Hubble’s Final Servicing Mission

Scientists predict that this last round of repairs will finally allow astronomers to realize the full potential of the orbiting telescope.

 

figureSTS-103 payload commander Steven L. Smith retrieved a power tool while standing on a mobile foot restraint during a Hubble servicing mission in 1999.

This month, if all goes well, a seven-person crew will blast off from NASA’s Kennedy Space Center in Cape Canaveral, Fla., for the fifth and final astronaut visit to the Hubble Space Telescope (HST). Astronauts from the space shuttle Atlantis will make some of the most technically challenging repairs they’ve ever attempted in low Earth orbit—but if they succeed, astronomers will have the best optical instruments flown in space.

“There’s still a lot of good science left in this old telescope,” said Preston M. Burch, the HST program manager.

According to senior HST project scientist David Leckrone, the $900 million servicing mission, officially designated STS-125, has two goals: science and maintenance. First, astronauts will install two new instruments, the Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), that take advantage of advances in detector technology since the Hubble program originated.

Second, the spacewalking team will attempt to fix the electronic systems of two other light-gathering instruments that have malfunctioned in the past few years. It will be the first time that the astronauts will attempt an in situ repair of instruments that weren’t originally designed to be fixed in microgravity. Finally, they will replace several spacecraft subsystems to extend Hubble’s lifetime for five years or more.

When they return to Earth, the space shuttle astronauts will bring home COSTAR, the original package of corrective optics that fixed the spherical aberration in Hubble’s primary mirror during the first servicing mission nearly 15 years ago. (COSTAR stands for Corrective Optical Space Telescope Axial Replacement.) Although COSTAR provided a useful solution to a vexing problem, once WFC3 and COS are installed, it simply won’t be necessary anymore—every imaging instrument aboard Hubble will have its own set of corrective optics.



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U.S. astronauts flew three subsequent missions to repair and upgrade Hubble between 1997 and 2002. Following the fatal accident of the shuttle Columbia in 2003, the space agency initially refused to send another crewed mission to Hubble, partly for safety reasons and partly to prioritize the construction of the International Space Station (ISS). After protests from both astronomers and the public and a change in NASA’s top leadership, the agency switched gears in the fall of 2006 and dropped plans for a robotic repair mission.

“When we left Hubble in 2002, I was convinced that it would be the last time I would see my friend Hubble the space telescope, and here we are planning to go back,” said John M. Grunsfeld, the lead extravehicular activity (EVA) mission specialist for STS-125.

Aboard Atlantis will be the most experienced telescope-repair crew NASA has sent to Hubble. Grunsfeld flew to HST aboard the 1999 and 2002 servicing missions, and STS-125 will be his fifth space flight. Astronaut Michael J. Massimino performed two Hubble-repair spacewalks in March 2002. Each veteran will team up with one of two rookie mission specialists, Andrew Feustel and Michael Good, for this trip’s EVAs.

Astronaut K. Megan McArthur will be in charge of grabbing Hubble with the shuttle’s robot arm and securing the aft end of the telescope to the Atlantis payload bay.

 

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New camera and spectrograph

A top priority of STS-125 is the replacement of the Wide Field/Planetary Camera 2 (WF/PC2), which was installed on the first servicing mission in 1993, with the WFC3. According to Leckrone, WFC3 will be the first Hubble wide-field camera that spans the range of wavelengths from the ultraviolet to the visible and the near infrared.

The grand-piano-shaped WFC will have about 30 times more “discovery power” than the Advanced Camera for Surveys (ACS), a malfunctioning instrument currently aboard Hubble. Discovery power is the product of a telescope’s sensitivity at a particular wavelength and its field of view on the sky. The higher the sensitivity, the faster a camera images its targets.

Thanks to advances in digital imaging since the early 1990s, the ultraviolet-visible (UVIS) channel of WFC3 will have a 16-megapixel CCD. According to WFC3 instrument scientist Randy Kimble, the near-infrared channel’s mercury cadmium telluride detector was custom-developed for the Hubble instrument.

