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Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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Hubble Space Telescope Servicing Mission 3A


Mission Overview

Advanced Computer
Crew Aids
Fine Guidance Sensor
S-Band Transmitter
Solid State Recorder
Space Telescope Control Center
Thermal Blanket
Voltage Temperature Improvement Kits
Cost to Taxpayers
Plans for the Future



Hubble's main computer is responsible for monitoring the health of its many systems, for controlling the movement of the telescope from target to target, and for holding the telescope steady when observing. The computer, called the DF-224, was designed in the late 1970's and its capabilities are much less than today's modern computers. Programming requires very specialized skills, unique to this computer, and maintaining the software is difficult and expensive.

The DF-224 computer has degraded over time and during the First Servicing Mission in 1993 it was augmented with an additional computer called a co-processor. The design of the co-processor was based on the Intel 80386 microchip.

Replacing Hubble's Main Computer

During the next servicing mission, astronauts will replace this DF-224/coprocessor combination with a completely new computer based on the Intel 80486 microchip. This new computer will be 20 times faster, and have six times as much memory, as the current computer on Hubble.

In a good example of NASA's goal of "faster, cheaper, better," commercially developed, commonly available equipment was used to build this new computer at a fraction of the price it would cost to build a specialized computer designed specifically for the spaceflight environment. NASA performed a battery of mechanical, electrical, radiation and thermal tests to guarantee that the computer would survive the trip to orbit, withstand bombardment by cosmic and solar radiation and work flawlessly in the extreme temperatures of space for the rest of Hubble's life. As a final check, NASA carried the computer to space in the Space Shuttle for 10 days in 1998. The computer worked perfectly. The greater capabilities of the new computer will increase productivity for the Hubble observatory by performing more work in space and less work by people on the ground. The computer software will be programmed in a modern programming language. The result will be decreased cost for software maintenance.


Size 18.8 x 18 x 13 inches
Weight 70.5 pounds



Servicing Hubble in Orbit

While other spacecraft have been retrieved or repaired by astronauts, the Hubble Space Telescope (HST) is the first designed with replaceable parts and instruments for planned servicing. To enable astronauts to change out parts and instruments, Hubble was built with 225 feet of handrails and 31 foot restraint sockets to give the astronauts safe, convenient worksites as they orbit Earth at 17,000 mph.

During the Fall 1999 mission, the seven-member crew of STS-103 will rendezvous with the Telescope, capture it with the Space Shuttle Discovery's robotic arm and dock it in the Shuttle bay. Working in teams of two, the four space-walking astronauts will outfit Hubble with new equipment, including six gyroscopes, a Fine Guidance Sensor, Solid State Recorder, new Main Computer, and a transmitter.

The astronauts will take more than 150 crew aids and tools on this service call. These range from a simple bag to sophisticated, computer-operated power tools. Some are standard items from the Shuttle's toolbox; others are unique to this mission. All are designed to accommodate and compensate for the astronauts' bulky, pressurized gloves and space suits.

Crew Aids

Crew aids are fixed-in-place or portable equipment items, other than hand tools, that assist astronauts in accomplishing their tasks. Crew aids permit the astronauts to maneuver safely or to anchor themselves while working in the weightlessness of space. Examples of crew aids are: handrails, hand-holds, transfer equipment, protective covers, tetheringdevices, foot restraint platforms, tool caddies,and stowage and parking fixtures. An example of a restraint is the Portable Foot Restraint. An astronaut places both feet in this restraint, holding him in place while he performs a task. The Translation Aid is a long, adjustable-length arm that the astronaut uses when moving between the payload bay and the Telescope.

Astronauts servicing Hubble use two different kinds of foot restraints to counteract their weightless environment. When anchored in a Manipulator Foot Restraint, an astronaut can be transported from one worksite to the next with the robotic arm. With the Portable Foot Restraint, a stable platform is established by mounting the restraint to any of the strategically placed receptacles on the Telescope.

Portable handles are attached to many larger pieces of replaceable equipment to aid in removal or installation.


Tools are hand-operated devices that allow space-walking astronauts to efficiently perform intricate, labor-intensive tasks. Some tools are used on each servicing mission for loosening and tightening bolts. Others are specially designed for specific tasks. For example, a tool called a fastener capture tool, will efficiently remove and hold the bolts removed from the transmitter.

Engineers anticipate that the aft shroud doors' latch mechanisms may have seized due to many years of extreme temperature fluctuations. To pre-pare for this, engineers developed special tools and replacement latches.

Power Tools

The astronauts' main power tools are the Power Ratchet Tool and the Pistol Grip Tool. Astronauts used the Power Ratchet Tool, a power

tool specifically developed for Hubble's first servicing mission in 1993. The experiences and recommendations gained from this mission led to the development of the smaller, more efficient Pistol Grip Tool. This newer tool has been used successfully on several missions for the Hubble Space Telescope and the International Space Station. Because of their different capabilities and limitations, both tools are used to service Hubble.

