Re: Re-entries

Alan Pickup (alan@wingar.demon.co.uk)
Sun, 15 Nov 1998 15:46:02 +0000

Jim Nix <nixj@bellsouth.net> writes
>    Can anyone pass on any hints on observing re-entering satellites.
>According to Sat-evo and the Latest OIG elset  I see a re-entry
>prediction for 87068 BJ at 02:02:37 on 11/12. I have passes starting
>here at 01:55:00 to 02:36:00 with trajectory's starting along the
>horizon and rising to zenith.  Would I just observe the sky along the
>Trajectories for thirty minutes and hope to spot something?  Is this
>object too small to make a large
>optical display on re-entry?

Put bluntly, you cannot take a single elset and expect SatEvo to give
you an accurate decay time. Even if the elset is perfect, I'd say that
the uncertainty in the predicted decay time is of the order of 10-20% of
the remaining lifetime. Too many factors come into play to make it much
better than this, the main ones being uncertainties in, and variations
of, the atmospheric density with time and position around the orbit.

Unfortunately, elsets for many of the debris objects are none too
frequent and appear less reliable than those for the larger objects. One
reason is that many of the debris objects have very low masses for their
cross-sectional areas, so they suffer high drag even at high altitudes
where the fluctuations in atmospheric density are also most pronounced.
The final elset for 87- 68 BJ, for example, has it in a 605x524 km orbit
from which it probably decayed within two days. Other more dense objects
can spend years orbiting happily at lower altitudes.

For each of the eight elsets I have for 87- 68 BJ over its final week,
SatEvo predicts decay dates between November 11 and November 16. The
final elset gives November 12.1 taken alone. When I combine the elsets
by carrying a SatEvo evolution through them, the evolution satisfies the
satellite's equator crossings to within +-20 seconds over the week. This
"fitted" evolution gives November 11.45 as the decay time, but the
uncertainty is such that I quoted only November 11 in my SatEvo decay
list just issued.

As to the observability of decays, I really do not know whether the
decay of such a small body would be noticeable or remarkable. Presumably
it would appear as a slow meteor, but I would not expect the brightness
and pyrotechnics associated with the re-entry of rocket bodies or
sizeable payloads. The accuracy in the timing and position of a re-entry
pass depends a great deal on the time since the last elset, and on the
change in orbital period during that time. Where the orbital period
change is large, as for 87- 68 BJ, the uncertainty is also large and
+-30 minutes in "along-track" error might be justified. In such a case,
though, remember that the orbital plane shifts quite a lot over 30
minutes or more, so allowance for this must be make when deciding where
to look in the sky. For other objects, such as the just-decayed Pegsat,
the time uncertainty may be much less (of the order of seconds only
along-track) so the orbital plane at decay is also pretty certain.
However, it could still appear several degrees high or low because of
uncertainties in its height above ground - it might be 120 km high and
still in orbit, or 70-80 km high and actively re-entering.

Alan
-- 
 Alan Pickup | COSPAR 2707:   55d53m48.7s N   3d11m51.2s W   156m asl
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