WFC3 will look at every type of object in the universe, but it will excel at studying the formation of galaxies and stars, Kimble said. WFC3’s IR channel will view the redshifted light of distant galaxies in the early universe.

WF/PC2 has been running strong for 15 years and has not experienced any significant problems. Indeed, it has been responsible for most of Hubble’s most popular images, and it’s being replaced only to upgrade the telescope’s capabilities. WF/PC2 has its own set of corrective optics and has never relied on COSTAR, said Leckrone, who believes that WF/PC2 has never gotten enough credit for the stunning photographs that captured the public’s interest after the first HST servicing mission.

Atlantis will bring WF/PC2 and COSTAR back to Earth, and perhaps the instruments will end up in the Smithsonian or another museum.

The removal of COSTAR will provide room for the second new Hubble instrument on this servicing mission: the COS. With two channels providing spectral coverage from 115 to 320 nm, it will be the most sensitive ultraviolet spectrograph flown in orbit.

“Spectrographs put the ‘physics’ in astrophysics,” Leckrone said, and COS will be a prime example. The instrument will collect photons at near and far ultraviolet wavelengths that can’t reach telescopes under Earth’s atmosphere. Just as they might do on a ground-based telescope, controllers can select different gratings to fit the type of observations an astronomer is making.

Every time a beam of light bounces off a surface in an optical system, some light is lost. COS developers, led by principal investigator James C. Green of the University of Colorado, designed the spectrograph with as few bounces as possible for maximum throughput.

 

 

figureInside wide field camera 3

Delicate repairs and replacements

Two important Hubble instruments, the ACS and the Space Telescope Imaging Spectrograph (STIS), have suffered electronics failures in recent years and have been mothballed for the time being. For the first time in the Hubble saga, the spacewalkers face the challenge of performing delicate repair operations on instrument components that were never designed for astronauts in their bulky gloves to handle.

Fixing these devices will be tricky, because the astronauts will have to remove dozens of tiny #4 screws—and not lose any of them inside the telescope. It’s not hard to imagine how difficult it would be to try to pick up small objects while wearing thick gloves. “It still makes me laugh that we actually have to try to do the task,” Grunsfeld said at a press conference in January.

In order to access the four failed circuit boards inside ACS, the astronauts also will have to cut through an electromagnetic interference gasket, leaving some sharp edges near the area where the astronauts will be reaching in at an odd angle. “We don’t like sharp edges—we’re in balloons,” Grunsfeld said, referring to the astronauts’ pressurized spacesuits.

The day after the ACS repair, spacewalkers will tackle the STIS repair. In order to get to this spectrograph’s innards, the astronauts have to remove 111 tiny screws—a feat that required ground-based technicians to invent a new technique for corralling the potential debris.

 

The astronauts will affix a “fastener capture plate” to the electronics access panel on the outside of the STIS, then use a drill-type tool to back the screws into the capture device. When they lift off the capture plate, the screws will remain embedded in it and won’t float away.

 

figureMission preparations: STS-125 crew members John Grunsfeld, Andrew Feustel, and Mike Massimino review the Rate Sensor Unit gyro assembly to be installed on the next Hubble servicing mission.

A robotic repair mission, as envisioned shortly after the 2003 Columbia disaster, would have installed WFC3 and COS, plus replaced the gyros and the batteries by a different method, Burch said. Some of the technology designs bandied about for the fully robotic repair mission found their way into the capture plate mechanism for the STIS work.

In addition to the ACS and STIS rehabilitation, the astronauts will perform several repairs, such as the installation of six new gyroscopes, which will extend the lifetime of Hubble for at least five years. Lately the telescope has been running on two gyroscopes, though ideally it needs three to operate properly. The spacewalkers also will replace the fine-guidance sensor that was installed on the December 1999 servicing mission and has since had problems.

Several equipment bays on the telescope are starting to get too warm, and so the HST team will install a new outer blanket of multi-layer insulation over the bays to avoid degradation of the safe-mode computer and other electronics. Hubble’s operators try to keep much of the inside of the telescope at roughly room temperature, but the team gets worried when the interior reaches roughly 40° C, Leckrone said. The new outer blanket layers will promote radiative cooling. “It’s something we wanted to do on prior missions, but we never had the time,” Leckrone said.

