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Pioneer 11's 'near miss' at Saturn
As old as Voyage...
post Mar 16 2007, 02:56 PM
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A couple of websites state that Pioneer 11 discovered a new moon of Saturn during its 1979 flyby and nearly collided with it.

http://www.uwgb.edu/DutchS/CosmosNotes/voyagr00.htm

The website says Pioneer 11 missed the previously unknown moon by only a few hundred km.

http://www.uwgb.edu/dutchs/planets/satslid.htm

Apparently the spacecraft inferred the moon's existance by the disturbances it created in Saturn's magnetic field.

Is this right? Did Pioneer 11 nearly end its mission in an unforseen Deep Impact style crash?


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As old as Voyage...
post Mar 16 2007, 03:28 PM
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Here's some more information on the near miss:

http://www.geocities.com/CapeCanaveral/Lau.../2099/9711.html

Its a shame Pioneer 11's 'camera' wasn't looking at the object. It may have provided one of the better satellite images if it could have been imaged without smearing.

A 200km moon. Anyone know which one it was?


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Ian R
post Mar 16 2007, 03:33 PM
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It was either Janus or Epimetheus, with the safe bet being the latter.


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belleraphon1
post Mar 16 2007, 03:38 PM
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Yes,

being older than Pioneer, I remember this and the Pioneer team even gave it an informal name....

quote from http://www.geocities.com/CapeCanaveral/Lau.../2099/9711.html

"Without warning, the outputs of several of Pioneer's radiation detectors suddenly plunged to nearly zero, held roughly steady for eight seconds, and then snapped back up to their former values. For the same eight seconds, the magnetometer recorded major disturbances in Saturn's magnetic field. Pioneer had streaked through the magnetic "wake" of a moon roughly 200 km across, at a distance of no more than a few thousand kilometers -- the closest it had come to any large object since leaving Earth. Later, the villain was tentatively identified as a moon discovered the previous day from Pioneer's imaging, and suspected from earlier observations by Earth-based telescopes. After the Voyager flybys, it became clear that there are two similarly-sized moons (now named Epimetheus and Janus) in the same orbit, and there is still some uncertainty about just which one was the object of Pioneer's near-miss. "

Will have to look up more references that do not exist online (books).

Craig
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dvandorn
post Mar 17 2007, 01:02 AM
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QUOTE (belleraphon1 @ Mar 16 2007, 10:38 AM) *
Yes,

being older than Pioneer, I remember this and the Pioneer team even gave it an informal name....

quote from http://www.geocities.com/CapeCanaveral/Lau.../2099/9711.html

The article doesn't give the informal name. Do you recall it? (I seem to recall the whole episode, but can't for the life of me remember the informal name.)

Good article, though -- especially as it's written by an old Usenet friend, Henry Spencer. As a proud virtual owner of several "I Corrected Henry" T-shirts from back in my Usenet days, I fondly recall his near-total and rarely-failing memory. (It's not true that I cribbed my writing style from Henry, though. When I encountered him on Usenet, I discovered that he and I have virtually identical writing styles -- another thing I enjoyed about him.)

-the other Doug


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GregM
post Mar 17 2007, 02:56 AM
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.
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Elias
post Mar 17 2007, 10:55 AM
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A rather detailed summary Pioneer 11 & Voyager observation of radiation absorption by rings & satellites can be found here:

http://ntrs.nasa.gov/archive/nasa/casi.ntr..._1991004777.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntr..._1990017468.pdf

The absorption effect of Janus that led to its discovery is also shown in some figures of these reports.

Also note that Cassini could have almost been destroyed when it nearly passed through the G-ring arc + Pioneer 11 passed through the core of the G-ring, but luckily the arc was not there...
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post Mar 17 2007, 03:00 PM
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Very interesting topic... thanks for sharing! cool.gif
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belleraphon1
post Mar 17 2007, 07:58 PM
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QUOTE (GregM @ Mar 16 2007, 10:56 PM) *
If I recall correctly, it was dubbed "Pioneer Rock", and the gap between the "A" and "F" rings was called the "Pioneer Gap". Don't hold me to it - my synapses are a little fuzzy after nearly 30 years.



Yes Greg.... my synapses are probably even fuzzier but that was the name I am sure.

