Science & Environment

'Zombie-sat' rises like a phoenix

Artist's impress of Galaxy-15 in orbit (Image: Orbital Sciences)
Image caption Galaxy-15 drifted past 15 other satellites in orbit 36,000km above the Earth

Intelsat says its once wayward Galaxy-15 spacecraft should deliver 10 years of service if put back into operation.

The satellite, which spent most of 2010 drifting out of control, is now under full command again and set for an intense test programme.

Assuming it is given a clean bill of health, Galaxy-15 will return to the business of relaying TV services to the Americas, says Intelsat.

The spacecraft's recent woes have had hardly any impact on its fuel reserves.

"The current end of life is 2022; that isn't going to change," said Tobias Nassif, Intelsat's vice president of satellite operations and engineering.

"We did save a little fuel in east-west station-keeping over the past eight months; however we'll now use that fuel to bring the satellite back into its station. So the net effect is essentially zero," he told BBC News.

The on-going investigation into the mishap suggests an electrostatic discharge on the spacecraft may have been at the root of the platform's problems.

'Dead but alive'

Galaxy-15 was launched into space in 2005 to a position high above the equator at longitude 133 degrees West.

Its mission was to re-distribute TV services to cable companies across North and Central America, and also to send navigation data to aeroplanes to improve the accuracy of their GPS receivers.

But on 5 April last year, it experienced a major glitch that left it incapable of sending telemetry or taking commands, and for Intelsat to keep it stationary in its assigned orbital slot.

In all other respects, though, Galaxy-15 retained a fully functional electronics payload and its capability to re-transmit on full power any TV signal it received of compatible frequency soon became a cause of concern among satellite operators.

With the platform drifting through the slots occupied by other telecommunications satellites, it risked interfering with those neighbours' services.

Galaxy-15's "dead-but-alive" condition earned the Intelsat spacecraft the unfortunate nick-name of "zombie-sat" in some quarters. But its return to full control means this moniker no longer applies. Indeed, Intelsat now describes Galaxy-15 and its likely reintroduction into service as a "phoenix".


The satellite ceased being an interference hazard once it lost the ability to orientate itself and lock on the Earth. This took longer than expected but when it did happen, the solar arrays on the spacecraft also then lost sight of the Sun and the electronics payload soon went into shutdown as the batteries drained of their energy.

A reset manoeuvre that followed allowed engineers to upload software patches and take back full command. Total control was re-established at the end of December.

Galaxy-15 has been instructed to move to a position at 93 degrees West. It should arrive on Saturday and when it does, Intelsat will undertake a series of tests not unlike those given to brand new spacecraft in their post-launch commissioning phase.

"Keep in mind the satellite for eight and a half months was fully functional; it was never during that period in any kind of state of stress," said Mr Nassif. "Therefore, we don't expect there to be any damage to the spacecraft. However, we're going through this whole testing."

The board of inquiry set up to investigate the incident should deliver its findings in February.

Harsh environment

Laboratory tests by spacecraft manufacturer Orbital Sciences have established that software issues left Galaxy-15 vulnerable to upset if there was an electrostatic discharge (ESD) of a particular energy within the satellite.

What might have caused such a discharge in Galaxy-15's case is not clear, but Intelsat said the suggestion that intense solar activity at the time of the outage has been dismissed.

"Many of the anomalies that operators experience onboard [their satellites] ultimately get attributed to ESD events," Mr Nassif told reporters. "Satellites are built to the best understanding of the space environment and as we learn more about operating in space and the harshness of space… satellite manufactures will apply those lessons to future designs."

Intelsat has yet to decide where to put Galaxy-15 if it does go back into service. It could return to 133 W, or perhaps 129 W, the slot vacated by Galaxy-12. This was the platform that moved to 133 W to pick up the services dropped in the incident.

It is possible Galaxy-15 may act as in-orbit spare for the rest of Intelsat's fleet.

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