For my master's thesis, I attempted to define the problem of identifying the forces behind the apparently varying spin periods of four inactive geosynchronous (GEO) telecommunications satellites. This sounds like a mouthful, and it is, but observing these four satellites for over a year showed to me that this problem would be much more difficult than anyone had thought.

One thesis and 252 pages later, the problem still persists. I did not imagine that I would single-handedly solve all of the problems, but I used some of my own equations and concepts to define at least what the problems were so that I could (semi) confidently launch into my PhD thesis.

When observing an inactive satellite spinning in space, many questions can be pondered, including:

1 - What force(s) caused the satellite to spin in the first place?;
2 - What is causing the satellite's spin period to vary?;
3 - Why are the spin periods of the four satellites so different from one another?;
4 - What are the individual orientations of the satellites' respective spin axes?;
5 - How much of the apparently varying spin period is real and how much is imaginary (synodic)?;
6 - How does the synodic effect change with the spin period and the spin axis orientation?; and
7 - Are some or all of the satellites' spin axes precessing?

The following is the story of all four satellites I studied for my master's thesis.

THE STORY OF SOLIDARIDAD 1

"Solidaridad 1" (English: Solidarity 1) is a Mexican telecommunications satellite launched on November 20, 1993. Its primary mission was to provide television, radio and telecommunications services to Mexico, the United States and the Caribbean islands. Its designed lifetime was 14 years, but it served for just half of that. In 1999 and 2000, tin solder whisker growths had shorted the satellite's main and backup satellite control processors (SCP). After the backup SCP had failed, the satellite was cut off from its controllers on the ground, thus ending the satellite's life. From 2000 onward, Solidaridad 1 was destined to drift in the geosynchronous belt indefinitely.

Artist's' Conception of the Solidaridad 1 Spacecraft - Boeing
 

When I first observed this satellite, it appeared to be spinning with a period of 970 seconds (just over 16 minutes). However, over time, this spin period decreased steadily until it reached about 850 seconds (just over 14 minutes). There it remained for over two months for reasons that I do not yet know. I had originally thought that this was a minimum and that the period would begin to increase. However, the period continued to drop to a minimum of just over 760 seconds (nearly 13 minutes). After that, it began to slowly increase again, but is appearing to reach another "plateau" of around 840 seconds.

At first glance, this appears to be very strange behaviour and could be the result of any number of physical processes and synodic effects. Right now, the spin period variation does not appear to have a periodic behaviour and only subsequent observations will clear up this question. It is possible that the period is much longer than one year. This is only one satellite of many, and so data from other box-wing GEO satellites was required to confirm what I had seen.

THE STORY OF TELSTAR 401

"Telstar 401" is an American telecommunications satellite launched on December 16, 1993 that once delivered satellite television and telephony to the continental United States. The satellite was designed to have a 15-year life span.

Artist's' Conception of the Telstar 401 Spacecraft - Lockheed Martin

On January 11, 1997, nearly one month after its 3rd year in orbit, the satellite was mortally damaged by a solar storm caused by higher than normal solar activity. The massive amount of charged particles from the storm rendered the power systems of the satellite permanently inoperative and therefore cut off all communications between the satellite and the ground station. The satellite ended up drifting within the geostationary belt threatening other payloads in its path. Today, the satellite has an inclined orbit, caused by the natural solar and lunar forces acting on it over the nearly 20 years of inactivity.

My first observation of Telstar 401 revealed that its spin period is much lower than Solidaridad 1, therefore Telstar 401 is spinning much faster. Its spin period was measured to be 145 seconds (2.5 minutes), but increased to just over 170 seconds (nearly 3 minutes) in about 5 months. The spin period was observed to decrease again to a minimum of about 147 seconds in about 5 months after the observed maximum spin period. The spin period of Telstar 401 is appearing to vary periodically from 145 seconds (minimum) to 170 seconds (maximum) with a period of about 300 days (10 months).

Unlike Solidaridad 1, there does not seem to be any "plateaus" in the spin period variation. If one looks very carefully between the maximum and minimum, one can just see what looks like a very small deviation (a "bump") in the normal sinusoidal behaviour, however it is too early to know if this trend will continue or become more pronounced with subsequent observations. The third satellite observed, Echostar 2, is a nearly identical twin of Telstar 401, since both satellites are of identical design (AS-7000).

THE STORY OF ECHOSTAR 2

"Echostar 2" is an American telecommunications satellite launched on September 11, 1996. Its primary mission was to provide television, radio and telecommunications services to the continental United States. The satellite was designed to have a 14-year life span.

Artist's' Conception of the Echostar 2 Spacecraft - Lockheed Martin

Like its nearly identical twin, Telstar 401, Echostar 2 suffered a catastrophic power failure due to a solar storm on July 14, 2008; nearly 12 years after its launch. At the time of this malfunction, the satellite was serving smaller markets in Alaska and was about to be retired.

