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(meteorobs) Re-entry vs fireball
The question of satellite vs meteor is raised regularly. Unfortunately
there is no single criterion that can differentiate between these possibilities
based on a single visual observation. The answer is to be found somewhere
in the following
a) Is the path consistent with a predicted satellite re-entry?
b) Is the duration longer or shorter than, say, 15 seconds?
c) Is it brighter than mag -8?
d) Does the object have many fragments?
e) What is the direction of motion?
A satellite re-entry will typically last many tens of seconds, whereas a
meteoric fireball would rarely last that long. Satellite usually have a
progressive breakup during re-entry and the fragments trail in the same line
as each other. It is certainly possible for a meteoric fireball to enter
the atmosphere at a shallow angle, but it would have to be massive to have
a duration longer than 15 seconds, and the brightness would therefore
usually be greater than, say, mag -8. Such objects often show a
progressive break-up but could probably be separated by the brightness (e.g.
Peekskill fireball, 40 sec durn, lots of fragments, mag brighter than -13).
Satellite re-entries are rarely, if ever, brighter than mag -8.
A single object of short (few seconds) duration will never be a satellite
re-entry and it would be rare for a re-entry to follow an east to west path.
Of course, determining the trajectory of the object will give a strong clue
as to the nature. Satellite re-entries will typically have a VERY shallow
trajectory (few degrees). A steep trajectory (3-D, not the 2-D apparent
trajectory as seen from certain directions) will invariably be a fireball.
Finally, if the velocity can be calculated, a satellite re-entry will be
below 8km/sec, whereas a meteoric fireball above 11 km/sec, often well above.
It should be noted that meteoric fireballs of mag -4 or brighter are some
hundreds of times more frequent than satellite re-entries. Perhaps this
realisation would make the NEO threat more real to people. An object has
to have a very long duration to have a reasonable chance of being a re-entry.
Hope this helps!
Cheers, Rob McNaught
(rmn@aaocbn.aaodot gov.au)