My article will explain how NASA engineers, anticipating the need to maintain the Hubble Space Telescope, designed it with the ability to be repaired in space. Through five shuttle missions from 1993 to 2009, astronauts replaced all major components except the mirrors and outer shell, which increased electrical power by 20% and restored the telescope’s ability to focus and perceive light without additional devices. This effort extended Hubble’s lifetime from a planned 15 years to nearly 35, demonstrating an outstanding feat of engineering and space technology.
Design features for on-orbit servicing
The Hubble Space Telescope was built to be easy for astronauts to service while orbiting the planet. On the surface of the telescope are bright yellow handles and foot restraints for the purpose of moving around it during upgrades. Inside are instruments and other components that can be replaced directly in space. In the event of engine or actuator failure, the solar panels that provide power to the telescope can be folded and unfolded. Hubble has 49 modular systems, from small fuses to large scientific instruments that can be accessed.
The observatory took more time and finances to build and launch than other NASA missions. Such spending was justified by the fact that the telescope would be in operation for years, and for this to happen, it had to be accessed regularly for repairs and upgrades.
Even in the 1970s before the launch, it was planned that the telescope would be serviced about every two and a half years for 15 years, and the timeliness of inspection and repair would prevent parts from failing abruptly. It was even envisioned that the telescope could be transported to Earth for repairs and then brought back. In this case, the cost of design and testing before each flight could be reduced.
The need for frequent maintenance of the space observatory became apparent when the implementation of the development program began to be delayed, and the budget significantly exceeded the original estimates. Interestingly, attempts to cheapen the design unexpectedly led to a positive result: engineers eliminated several orbital replacement units, making subsequent repairs more convenient.
After the telescope was deployed in 1990, a serious manufacturing defect was revealed – the main mirror element was improperly manufactured. Due to a polishing error, the mirror had a deviation of only 2.2 microns, but this was enough to noticeably deteriorate the quality of images. In addition to this, the gyroscopes, which provide accurate orientation of the telescope, and other key components began to fail.
As a result, planned routine maintenance missions turned into large-scale repair operations, without which Hubble’s work would have been impossible.
The observatory’s major components
At 13.2 meters long, Hubble is comparable to the size of a school bus. It consists of the following major elements:
Optics. The telescope’s main mirror has a diameter of 2.4 meters, is coated with a layer of aluminum and is capable of collecting 40,000 times more light than the human eye. Light hitting the main mirror is reflected onto a secondary mirror with a diameter of 30.5 centimeters, and then returned through the central opening of the main mirror to scientific instruments for analysis.
Initially, five scientific instruments and one of the three guidance sensors were mounted behind the main mirror. The latter was also used for data collection. , some instruments were replaced or upgraded during maintenance missions to adapt the telescope to modern scientific tasks.
Batteries. Two solar panels power all of the observatory’s electrical equipment. They generate about 5,000 watts, converting sunlight into electricity, which is sent to six batteries. All six batteries store about as much energy as 20 car batteries. This design allows the device to operate even when in the Earth’s shadow.
Antennas. Two high-gain antennas are used to send data. There are also low-gain antennas that receive incoming commands and are activated during accidents to transmit information. The observatory uses satellites to communicate with the Earth, but in an emergency it can contact the ground station on its own.
Pointing control system. It consists of reaction wheels, gyroscopes, sensors, and two actuators designed to physically rotate the telescope.
Jet wheels. Four 45 kg thrust wheels are located in the center of the craft and are used to reorient the telescope. Three of them are sufficient for operation, with the fourth serving as a spare. When a wheel is rotated in one direction, the entire observatory is rotated in the opposite direction, ensuring accurate pointing. Commands to operate the wheels are transmitted to the telescope from the control center.