There was no possibility of getting good imaging of this moon during this event. First, it was unexpected and second the Pioneers used spin-scan for its camera..... there was a certain bit of rivalry between the science teams of Pioneer and Voyager, due to the simpler design of Pioneer and the heavy emphasis on particle and fields data from the Pioneers.

Pioneers were simpler spacecraft, but did a magnificent job by being first to directly probe the asteroid belt (no no knew back then whether the dust in the belts would be so dense as to damage spacecraft), first to direclty sense the radiation belts of Jupiter and Saturn, as well as determing whether the ring flyby at Saturn was safe for Voyagers (if one of these craft was go on to Uranus and Neptune). At one point in the Saturn flyby planning, one group was lobbying to send Pioneer 11 right thru the Cassini Division.

Fun thread.... brings back memories... once I get home I will check a recent book published about Pioneer...cannot remember the title right now, but I think participants of this thread will enjoy it.

PIONEER ROCK... I like that name.....

Craig
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belleraphon1
post Mar 17 2007, 10:33 PM
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All....

my apologies.... some on the Pioneer team had planned a trajectory through the Guerin Division, not the Cassini division....

The moon that was detected and barely missed was dubbed the Pioneer Rock.... see

"
Based on these readings and others that showed changes in the surrounding magnetic field, scientists concluded that the spacecraft had passed within about 2,500 km (1,560 miles) of what appears to be a previously undiscovered moon* with a diameter as large as 600 km (370 miles). "The object was very close," says Physicist John Simpson of the University of Chicago. "It could be rocky or composed largely of ice. Either material will effectively block high energy particles." The moonlet, in orbit about 90,000 km (56,000 miles) above Saturn's cloud tops, was nicknamed "Pioneer rock" by the scientists, and it is being officially designated as 1979 S-l (for the first new moon of Saturn discovered this year). "

from http://www.time.com/time/magazine/article/...,920677,00.html

A good book on these missions is
"Pioneer Odyssey (NASA SP-396) by William Swindell and Eric Burgess Richard O. Fimmel"

And the book I referred to in my earlier post is "The Depths of Space" by Mark Wolverton.
This book is actually a chronicle of the entire Pioneer program run from Ames.

Craig
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As old as Voyage...
post Mar 18 2007, 02:40 PM
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Thanks for the info beleraphon1, I'll also be sure to check that book out.

It's interesting to note that Pioneer 10 also had a similar encounter with a previously unknown object:

http://news.bbc.co.uk/1/hi/sci/tech/460095.stm

This KBO near miss has been discussed here on UMSF before but I cannot find a link to it at the moment.


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scalbers
post Mar 18 2007, 06:05 PM
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QUOTE (Elias @ Mar 17 2007, 10:55 AM) *
The absorption effect of Janus that led to its discovery is also shown in some figures of these reports.


I think however that Janus was originally discovered from Earth based photography (still with some uncertainty) back in 1966, so the possibility of such an "encounter" with Pioneer 11 might have been anticipated?


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PhilHorzempa
post Jun 1 2007, 02:43 PM
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QUOTE (Phil Stooke @ May 30 2007, 05:39 PM) *
The team posted a pic of Janus today, looking tiny against the clouds of Saturn. The caption said a few craters were visible. I thought I'd check it out. Here's an extreme blow-up and specially processed version showing that, yes indeed, a few craters are in fact visible.

Phil.



Great work Phil! However, can you, or another UMSF member,
answer a niggling question of mine?
Is Janus the "Pioneer Rock" with which Pioneer 11 almost collided, or was it one
of the other small icy moons?


Another Phil
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Ian R
post Jun 1 2007, 03:03 PM
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Pioneer Rock was also imaged by Pioneer 11, orbiting in the region where Janus and Epimetheus are today.

I'm 99% certain that Epimetheus is the moon in question, although I'd be interested to know what Phil has to say about it.

Ian.


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Phil Stooke
post Jun 1 2007, 05:02 PM
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A bit of detective work. Sky and Telescope (Nov. 1980, p. 360) says Pioneer 11 imaged and nearly collided with 1980S3. The IAU at this URL:

http://www.johnstonsarchive.net/astro/moonlist.html

says 1980S3 is Epimetheus.

Phil


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