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'Whiskers' Caused Satellite Failure: Galaxy IV Outage Blamed On Interstellar Phenomenon
By Bruce Felps
WirelessWeek - May 17, 1999

Contrary to popular belief that solar flares damaged the Galaxy IV satellite one year ago this week, no amount of sunblock could have saved the doomed satellite.

As it turns out, an orbital "5 o'clock shadow" caused the satellite failure. Tiny strands of tin formed by pulling away from the thin layer of the metal that plates other surfaces, causing a phenomenon called "tin whiskers." This resulted in a shorting out of the satellite's main control computer.

Hughes Space and Communications Co., which manufactured the model HS 601 spacecraft christened Galaxy IV, reached this conclusion during its investigation into the satellite's failure. The inquiry, similar to a forensic investigation, according to Michael Bodeau, manager of survivability activity at Hughes, dismissed solar flares as the source of damage.

"There was circumstantial evidence that a solar storm afflicted the satellite. The sun has its seasons that run on an 11-year cycle, but we weren't in the bad part of the cycle last year. We identified the [primary control] computer as the component that failed. Space weather looked possible, but the review of data didn't back up the theory."

The data eliminating solar flares was collected from the satellite's telemetry transmissions. The same telemetry data also identified the main computer, called SCP No. 1, as the likely source of the failure because the computer shorted out. The Hughes investigation team replicated conditions on the satellite and concluded tin whiskers came in contact with a relay switch on the computer causing a circuit to malfunction.

Tin whiskers occur when the conformal coating used to plate the tin surfaces and keep them in place gave way to the vacuum of space. "If there is any breach in the conformal coating, tendrils will grow," said Don O'Neal, a spokesman for Hughes. "In this case, they made contact with a grounded surface causing a circuit to blow. The computer's functions should have toggled over to a redundant unit, but it already malfunctioned for a different, unknown reason."

The conformal coating is a wax-like substance applied on top of the tin plating. O'Neal said it must cover the tin completely and smoothly to eliminate the possibility of tin whiskers. However, the application process could cause a breech in the coating. "Usually, it's applied after a piece of equipment is mounted in the spacecraft. It might have to go in to a small opening that would be difficult for the technician to reach."

To combat tin whiskers, O'Neal said Hughes could either stop using tin plating or improve the conformal coating process. The satellite maker chose to use nickel plating during the manufacturing process instead. "We've taken tin out of our inventory. Now we use nickel." Hughes initially opted for tin because it weighs less than nickel.

Using nickel adds an extra 100 pounds to 200 pounds per payload.

Today, the derelict Galaxy IV, which PanAmSat moved farther into outer space, spins irreparably crippled.

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