The Power Ratchet Tool is a 3/8" right angle drive power tool used for tasks requiring controlled torque, speed or turns. It consists of awrench, controller, umbilical and battery module. It is capable of 0.5 to 25 foot-pounds (an average man or woman usually exerts about 2 to 5 foot-pounds of torque by hand with a regular screw-driver) of motorized torque and 75 foot-pounds of manual torque. The Power Ratchet Tool weighs 8 pounds and is powered by a 28-volt silver zinc bat-tery.

The Pistol Grip Tool is a self-contained, computer-controlled, battery-powered 3/8 inch drive power tool with a pistol-style handle. Numerous torque, speed and turn limits can be programmed into the tool for mission-specific applications. A light emitting diode display on the tool tells the astronaut what torque he or she is applying, at what speed, and how many turns the motor has made. It also displays error messages. The motor-ized torque ranges from 2 to 25 foot-pounds, the speed from 5 to 60 rotations per minute, and the number of turns from 0 to 999. In manual mode, the Pistol Grip Tool can apply 38 foot-pounds of torque.

The Hubble crew aids and tools have proven in previous servicing missions that they are key components in achieving safe, efficient tasks by the space-walking astronauts.


The Fine Guidance Sensor (FGS) is an optical sensor used on the Hubble Space Telescope to provide pointing information for the spacecraft and also as a scientific instrument for astrometric science.

A FGS consists of a large structure housing a collection of mirrors, lenses, servos, prisms, beam-splitters and photomultiplier tubes. There are three fine guidance sensors on Hubble located at 90-degree intervals around the circumference of the telescope. Two FGSs are used to point the telescope at an astronomical target and hold that target in the scientific instrument's field of view. The third FGS can then be used as a scientific instrument for astrometry.

Pointing Control

The fine guidance sensors are one of the sensors used by Hubble's pointing control system to point the telescope at a target with an accuracy of 0.01 arcsec. What is an arcsec? An arcsec is the width of a paperclip wire viewed from the distance of two football fields. With this fine precision, the guidance sensors lock on to a star and then measure any apparent motion to an accuracy of 0.0028 arsec. This gives Hubble the ability to remain pointed at that target with no more than 0.007 arc-sec of deviation over long periods of time. This level of stability and precision is comparable to being able to hold a laser beam focused on a dime that is 200 miles away (the distance from Washington D.C. to New York City).

Astrometry Science

Astrometry is the science that deals with the determination of precise positions and motions of stars. The FGS's can provide star positions that are about 10 times more precise than those observed from a ground based telescope. When used for astrometric science the fine guidance sensors will let Hubble:

• Search for a wobble in the motion of nearby stars that could indicate the presence of a planetary companion.

• Determine if certain stars really are double stars.

• Measure the angular diameter of stars, galaxies and other celestial objects.

• Refine the positions, distances and energy output of stars.

• Help determine the true distance scale for the universe.


During the Hubble Servicing Mission 3A, astronauts will exchange a FGS with a refurbished unit that has an enhanced on-orbit alignment capability. The refurbished FGS on SM3A is the same unit that was returned from Servicing Mission 2. The FGS returned from SM3A will be refurbished and upgraded for re-use on Hubble's 4th Servicing Mission.

The astrometry science program is managed through the Space Telescope Science Institute, Baltimore, Md., and is open to scientists throughout the world in the same manner as all other Hubble science. The Hubble Space Telescope operations an servicing are the responsibility of NASA's Goddard Space Flight Center, Greenbelt, Md.


Size 5.5 x 4 x 2 feet
Weight 478 pounds
Power 19 Watts



The gyroscopes, or gyros, on Hubble are needed for pointing the telescope. They measure attitude when Hubble is changing its pointing from one target (a star or planet, for example) to another, and they help control the telescope's pointing while scientists are observing targets. Three gyros must operate simultaneously to provide enough information to control Hubble. There are a total of six gyros on board — three serve as backups. Each gyroscope is packaged in a Rate Sensor assembly.

The Rate Sensors are packaged in pairs in boxes called Rate Sensor Units (RSUs). It is the RSU that astronauts change when they replace gyros, so gyros are always replaced two at a time.

How do gyros work?

The gyros work by a scientific principal called the gyroscopic effect. This effect can be demonstrated by holding a bicycle wheel by the axle and asking someone to spin the tire. If you try to move the axle of the spinning wheel, you would feel a movement in a direction different from the way you were attempting to move it. This movement is similar to the way the gyros move when Hubble moves.

The gyroscopic movement is achieved by a wheel inside each gyro that spins at a constant rate of 19,200 rpm on gas bearings. This wheel is mounted in a sealed cylinder, which floats in a thick fluid. Electricity is carried to the motor by thin wires (approximately the size of a human hair) which are immersed in the fluid. Electronics within the gyro detect very small movements of the axis of the wheel and communicate this information to Hubble's central computer.

