassini made a much closer flyby of Titan. Wouldn't it have helped to fly past Titan several times to scout for some potential locations before releasing the probe?


Attempting to answer:
One of my buddies at space.com suggested that hanging on to Huygens even for one extra orbit burned up a significant amount of reserve fuel, and there was a lot of other exploring for Cassini to do around the Saturn system. I read a status report from soon after the probe was deployed that supports this point.

I appreciate if anyone can run this question by someone on the Cassini-Huygens team!
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