Gyroscopes. Make 19,200 revolutions per minute. They are used to determine the speed and direction of motion in three-dimensional space. They also record the direction and speed of rotation. Six gyroscopes were initially installed, three of which are in reserve. As of June 2024, two working devices remained, the others failed. The telescope now operates with one gyroscope, with the second still in reserve.
Two precision pointing sensors to focus on the target. Deviation is possible no more than seven angular milliseconds per day. A total of three sensors are installed: two of them fix the target and the third is used to collect scientific data.
Solar sensors. Determine the telescope’s position relative to the Sun to protect sensitive instrumentation and to aid in targeting.
Magnetic sensors. Determine the position of the observatory relative to the Earth’s magnetic field.
Scientific instruments. Initially installed cameras, spectrograph, high-speed photometer, and precision pointing sensors.
The two main camera systems provide wide-angle imaging of a range of wavelengths.
A spectrograph is an instrument for studying the temperature, density, and chemical composition of any object that absorbs and emits light.
Interferometers, or precision pointing sensors allow you to focus on a target and measure the relative position and brightness of stars. One of the three sensors is used to collect scientific data.
Only one instrument remains on Hubble, which was launched with the telescope in 1990. It is the third precision pointing sensor. The other instruments were replaced during maintenance missions, increasing the observatory’s capabilities. The ACS camera and STIS spectrograph were also refurbished during the last mission in 2009.
Engineers anticipate that the observatory’s instruments and systems can operate for another five or more years into the 2030s.
Instruments to work with
For almost 35 years, astronauts have kept the observatory running smoothly and equipped it with ever more powerful equipment. To maintain the telescope, NASA engineers have developed more than three hundred special instruments designed to work in zero gravity.
Only for the last, the fourth mission was created 116 such tools. Among them, for example, a special cordless wrench for work in space, a telescopic handle for manipulation at a distance, as well as an adapted illuminator to help astronauts work in the shadow of Hubble.
Back in 1976, astronaut engineer Story Musgrave was designing and developing equipment for service in space. He was one of the main scientists involved in preparing instruments for the observatory.
It was Story Musgrave who, in May 1993, just seven months before the first mission, found out that the astronauts would have to work in extremely low temperatures that made the task nearly impossible. During a thermovacuum test, he experienced severe pain and suffered frostbite on eight fingers without even noticing it. This incident demonstrated the need to revise equipment and procedures. Initially, the orbiter’s position was calculated solely for protection from sunlight. However, after the incident, it was changed to also maintain an optimal temperature for the astronauts. In addition, warmer gloves were developed.
Hubble’s maintenance tools were modified to accommodate bulky spacesuits and stiff gloves. They were adapted to the conditions of limited space, weightlessness, and minimize the risk of loss of materials and equipment.
The Pistol Grip Tool (PGT), is one of the best known designs. It was first used on the second maintenance mission. It is similar to a mechanical screwdriver. You can set speed, RPM, direction, and other characteristics on it.
It also makes a record of information about the work being performed. If the grip needs to be loosened in the future, the astronaut can adjust his actions by comparing it to the existing record. Today, this tool is used for work in space, including on the ISS.
During the last fourth service mission, 111 small screws had to be loosened to repair the STIS spectrograph. To do this, they created a similar Mini Power Tool (MPT) device, but smaller, faster and with LED lights.
They came up with a special plate to grab the fasteners for the 111 screws, which allowed them to stay in place rather than flying off into space or getting into other parts of Hubble and damaging them. The plate is also visible in the photo. Holes in it held the screws in place and at the same time gave access to each screw with a power tool. Subsequently, the MPT was also used on the ISS.
To make working in space more comfortable, engineers designed a leg lock, which was first used during the first mission in 1993. This tool helps astronauts to fix themselves in weightlessness, providing stability and precision when performing complex tasks.
It provides a stable platform to secure the astronaut’s boots while allowing the person to push off and pivot while working. Nests for the retainer are located around the telescope and in the cargo bay of the space shuttle. The boot mount requires the astronaut to first place the toe in one of the stirrups and then lock the heel in place. The tilt of the platform can then be adjusted.