The gyros are extraordinarily stable and can detect extremely small movements of the Telescope. The gyros are the most accurate in the world and, combined with other fine pointing devices, keep HST pointing for long periods of time to collect spectacular images of very faint galaxies, planets and stars not visible from Earth.

What is the status of the gyros on HST?

Three of the Hubble's six gyros are not work-ing, leaving only the minimum number needed to continue its science program. At present the Telescope continues to operate normally with no impact to its mission. However, should another gyro go offline, Hubble will automatically place itself into a protective safe mode. In this mode, ground controllers will still have complete control of the Telescope, but science operations would be suspended until the Fall 1999 servicing mission. During that mission, astronauts will install three new RSUs, leaving Hubble with six fresh gyroscopes, three of which will serve as spares. Four new gyros were installed during the First Servicing Mission in 1993. All six gyros were working during the Second Servicing Mission in 1997. Since then, a gyro failed in 1997, the second failed in 1998 and the third failed in 1999.

Why aren't the gyros working?

The Hubble team believes they understand the cause of the failures, although they cannot be certain until the gyros are returned from space and taken apart. Based on nearly one and a half years of intensive chemical, mechanical and electrical investigations, the team believes that the thin wires are being corroded by the fluid in which they are immersed and ultimately this corrosion causes them to break. The fluid is very thick (about the thickness of 10W-30 motor oil), and in order to force this fluid into its float cavity, pressured air was used. The team believes that eventually, oxygen in the air interacted with the fluid to create a small amount of corrosive material and the wires were partially eaten away. Sometimes the wires were strong enough to carry electricity and some-times they were not and they broke. Pressurized nitrogen is now used instead of pressurized air. Using pressurized nitrogen eliminates the introduction of oxygen into this fluid.


Size 12.8 x 10.5 x 8.9 inches


Size 2.75 x 6.5 inches
Weight 6 pounds


The Hubble Space Telescope Third Servicing Mission, originally scheduled for June 2000, has been divided into two missions. The first part, called SM3A, will be this fall, and the second part, SM3B, is tentatively scheduled for late 2000. This is in response to recent problems with several of Hubble's gyro-scopes, which are required to accurately point the telescope at its scientific targets around the sky. During SM3A, astronauts will replace the gyroscopes, upgrade other telescope subsystems and perform sched-uled

preventative maintenance. In late 2000 a crew of astronauts will complete the remaining upgrades planned for Servicing Mission 3, including the insertion of a new, technologically advanced camera.

Hubble's Mission

The Hubble Space Telescope's mission is to spend 20 years probing the farthest and faintest reaches of the cosmos. This unique observatory operates around the clock, above the Earth's atmosphere, to gather information for teams of scientists who study virtually all the components of our universe, including planets, star-forming regions of the Milky Way galaxy, distant galaxies and quasars.

Crucial to fulfilling this mission is a series of scheduled, on-orbit manned servicing missions. During these servicing missions, astronauts per-form a number of planned repairs and mainte-nance activities to upgrade the observatory's capa-bilities.

The First Servicing Mission took place in December 1993, the Second Servicing Mission in February 1997. Servicing Mission 3A is scheduled for the fall of 1999 and 3B is scheduled for December 2000. The Fourth and final Servicing Mission is currently planned for 2003.

Mission Overview

The nine-day mission, STS-103 is scheduled for the fall of 1999. Members of the STS-103 flight crew are: Commander, Curtis L. Brown;

Pilot, Scott J. Kelly; Payload Commander, Steven L. Smith; and Mission Specialists C. Michael Foale, John M. Grunsfeld, Claude Nicollier (European Space Agency), and Jean-Francois Clervoy, Mission Specialist (European Space Agency).

Working in pairs on alternating days, the four Extravehicular Activity (EVA) Crew members will replace all six of the Telescope's gyroscopes, a guidance sensor and Hubble's main computer. They also will fit Hubble with a new transmitter and solid state data recorder, and they will attach voltage/temperature improvement kits to Hubble's six batteries. The EVA crew also will add new thermal coverings to Hubble's exterior. Astronauts will complete these tasks in four scheduled EVA days. One additional EVA day can be added if necessary. This service call will leave Hubble renewed and refurbished to continue its 20-year science mission.

What's Being Replaced

Rate Sensor Units:

The Rate Sensor Units allow the Telescope to point at stars, planets and other celestial targets. Three are aboard Hubble, and each unit contains two gyroscopes. Hubble needs three of these six gyroscopes to meet its very precise pointing requirements, and the other three are spares. Gyroscopes have limited lifetimes, and currently only three of the six are working proper-ly the minimum number needed to continue sci-ence operations. Astronauts will replace all three units, leaving Hubble with six fresh gyroscopes.

Fine Guidance Sensor:

This is the second in a "round-robin" series of changeouts and refur-bishments of the three fine guidance sensors, which allow fine pointing and keep Hubble stable. The SM3A refurbished Fine Guidance Sensor is the same unit that was returned from Servicing Mission 2. The Fine Guidance Sensor returned from this mission will be refurbished and upgrad-ed for re-use on Hubble's fourth Servicing Mission.

