Making Hubble Even Better

The Hubble Space Telescope is both a national asset and a complex machine, so NASA astronauts visit it regularly to keep it running smoothly and extend its life. On-orbit servicing ensures that this unique scientific resource will continue making exciting discoveries in our exploration of the universe.

Shuttle astronauts visit the Hubble Space Telescope approximately every three years. During these service calls they replace gyroscopes, electronic boxes, and other limited-life items and install state-of-the-art science instruments — creating, essentially, a more capable observatory.

Because the Hubble Space Telescope was designed for periodic servicing, the items to be replaced are easily accessible. Ranging in size from a shoebox to a telephone booth, most of these items can be removed or installed using special wrenches and power tools.

Servicing Mission 3B

Better Than Ever

On March 1, 2002, NASA launched the space shuttle Columbia into an orbit 360 miles above Earth, where its seven-member crew met with the Hubble Space Telescope to perform a series of upgrades. Servicing Mission 3B, also known as STS-109, was the fourth visit to Hubble. NASA split the original Servicing Mission 3 into two parts and conducted the first part — Servicing Mission 3A — in December 1999.

A Bigger, Sharper View

The highly trained astronauts performed five spacewalks. Their principal task was to install a new science instrument called the Advanced Camera for Surveys, or ACS. The first new instrument to be installed in Hubble since 1997, ACS brought the nearly 12-year-old telescope into the 21st century. With its wide field of view, sharp image quality, and enhanced sensitivity, ACS doubled Hubble's field of view and collects data ten times faster than the Wide Field and Planetary Camera 2, the telescope's earlier surveying instrument.

A More Efficient Power System

Hubble gets its power from four large flexible Solar Array Panels. The 8-year-old panels were replaced with smaller, rigid ones that produce 30 percent more power. Astronauts also replaced the outdated Power Control Unit, which distributes electricity from the solar arrays and batteries to other parts of the telescope. Replacing the original unit, which has been on the job for nearly 12 years, required the telescope to be completely powered down for the first time since its launch in 1990.

Restoring Hubble's Infrared Vision

During the last spacewalk, astronauts installed a new cooling system for the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS, which became inactive in 1999 when it depleted the 230-pound block of nitrogen ice that had cooled it since 1997. The new refrigeration system, which works much like a household refrigerator, chills NICMOS's infrared detectors to below —315º F (—193º C).

New Steering Equipment

Astronauts replaced one of the four Reaction Wheel Assemblies that make up Hubble's Pointing Control System. Flight software commands the reaction wheels to "steer" the telescope by spinning in one direction, which causes Hubble to spin in the other direction.

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Servicing Mission 3A

Long Distance House Call

On December 19, 1999, seven astronauts boarded the space shuttle Discovery to pay the Hubble Space Telescope a special holiday visit. After a successful launch and several trips around Earth, the crew caught up with Hubble and hauled it into the shuttle's cargo bay. Six days and three 6-hour spacewalks later, the crew had successfully completed Part A of the two-part Third Servicing Mission, which had them replacing worn or outdated equipment and performing several critical maintenance upgrades.

Servicing Mission 3A (STS-103) was a busy one. The most pressing task was the replacement of gyroscopes, which accurately point the telescope at celestial targets. The crew, two of whom were Hubble repair veterans, replaced all six gyroscopes — as well as one of Hubble's three Fine Guidance Sensors (which allow fine pointing and keep Hubble stable during observations) and a transmitter.

The astronauts also installed an advanced central computer, a digital data recorder, an electronics enhancement kit, battery improvement kits, and new outer layers of thermal protection. Hubble was as good as new.

The Discovery crew deployed Hubble back into orbit on Christmas Day and returned to Earth, landing safely on December 27. Once the mission ended, scientists "turned on" the telescope's science instruments, which were taken offline during the servicing mission, and took "test" pictures to make sure the instruments were functioning properly.

Why the Rush?

NASA decided to split the Third Servicing Mission (SM3) into two parts, SM3A and SM3B, after the third of Hubble's six gyroscopes failed (Hubble needs three gyroscopes to observe a target). The second part of the mission, SM3B, took place March 1-12, 2002.

On November 13, 1999, the Hubble Space Telescope was placed into "safe mode" after the failure of a fourth gyroscope. In safe mode Hubble could not observe targets, but its safety was preserved. This protective mode allows ground control of the telescope, but with only two gyros working, Hubble cannot be aimed with the precision necessary for scientific observations of the sky. Controllers closed the aperture door to protect the optics and aligned the spacecraft to ensure that Hubble's solar panels would receive adequate power from the sun.

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Servicing Mission 2

Expanding Hubble's Universe

There is no question that Hubble's "first generation" cameras gave us remarkable views of very distant galaxies. However the light from the most distant galaxies is shifted to infrared wavelengths by the expanding universe. To see these galaxies, Hubble needed to be fitted with an instrument that could observe infrared light.

During the 10-day Second Servicing Mission (STS-82) in February 1997, the seven astronauts aboard the space shuttle Discovery installed two technologically advanced instruments. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) would be able to observe the universe in the infrared wavelengths. The second instrument — the versatile Space Telescope Imaging Spectrograph (STIS) — would be used to take detailed pictures of celestial objects and to hunt for black holes.

Both instruments had optics that corrected for the flawed primary mirror. In addition, they featured technology that wasn't available when scientists designed and built the original Hubble instruments in the late 1970s — and opened up a broader viewing window for Hubble.

The new instruments replaced the Goddard High Resolution Spectrograph and the Faint Object Spectrograph.

Also installed during the Second Servicing Mission were:

• A refurbished Fine Guidance Sensor — one of three essential instruments used to provide pointing information for the spacecraft, to keep it pointing on target, and to calculate celestial distances
• A Solid State Recorder (SSR) to replace one of Hubble's data recorders (An SSR is more flexible and can store 10 times more data)
• A refurbished, spare Reaction Wheel Assembly — part of the Pointing Control Subsystem.

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Servicing Mission 1

Restoring Hubble's Vision

As the first in a series of planned visits to the orbiting Hubble Space Telescope, the First Servicing Mission (STS-61) in December 1993 had a lot to prove and a lot to do. The mission's most important objective was to install two devices to fix Hubble's vision problem. Because Hubble's primary mirror was incorrectly shaped, the telescope could not focus all the light from an object to a single sharp point. Instead, it saw a fuzzy halo around objects it observed.

Once astronauts from the space shuttle Endeavour caught up with the orbiting telescope, they hauled it into the shuttle's cargo bay and spent five days tuning it up. They installed two new devices — the Wide Field and Planetary Camera 2 (WFPC2) and the Corrective Optics Space Telescope Axial Replacement (COSTAR). Both WFPC2 and the COSTAR apparatus were designed to compensate for the primary mirror's incorrect shape.

Once Hubble received its corrective "eyeglasses," it began seeing more clearly.

Also installed during the First Servicing Mission were:

• New solar arrays to reduce the "jitter" caused by excessive flexing of the solar panels during the telescope's orbital transition from cold darkness into warm daylight
• New gyroscopes to help point and track the telescope, along with fuse plugs and electronic units.

This successful mission not only improved Hubble's vision — which led to a string of remarkable discoveries in a very short time — but it also validated the effectiveness of on-orbit servicing.

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