Finally, the astronauts will add some docking hardware to the aft end of Hubble. Someday, when Hubble reaches the end of its useful lifespan, an unmanned rocket will be sent up to grab the telescope robotically and guide it to a breakup and landing somewhere over a remote stretch of ocean.

 

figureA technician works with Hubble’s new WFC3 Camera to be installed on the next servicing mission.

Once the astronauts leave

From the ground, controllers will perform a quick “aliveness test” to ensure the instruments are properly connected about 10 to 15 minutes before the astronauts close out the work site at the end of each EVA. After the shuttle returns to Earth, the ground crew at NASA will put the refurbished Hubble through a “service mission orbital verification period” lasting several months, Leckrone said.

The systemic verification doesn’t start immediately because the new instruments and subsystems need to outgas water vapor and other substances clinging to the surfaces. The controllers allow about 21 days for outgassing, said Keith Walyus, Hubble servicing mission operations manager at NASA Goddard Space Flight Center in Greenbelt, Md.

Controllers will check out and calibrate the instruments. Mirrors and gratings in the light path are equipped with remote-controlled actuators that provide side-to-side and focus motion, so that the optics can be collimated after their rough ride to orbit, said H. John Wood, a NASA Goddard optical scientist who has worked on HST for many years.

Astronomers on the Hubble team hope to make their first set of observations with the new and refurbished instruments in early December, and show the results at the next American Astronomical Society meeting in January 2009.

Preparations for the mission

Before any instrument flies up to Hubble, “we have to prove to the shuttle program that it’s not going to fall apart and damage the shuttle,” said HST deputy project manager E. Michael Kienlen Jr. At NASA Goddard, the instruments are subjected to a variety of tests that simulate the stresses of the 8-min. ride from sea level to Earth’s orbit. Engineers blast each instrument with 150 dB of sound, spin it up in a centrifuge that pulls up to 30 g’s, enclose it in a high-vacuum chamber that alternates between extremes of heat and cold, and shake it up on a device that technicians call “the rack.”

When not undergoing launch-simulating torture, the instruments and the astronauts’ tools sit in NASA Goddard’s gigantic clean room to keep dust from contaminating the optics. According to NASA officials, it’s the largest Class 10,000 clean room in the world; the “class” designates the maximum number of 0.5-µm-sized particles permitted per cubic foot of air.

Inside the clean room, technicians use two large mockups of the space telescope’s aft bays to check the mechanical fit and electrical connections of the new instruments. When the spacewalking astronauts visit NASA Goddard, they garb up, enter the clean room and handle the tools they will be using in microgravity. A crane will lift up the instruments in front of the life-size HST mockups so that the astronauts can get a feel for sliding the devices in and out of the instrument bays. (The spacewalkers also spend many practice hours in NASA’s neutral buoyancy tanks at Johnson Space Center in Houston.)

The “launch readiness date” for STS-125 is October 8, but that date could slip backward if NASA faces delays in launch-pad repairs or problems with either of the two space shuttles assigned to the mission. The launch could also be moved ahead to October 4 or 6 to give the space agency more time to send Endeavour on its November mission to the ISS.

 

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Mission logistics

The schedule for the 11-day STS-125 mission is jam-packed. Atlantis will spend its first two days in space just traveling to reach Hubble’s orbit, which, at an altitude of 569 km, is significantly higher than the shuttle’s usual destinations (for example, the ISS’s mean altitude is about 336 km).

The EVAs are tightly choreographed. According to Burch, a sixth EVA could be squeezed in just before the robot arm releases Hubble into its free-flying orbit, but only if the astronauts run into unexpected equipment problems that can’t be fixed during the five scheduled spacewalks.

The controllers at NASA Goddard’s Space Telescope Operations Control Center (STOCC) are repeatedly practicing their scripts for the spacewalks, Walyus said. STOCC operates the Hubble year round.