New Spacecraft Computer:

The radiation-rugged computer will replace Hubble's original, outdated main computer. The new computer will dramatically increase operational capabilities, reduce the burden of flight software maintenance, and significantly lower operational costs.

Voltage/Temperature Improvement Kits:

As Hubble's batteries age, they become more sus-ceptible to overheating if overcharged. The Voltage/Temperature Improvement Kit compensates for this by lowering the battery's charge termination voltage. Astronauts will install one kit for each of Hubble's six batteries.

Spare S-Band Single Access Transmitter:

The transmitter replaces an aged and failed unit. That unit will be removed, returned to Earth and refurbished for a later flight.

Spare Solid State Recorder:

The digital data recorder will serve as a high capacity backup to the Solid State Recorder that replaced a mechani-cal tape recorder in 1997. It is essential for effi-ciently handling the high volumes of data from Hubble's newest instruments and for maintaining high science productivity.

New Outer Blanket Layer:

Stainless steel sheets will be installed in various locations on the Telescope to help control Hubble's internal tem-perature. Covered with a protective thermal coat-ing, these sheets will fit over existing insulation that has degraded.

The following tasks will be performed on SM3A, time permitting. If there is insufficient time, these tasks will be completed on SM3B:

Shell/Shield Replacement Fabric:

Flexible aluminized Teflon sheets will be added to the exte-rior surfaces of Hubble's forward shell and light shield. This protective covering provides addition-al insulation against the harsh space environment.

Aft Shroud Latch Repair Kit:

Astronauts will replace latches on Hubble's bay door. During Servicing Mission - 2, Astronauts observed galling on these latches. The galling was caused by high torque.

Handrail Covers:

Fiberglass cloth, called beta cloth, will be fitted like sleeves around the handrails above the Fine Guidance Sensors bay to prevent contamination to the Aft Shroud area. Flaking paint was observed on these handrails dur-ing Servicing Mission - 2.


The purpose of the Hubble Space Telescope is to gather light from cosmic objects so scientist can better understand the universe around us.

Hubble is in space, astronomers are on Earth.

How does the information get to them? The simple answer is a transmitter, called the S-Band Single Access Transmitter (SSAT) sends the data from Hubble to the ground by radio. But it is more complicated than that.

The light that is gathered and magnified by the Telescope is first sent to one of Hubble's scientific instruments. Devices similar to those in digital cameras convert the light to computer bits of information for processing. The computerized science data are then either transmitted immediately from the spacecraft or stored in one of Hubble's on-board recorders for future transmission.

Where do these transmissions go?

The transmissions are directed by one of Hubble's two big communications dishes. Strangely enough Hubble's transmissions don't go down to the ground, only about 360 miles below. They go up! Approximately 22,000 miles above the Earth, nestled among commercial communications satellites, are a few NASA communications satellites. NASA uses these satellites to collect transmissions from its many scientific satellites (including Hubble) and beam them down to a ground station in New Mexico. From there Hubble's scientific data ultimately ends up at Hubble's Space Telescope Science Institute in Baltimore, Md. After processing, the data are

available to astronomers.

There are two identical S-Band Single Access Transmitters on-board Hubble. "S-Band" identifies the radio frequency and "Single Access" specifies a type of antenna on NASA's communications satellites. One of the two SSATs failed in 1998. The other SSAT has been able to shoulder the load and Hubble's observing program has not

been affected. Hubble can operate with one transmitter by making additional commands to rotate the telescope. Optimally, the telescope operates with two SSATs.

What is planned for SM3A?

During Servicing Mission 3A astronauts will replace the faulty transmitter with a spare. Some of the cables and connectors are smaller versions of the ones on the back of television sets. Because the connectors are difficult to handle with bulky space suit gloves, special enhancements have been made to the new unit to aid the Astronauts. In addition, a special connector tool helps to remove and install the connectors. The failed transmitter will be returned to Earth and refurbished for a later



Size 14 x 8 x 2 3/4 inches
Weight 8.5 pounds



Inside the Hubble Command Center

Working 24 hours a day at NASA's Goddard Space Flight Center in Greenbelt, Md., ground controllers command and control the Hubble Space Telescope. From the Space Telescope Operations Control Center or STOCC, as it's bet-ter known, commands are sent to the Telescope to direct the observation of astronomical targets, one after another, all across the sky. The Telescope sends information to the STOCC that engineers use to make sure everything is going as planned. The STOCC is the focal point of all Hubble Space Telescope operations.

Here Hubble's operators monitor the Telescope's health and safety while they control flight operations and engineering and science activities. One section of the STOCC supports the preparation, test and simulation for the next ser-vicing mission, while routine operations continue simultaneously in an adjacent area. In another area, engineers perform in-depth subsystem analy-sis, conduct simulated subsystem tests, integrate new databases, and validate new ground software and updates to flight software.