A grease applicator was used to loosen the bolts of the door latches to make them easier to open. The trigger had to be pulled to apply the grease.
Astronaut Training
During the five missions to Hubble, astronauts spent 57 days 15 hours 48 minutes and 8 seconds in space.
Training was conducted at three centers: the Kennedy Space Center in Florida, the Johnson Space Center in Texas, and the Goddard Space Flight Center in Maryland.
Training included:
- simulation of shuttle and observatory operations at Johnson and the Space Telescope Operations Control Center at Goddard;
- testing and preparing instruments and equipment for the flight at Goddard;
- preparing operations for the launch, flight, and landing of the space shuttle at Kennedy.
Accurate replicas of Hubble and its components allowed astronauts to practice all movements and anticipate nuances. Missions were based on eliminating possible surprises.
Methods and instruments were perfected through testing in the neutral buoyancy simulator at the George C. Marshall Space Flight Center. Marshall Space Center and the weightlessness training center at Johnson Space Center.
The buoyancy simulator created microgravity conditions and helped estimate approximate time costs for specific tasks.
In training, the astronauts practiced every possible movement and predicament that could be encountered in orbit. They knew every movement of their hands, body turns, manipulations with instruments. For example, astronaut John Grunsfeld practiced for a year on a copy of the power control unit to carry out its replacement during mission 3B. And astronauts Bruce McCandless and Kathy Sullivan practiced various emergency situations for years.
Just before the flight for several weeks astronauts exercised every day for 6-7 hours in the simulator, then discussed and analyzed the work, and after that went to the gym for an hour and a half or two hours.
The missions were successful because they were the result of long and hard training, constant analysis and improvement of actions and equipment.
Maintenance mission 1: fixing a defect in the optical system
- Period: December 2-13, 1993
- Number of spacewalks: 5 spacewalks totaling more than 35 hours.
- Shuttle: STS-61
- Space Shuttle: Endeavor
From the early days of Hubble’s launch, it became clear that the images it was taking were of lower quality than the calculations suggested. In addition, several systems soon began to fail: failures occurred in the gyroscopes used to point the telescope, as well as in the solar panels, which began to vibrate due to thermal expansion.
The reason for the poor image quality is spherical aberration caused by an error in the primary mirror, which occurred at the stage of its manufacture. The main instrument for parameter control was assembled with miscalculation. In this regard, the mirror itself was produced with a deviation of two microns. The error required correction. Without it Hubble could work further, collect other data on the instruments, but in this case, the financial costs of its construction and launch would not be justified.
The Corrective Optics Space Telescope Axial Replacement (COSTAR) is a system of mirrors to correct the focus of light reflected from the main mirror. It includes 5,300 parts along with the mirrors themselves, namely mechanical components and electronics controlled from Earth. The optics compensated for the mirror’s deficiency, much like glasses correct vision problems. But installing the new system required removing a high-speed photometer that could observe objects with changing brightness and take 10,000 measurements per second.
The COSTAR mirrors facilitated rays into all scientific instruments except the wide-angle camera. It was replaced by another camera, the WFPC2, with built-in corrective optics. The new instrument improved the telescope’s sensitivity in the ultraviolet.
All future equipment over the course of five missions was upgraded to account for spherical aberration, and the COSTAR system was eventually removed in 2009.
The solar panels were also replaced during Service Mission 1 (SM1). The original solar panels flexed during heating and cooling, which caused a slight wobble. But such wobble was reflected in the guidance sensors. They would occasionally lose their target while making observations. The new batteries were supposed to solve this problem.
Three of the six gyroscopes had failed by the time of the mission. In those days, Hubble needed three gyroscopes for pointing and tracking an object; the others were spares. With the loss of one more gyroscope, Hubble would not have been able to collect scientific data. Located in pairs, the astronauts replaced two pairs of these devices.
