Re: Moln 3-11

Alan Pickup (alan@wingar.demon.co.uk)
Wed, 2 Dec 1998 23:18:09 +0000

Jim Nix <nixj@bellsouth.net> writes
>I just downloaded this from OIG at 09:25:00 UTC day Dec 2 (336).

                                             ^^^ I think not :)
>
>MOLNIYA 3-11    Decayed: 1998/12/02
>1 11240U 79004A   98336.62432160  .99999999  99048-5  24553-2 0   855
>2 11240  61.9440 334.5503 0463804 285.5206  69.9709 15.51169661149689

This is probably the final elset for the Molniya, though the object
probably survived for another couple of orbits. From the logs of my
automated OIG-login sessions, I see that USSPACECOM (or OIG?) put the
"Decayed..." announcement in the header line between 18:10 and 19:10
UTC. My analysis earlier this (UK) evening suggested that it would decay
at ~19:15 UTC, or possibly ~17:45; if the announcement is taken at face
value, then the 19:15 time is wrong. However, in the absence of further
elsets or other evidence, I will hold to the 19:15 time. I am suspicious
of the above elset, in any case, since it indicates a perigee height of
98 km, some 10-15 km higher than most other recent elsets. Even without
it, however, a number of SatEvo evolutions through the previous orbits
today had mostly indicated decay at (or near) the ~19:15 perigee.

>Does the "Decayed" indicate decay of orbit or re-entry?

I assume "Decayed" in USSPACECOM's parlance implies that the object is
no longer in orbit.

I must admit to being lax in my application of the term "decay" and "re-
entry". Strictly speaking, all satellites in LEO are "decaying" in that
they are subject to atmospheric drag which could eventually lead to
their re-entry. I use "decay" and a synonym for "re-entry" - normally
the "point" at which the drag reaches such a peak that the velocity
falls below that required to stay in orbit. That "point" may stretch
over 100s if not 1,000s of km, of course, and may correspond with a
visible event as the object becomes incandescent through frictional
heating and disintegrates. For many objects, it is convenient to take an
altitude of 90-100 km as the "decay height". The Molniya, however, was
possibly dipping below this height during every one of its final orbits,
but with enough energy remaining to climb once again out towards its
next (much lowered) apogee. Eventually it could not...

As Granat should demonstrate next June, it is possible for an object to
decay or re-enter having experienced little or no atmospheric drag until
it plunges for the first and last time into the atmosphere. In Granat's
case, luni-solar gravitational effects will cause it to "fly" into the
Earth.

Does anyone have a more precise distinction between "decay of orbit" and
"re-entry"?

Alan
-- 
 Alan Pickup | COSPAR 2707:   55d53m48.7s N   3d11m51.2s W   156m asl
 Edinburgh   | Home:   alan@wingar.demon.co.uk    +44 (0)131 477 9144
 Scotland    | SatEvo page:     http://www.wingar.demon.co.uk/satevo/