Discriminating re-entries from meteoric fireballs

Robert H. McNaught, Anglo-Australian Observatory (RMN@aaocbn2.aao.gov.au)
Fri, 12 Jul 1996 16:39:36 +1000 (EST)

As I understand it, most decaying satellites circularise their orbits before
burn-up. This produces long duration fireballs that last over a minute and
cover very long ground tracks of several hundreds of kilometers.

Whilst solar-system debris can collide with the atmosphere tangentially, this
is a relatively rare event and few meteoric fireballs would last more than
about 30 seconds.  The vast majority lasting under 10 seconds.

Velocity is not a good discriminant to the visual observer, as the decaying
satellite is moving at 8 km/sec, whereas the SLOWEST entry velocity for
a meteoric object is 11 km/sec. Although velocities of over 70 km/sec are
possible, most of the LARGE debris that causes meteoric fireballs is in
low inclination direct orbits which catch up with the Earth from behind.  This
is the reason for the excess of slow moving evening meteoric fireballs over
faster moving morning objects (that are met head on).

Due to the problems in discriminating the lower velocity fireballs on angular
motion alone, and as the higher velocity objects must have short durations,
it would seem that the duration of the event may be the best discriminant.
Without any firm data, I would suggest a duration of around 30 seconds could
separate the vast bulk of re-entries from meteoric fireballs.

Over a period of a decade or so some friends and I operated all-sky cameras for
recording fireballs.  We recorded over 50 events brighter than about mag -6,
but only one event was known to be a satellite re-entry.  This event passed
from horizon to horizon with a maximum elevation of some 40 degrees.  In terms
of its angular path across the sky (and thus duration) it stood out
dramatically from all the other trails.

Of course, if it is possible to calculate the trajectory and velocity from
observations, the nature of the event should be fairly clear.

I would be interested to know what sort of fireball durations have been
produced be re-entring debris of various sizes and at what heights they
typically start to burn and extinguish.  I guess this is well documented
for the Shuttle.

Cheers, Rob McNaught