As a result, the following components were upgraded and installed:
- COSTAR;
- WFPC2 wide angle camera;
- SA2 solar panels and one solar panel actuator;
- two electronic gyroscope control units;
- magnetometers;
- Goddard High Resolution Spectrograph (GHRS);
- main and flight computer memory units (these are different units).
Maintenance Mission 2: Increasing the efficiency of the observatory
- Number of spacewalks: 4 planned and 1 unplanned spacewalks
- Shuttle: STS-82
- Space Shuttle: Discovery
Service Mission 2 (SM2) increased Hubble’s efficiency and productivity, and extended the range of visible wavelengths to the near-infrared by installing two new instruments. The telescope was now able to explore distant parts of the Universe.
In total, there were four planned spacewalks and a fifth unplanned one. The last spacewalk was needed to restore Hubble’s thermal insulation in some areas.
The Goddard High Resolution Spectrograph and Faint Object Spectrograph were replaced during the mission by the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer, or Near Infrared Camera and Multi-Object Spectrometer (NICMOS).
The STIS spectrograph analyzes the composition, temperature, motion, and other chemical and physical properties of a target. It could simultaneously cover a group of objects, such as the Galaxy, while previous instruments were only capable of observing a single point. The device is sensitive to ultraviolet, near-infrared and other wavelengths. It can also block light from bright objects to examine dimmer targets.
The STIS spectrograph failed in August 2004 due to a power failure. It was repaired during the last mission in 2009.
Thanks to the camera and the NICMOS spectrometer, it was possible to study dust clusters, star and planet formation processes. NICMOS, in turn, consisted of three cameras for infrared imaging and spectroscopic observation. It provided a near-infrared view of the universe for the first time.
NICMOS operated until 1999. In 2002, astronauts on Mission 3B installed a cryocooler and it came back online. In 2009, the Hubble Wide Field Camera 3 (WFC3) took over its functions. NICMOS is currently not operational.
They also added the Optical Control Electronics Enhancement Kit (OCE-EK) retrofit unit. It increased the capabilities of the precision guidance sensor by electrically controlling the mechanism.
During the expedition, the astronauts installed a SSR solid-state digital recorder to replace one of the three engineering ESTR reel-to-reel recorders. Ten times more data could be stored with the new recorder. Another ESTR was replaced with a spare unit of a similar device. And during mission 3A, the reel-to-reel devices were replaced with solid state recorders.
In summary, the following components were upgraded and installed:
- STIS spectrograph;
- NICMOS camera and spectrometer;
- FGS precision pointing sensor;
- OCE-EK unit;
- SSR solid state digital recorder;
- ESTR reel-to-reel tape recorder;
- one of four RWA reaction wheels;
- DIU data processing unit;
- a second SADE solar panel electric actuator;
- BIK battery improvement kit;
- magnetometer covers;
- thermal insulation.
Maintenance mission 3A: gyroscope repair
- Period: December 19-27, 1999
- Number of spacewalks: 3 spacewalks
- Shuttle: STS-103
- Space Shuttle: Discovery
The SM3A servicing mission was originally planned for June 2000 as a preventive maintenance mission. But on November 13, 1999, the fourth of six gyroscopes failed, and Hubble went into safe mode pending repairs. The mission was decided to be conducted ahead of schedule and carry out some of the work of the third servicing program, so it was called 3A
The astronauts replaced all six gyroscopes during the first servicing mission. After that, contingency plans were developed in case the gyroscopes failed. The first to be introduced was a dual gyro mode of operation, which allowed scientific observations to continue with a minimum number of operating devices. This mode was first used in 2004, when the servicing mission was canceled due to the shuttle Columbia disaster.
, engineers developed a single gyroscope mode to keep the latter in reserve and extend the telescope’s life. During the last expedition in 2009, astronauts replaced all six gyroscopes. However, by 2024, four of them had already failed, and Hubble is operating in single gyro mode while continuing to perform science missions.
