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Preparing for One Last Visit to Hubble
STS-125 mission patch. Small picture of the Hubble and the shuttle with a cone of galaxies that originates at the open end of the Hubble.STS-125 mission patch Credit:NASA
When astronauts visit the Hubble Space Telescope later this year, they will perform history-making, onorbit “surgery” on two important science instruments aboard the telescope. With the Space Telescope Imaging Spectrograph (STIS) and Advanced Camera for Surveys (ACS) still in place in Hubble, spacewalkers will—for the first time ever—attempt to repair an instrument on orbit. In this case, they’ll be repairing two, and neither was designed to be fixed in space.

Because neither instrument was designed to be fixed on orbit, neither has astronaut-friendly features. Hubble engineers and the astronauts worked diligently to design special tools, crew aids and procedures to accommodate this situation.

“The repair of STIS, and of ACS in particular, involves techniques that the astronauts have never done before on Hubble, possibly never before anywhere,” explained Dr. Dave Leckrone, Senior Project Scientist for Hubble Space Telescope. “That is, to open up an instrument that was not designed to be opened up and actually pull out electronic printed circuit boards and replace them with new boards.”

Work is progressing well as the Hubble Team prepares for this final service call. The 11-day mission, which will leave the telescope more capable than ever before, includes five spacewalks. In addition to the attempted repair of STIS and ACS, spacewalking astronauts will install the powerful new Wide Field Camera 3 (WFC3) and Cosmic Origins Spectrograph (COS), replace a fine guidance sensor, all six batteries, and all six of the telescope’s gyroscopes, add new thermal coverings, and install a soft capture mechanism on Hubble’s aft bulkhead.

image of the hubble attached to the space shuttleHubble will be retrieved from its orbit by the Shuttle’s robotic arm and placed in the payload bay for servicing.  Credit: NASA
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“This is the granddaddy of everything we’ve learned over 25 years, and we’re putting it all together into one great mission,” explained Frank Cepollina, Deputy Associate Director of the Hubble Space Telescope Development Project.

Crew Preparations

During the mission, the Space Shuttle Atlantis and her seven crew members will rendezvous on orbit with the Hubble Space Telescope, capture it with the Shuttle’s robot arm, and place it in the Shuttle’s payload bay. Here it will be serviced by two teams of spacewalking astronauts during five planned spacewalks.

The SM4 astronaut crew includes Hubble veterans and first time flyers. Commander Scott “Scooter” Altman (Captain, USN) is a veteran of three previous flights, including serving as Commander on STS-109, Hubble Servicing Mission 3B in 2002. Pilot Gregory C. “Ray J.” Johnson (Captain, USNRC) will be making his first spaceflight. This is also the first flight for Shuttle Arm Operator Dr. K. Megan McArthur. Payload Commander and Extravehicluar Activity (EVA) Astronaut Dr. John Grunsfeld will be visiting Hubble for the third time on his fifth spaceflight. EVA Astronauts Dr. Andrew Feustel and Michael Good (Colonel, USAF) will be making their first flight. This is the second trip to Hubble for EVA Astronaut Dr. Michael Massimino, as well as his second spaceflight.

Training is on schedule for this complex mission. In addition to practicing down at Johnson Space Center (JSC), the astronauts also spend time at Goddard working with the actual flight instruments, tools, and other hardware and the high-fidelity Hubble simulators. As of May 1, 2008, the SM4 crew will have spent their fourth intensive crew familiarization, or “crew fam” at Goddard working closely with the Hubble Team.

Heaviest Hubble Servicing Mission

“Servicing Mission 4 will be the heaviest servicing mission to date,” explained Preston Burch, Associate Director of the Astrophysics Projects Division and Program Manager for Hubble Space Telescope. “It will be carrying approximately 22,000 pounds of hardware onboard. We’ll be using four carriers inside the Shuttle cargo bay to carry all the new science instruments, replacement hardware, tools for the astronauts, and to attach Hubble to the Shuttle while the astronauts are working on it.” In previous missions, only three carriers were needed. One of the four SM4 carriers utilizes an advanced design and composite materials to save weight so Atlantis can carry more to orbit.

Servicing Mission 4 is actually the fifth visit to Hubble. The First Servicing Mission took place in December 1993, the Second Servicing Mission in February 1997, Servicing Mission 3A in December 1999, and Servicing Mission 3B in March 2002. (NASA split Servicing Mission 3 due to a critical need to replace gyroscopes in 1999.)