Since the space shuttle Columbia’s fatal re-entry in 2003, NASA has required some sort of backup plan to rescue the astronauts in the event of damage to the shuttle’s wings. On other flights, the astronauts could simply stay at the ISS until another spacecraft arrived, but Atlantis wouldn’t be able to reach the ISS orbit from Hubble’s orbit. Thus, NASA is taking the unprecedented step of readying the shuttle Endeavour on a neighboring Kennedy Space Center launch pad during STS-125 in case it needs to ferry the Atlantis astronauts home.

The astronomical benefits

Thousands of astronomers around the world have used Hubble images and spectra for their research. HST’s oversubscription rate, or the ratio of submitted to granted requests for telescope time, continues to be about five-to-one. NASA aims to keep providing them with cutting-edge scientific tools to do their job by preserving the cutting-edge nature of HST.

Once STIS is repaired, astronomers will be able to use it to study the chemical composition of the atmosphere of extrasolar planets; chemical composition and other properties of stars and the interstellar medium; the death throes of Eta Carinae, a massive and unstable star that might become a supernova; supermassive black holes at the centers of galaxies; and properties of active galactic nuclei.

WFC3 was purposely designed to complement ACS in its capabilities, and the latter will be used to study such exotic phenomena as dark matter and gravitational lenses. “The two instruments working together is a gangbusters combination,” Leckrone said.

According to Kenneth Sembach, HST project scientist at the Space Telescope Science Institute in Baltimore, Md., COS will help astronomers study how the galaxies formed out of the intergalactic matter. This matter is so thin that a cubic meter of it, according to some theories, would contain only one atom. Galaxies accrete intergalactic matter and then expel dust, gas and heavy elements back into intergalactic space.

“With just a few weeks of observing time, COS will probe more of the cosmic web than all previous Hubble spectrographs combined,” Sembach said. Spectra can tell astronomers not just the chemical composition of a distant object but also its state of matter (molecular, ionic or atomic), temperature, quantity, velocity and location (due to its redshift).

After STS-125, the refurbished Hubble will be 90 times more powerful than when it was launched, said Sandra M. Faber, an astronomer at the University of California, Santa Cruz. The telescope will capture images more quickly and be more sensitive to the faintest light from ever more distant galaxies.

“Every wavelength is a new window on the universe,” Faber said. Hubble’s working range will go from the far ultraviolet to the near infrared, about a factor-of-10 difference in wavelengths, and that is enough to show dramatically different aspects of each sky object.

We live in a wonderful time for astronomy, Leckrone said: “We can ask really profound questions and have a reasonable chance of getting answers within a reasonable time span.”

“It’s amazing to me how we’ve been able to reinvent the Hubble Space Telescope with each of these missions,” Grunsfeld said. Thanks to NASA’s manned spaceflight capability, astronomers and technicians are able to use HST the way they would operate one of the large ground-based optical telescopes in Arizona, California, Hawaii, Chile or the Canary Islands.

Added Grunsfeld, an astrophysicist by training: “The best discoveries will be the ones we make after this mission.”


Patricia Daukantas is the senior writer/editor of Optics & Photonics News.

References and Resources

>> OPN published a special issue on the first Hubble servicing mission in November 1993 (Vol. 4, No. 11). Other past coverage of the space telescope in this magazine:

  • A. Boggess and D.S. Leckrone. “The History and Promise of the Hubble Space Telescope,” Opt. Photon. News 1(3), 9 (March 1990).
  • E. Cheng et al. “Second HST Servicing Mission: Expanding the New Frontier,” Opt. Photon. News 8(1), 23 (January 1997).
  • H. J. Wood. “Hubble Space Telescope: Mission Update,” Opt. Photon. News 5(8), 8 (August 1994).

>> In addition, Applied Optics devoted its issue of April 1, 1993 (Vol. 32, No. 10), to the error correction of the Hubble mirror and other new optical technologies for space science.
>> NASA maintains a number of Web sites on HST and the 2008 servicing mission, including hubble.nasa.gov and sm4.gsfc.nasa.gov. Hubblesite.org is the general-interest portal to thousands of HST’s best images.

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Hubble’s Final Servicing Mission

Scientists predict that this last round of repairs will finally allow astronomers to realize the full potential of the orbiting telescope.

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