The on-board computer was also replaced with a more powerful one. The new unit uses an Intel 80486 processor with increased resistance to radiation. After that, it became possible to transfer some of the calculations previously performed on Earth, directly to the onboard system of the telescope, increasing its performance by 20 times.
Astronauts also installed six sets of voltage and temperature improvement VIK to regulate the level of charge of batteries and protection against overheating and overcharging of batteries.
As a result, the following components were upgraded and installed:
- six gyroscopes;
- an on-board computer;
- precision-guidance sensor;
- SSR solid state digital recorder;
- a VIK voltage and temperature improvement kit;
- a transmitter for sending scientific data.
Maintenance Mission 3B: battery and camera replacement
- Period: March 1-12, 2002
- Number of spacewalks: 5 spacewalks totaling more than 35 hours.
- Shuttle: STS-109
- Space Shuttle: Columbia
Maintainance Mission 3B (SM3B) increased the telescope’s science capabilities and performance.
The SA2 solar arrays were replaced with SA3 solar arrays. They were first upgraded during the first mission. The SA3 batteries are more robust, more powerful, more resistant to extreme temperatures, and a third smaller than the previous batteries. Their performance has improved by more than 20 percent. The reduced panel size contributed to lower drag and better orbit retention. The SA3s are still installed on Hubble today, in 2025.
The original Power Control Unit (PCU) was replaced with a new one that utilized the full power of the upgraded solar panels. The PCU is responsible for controlling and distributing power to other parts of the telescope through four Power Distribution Units (PDUs). The replacement procedure required a complete shutdown of Hubble, which lasted several hours and was carefully planned.
In place of the last remaining original instrument – a camera for taking pictures of dim objects – was installed a more powerful Advanced Camera for Surveys (ACS), which is 10 times more powerful than the previous one. With its help, scientists were able to start studying planets of neighboring star systems, which was previously considered unattainable.
The camera consists of three specialized channels:
- High-Resolution Channel (HRC). Designed for detailed studies of planets in neighboring stars and the inner regions of galaxies.
- Wide Field Channel (WFC). Provides a view twice as large as the previous camera, allowing you to study the distribution of galaxies and conditions on planets.
- Sun-Blocking Channel. Designed for observing objects close to bright stars.
In 2007, two of the camera’s three science channels, used to capture different types of images, failed. During the fourth mission, engineers were able to repair the most needed channel, the wide-field channel WFC, which provides high-resolution wide-angle images. However, the high-resolution HRC channel, aimed at detailed images of planets and galaxies, could not be repaired despite attempts.
The astronauts restored the NICMOS camera by installing a cryocooler for its infrared detectors. This extended its service life by several years, until 2008.
As a result, the following components were upgraded and installed:
- SA3 solar panels;
- PCU power control unit;
- an expanded ACS research chamber;
- one of the four RWA reaction wheels.
Maintenance Mission 4: Scheduled repairs and modernization
- Period: May 11-24, 2009
- Number of spacewalks: 5 spacewalks for a total of almost 37 hours.
- Shuttle: STS-125
- Space Shuttle: Atlantis
The fifth and fourth named SM4 mission was scheduled to take place in 2004, but was canceled due to the loss of the shuttle Columbia. It was then rescheduled for 2008 and postponed again due to technical problems. It finally took place in 2009 and helped prepare Hubble for years of further work.
Astronauts were able to repair one of the ACS camera channels and the STIS spectrograph, which had failed back in 2004. These were the first repairs in orbit, and they came with challenges such as frozen bolts and stripped screws.
The STIS repair tools already developed were adapted for work on the ACS, which saved a lot of time. The astronauts worked under strict time constraints, as each spacewalk required careful coordination and left no margin for error. ACS repairs included replacing four boards and a new power supply, while STIS recovery required replacing an electronic board.
