Three gyros must be operational at any given time to provide HST with a highly precise, three-dimensional frame of reference within which to slew and to point to astronomical targets.

Six gyros are on board HST to provide a high degree of redundancy. If fewer than three gyros are functioning, there is no physical risk to the observatory. Rather, HST drops into a "zero-gyro" safemode, orients its solar arrays to the Sun, and awaits a service call. However, under such conditions it is no longer possible to carry out observations -- the science program ceases until 3-gyro operations can be restored.

At the time of the second servicing mission (SM2), in February, 1997, HST had six properly functioning gyros. No gyros were replaced on that mission, given our servicing philosophy of "if it isn't broken, don't fix it."

Moreover, the mean time to a gyro failure, calculated at that time on the basis of prior history with the HST gyros (an admittedly very small statistical sample) was 19 years.

Unfortunately, since SM2, three gyros have failed. The failure mechanism appears well understood. Each gyro package contains a viscous fluid surrounding the gyro rotor housing; electrical current to and from the gyro motor is carried by two pairs of thin wires (each about the thickness of a human hair) called "flex leads", immersed in the fluid. These wires are composed of a mixture of copper and silver.

When the gyros were filled with fluid during their original construction, oxygen contamination from the air resulted in the release of bromine in the fluid. The latter reacts with copper, effectively leaching it out of the flex leads and leaving behind the silver.

The resulting silver filaments are brittle and thus are more likely to break. Whether or not they do break is apparently a function of how coarse or fine the original copper granules are within the substructure of the individual flex lead -- i.e. there is an element of luck in all of this.

The signature of all three gyro failures is an enhanced current powering one phase of the gyro motor, indicating that the other circuit has been interrupted by a broken flex lead. In the first two gyro failures, in 1997 and 1998, a second flex lead broke, and the gyro ceased all operation, within a few hours to a few months.

This past January one flex lead on Gyro #3 broke. We do not expect the second flex lead circuit to remain intact for very long. In any event the performance of Gyro #3 is now erratic and it would be difficult to use in routine science operations, even if the remaining flex lead survives. Therefore, we are now down to the minimum set of gyros required with no additional redundancy on board HST.

Servicing Mission 3 (SM3), originally scheduled for December, 1999, had recently been rescheduled to June, 2000, and reassigned to another shuttle orbiter, because of problems with the availability of our original launch vehicle (Columbia), which is also being used by the Chandra Observatory (formerly AXAF).

We believe there is only about a 50% chance of reaching June, 2000, without another gyro failure. In fact, there is a 10 to 20% chance of another failure by June, 1999. It was this consideration, the possibility of up to one year of scientific downtime for HST, that led NASA Administrator Goldin to conclude that an "emergency" servicing mission to replace HST's gyros is necessary.

Servicing Mission 3 has now been broken into two parts, SM3A and SM3B. SM3A is scheduled for launch on October 14, 1999 (at 5:42 AM, eastern time). It will consist of 3 EVAs, during which astronauts Steve Smith (a veteran of SM2), John Grunsfeld, Mike Foale and Claude Nicollier (of SM1 fame) will insert six new gyros, a new spacecraft central computer, a replacement fine guidance sensor (FGS2-R), a backup S-band trnasmitter, a backup solid state recorder, and a battery thermal protection kit. They'll also work on covering some of the multi-layer insulation on the exterior of the spacecraft, which was seen to be degrading during SM2.

Launch of the Advanced Camera for Surveys (ACS), the aft-shroud cooling system, the NICMOS cooling system (better known as the "cyrocooler"), and new solar arrays must unfortunately be deferred to SM3B, which we hope to launch around December, 2000. The fourth and final servicing mission is scheduled no sooner than July, 2003.

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