# Velocity/duration of re-entries

From: Ed Cannon (ecannon@mail.utexas.edu)
Date: Mon Dec 03 2001 - 00:54:39 EST

• Next message: Ted A. Nichols II: "JERS-1 Reentry Ground Trace Map...."

```A while ago I got to see local TV news show video of the Proton
re-entry yesterday.  I don't know if they were playing the film
in slow motion, or if it was really that slow.  Also, in this
video, there appeared to be two parallel and quite separate
tracks, both of them large and fragmenting.

I'm puzzling over some of the re-entry reports and want to pose
a question that I'm definitely not qualified to answer.  Here's
a sample from about 32 km south of Lincoln, Nebraska, USA, that
was forwarded to the Meteorobs list by Gary Kronk:

> Starting at the SW horizon we followed with our eyes no less
> than 50 "shooting stars" streaking  across the sky above us
> to the NE horizon where the earths' curvature prevented
> further observation of light show. Estimated total time of
> sighting from our position was 20-30 seconds.

I am trying to figure out how a track from SW horizon to NE
horizon could only last 20-30 seconds.  Taking it a different
way, if the fiery objects were at the expected height for the
observing site, how high above the ground would they have been,
and how long would it have taken them to traverse the sky from
horizon to horizon?  Another question is at what height above
the ground would we expect such objects to slow down enough to
stop being visible?  I'm wondering if a very low height could
be consistent with a fast traverse, but doubting that objects
that low would still be visible.  I haven't seen any reports
of sonic booms.

For reference, the Space Shuttle takes something like two
minutes to cross from horizon to horizon on a re-entry that's
above 30 degrees above the horizon.  I believe at that time
that it's something like 80 km up, when it's due north or
south of us here.  (Or maybe the range is 80-90 km.)

One correction to an earlier post.  I wrote that I had seen a
report of "east to west" direction on the re-entry over
England-France-Belgium, but it was west to east, from an
observer in England.

Ed Cannon - ecannon@mail.utexas.edu - Austin, Texas, USA

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