The astronauts replaced the WFPC2 camera with an improved Wide Field Camera (WFC3), which has greater sensitivity. This instrument was designed to operate in the ultraviolet, visible and infrared spectra, greatly expanding the telescope’s capabilities. The WFC3 perfectly complemented the previously installed ACS, enhancing the telescope’s research capabilities.
Hubble now has two main cameras on board: the ACS and WFC3. Together, they capture images over a wide range of wavelengths and provide access to studying the most distant galaxies. Their combined operation has given scientists the opportunity to study the structure of the Universe in unprecedented detail.
The second scientific instrument installed during this mission was the Cosmic Origins Spectrograph (COS) ultraviolet spectrograph. With its help, scientists began to study the processes of formation and evolution of galaxies, to study the origin of stellar systems and features of the interstellar medium. The COS spectrograph complemented the functionality of STIS. To install it, the astronauts dismantled the COSTAR optical device, which was no longer relevant, since all the new instruments had already been designed with the original defect in mind.
The original set of six batteries was replaced with a second set that still works today.
As a result, the following components were upgraded, repaired and installed:
- WFC3 wide angle camera;
- ACS camera channel;
- STIS spectrograph;
- COS spectrograph;
- battery pack;
- gyroscopes;
- scientific instrument control and data processing unit;
- one of three precision pointing sensors;
- thermal insulation panels.
Results of five maintainance missions
The STIS spectrograph and the ACS camera were the first on-orbit repairs in human history. The development and success of such a program was the first step toward the ability to service spacecraft in outer space.
Overall, the five missions resulted in improved performance, refined design of individual parts, and enhanced Hubble’s scientific capabilities. All of the new instruments had built-in corrective optics, which eventually made COSTAR unnecessary and the astronauts were able to remove it.
The telescope was expected to be able to operate until at least 2014, but it is still collecting scientific data even after another 11 years.
During its lifetime, Hubble has made more than 1.6 million observations. Here are some of its discoveries and research.
Two of the most significant discoveries:
- Expansion of the Universe. Hubble showed that the universe is expanding at a constant acceleration – a discovery that became the basis for the study of dark energy.
- Detection of the most distant star. The telescope detected the light of the star Earendel, 12.9 billion light-years away from Earth.
Research:
- Formation of exoplanets. Telescope observes how new planetary systems nucleate.
- The Great Red Spot on Jupiter. Hubble studies the giant atmospheric vortex and also helps track the evolution of storms.
- Black Holes. The telescope studies the effects of black holes and helps find supermassive black holes at the centers of galaxies.
Applications of technology in everyday life:
- Medicine. Mirror polishing techniques used for Hubble have been adapted to create high-precision surgical instruments.
- Human Genome. Algorithms developed to analyze telescope data were used in sequencing the human genome.
- Endangered Species Monitoring. A modified star recognition algorithm helps track whale sharks by the individual pattern of their spots.
Hubble’s future
The Hubble Telescope remains a unique scientific instrument that has enabled mankind to make many discoveries. As of 2025, it is still functioning, even though the last servicing mission took place more than 15 years ago.
However, time and wear and tear take their toll. Over the years, the telescope has had various systems fail, and it has entered safe mode several times – a special mode of operation in which all scientific instruments are turned off and the telescope operates only to minimally stabilize and maintain communication with Earth. This helps prevent damage from technical malfunctions.
Since 2024, Hubble has been operating in the mode of one gyroscope, while the second remains in reserve. This approach helps extend the telescope’s lifespan into the 2030s, at least that’s what engineers envision.
Meanwhile, NASA has contracted to operate the telescope until June 30, 2026, but scientists are already discussing options for safely ending the mission. Hubble’s orbit is gradually declining, and in the future NASA plans to organize a controlled descent so that the telescope’s debris, which will not burn up in the atmosphere, will fall to a predetermined location that eliminates the threat to humans.