Fulfilling a Vision

In February 1999, the mission operations team began using the new Control Center System (CCS) to command and control Hubble. CCS is the culmination of a massive reengineering of both the flight and ground system begun in February 1995 to improve Hubble's overall per-formance and drastically reduce the cost of opera-tions and systems maintenance. This effort, called Vision 2000, goes well beyond replacing outdated software and hardware. It streamlines the func-tional flow, eliminates redundant systems and pro-vides the operators and spacecraft engineers with a user-friendly interface. Architectural complexity is greatly reduced by consolidating the functions of the five original systems of spacecraft control, analysis, data management, command manage-ment and subsystem calibration into a single sys-tem. CCS is very portable. It may be operated in a single computer mode at a user facility or in a high performance multi-computer mode, the configura-tion used in the STOCC. Currently 25 CCS strings support a broad range of activities including: operations; servicing mission simulations; pay-load and spacecraft flight software development; operator training; procedure development and ver-ification; software maintenance and test; new spacecraft hardware test and integration at Goddard Space Flight Center and science instru-ment development at Ball Aerospace in Colorado. Users can customize their string's databases to suit their particular application. Some of the advanced performance features of CCS are identified below. CCS automates engineering telemetry man-agement, including the merging of real-time and stored spacecraft data, thereby reducing the delay in obtaining this data for analysis and trouble shooting from 6-48 hrs to 0-8 hrs. In the past, this process has involved as many as sixteen people; that number is now reduced to zero.

At all times, controllers have instant access to six months of telemetry data, stored either on-line or in data warehouse. CCS provides approximate-ly 300 users concurrent access to Hubble Space Telescope data via an advanced, interactive graph-ical interface to perform monitoring and analysis. The operators can obtain visual displays of any requested telemetry, produce graphs and perform analysis on the data in near real-time at the con-soles.

The Team Behind the Telescope

Just as the astronauts have trained extensively for the upcoming 1999 servicing mission, so too has Hubble's Flight Operations Team in the STOCC. Shortly after launch, this team will pre-pare Hubble for the on-orbit service call. They will begin by transitioning the Telescope from normal science operations to a "ready for servicing" condition. The team will command Hubble to its capture attitude and configure it for rendezvous with the Space Shuttle Discovery. They will command Hubble's aperture door to close and the high gain antennas to be stowed in preparation for capture by the Shuttle's robotic arm and berthing in the payload bay. The solar arrays will remain deployed and following berthing, Hubble's power will be provided by the Shuttle. To aid astronauts in their servicing tasks, the Telescope will then be rotated to several different positions on its berthing platform to allow easy access to the equipment bays.

After the new equipment is installed, STOCC ground controllers will command tests on the newly installed items. These tests will be done immediately after installation, with the crew positioned at a safe location, to determine if the installed equipment will require any further astronaut activity. Later, while the crew sleeps, the STOCC team will perform more detailed function-al checkouts of the installed equipment to deter-mine if further work is necessary.

After all servicing is completed, Hubble and Discovery will be configured for battery charging. The Shuttle crew will transfer the Telescope to internal power, disconnect the power feed, and using the robotic arm, position Hubble for deployment. The STOCC will command the deployment of the high gain antennas and opening of the aperture door. All equipment powered off for servicing will be reactivated and checked out. Hubble will then be released. Operational re-commissioning of the telescope will take place, and normal science operations will resume.



To communicate with its operators on the ground, the Hubble Space Telescope uses a group of NASA satellites called the Tracking and Data Relay Satellite System (TDRSS). By way of TDRSS, Hubble sends the data from its science instruments and spacecraft systems to the Space Telescope Operations Control Center at NASA's Goddard Space Flight Center in Greenbelt, Md. When the TDRSS link is not available, Hubble stores its science and engineering data in onboard recorders for playback at a later time. Hubble records all of its science data to prevent any possible loss of unique information.

Prior to the Second Servicing Mission, Hubble used three 1970s-style, reel-to-reel tape recorders. In February 1997, Astronauts replaced one of these mechanical recorders with a digital Solid State Recorder. During Servicing Mission 3A Astronauts will remove a second mechanical tape recorder and install a second Solid State Recorder.

More Storage, No Moving Parts

Unlike the reel-to-reel recorder it replaces, the Solid State Recorder has no reels, no tape, and no moving parts to wear out and limit lifetime. Data is digitally stored in computer-like memory chips until Hubble's operators command its playback. Although the Solid State Recorder is about the same size and shape as the reel-to-reel recorder, it can hold approximately ten times as much data. It stores 12 gigabits of data, while the tape recorder it replaces can hold only 1.2 gigabits. This ten times greater storage has proven essential in allowing Hubble's new, high-tech scientific instru-ments to be fully productive.

