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Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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Servicing Mission 3A Overview
STS-103 on Space Shuttle Discovery, landed at 7:01 EST, Monday, December 27, 1999, 12:01 am GMT, December 28, 1999.
Mission Elapsed Time: 7 days, 23, hours, 10 minutes.
Launch Date: 7:50 pm EST, December 19, 1999; 12:50 am, GMT December 20, 1999.

STS-103 Launch/Landing Opportunities

The failure of a fourth gyroscope on November 13, 1999 placed Hubble in safe hold until the SM3A rendezvous and grapple on December 20, 1999.

Servicing Mission 3A successfully replaced equipment and performed maintenance upgrades to the Hubble Space Telescope. Although no new scientific instruments were installed, many activities took place over 3 EVA days. The originally planned 4 days of EVA were changed to 3 days because of the weather-delayed launch. The deorbit time was fixed for this mission in order to avoid any possible Y2K problems.

Listed below are a few of the main activities that have taken place.

More Mission Information
Mission Objectives
Mission Success Criteria (pdf) (text only)
Media Guide
Hubble Fact Sheets
Servicing Mission Launch Times
Hubble Highlights

What was Installed

Advanced Computer Advanced Computer: The new main computer is 20 times faster with six times more memory than its predecessor. This computer, which was successfully tested aboard STS-95 in 1998 on the HOST mission, will dramatically increase capabilities, reduce maintenance, and significantly lower operational costs.

Aft Shroud Latches Repair Kit (ASLRK): This kit will fix door latches in the aft shroud area that have been damaged by extreme temperature changes and high torques. (Planned but not installed because of the shortened mission)
Fine Guidance Sensors

Fine Guidance Sensor (FGS): Hubble uses three Fine Guidance Sensors for steady pointing and measurement. Astronauts will exchange one of these optical sensors with a refurbished unit that has enhanced, on-orbit alignment capability. The new unit is a refurbished version of the one that was returned from Servicing Mission 2 in 1997. The unit it replaces will be refurbished and reinstalled during Servicing Mission 4 in 2003.

Handrail Covers: Fiberglass fabric, called beta cloth, will be fitted like sleeves around the handrails above the Fine Guidance Sensor bay. These covers prevent possible contamination to the Aft Shroud area from flaking handrail paint.
New Outer Blanket Layer (NOBL): The NOBL consists of stainless steel panels covered with a protective thermal coating. These panels fit over existing, degraded insulation on Hubble's exterior surface, to control Hubble's internal temperature. In 1997, astronauts on Servicing Mission 2 discovered damaged areas of insulation and performed temporary repairs. The NOBL will be permanently mounted. (Planned but only partially installed because of the shortened mission)

Rate Sensor UnitGyroscopes (Rate Sensor Units): The gyroscopes are part of Hubble's pointing system. They provide a frame of reference for Hubble to determine where it is pointing and how that pointing changes as the telescope moves across the sky. They report any small movements of the spacecraft to Hubble's pointing and Control System computers. The computers then command the spinning reaction wheels to keep the spacecraft stable or moving at the desired rate. The gyroscopes work by comparing Hubble's motion relative to the axes of the spinning masses inside the gyroscopes. In the absence of external torques, these axes remain stable relative to the fixed stars in the sky. By keeping Hubble fixed relative to these axes, Hubble stays stable relative to the stars.

Each gyro contains a spinning wheel inside a sealed cylinder. This cylinder is immersed in a thick, motor-oil-like fluid. Fine, hair-like wires, surrounded by this thick fluid, carry electricity to the motor. Gyroscopes, Exploded ViewOxygen pressurized air, used to force the thick fluid into the float cavity which contains these wires, has corroded the wires and caused them to break. Pressurized nitrogen, used in the new gyroscopes, will eliminate the introduction of corrosive oxygen.

Hubble has a total of six gyroscopes grouped in pairs inside three Rate Sensor Units (RSUs). They are arranged in such a way that any three gyroscopes can keep Hubble operating with full accuracy. At the end of Servicing Mission 2, all six gyroscopes were working normally. From January 1999 until November 13, 1999, Hubble had been operating with only 3 functional gyroscopes, making it highly desirable to restore the remaining three. Now that there are only two functional gyroscopes remaining, Hubble is in safe-hold mode and the science program has been suspended until the completion of Servicing Mission 3A.

s-band single access transmitter

S-Band Single Access Transmitter (SSAT): This replacement transmitter uses radio waves to send data to the ground. The older unit it replaces will be returned to Earth and refurbished for a later flight.


Shell/Shield Replacement Fabric (SSRF): The SSRF consists of sheets of flexible, aluminized Teflon® fabric that fit over the original Multi-Layer Insulation on Hubble's forward shell and light shield to add thermal protection. In 1997, astronauts on Servicing Mission 2 discovered damaged areas of insulation and performed temporary repairs. The SSRF will be permanently mounted over these temporary coverings. (Planned but not installed)
Solid State Recorder Solid State Recorder(SSR) Installation: Hubble's original data recorders were mechanical, reel-to-reel tape recorders with many moving parts that wear out over time. The digital SSR has no moving parts or tape to break, so it is much more robust. This next-generation recorder is faster and more reliable, and it can store 10 times as much data as a mechanical recorder. The SSR will replace one of the two remaining tape recorders aboard Hubble. Another SSR, installed in 1997, continues to perform well.


Voltage/Temperature Improvement Kits (VIKs)Voltage/Temperature Improvement Kits (VIKs): These protect Hubble's batteries from overcharging and overheating when in safe mode.


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