Tim Furniss/LONDON
THE DATE, 20 JANUARY 1994, does not immediately spring to mind as memorable in space history, but it was a nightmare day for the communications-satellite industry, particularly in Canada. Without warning, the gyroscopic guidance systems on board the country's primary-communications satellites, the Anik E1 and E2, failed. Cable television, telephone, news-wire and data-transfer services were interrupted. Transcontinental hotel and airline bookings were halted, and financial services were plunged into chaos.
Both satellites had been subject to a blast of space plasma following a geomagnetic storm, which had begun seven days earlier. The plasma had been discharged into the momentum-wheel assemblies of both craft, the electrostatic discharge creating electromagnetic impulses within the guidance-system circuitry. Although the Anik E1 was restored to service after 8h by a back-up guidance system, the Anik E2 was inoperable. Its operator, Telesat, was faced with the loss of a $228 million satellite and $3 billion in revenue.
Order was restored by innovative thruster firings, however, and the satellite was back in service by August 1994. Telesat spent $70 million on recovery and repair costs and a one-year decrease in the craft's ten-year lifetime, because of the consumption of on-board fuel.
INTELSAT WOBBLE
While the Aniks were malfunctioning, the Intelsat K communications satellite had also been hit and its gyros malfunctioned and wobbled the craft temporarily. While the location of the momentum-fly wheel assemblies on the Aniks had caused them to be more susceptible to damage, the assemblies on the Intelsat K were in a different position, and so more serious damage was avoided. The Aniks and the Intelsat K were former Martin Marietta Astro Space Satcom 5000 buses.
The 1994 event was not significantly more intense than other previous occurrences. Many communications and other satellites before the Aniks had experienced electrostatic discharges and ensuing service interruption. Some spacecraft had been lost (see panel). Other satellites since the Aniks have suffered malfunctions, some displaying similar symptoms.
"These have not been recorded officially as electrostatic discharges, neither have the incidents before and after been advertised by satellite manufacturers who don't like to publicise failures," says Dr Gordon Wrenn, team leader of the spacecraft environment and protection office in the Space Department of the Defence Research Agency (DRA), in Farnborough in the UK. Wrenn has made a detailed study of these phenomena.
He says that the threats to most of the satellites have resulted from interaction of the Solar wind with the Earth's magnetic field, but "-it is surprising that they occurred near to the period of minimum [rather than maximum] activity of the Sun during its 11-year solar cycle".
He has produced conclusive proof that satellites are at risk from electrostatic discharge while in orbit, especially from the build-up of energetic electrons linked to the evolution of coronal holes on the Sun, which are usually more long-lasting during this declining phase, when a minimum solar activity is approaching.
The attitude of manufacturers is that, if the only result of such incidents is a phantom command or similar momentary glitch, "nobody need know", says Wrenn, but, if every incident is catalogued and reported, it will make understanding of the problem and its adverse effects much easier to avoid.
Modern, miniaturised, electronic systems, on satellites - many of them off-the-shelf components - use low voltage and low currents and are packed into small areas. They are more sensitive to space-charging effects than older-generation equipment.
Earlier generations of electronics, because of their size and more distributed layout, provided a certain robustness against the effects of charging. Modern, integrated, component technology does not provide the same protection. The increased use of dielectric thermal coatings and composite spacecraft structures increases the risk of spacecraft charging. Different plasma-energy levels affect different parts of a craft. The effect of the flux with energy, ranging from 0.1-5MeV (million electron volts energy) depends on electron number (measured in per cm2/s, on energy distribution and directionality. While lower-energy plasma may result in surface charging, this higher energy plasma can penetrate through typical 0.2mm-thick aluminised thermal blankets and deposit a charge, perhaps on a critical component.
These problems are confined largely to the outer shell of the craft under the thermal blankets (rather than within the shell of the craft), where insulated, co-axial, cables are strung - "sometimes carelessly", says Wrenn - around the craft and conduct current into sensitive satellite parts.
Detailed studies by the DRA and engineers at NASA and other research centres have clearly indicated that the internal charging (which results in operational anomalies) occurs more regularly and with more significant effect during this period during the Sun's activity cycle.
BETTER PROTECTION
Wrenn's work has highlighted the need to equip satellites, with better protection against the particularly invasive radiation, which penetrates at just the right distance into a spacecraft, to discharge its power with the most effect, during the identified period approaching minimum activity, which has occurred in 1974, 1984 and 1994 (see graph)."
There was nothing particularly significant about the Sun's activity in January 1994," says Wrenn. "Electron enhancements were simply tied up with the fast solar-wind streams, from the Sun's coronal holes, that seem to persist during this period of the solar cycle."
He adds: "It is not clear what additional factor made 20 January such a dark day for the Aniks, but it is likely that the electron energy caused penetration to a critical depth."
In geostationary orbit, "-where the radiation environment is relatively benign", says Wrenn, spacecraft can be susceptible to such enhancements. In 1992, the frequency of these events started to increase, occurring for 20 days each month.
The main actions required by manufacturers to protect satellites should be to desensitise vulnerable circuits and to identify potential charge sites, for example co-axial cables, so that shielding can be increased locally, says Wrenn. Such measures have been successfully implemented in the attitude control systems aboard specific spacecraft, although he will not specify what they were.
"Manufacturers have been slow to change, but Intelsat has seen the light." Wrenn adds that Intelsat is telling manufacturers to fit plasma monitors on its satellites. The National Geophysical Data Center at Boulder, Colorado, initiated a Space Anomaly Database of declared environmental anomalies so that open reporting might eventually permit a comprehensive analysis of space-weather effects and enable a cost-effective protection strategy to be employed. NASA's Goddard Spaceflight Center in Greenbank, Maryland, also produces annual anomaly reports.
The next critical period in a declining solar cycle will be in 2003. The communications satellite industry will await the time with interest. "The jury is still out," says Wrenn.
Source: Flight International