Flexibility and Multi-tasking

State-of-the-art electronics provide the Solid State Recorder with more capability and flexibility than reel to-reel recorder. This digital recorder is designed to perform the tasks of two separate mechanical recorders. Unlike a mechanical recorder, the Solid State Recorder can record and play back data simultaneously.

Another advantage is its ability to record two data streams at the same time, allowing both the science and engineering data streams to be captured on a single recorder. Unlike the reel-to-reel recorders, data can be played back without having to rewind the tape, and information can be instantly accessed.

Resilient and Long-lasting

Reel-to-reel tape recorders can fall victim to single-point failures, such as a break in the tape or a mechanical defect. The Solid State Recorder does not have mechanically moving parts, and its memory modules have a very low failure rate. This digital recorder is designed to grow old gracefully, compensating for situations such as a bad chip or a bad module.

The recorder automatically detects, corrects and reports random errors in memory. If the failure is too difficult to correct, the affected area can be isolated and skipped over, leaving the rest of memory fully functional. The old, mechanical recorders also had to be sealed in a pressurized enclosure to protect the tape and the delicately lubricated mov-ing parts from the hazards of a space vacuum. This special packaging is not necessary for the Solid State Recorder.

NASA successfully tested this Solid State Recorder during a 10-day Space Shuttle Mission (STS-95) in October 1998. Based on these results and the on-orbit performance of the unit already aboard Hubble, NASA expects this new recorder to last the life of the Telescope. The Solid State Recorder was developed at Goddard.


Size 12 x 9 x 7 inches
Weight 25 pounds



During the Hubble Space Telescope Second Servicing Mission in 1997, astronauts detected damage to some of the Telescope's thermal insulation. Years of exposure to the harsh environment of space had taken a toll on Hubble's protective multi-layer insulation, and some areas were torn or broken. This multi-layer insulation protects the Telescope from the severe and rapid temperature changes it experiences as it moves through its 90- minute orbit from very hot sun to very cold night.

Although the cracks looked dramatic, the dam-age was limited to the outermost layer and did not affect the insulation's protective function or Hubble's operation. With help from ground controllers, the astronauts used materials aboard th Space Shuttle Discovery in 1997 to fashion temporary patches. They installed them over the most critically damaged areas, mostly on Hubble's sun-facing side.

Permanent Repair in 1999

During the 1999 servicing mission, astronauts will cover Hubble with permanent sheets called the New Outer Blanket Layer, or NOBL. The crew also will carry a special fabric, called the Shell/Shield Replacement Fabric, or SSRF. The SSRF is scheduled for installation on Servicing Missions 3A and 3B. During SM3A astronauts will install the SSRF on Hubble's forward shell and light shield if time is available. The NOBL covers and SSRF pieces are designed to protect Hubble's external blankets. They will prevent Hubble's insulation from fur ther degradation and maintain normal operating temperatures. NASA tested the materials to ensure that they can withstand exposure to charged particles, X-rays, ultraviolet radiation, and thermal cycling for at least ten years. Astronauts will install seven NOBL covers on Hubble's electronics bay doors. These covers are specially coated stainless steel foil trimmed to fit each particular door. Each cover is supported by a steel picture-frame structure. Expanding plugs, like common kitchen bottle stoppers, fit into door vent holes to allow quick installation.

The SSRF pieces are designed to cover the Telescope's forward shell and light shield. The fabric is composed of flexible, aluminized Teflon© with rip-stop material bonded to the backside. Astronauts will use wire clips to attach each SSRF piece to convenient attachment points such as handrails, brackets and struts. Seven pieces up to 22 feet (7 meters) long will cover 80 percent of the sun-side light shield and forward shell. The fabric pieces are stored in rolls for their trip to orbit.



Hubble Space Telescope circles the Earth at approximately 17,000 miles per hour, every 90 minutes. On the ground we see a day and a night every 24 hours. In space Hubble sees daylight and night every hour and a half. Hubble uses solar energy, collected by the solar arrays, to power its computers and science instruments. At night when there is no sun, Hubble uses its batteries for power. These batteries are then recharged during Hubble's next day.

Charging Hubble's Batteries

An automobile has a voltage regulator to con-trol the rate of charge into its car battery.Similarly, Hubble has charging regulators for its batteries. Hubble's regulators use battery voltage and battery temperature to control the rate of charge into the batteries. The batteries aboard Hubble are almost 10 years old. They still do the job well but, as they age, they become more sensitive to the way they are charged and become susceptible to overheat-ing. To compensate for the effects of aging, astronauts will install a battery Voltage/Temperature Improvement Kit (VIK) on each of Hubble's six batteries. Hubble's batteries are fully charged each orbit by the Solar Arrays. Each battery is fully charged when its respective charge controller senses a specific charge cutoff voltage. The VIK modifies the charge cutoff voltage to a lower level to prevent battery overcharging and associated overheating. In the Telescope, the batteries are located in two compartments, called bays, three batteries to a bay.

The VIK is a simple device, about the size of a cell phone, weighing about three pounds.