Initial observations of this satellite revealed that it was spinning with an apparent period of 503 seconds (8.4 minutes). Subsequent observations revealed that this satellite's spin period was also varying; possibly in a periodic fashion. The first minimum spin period was observed to be about 375 seconds (6.2 minutes). The maximum spin period after this minimum was observed to be about 535 seconds. However, the second minimum (not yet observed) seems to be less than the first; below 360 seconds. This behaviour is much different than what was observed for Telstar 401.

Echostar 2's spin period variation appears to be much different in shape than either Solidaridad 1 or Telstar 401. This suggests that each box-wing GEO satellite has a very unique spin period variation, even when comparing satellites of identical design.

The Echostar 2 spin period variation shows three "plateaus" that are located in between the maximum and minimum spin periods. This seems to be the same observed behaviour as seen in Solidaridad 1 and (possibly) Telstar 401. A trend also seemed to emerge from the observations of these three satellites. A satellite with a lower spin period seems to exhibit a "stable" spin  period variation; nearly sinusoidal behaviour. As the spin period increases, the spin period variation becomes increasingly distorted away from the sinusoidal because of the "plateaus". As the average spin period increases, so do these distortions. Currently, the cause of these distortions is unknown. However, synodic effects will increase as the spin period increases, therefore these distortions might not be real, but synodic effects caused by the orbit motion of the satellite.

THE STORY OF HGS-1 (ASIASAT 3) (PAS 22)

 "HGS-1" is an American telecommunications satellite launched on December 24, 1997. The satellite was originally named "Asiasat 3" and was originally to be owned by the AsiaSat corporation to service eastern Asia with radio, television and telecommunications. However, just after its launch into the geosynchronous transfer orbit (GTO), the fourth stage of the rocket fired for an insufficient amount of time, thus stranding the satellite in the GTO instead of a true geosynchronous orbit. The satellite's perigee was only several hundred kilometres, while it apogee was over 36,000 kilometres.

Normally, this is where the story would end. The satellite would be left stranded in the GTO and the insurance company would have compensated the owner's loss. However, the original builders of the satellite, Hughes Aerospace, decided to use the satellite as a test bed for a radical idea. Hughes bought the satellite from the insurance company and decided to attempt to manoeuvre the satellite into a geosynchronous orbit using only the fuel onboard the satellite and the Moon's gravity. The name of the satellite was changed from "Asiasat 3" to "Hughes Global Services 1" (HGS-1 for short).

Artist's' Conception of the HGS-1 (Asiasat 3) Spacecraft - Boeing

The daring manoeuvre would involve raising the satellite's apogee to several hundred thousand kilometres in order for the Moon's gravity to influence it. The satellite was successfully manoeuvred using the Moon's gravity in a series of manoeuvres requiring a total of 6 months to complete. The orbit was not a perfect geostationary orbit, however it could be used as a geosynchronous satellite. When the new owners attempted to unfold the solar panels from their stored position, a problem occurred. Only one of the two solar panels successfully deployed, leaving the other one stowed indefinitely. This fact is extremely important, as the light curve of such a satellite should be vastly different from that of a spacecraft with two identical deployed solar panels, such as Solidaridad 1, Telstar 401 and Echostar 2.

After the satellite was deemed to be in working order (albeit have one deployed solar panel) it was successfully sold to PanAmSat (PAS) and was renamed yet again to PAS-22 in 1999. The satellite was parked over the mid pacific ocean where it worked for nearly 3 years until 2002, when it was parked into the junkyard GEO orbit. Since much of its onboard fuel was used for the rescue attempt, its originally designed lifetime of 15 years could not be realised. However, Hughes had shown that a geosynchronous satellite can be rescued using the Moon's gravity for assistance.

When first observing this satellite, it was apparent that its spin period was much longer than even Solidaridad 1. Its first spin period was measured to be about 1650 seconds (27.5 minutes); nearly double that of Solidaridad 1. As expected, its light curve behaviour looks much different than the other three. Its light curve normally has two larger spikes (possibly the solar panel) separated by 16% of the total spin period. This apparent behaviour seems consistent throughout the year the measurements were made.

The spin period variation was observed to be very choppy, but had a definite overall trend. The trend was much different than the other three satellites. At the present time, there is little evidence of periodic behaviour in the spin period variation. Further observations are required, maybe for another several years. So far, the satellite's spin period ranges from 1250 seconds to 1850 seconds.

The four satellites have significantly different light curves and spin period variations. At the present time, it is unclear why satellites with similar characteristics (with the exception of HGS-1's one deployed solar panel) have such diverse behaviours. However, the knowledge that the four satellites exhibit such different light curves is significant knowledge that might not yet be widely known. Given the observations collected so far, it is obvious that much more observations are required to explain what is actually happening when these satellites are observed.