The following sections describe the tasks for SM4, as well as the hardware’s status to date:

Wide Field Camera 3 (WFC3)

WFC3 will study early and distant galaxies that are currently beyond Hubble’s reach, as well as galaxies in our cosmic neighborhood. This powerful instrument will help astronomers understand more about galactic evolution and star formation, unlock secrets about the planets in our solar system, and probe the mysteries of dark energy. WFC3’s key feature is its ability to span the electromagnetic spectrum from the near ultraviolet (NUV), through the optical and into the near infrared (NIR). WFC3 is the only Hubble instrument with this panchromatic capability.

This next-generation imaging instrument builds on the capabilities of its predecessors, Wide Field and Planetary Cameras 1 and 2 (WFPC1 and 2), as well as Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and Advanced Camera for Surveys (ACS). WFC3 is superior to WFPC2 in resolution and field-of-view. Its “UVIS” detector—sensitive to near ultraviolet and optical light—will provide a 35 times improvement in discovery efficiency (the product of the field of view multiplied by the optical throughput) in near ultraviolet and blue light over ACS. Its near-infrared detector will provide a 15 to 20 times improvement in discovery efficiency over NICMOS.

WFC3, a Goddard in-house instrument, has just completed its third and final full-up thermal vacuum test. Already in flight configuration, it has beaten the specifications in all cases and there are no liens against the instrument. The WFC3 Team is preparing for “Crew Fam 4,”and will then deliver the instrument to the Project.

Cosmic Origins Spectrograph (COS)

COS will study the large-scale structure of the universe and how galaxies, stars, and planets formed and evolved. It will also help determine how elements such as carbon and iron, which are needed for life, first formed. One primary science objective is to measure the structure and composition of the ordinary matter that is concentrated in what scientists call the "cosmic web"—long, narrow filaments of galaxies and intergalactic gas separated by huge voids.

The instrument has two channels, the far ultraviolet (FUV) and the near ultraviolet (NUV). A key feature of COS—the one which makes it unique among Hubble spectrographs—is its maximized efficiency, or “throughput.” Hubble’s other spectrograph, STIS, which was installed in 1997 during Servicing Mission 2, is highly complementary to COS in its capabilities. STIS is a highly versatile, “all purpose” spectrograph. By design, the COS does not duplicate all of STIS’s capabilities, but by having more than 30 times the sensitivity of STIS for FUV observations of faint objects such as distant quasars, COS will enable key scientific programs which would not be possible with STIS. If the STIS repair is successful, the two spectrographs working together will provide a full set of spectroscopic tools for astrophysical research.

COS, which is primarily a Ball Aerospace instrument, has completed all of its major tests and is ready to ship to Kennedy Space Center (KSC) in preparation for launch.

STIS: Repairing a Black Hole Hunter

Astronauts installed STIS on Hubble in 1997 during Servicing Mission 2. Its main function is spectroscopy—the separation of light into its component colors, or wavelengths, to reveal information about the chemical content, temperature and motion of stars and gas. STIS’s many accomplishments include confirming the existence of supermassive black holes, surveying and weighing supermassive black holes at the centers of galaxies, and being the first instrument to detect and analyze the atmosphere of a planet orbiting another star. STIS performed brilliantly until a power supply failure in 2004 caused it to stop working. The spacewalkers will replace a low voltage power supply board that contains a failed power converter. They will first remove four screws to install a see-through “capture plate” over the top of a STIS electronics access panel. This plate will hold an additional 107 tiny screws that attach the panel to STIS. By capturing the small screws in the plate, astronauts avoid having to handle them through bulky gloves.

The astronauts will use a specially made, miniature power tool to remove the screws. After detaching the panel (with the attached capture plate holding the screws), they will remove the failed power supply card. They will then insert the new one, much like replacing a computer circuit board. Finally, they will replace the existing panel with a new, simplified version not requiring any screws. Instead, two lever-like latches will lock the new panel into place. Preparations for this repair are going well, and engineers expect it to return STIS to its mission of exploration.

