Seeking assistance with a HEO orbit decay propagation

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Wed, 7 May 2014 10:12:52 -0400
I am seeking advice regarding the re-entry of Explorer 6 (59004A / 15), which I suspect may account for a slow-moving,
meteor-like fireball, seen on a horizontal trajectory over more than 1000 km of Northern Australia, on 1961 July 27 near
10:25 UTC. The sighting reports are in an the RAAF's archives. The RAAF speculated that one or more meteors had been
seen, but the re-entry of an object from Earth orbit seems to me at least as likely, probably more so.

My plot of the sighting locations suggested a southeast-bound object, inclined ~50 deg. The following is a good "fit" to
what I judge to be the sightings with the most useful trajectory data:

1 70000U          61208.43402778  .00000000  00000-0  00000-0 0    02
2 70000  46.8000  36.5000 0010000 300.6167 263.0000 16.75000000    05

The mean motion is a guess intended to place the altitude within the range of the luminous portion of a re-entry. I
believe that the inclination and RAAN are accurate to within about 0.5 deg, for the stated epoch. The uncertainty of the
epoch is at least several minutes, which adds to the RAAN uncertainty.

Here is a plot of the track of the above orbit in relation to all of the likely sighting locations:

http://satobs.org/seesat_ref/misc/UNID_re-entry_track.png

As of the date of the fireball, very few objects had been launched into that approximate inclination, and the only ones
expected to have been close to decay were Explorer VI (aka Explorer 6) and its rocket.

They were launched from Cape Canaveral on 1959 Aug 7 aboard a Thor Able III launch vehicle, into a highly elliptical
orbit: 46.9 deg, 249 X 42327 km. The satellite's largest dimension was about 0.75 m, and its mass was 64 kg. It was
initially assigned the international designation 1959 Delta 2. The final stage of its rocket entered a similar orbit.
The RAE (Royal Aerospace Establishment) Table of Earth Satellites describes it as a cylinder of 1.5 m length and 0.46 m
diameter, of mass 24 kg. It was initially catalogued as 1959 Delta 1. Eventually, the international designations of the
objects were swapped, so that the payload became object 1. Today, the international designation and USSTRATCOM catalogue
number of payload and rocket are respectively 1959-004A / 15 and 1959-004B / 17.

SAO Special Report No. 30 (Nov 1959) included "Anticipated Orbital Perturbations Of Satellite 1959 Delta Two" (Y. Kozai,
C. A. Whitney), that predicted decay within about two years of launch, due to depression of the perigee by luni-solar
perturbations. Plots of the numerically integrated evolution of key orbital elements, based on two different values of
A/m (area to mass ratio) indicated decay in late February or May of 1961. The analysis was described as preliminary.
This launch inspired several other papers on luni-solar perturbation, but I have yet to locate any more useful estimate
of the decay of the 1959-004 objects.

The satellite tracking network of the day was unable to keep track of either object for long. The payload's orbit was
determined by analysis of the Doppler shift of its radio signal. The latest radio observations for the purpose of orbit
determination occurred on 1959 Sep 30. The spacecraft ceased transmitting on 1959 Oct 6. Just seven (7) TLEs of
1959-004A / 15 exist, the latest, of epoch 1959 Sep 29 UTC.

The rocket reportedly was imaged by the Baker-Nunn camera at Arequipa, Peru, shortly after it entered orbit, but a
precise initial orbit appears not to have been determined. Only one TLE of 1959-004B / 17 exists, of epoch 1961 Jan 8
UTC. It was numbered 17, suggesting that there had been 16 earlier ones. Reliability of the existing TLE is uncertain.

NASA/GSFC's Satellite Situation Report, Vol. 1, No. 11, dated 1961 Jul 5, listed only the payload among the objects in
orbit, but in place of the orbital data, it stated "Position Uncertain". Vol. 1, No. 22, dated 1961 Dec 5, announced the
addition of both objects to the decayed objects list, with the date of decay "Presumed Prior Jul 61". Since the objects
had been lost, their date of decay could only be estimated. USSTRATCOM's satellite catalogue lists the date of decay of
both as 1961 Jun 30. The RAE Table of Earth Satellites lists both decay dates as "1961 Jul?".

As a preliminary test of the possibility that one of the 1959-004 objects could account for the fireball sightings in
question, I propagated the last available TLE of both using Mike McCants' int2 program, making trial and error
adjustments to the initial ndot/2 to bring about decay on or about 1961 Jul 27 UTC. The resulting RAAN at re-entry was
sufficiently close to the 36.5 deg of my estimated re-entry TLE to be interesting, especially for the payload (within a
few tens of degrees); therefore, I would appreciate learning the results of propagations using other, likely more robust
gravity and especially atmosphere models.

If either object reached final decay (mean motion ~16.5 rev/d) on 1961 Jul 27 near 10:25 UTC, would its RAAN have been
close to 36.5 deg? An estimate/discussion of uncertainty would be appreciated.

I doubt that the available TLEs are as reliable as present-day ones for similar orbits, but perhaps they are sufficient
for the requested analysis. A definitive orbit was published (Final Report On Explorer VI Definitive Orbit,
8650-6001-RU-000, Space Technology Laboratories, 25 June 1962), but the only copy I have located so far is in a
collection of papers at University of Chicago Library. I do not propose to seek a copy unless it is absolutely necessary
for the analysis.

Ted Molczan


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Received on Wed May 07 2014 - 09:13:55 UTC

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