Cassini records streak at Saturn

Either there's an interplanetary "hot rodder" or there's some
co-orbiting debris floating alongside Cassini.

http://saturn.jpl.nasa.gov/multimedi...eiImageID=8552

wrote in message . ..
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=8552
Interesting! When you click on the full-res version of the photo the
"blob" looks like an arrowhead or a windows-like pointer. Also, I
wonder what would be cause of the luminescence in the streak?

Bob Carlson

Bob Carlson wrote:

wrote in message . ..
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=8552
Interesting! When you click on the full-res version of the photo the
"blob" looks like an arrowhead or a windows-like pointer. Also, I
wonder what would be cause of the luminescence in the streak?

Bob Carlson

Remember that you're looking at a .jpg image, which (as I understand it)
has been subjected to some sort of lossy compression. The original,
uncompressed image may well look different. This includes the apparent
shape of Mimas, which would present roughly a half-moon appearance with
its terminator at about the same angle as Saturn's. Mimas's big crater
would not have been visible in this picture, so we can't blame the
strange shape of the terminator on it.

There are actually *two* streaks in the image -- if you put a ruler up
to one of them, the other doesn't lie in the same line. These are
caused
by cosmic rays or other charged particles impinging on the camera's CCD
detector during the exposure or during readout. They deposit some of
their energy into the CCD by ionizing some of the silicon atoms. The
electrons thus liberated look no different from the electrons liberated
by the light that hits the chip during the exposure. This happens all
the time -- you can see lots of little white dots all over Saturn --
it's
just that these two happened to come in almost parallel to the chip's
surface (at "grazing incidence") and left long trails as a result.

-- Bill Owen

wrote:

Either there's an interplanetary "hot rodder" or there's some
co-orbiting debris floating alongside Cassini.

http://saturn.jpl.nasa.gov/multimedi...eiImageID=8552

That's Mimas in the lower left corner, presuming that the picture was
taken at about 20:00 UT on July 25. The streak running diagonally just
off the south pole is undoubtedly a cosmic ray hitting the CCD at
grazing
incidence. I've seen some spectacular cosmic ray trails in my images --
one went the entire width of the field.

-- Bill Owen, Cassini optical navigation team

-- Bill Owen, Cassini optical navigation team
Thanks for the good info. Pardon my dumbness in this area but could
the speed of the cosmic ray be determined by using a very short
exposure time? And why would the cosmic ray only affect the ccd
during the exposure? I thought cosmic rays would penetrate to the ccd
at all times regardless of exposure time.

Bob Carlson

Bob Carlson wrote:


-- Bill Owen, Cassini optical navigation team
Thanks for the good info. Pardon my dumbness in this area but could
the speed of the cosmic ray be determined by using a very short
exposure time? And why would the cosmic ray only affect the ccd
during the exposure? I thought cosmic rays would penetrate to the ccd
at all times regardless of exposure time.

Bob Carlson

Cosmic rays (and other charged particles ... remember, Cassini is inside
Saturn's magnetosphere) are continually impinging on the CCDs. But the
accumulated charge is flushed out immediately before each exposure
begins
(just like any CCD camera on a ground telescope), so whatever happens
before that is gone. During readout, the picture is slowly moved up
the chip into a "serial register", and any cosmic rays which hit the
remaining part of the picture will also get recorded. You do see more
CR hits at the bottom of a Cassini image than at the top. But after
the readout is completed, the numbers are (more or less) safely tucked
away in the solid state recorder, and cosmic ray effects there are much
rarer. Meanwhile, the CCD itself continues accumulating hits until it
gets cleared again before the next picture.

As for determining the speed, no, this is not the kind of instrument
you'd want to use for that.

-- Bill