ACS: Restoring Hubble’s Workhorse Camera

Installed during Servicing Mission 3B in 2002, the Advanced Camera for Surveys quickly became Hubble’s workhorse camera and was responsible for many of the most popular and dramatic images over the last few years. However, the science instrument recently experienced two separate power failures, one on each of its two redundant sides of electronics.

The first failure, in 2006 on Side 1, rendered two of the camera’s three channels—the Wide Field Channel and the High Resolution Channel—unusable on that side. Only the circuitry of the Solar Blind Channel, which images ultraviolet light, was unaffected. Ground controllers switched to Side 2, but when a fuse blew on Side 2 in 2007, all three channels on that side failed, and on-orbit repair became necessary.

With less than 18 months to launch, a repair concept was rapidly conceived and developed. The list of tasks for the Hubble Servicing Mission’s five spacewalks was already jampacked, so any new tasks would have to somehow fit into the existing schedule. The team quickly determined that there was no time in the EVA plan to perform the work necessary to access the Side 2 electronics. Instead, they focused on Side 1, analyzing the ways the power supply could have failed and developing a method of bypassing suspect subsystems and components.

The team agreed that while the failure was likely confined to the low-voltage power supply, a direct repair of that subsystem would require too much of the astronauts’ spacewalking time. It would also run the risk of not fixing everything that was damaged. Instead, the team devised an ingenious “bypass” solution: Astronauts will remove the four boards that drive the Wide Field Channel detectors by going into a small panel outside of the instrument. They will then replace these boards with a new device that performs the same function but takes power from an externally mounted box attached to ACS’s existing main power supply.

The only way to supply external power to the boards is by removing and replacing them entirely. The team realized that replacing the boards individually is too time consuming, so they devised a cartridge that holds all four boards, allowing them to be mated with a single action.

While formulating the bypass solution, engineers realized that the wires that power the old Wide Field Channel electronics are also connected to the High Resolution Channel electronics. By applying power to these wires, the High Resolution Channel electronics could be powered without direct astronaut manipulation. The Wide Field and High Resolution channels, along with the still operable Solar Blind Channel, will provide a fully functioning Side 1.

As in the STIS task, astronauts will use the “capture plate” to hold screws—in this case, 32—that come from the access panel. And again, astronauts will replace the old panel with one that has two lever-like latches to hold it in place. One side benefit of the ACS repair is lower power usage. Whereas the WFC detector electronics previously consumed 21 Watts, the newly repaired WFC will use approximately 9 Watts. If the repair goes as planned, the noise level of the detectors could also be reduced.

Preparations for this task continue. The development program went very well and all technical issues have been resolved. High fidelity engineering units are now running and meeting all the requirements and goals established for that hardware. Fabrication of the flight hardware has begun, as has assembly of the cards for Flight Units 1 and 2. The replacement Low Voltage Power Supply (LVPS-R) has completed final qualification testing and is now at Goddard.

“There are only three words that can best describe the efforts of the ACS Repair Team,” explained Cepollina. “Blood, sweat, and tears. To do what they’ve done—and they’re almost there now, they’re almost home, ready for the final flight acceptance test program—but in order to do it they’ve had to overcome heartache after heartache after heartache in terms of failures, problems, issues, schedule challenges, only to be invigorated after each of those kinds of things by successes.”


Spacewalking astronauts will replace all six of Hubble’s original 125-pound nickel hydrogen batteries, which provide electrical power to Hubble during its nighttime to support the telescope’s functions. Now 18 years into the mission, Hubble’s nickel hydrogen batteries have lasted more than 13 years longer than their design orbital life—longer than those in any other low Earth orbit spacecraft. This was possible partly because the batteries are built to very exacting standards using an extremely robust design.

Another reason for the batteries’ longevity is the careful, daily, on-orbit management by Electrical Power System engineers at Goddard to ensure long-term on-orbit performance. However, due to aging and cycling, the batteries are showing a slow loss in capacity. If not replaced, they will eventually be unable to support Hubble’s science mission during the orbit night. The replacement batteries are also nickel hydrogen, but they are superior to the old ones in several ways. The new batteries are made using a process called wet slurry, which makes them physically stronger and better performing than the dry sinter batteries they replace. Each new battery also has the added safety feature of a battery isolation switch that electrically dead faces each connector. “Dead face” means no electrical power is present at the connectors while the switch is in the “off” position. This creates a safer environment for astronauts installing the battery modules.