NASA's Hubble Space Telescope is the first obser-vatory designed for routine maintenance, upgrade, and refurbishment on orbit. The program is planned as a 20- year mission with periodic servicing by Shuttle astronauts. Hubble's modular design allows for more than 90 spacecraft components and all of the scientific instruments to be replaced on orbit. Servicing main-tains the spacecraft, ensures operation at maximum scientific efficiency and allows for incorporation of new technology.

Hubble was launched on April 24, 1990 with a full complement of six scientific instruments. At that time, an inventory of spare HST hardware was available to support future servicing missions. Since launch, HST budgets have been sized to develop new instruments, to maintain the spare hardware, to sustain hardware exper-tise, to plan and develop servicing activities, and to test and integrate the payloads with the Shuttle.

Due to gyroscope failures and the potential for interruption of the science program should another gyroscope fail, a servicing mission was needed as soon as replacement gyroscope hardware could be ready. Servicing Mission 3 (scheduled for June 2000) was divided into two flights—Servicing Mission 3A (SM3A), scheduled for Fall 1999, and Servicing Mission 3B (SM3B), scheduled no earlier than 2001. Much of the hardware planned for Servicing Mission 3 will not be ready in time for the first flight, and will therefore be installed on SM3B.

The cost to carry out the 3A mission is $136 million. This includes $19 million of HST costs for the additional servicing mission, $7 million of HST costs to switch from Columbia to Discovery, and $110 million for the Shuttle to carry out the mission and replace Space Shuttle flight hardware previously assigned to another mission. NASA has also spent approximately $69 million on Servicing Mission 3A, in addition to the $136 million, reflecting the costs of building and testing the planned replacement hardware, ground operations and other related activities.

All 6 gyroscopes will be replaced during the Fall 1999 SM3A mission. In addition, the crew will replace a guidance sensor (a unit brought back to the ground after the last mission and refurbished) and the space-craft's computer. The new computer will reduce the burden of flight software maintenance and significantly lower operating costs. Voltage/temperature kits will be installed to protect spacecraft batteries from over-charging and overheating in certain, infrequent spacecraft modes of operation. A new radio frequency transmitter will replace a failed spare one currently aboard the spacecraft, and a spare solid state recorder will be installed to allow more efficient handling of high-volume data.


HST Programs & STS-103 Costs

Servicing Mission Costs - HST

Planned Servicing Mission Hardware & Software
Gyroscopes 8
Fine Guidance Sensor 13
Advanced Computer 7
Other Flight Hardware 11
Simulators/Testing 6
Ops/Software Development 24

Sub-total 69

Costs for Flight Changes
HST Cost for Additional Servicing Mission 19
HST Cost for Switching from Columbia to Discovery 7

Sub-total 26

HST Total 95

Million Servicing Mission Costs - Shuttle
Shuttle Flight Costs 110 Million

Total STS-103 Mission Costs
Shuttle 110
HST 95
Total 205 Million


For additional information contact:

Nancy Neal
Goddard Space Flight Center
Office of Public Affairs
(301) 286-0039

Don Savage
NASA Headquarters
Office of Public Affairs
(202) 358-1600


The Hubble Space Telescope’s purpose is to spend 20 years probing the farthest and faintest reaches of the cosmos. Crucial to fulfilling this objective is a series of on-orbit servicing missions. Hubble was placed in orbit on April 25, 1990, by the shuttle Discovery and subse-quent servicing followed in December 1993 and February 1997. The third in the series of planned servicing missions for the Hubble Space Telescope was scheduled for June 2000. This third Servicing Mission has been separated into two flights. The first of these flights, Servicing Mission 3A, is scheduled for December 1999, and the second, Servicing Mission 3B, is scheduled for 2001. The fourth Servicing Mission is scheduled for 2003 with a "close-out" Mission in 2010.

Three instruments are currently in active scientific use on Hubble – the Wide Field and Planetary Camera 2, the Space Telescope Imaging Spectrograph, and Fine Guidance Sensor 1R, which has been designated as the prime FGS for astrometric science. Other instrument bays are occupied by the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which is now dormant due to the depletion of its solid nitrogen cryogen, the Faint Object Camera, which has been decommissioned, and the corrective optical device called COSTAR, which is no longer needed.

Servicing Mission 3A

Three gyroscopes are the minimum required for normal science operations. After three of Hubble’s six gyroscopes failed, NASA managers were concerned that another gyroscope might soon fail, leaving Hubble unable to perform its science mission. An early servicing mission, Servicing Mission 3A, was sched-uled for December 1999 to avoid an extended down period. Although no new Science Instruments will be installed, there are many activities planned for this mission which are important to Hubble’s scientific performance.

Working in pairs on four alternating days, four EVA crewmembers will replace all six gyroscopes, a guidance sensor and Hubble’s main computer. Astronauts will install a new transmitter, a solid state data recorder and attach voltage/temperature improve-ment kits to the six batteries. The task of applying new thermal coverings to the exterior will be started. This service call will leave Hubble repaired and improved.