The flight batteries are at Goddard and have already undergone performance testing, as well as pre-launch testing and verification. The batteries meet specifications and are in great shape.


Hubble’s six gyroscopes are packaged in pairs within three Rate Sensor Units (RSUs). They are part of the system that allows it to point at stars, planets, and other celestial targets. All of the current gyros were installed in December 1999, and all are approaching the end of their limited lifetimes. The telescope was originally designed to use three at a time, with the other three held as spares.

After thorough analysis and testing, engineers determined that Hubble could conduct science on two gyros. With new control modes added to Hubble’s main computer, and major changes made to Hubble’s planning and scheduling system, two-gyro operations began in 2005. Currently, three gyros are still operational. Two gyros are in use, and the third is turned off and is being held in reserve. Astronauts will install a fresh set of six new gyros during Servicing Mission 4 to keep the telescope in peak condition through 2013.

The team has completed all testing on the three flight RSUs and one flight spare, and all are ready to fly. One interesting tidbit is that RSU 1004 has always been considered the “hangar queen”—it has flown as a spare on SM1, SM3A and SM4, but it has never been installed. Now, because the team has rebuilt it and it has new flex leads in it, we will install the hangar queen.

Fine Guidance Sensor (FGS)

Along with the gyroscopes, the FGSs are part of Hubble’s pointing control system. The FGSs and gyroscopes together produce extraordinary stability—0.007 arcseconds of “jitter”—which is like holding a laser beam on a dime 350 miles away. The FGSs also provide capability for astrometry, the detailed study of stellar dynamics and motions, enabling the detection of close binary stars and star-planet systems.

Over the past servicing missions, astronauts have been replacing Hubble’s three FGSs with refurbished units one at a time in “round robin” fashion. A refurbished unit returned from the 1999 mission will replace FGS 2 on SM4. Only two units are needed to point Hubble; the third FGS provides additional target pointing efficiency and redundancy.

The Hubble Team has completed all testing, and the FGS is ready to fly.

Soft Capture Rendezvous System

To prepare for the end of Hubble’s life, engineers developed the Soft Capture and Rendezvous System, which will enable the future rendezvous, capture, and safe disposal of the telescope. The Soft Capture and Rendezvous System is comprised of the Soft Capture Mechanism (SCM) system and the Relative Navigation Sensor (RNS) system.

The SCM is a ring-like device that attaches to Hubble’s aft bulkhead. It provides a Low Impact Docking System (LIDS) interface and associated relative navigation targets for future rendezvous, capture, and docking operations. The SCM will launch attached to the turntable-like Flight Support System (FSS), which serves as the berthing platform for Hubble and provides all electrical interfaces between the Shuttle and the telescope while Hubble is docked. About 72 inches in diameter and 2 feet high, the SCM will sit inside the FSS berthing and positioning ring without affecting the normal FSS-to-Hubble interfaces. It will be fitted onto the telescope by three sets of jaws, which will be controlled by the astronauts.

The RNS system consists of optical and navigation sensors, as well as supporting avionics and processors. It will collect data on Hubble during capture and deployment. This information will be used for developing the navigation systems of the spacecraft that will de-orbit Hubble when the telescope reaches the end of its useful life.

All of the preliminary tests for the SCM engineering unit hardware went well. The flight unit has been delivered and has been integrated onto the FSS.

IMAX Camera

The IMAX community approached NASA with a desire to fly a camera on SM4. IMAX cameras have been used on previous Hubble missions. Now, the IMAX people are making a new movie about Hubble servicing and currently targeting 2010 for release. It will use the footage from the previous missions along with new footage shot during this mission. The IMAX folks are also working with the Space Telescope Science Institute to make certain Hubble images are three dimensional, to give the effect of flying through the Universe. They’re planning to do specific observations with Hubble for the purpose of creating images to be used in the IMAX movie.

The IMAX camera is going to fly on the ORUC carrier and look up at Hubble. This unique perspective has never been shot before. This time, there will be no IMAX hand-held camera inside the crew cabin, as in previous missions. The only IMAX camera will be the one on ORUC, and it will carry three lenses for versatility. “Our crew will be the producers on orbit when it comes time to take the pictures,” explained Michael Kienlen, Deputy Project Manager for the Hubble Space Telescope Development Project.