Servicing Mission 3B

This servicing mission will focus on installing the Advanced Camera for Surveys and more efficient rigid solar arrays. Astronauts also will install the aft shroud cooling system. In addition, an advanced cooling system will be installed on NICMOS, which became dormant after its solid nitrogen coolant was exhausted in January 1999. The application of new external thermal coverings will be completed, if necessary.

Advanced Camera for Surveys

During this mission, astronauts will install a new science instrument, the Advanced Camera for Surveys (ACS). The Advanced Camera for Surveys will physi-cally replace the Faint Object Camera. This new instru-ment is designed for survey mode imagery and discov-ery. It is estimated that the survey capability of the Telescope will be increased tenfold. A major objective for the ACS is mapping the distribution of dark matter throughout the universe. Several other maintenance activities are planned over four EVA days. A few of the activities are discussed below.

Solar Array III

Two large flexible solar array (SA) wings provide power to Hubble. During Servicing Mission 1, the original European Space Agency arrays (SA1) were replaced with a new upgraded set of solar arrays, called SAII. These arrays consist of silicon cells installed on a thin layer of Kapton blanket. When the SAII wings are replaced they will have powered the Telescope for nearly 7 years.

The newest arrays (SAIII) are rigid arrays, which do not roll up and therefore are more robust. They are also smaller and more efficient and will slightly reduce the effects of atmospheric drag on the spacecraft. SAIII has several enhancements and incorporates new tech-nology: The cells are made from gallium arsenide which are more efficient than the original silicon cells. The frames are made of lightweight Lithium Aluminum alloy tubes, in an "H"-shaped configuration. Each wing can be folded for transport, and then easily locked into place when fully deployed.

NICMOS Cooling System

The NICMOS Cooling System, an experimental mechanical cooling system, will be connected to NICMOS to return it to normal operation.

Aft Shroud Cooling System

This new system is designed to carry heat away from scientific instruments in the Aft Shroud area of the Telescope assembly and to allow the instruments to operate better at lower temperatures. The cooling sys-tem allows multiple instruments to operate simultaneously, helping the science team maintain the program's high productivity

Possible Reboost

Although the atmosphere is quite thin at satellite altitudes, it is not a perfect vacuum. Over time, all low Earth orbiting satellites feel the effects of atmospheric drag and lose altitude. If the altitude is not restored, the Telescope eventually will re-enter Earth’s atmosphere. Hubble has no on-board propulsion, so the only way to restore lost altitude is by the creative use of shuttle jets. If nec-essary, Hubble will be reboosted to a higher alti-tude. This was done on both Servicing Missions 1 and 2.

Servicing Mission 4

Plans for the Fourth Servicing Mission are very preliminary at this time, but two Science Instruments are in development. COSTAR will be removed during this servicing mission to make room for the Cosmic Origins Spectrograph. Wide Field Camera 3 will replace the Wide Field Planetary Camera 2. Also a re-furbished Fine Guidance Sensor will be installed leav-ing Hubble in optimum condition.

Cosmic Origins Spectrograph

The Cosmic Origins Spectrograph (COS) is a medium resolution spectrograph specifically designed to observe into the near and mid ultraviolet. COS cou- pled to Hubble’s optics will be the most sensitive spec-trograph ever flown in space. The ultraviolet region is particularly interesting for observing hot objects such as new hot stars and quasars. It is also a good region for viewing the composition and character of interstellar and intergalactic gas. COS will measure the chemical composition of the gas between the galaxies at great distances, as it was when the universe was very young.

Wide Field Camera Three

Wide Field Camera Three (WFC3) will be the last imaging camera mounted on HST. WFC3 will replace the current workhorse of Hubble, Wide Field and Planetary Camera 2. WFC3 will be a "panchromatic" camera, extending Hubble’s imaging capability over an enormous range of wavelengths from the ultraviolet to the near-infrared. It will provide important backup to the ACS in visible light and will supersede the near-infrared capability of the aging NICMOS. This up-grade will allow Hubble to maintain good imaging capabilities throughout the remainder of its mission.

Fine Guidance Sensor

The Fine Guidance Sensors are systematically refurbished and upgraded. In "round-robin" fashion, one FGS per servicing mission is being replaced. The returned FGS is disassembled and refurbished, and then taken back to Hubble on the next servicing mission to replace the next FGS. By the conclusion of SM4 all three FGS’s will have been brought up to optimum condition in this manner.

Closeout Mission

NASA will determine the best approach to secure the Telescope, upon the completion of Hubble’s 20- year mission. Currently there are several options being considered, ranging from staying in orbit indefinitely through a large reboost, to a return to ground.

For additional information contact:

Nancy Neal
Goddard Space Flight Center
Office of Public Affairs
(301) 286-0039

Don Savage
NASA Headquarters
Office of Public Affairs
(202) 358-1600

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