The Flight Support System (FSS) completed its system level thermal vacuum test and carrierlevel EMI test, and will soon undergo acoustics testing. Although the FSS has flown on every Hubble mission, Kienlen explained, “FSS is a new carrier for SM4. A lot of effort went into to getting the FSS ready for this flight. We have new or rebuilt motors, a completely recertified electrical system, and all the mechanisms have been taken apart and rebuilt.”

The Orbital Replacement Unit Carrier (ORUC) will hold COS and an IMAX camera, which will document the mission for a future film on Hubble. The ORUC is in the process of electrical integration, and the majority of its components mechanically integrated. Because of the late addition of the IMAX camera, some significant structural changes had to be made to the carrier, and that engineering is complete. The hardware to hold the IMAX camera is at Goddard and testing is complete. No major tests are left except for supporting the acoustics test for COS, and possibly a stand-alone acoustics test for ORUC.

The Super Lightweight Interchangeable Carrier (SLIC) will contain the WFC3 and Hubble’s new batteries. SLIC is the first all-composite carrier ever to fly in the Manned Space Flight Program. All structural testing is complete, and electromagnetic interference (EMI) and acoustics tests will be conducted soon.

The Multi-Use Logistic Equipment Carrier (MULE) , which will hold contingency hardware, is undergoing thermal vacuum testing in May to validate the carrier and the RNS hardware. Afterwards, the team will conduct EMI testing on this carrier.

Crew Aids and Tools

The Hubble Team is flying significantly more crew aids and tools for this mission than ever before. More than 60 Hubble engineers are working on tool development right now. Every servicing mission continues to advance the technology, developing more complex tools that enable the astronauts to accomplish increasingly more difficult tasks. The ACS and STIS repairs are prime examples.

Two specialized tools developed for this mission are the Mini Power Tool and the Fastener Capture Plates. The Mini Power Tool is a small, high speed, low torque power driver that astronauts will use on all of the fasteners described in the ACS and STIS repair tasks. The tool’s low torque is an advantage, because higher torque risks breaking a fastener. The high speed is also essential. “Speed is very crucial for getting the STIS task in the box in the time frame that we’re allocated, explains EVA Tool Engineer Justin Cassidy. “So, in this case faster is better.”

Astronauts working on the Hubble in space while Hubble is attached to the space shuttle.Astronauts will perform the fifth and final visit to Hubble
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The Fastener Capture Plates, which will also be used for the STIS and ACS repair tasks, are transparent plates that capture the many, tiny fasteners as they are removed from the instruments’ covers. These ingenious plates will prevent the fasteners from floating away, and they will also preclude the need for astronauts to handle very small fasteners with bulky EVA gloves.

At the Apex

The improvements of SM4 will add about five extra years of science to Hubble’s mission. They will also provide a full toolkit of cutting edge research tools to astronomers around the world. “Personally, I think that’s where the more exciting results will come from after the servicing mission,” explained Leckrone. “What will ‘blow us all away’, I predict, will be the unexpected discoveries that come from the completely new ideas that astronomers will have about how to use this wonderful set of cutting-edge tools. After SM4 our instrument “toolbox” will be complete for the first time since 1993.”

“That just demonstrates the role and the value of having astronaut servicing of a spacecraft like this, that we can keep it fresh and up-to-date and basically it becomes a new observatory each time it’s serviced,” Leckrone added. “At the end of Servicing Mission 4, when the astronauts leave Hubble for the last time, we have a very good prospect that Hubble will be at the apex of its capabilities. It will be better than it’s ever been before, which is quite awesome when you realize that it will be over eighteen years old as an observatory,” Leckrone said.

“Servicing Mission 4 marks the apex of not only the scientific capabilities of Hubble, but the apex of the capabilities of NASA,” added Burch. “In addition to making enormous gains in our understanding of the universe we live in, we have learned a lot in the areas of technology development, engineering, and management. The long duration of the Hubble mission, and its symbiotic relationship to the human spaceflight program, has taught us much about how to build better spaceflight systems and components, how to best utilize human capabilities in space, and how to work together with multiple NASA Centers and with industry and academia.”

“Today, those of us who have been privileged to work on this great mission are standing on the shoulders of the giants who preceded us, who envisioned the Hubble mission and built this program over several decades,” concluded Burch.

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Ann Jenkins/SGT 442
Hubble Space Telescope Development Project